Plants

The plant kingdom is eukaryotic, multicellular, autotrophic and usually live fixed to the substrate and with conductor vessels

In biology, plants refer to organisms that are mostly photosynthetic without locomotor capacity (that is, without the ability to move, although they can move) whose cell walls are composed mainly of cellulose. Taxonomically they are grouped in the kingdom Plantae and, as such, constitute a eukaryotic monophyletic group made up of terrestrial plants and the algae that are related to them; however, there is no agreement among the authors on the exact delimitation of this kingdom. The branch of biology that studies plants is botany; also known as phytology.

In its more restricted circumscription, the kingdom Plantae (from the Latin: plantae, "plants") refers to the group of land plants, which are photosynthetic multicellular eukaryotic organisms, descendants of the first green algae that managed to colonize the earth's surface and are what we most commonly call "plant" or "vegetable". In its broadest circumscription, it refers to the descendants of the Primoplantae, which involves the appearance of the first photosynthetic eukaryotic organism by acquisition of the first chloroplasts.

They obtain energy from sunlight, which they capture through the chlorophyll present in the chloroplasts, and with it they carry out photosynthesis, through which they convert simple inorganic substances into complex organic matter. As a result of photosynthesis, they release oxygen (although, like animals, they also need it to breathe). They also explore the environment around them (especially through roots) to absorb other essential nutrients used to build, from the products of photosynthesis, other molecules that they need to survive.

Plants have alternation of generations determined by a haplodiplontic life cycle (the ovule and antherozoid develop asexually until they are multicellular, although in many plants they are small and masked by structures of the diplontic stage).

In general, land plants as we normally recognize them are just the diplontic stage of their life cycle. In their diplonte stage, plants present cells of plant cell type (mainly with a rigid cell wall and chloroplasts where photosynthesis occurs), their cells being grouped into tissues and organs with work specialization. The organs that they can possess are, for example, the root, the stem and the leaves (or similar structures), and in some groups, flowers and fruits.

The importance that plants have for humans is indisputable. Without them we could not live, since plants participated in the composition of the gases present in the Earth's atmosphere and in ecosystems, and are the primary source of food for heterotrophic organisms. In addition, plants are directly important to man: as a source of food; such as construction materials, firewood and paper; as ornamentals; as substances that aggravate or improve health and therefore have medical importance; and as a consequence of the latter, as a raw material for the pharmacological industry.

Role of plants as photosynthesizers

Light energy and carbon dioxide are first taken by plants to make their food (sugars) through photosynthesis. That is why plants are considered the producers of an ecosystem.

Circumscriptions of Plantae

The circumscription of the Plantae kingdom and the definition of plant changed over time, although the definition always included terrestrial plants, the most important for humans and the most studied group.

Etymology

The Neo-Latin term 'Plantae' and the Spanish 'plantas' They derive from the Latin planta (shoot, shoot), plantare (to stay where the soles of the feet touch the ground). From this point of view, we could call any being planted on the ground or in a substrate a plant; however, we can see exceptions in some bryophytes and in the angiosperm duckweed, which are not fixed but we consider them plants.

On the other hand, the modern concept of plant comes from Haeckel's classification system, who discarded the old plant kingdom, creating a new kingdom Plantae with a more natural sense and with its common characteristics, so this new kingdom it does not admit as members other organisms that are not related to terrestrial plants, as is the case with bacteria, fungi and protists in general.

Concepts

There are several concepts that define plants, some are polyphyletic and others are monophyletic. There are two polyphyletic concepts: First, that of the plant as equivalent to the ancient vegetable kingdom where terrestrial plants were grouped with algae and fungi (Jussieu 1774); and the second concept is from Haeckel and Whittaker, also called Metaphyta by other authors and which grouped terrestrial plants with multicellular algae (green, red and brown).

There are three monophyletic concepts:

The different concepts about Plantae (equivalence in bold) can be summarized in the following cladogram:

Finally, it can be said that there are times when "plant" It has a different meaning from the ones described here, when it is so it should be defined at the beginning of the text. For example, it could mean "eukaryote with chloroplasts", "eukaryote that performs photosynthesis", and other meanings.

History

From ancient Greece: the plant kingdom

The term plant (regnum Vegetabilia) has a very broad definition and is from the time when organisms were only divided into animals and plants, this definition derives from the ancient Greeks and was kept in textbooks until beyond the mid-XX century. In its broadest circumscription, Vegetabilia includes many clades of unrelated organisms, which almost do not have any character in common except for the fact that they do not have mobility, which is why they basically grouped together terrestrial plants, fungi and algae.

Etymologically, vegetable comes from the Latin vegetare (to grow), and is traditionally defined as living beings without movement, that is, all those that are not animals. This wide circumscription was part of the beginnings of the science of Botany. Linnaeus adopts it in his system of three kingdoms (animal, vegetable and mineral), defining plants because they grow, but do not feel or move. That classification endured for a long time in our culture. As a result of the invention of the microscope, microorganisms were discovered, initially considering those endowed with movement as animals and those without movement as vegetables. In 1875 Cohn included bacteria within the plant kingdom under the name of Schizophyta.

Vegetables are still considered today, since they are the ones that define the limits of the study of the science of Botany, and they are used in the scientific field only in that sense, although today, prokaryotes, only cyanobacteria are studied because they are similar to the photosynthetic ancestors of chloroplasts, and those photosynthetic protists (which may look like animals or plants) that entered the broad definition of algae that is used today (algae can be in his own Phycology department); In addition, fungi are studied (today more and more in their own department of Mycology), and terrestrial plants, the most studied and most important group for humans.

Haeckel: the kingdom of plants

By the 19th century, the division into just two biological kingdoms: animal and plant, was no longer satisfactory for encompassing to all known organisms. Microorganisms could not be clearly classified as animals or plants, so Owen proposed the kingdom Protozoa in 1858 and Hogg proposed the kingdom Protista in 1860. Haeckel, on the other hand, proposed in 1866 to divide the plant kingdom into two new kingdoms: Protista and Plantae, grouping unicellular microorganisms such as microalgae, protozoa and bacteria under Protista, and multicellular microorganisms such as land plants under Plantae., multicellular algae and fungi.

However, in successive publications, Haeckel made corrections to his classifications: he determined that fungi could not belong to the kingdom Plantae and placed them in Protista, while blue-green algae that appeared to be multicellular like Nostoc went to Protista along with bacteria and unicellular green algae like volvocales, he took them from Protista to Plantae. Thus, although Haeckel began by simply distinguishing between multicellular and unicellular beings, he later progressed to a more coherent, monophyletic classification, closer to what is today we know as plants.

Copeland: green plants

When Herbert Copeland postulated his system of four kingdoms, he defined Plantae or Metaphyta as organisms whose cells contain bright green chloroplasts, which in turn contain pigments such as chlorophyll a, chlorophyll b, carotene and xanthophyll, and not others; and that produce substances such as sucrose, starch, and cellulose. This definition is equivalent to the clade Viridiplantae (green plants), which groups land plants and green algae.

Whittaker: multicellularity

A circumscription that was very successful in textbooks was given by Robert Whittaker (1969), a classification whose outlines already appeared in previous publications (such as Whittaker 1959) and who divided life into five kingdoms: Plantae, Monera, Fungi, Protista and Animalia. In this classification, Whittaker grouped into Plantae all groups that had multicellular photosynthetic members: green plants (land plants and green algae), red algae, and brown algae (see description in Plantae after Whittaker, 1969). Whittaker also did not believe that these three groups of multicellular autotrophs were particularly related to each other, but he grouped them into the Plantae because they were multicellular with the same mode of nutrition.

Whittaker defines the kingdom Plantae as multicellular organisms with eukaryotic cells with cell walls and often vacuolates, with photosynthetic pigments in plastids, together with closely related organisms that lack pigments or are unicellular, or syncytial (multinucleate). They are mainly photosynthetic and immobile, anchored to a substrate. They have structural differentiation leading to photosynthesis and support organs, and in higher forms to specialized photosynthesis, vascular and covering tissues. Reproduction is mainly sexual with alternating cycles of haploid and diploid generations, which are reduced in the higher members of the kingdom.

It should be noted that this circumscription leaves unicellular algae outside the kingdom Plantae, so Plantae defined in this way is polyphyletic, since the 3 types of multicellular algae (green, red and brown) have their corresponding unicellular ancestor.

Margulis: land plants

Whittaker's system was modified by Margulis, who in 1971 proposed that groups with multicellular algae ("green algae", red algae, brown algae) be transferred to kingdom Protista, so that in Plantae only terrestrial plants are grouped. In this modification of the kingdom carried out by Margulis, he defines it by the development of tissues for autotrophic specialization (photosynthetic mode of nutrition), where the most significant environmental selection factor was the transition from an aquatic environment to a terrestrial one.

Cavalier-Smith: the first plate

For Cavalier-Smith (1998) and others, the most important factor in the evolution of plants is the origin of the first plant cell, which occurred by symbiogenesis between a biflagellate phagotrophic heterotrophic protozoan and a cyanobacterium. This first photosynthetic eukaryotic organism represents the ancestor of the kingdom Plantae and is called by other authors Primoplantae or Archaeplastida, whose monophyletic clade includes green plants, red algae and glaucophytes. Initially, Cavalier-Smith suggested that the Viridiplantae could have kingdom status, until the relationship between green plants and red algae and glaucophytes was established. Currently it is common to use Plantae as the "supergroup" where the first plastus appears, before considering it a kingdom.

With the new characters and analysis methods that have appeared in recent years, the relationships of land plants with algae have been broadly resolved, indicating that everything we know as land plants and related algae ("green algae", red algae and glaucophytes), have a common ancestor, which was the first eukaryotic ancestor that incorporated what would become the first chloroplast on Earth, in a process of endosymbiosis with a cyanobacterium. Today, this grouping of organisms is recognized as the Plantae by many scientists (sometimes calling it the "clade Plantae", because their organisms have a common ancestor). Alternative names for this clade, which are "Primoplantae" (first "plant" on Earth) and "Archaeplastida" (the ancient plast), refer to the oldest photosynthetic eukaryotic ancestor that incorporated the first chloroplast. It must be taken into account that after that event other types of eukaryotes with chloroplasts appeared, which were not direct descendants of this clade, due to repeated endosymbiosis events in which another eukaryote not related to this clade gobbled up a green alga or a red alga and incorporated its chloroplasts into its organism in a process called "secondary endosymbiosis." For this reason, many organisms with chloroplasts (for example, brown algae) are left out of the Plantae taxon, because they are not direct descendants of those that acquired the first chloroplast, but rather acquired their chloroplasts "secondarily", when they incorporated a green alga or a red alga to its cell, and today they are therefore placed in other taxa, despite being multicellular eukaryotes with chloroplasts (see under "Origin of all plants").

This broad circumscription of Plantae (Archaeplastida supergroup) was accompanied by a whole new eukaryotic classification into various supergroups, in which, for example, the "traditional" Fungi and Animalia, which were related, were grouped into the Unikonta supergroup, and brown algae were placed in the supergroup containing those with chloroplasts derived from a red alga (Chromalveolata).

Evolution and phylogeny

Philogeny of plants showing the main nails and traditional groups. The monophylatic groups are in black letters and paraphylatics in blue.

The attached image is an updated (2015) phylogenetic tree of living plants. This diagram charts the endosymbiotic origin of plant cells, and the phylogeny of algae, bryophytes, vascular plants, and flowering plants.

• Plane sensu lato (Haeckel 1866, emend. Caval.-Sm. 1998) or Archaeplastida and the origin of the first plant cell by symbiogenesis between a cyanobacteria and a biflagelated protozoo of the Corticata clado.
  • Rhodelphis
  • Glaucophytes (Glaucophyta)
  • Rhodophyta is divided into two nails: cyanidephiceas and red algae. By secondary endosymbiosis, chromophyte algae originates.
  • Green plants (Viridiplantae) or Plantae sensu stricto (Copeland 1956)
    • Green algae: chlorophytes and carophytes
    • Land plants (Embryophyta) or Plantae sensu strictissimo (Margulis 1971)
      • Briofites: liver, mosses and craving
      • Vascular plants (Tracheophyta)
        • Pteridophytes: Licopods, equisetos and ferns.
        • Seed plants (Spermatophyta)
          • Gimnospermas: cicadas, ginkgo, conifers and gnetales.
          • Plants with flowers or angiospermas: monocothylenes and dicotylenes (grade ANA, magnólidas and eudicotas).

Phylogeny

Molecular analyzes give approximately the following phylogeny among plant groups:

The relationships so far are a matter of discussion in the monophyly or paraphyly of bryophytes, although most molecular analyzes have supported bryophytes as a monophyletic group, and in the diversification of green algae, red algae, glaucophytes and the genus of proto-algae Rhodelphis.

Origin of all plants and chloroplasts

Main article: Serial Simbiogenesis

Illustration of the endosimbiosis process of a cyanobacteria by an eukaryote, such as the one who formed the first chloroplast, in the ancestor of the Archaeplastide taxon or Primoplantae.

The appearance of plants on Earth occurred through a process of symbiogenesis between a protist and a bacterium. Bacteria are generally prokaryotic organisms, with small and circular DNA, without a cell nucleus or organelles, where their only membrane is the cell membrane and they reproduce by binary fission (the cell grows and divides in two); They are microscopic without mobility or with little mobility that reproduce very quickly. Of the bacteria, we are interested in the group of cyanobacteria (also called "blue-green algae"), which are one of the bacterial groups in which photosynthesis occurs. Protists are mostly microscopic unicellular eukaryotes, they have larger and more complex cells: with multiple linear DNA chromosomes confined in the nucleus, with membranous organelles with specialization of work, a rigid internal structure called cytoskeleton and reproduction by mitosis or meiosis. All eukaryotes come from an ancestor that had mitochondria, since ancestrally it was incorporated by endosymbiosis with a bacterium and is in charge of cellular respiration. In addition, all eukaryotes capable of photosynthesis do so thanks to another particular organelle called the chloroplast, which was ancestrally an ancient cyanobacterium that was also incorporated by endosymbiosis. That they have been incorporated by endosymbiosis means that the organism originally ingested the bacterium (probably with the primary purpose of feeding on it or as a parasite), but instead of degrading it, it began to live with it, initiating a symbiotic relationship, where the bacterium continues reproducing on its own but integrating into the host cell, losing its ability to live freely. Today, although in some lineages some of these organelles may have been lost, in general they are essential for the plant. Mitochondria and chloroplasts, like the bacteria from which they originated, possess prokaryotic-like (small and circular) DNA, similar reproduction (binary fission), and their own ribosomes are prokaryotic in size (70S). The time a protist gobbled up a cyanobacterium and turned it into a chloroplast, a new lineage was formed, together with all its descendants would form the Primoplantae or Archaeplastida clade, which contains all land plants and the algae related to them.

Detail of the ultrastructure of a cyanobacteria like the one that became the first chloroplast.

Evolution of the ultrastructure of chloroplast that resulted in glaucophyte chloroplasts, red algae and green plants (green algae and terrestrial plants).

The other eukaryotes that have chloroplasts acquired them by ingesting not a cyanobacterium but a "green algae" or red algae that already had chloroplasts (they acquired them "by secondary endosymbiosis"). Thus, chloroplasts are all derived from a single cyanobacterium that was the first to be incorporated as a chloroplast, but the eukaryotes that possess them, having endosymbiosis several times independently, are not phylogenetically related.

Keep in mind that the acquisition of mitochondria and chloroplasts were not the only endosymbiotic events, many modern organisms have symbiotic intracellular bacteria, indicating that these relationships are not difficult to establish and maintain.

Evolution of algae

CrucifixAn algae Chlorophyceae. Unicellular algae can be considered the most primitive plants.

The evolutionary history of plants begins with the origin of the first plant cell. This constitutes at the same time the first algae, that is, the first photosynthetic eukaryotic being that has already acquired its first plast (or chloroplast). This beginning seems to have been the product of symbiogenesis between a cyanobacterium and a biflagellate protozoan.

It is estimated that the first algae (Archaeplastida) would be from the Paleoproterozoic, with a little more than 1900 Ma (million years) and the red algae (Rhodophytina) diversified into the current groups more than 1800 Ma. Fossils of The oldest multicellular algae are (Rafatazmia and Ramatallus), with about 1600 Ma_and the diversification of green algae (Chlorophyta, Charophyta) in their current subgroups has about 1800 Ma starting the Neoproterozoic. On the other hand, an estimate of molecular clocks calibrated using the oldest known fossils of each group, estimates the appearance of Archaeplastida at 1.9–2.1 billion years ago.

The evolution of multicellular forms occurred several times, as can be observed in red algae, chlorophytes, and charophytes. From red algae and their interaction with other protists, a new type of algae appeared: Symbiogenesis again acts as an important factor in evolution, since chromophyte algae (Chromista like brown algae) originated by secondary endosymbiosis between a biflagellate protist and a red alga, although it is also possible that they were several independent endosymbiotic events.

Colonization of the land

Fossombronia. Hepatic plants are considered to be the most primitive ground plants.

Terrestrial plants (Embryophyta) appeared as descendants of freshwater multicellular green algae (from Charophyta), and upon populating the earth marked the most important milestone in terrestrial evolution and biological diversity. The presence of fossil spores with affinities to the spores of the current liverworts constitutes the fossil evidence that indicates that these plants colonized the earth during the Middle Ordovician, about 472 Ma (million years) ago and began in the west of the Gondwana continent (Argentina).

The first terrestrial plants are called bryophytes, an evolutionary degree that implies the appearance of the multicellular sporophyte, which constitutes the diploid phase of the alternation of generations, and adaptations to terrestrial life are achieved such as the development of a cuticle that protects to the sporophyte, sporopolenin that protects the spore, and flavonoids that protect against ultraviolet radiation, which is more intense outside of water.

Vascular Plants

In the Devonian vascular plants mark the beginning of the extensive colonization of the earth.

The first vascular plants, such as rhiniophytes and lycopods, appear in the Upper Silurian and in them the sporophyte becomes the dominant phase with development of vascular and supporting tissues, roots and photosynthetic stem with dichotomous growth.

In the Devonian there is a great evolutionary radiation of vascular plants. The first forests appear in swampy areas and are formed by Pseudosporochnales (Cladoxylopsida). Horsetails, ferns, woody plants (progymnosperms) and the first seed plants appear, which resemble ferns (pteridosperms)

In the Carboniferous, spermatophytes (seed plants) diversified into the clade of modern gymnosperms and flowering plants (Anthophyta). Among the gymnosperms, the oldest are conifers such as Cordaitales and in the Permian, ginkgos, cycads and gnetales clearly appear. By contrast, angiosperms appear much later, beginning in the Cretaceous, as descendants of flowering plants of the Anthophyta clade, and the oldest fossils would be magnolids from about 140 million years ago.

Evolutionary relationship with other living things

Symbolic tree of living beings. The symbiogenetic origin of plants by fusion between a biflagelado protist and a cyanobacteria is currently considered to be demonstrated. Later the symbiogenesis between a red algae and another protist originated the chromophyte algae.

The explanation is synthesized in the following phylogenetic trees elaborated according to the ideas of Cavalier-Smith, which show the 3 lines of life (bacteria, archaea and eukaryotes), with the divisions that eukaryotes later suffered, and in blue arrow how a bacterium joined a line of eukaryotes (of the Corticata clade) to form the first chloroplast in the taxon that was named Archaeplastida or Primoplantae, and in green and red arrows how two of those algae (perhaps more) joined other different eukaryotes at some point in the formation of the groups Chromalveolata, Rhizaria and Excavata, which complete all taxa of eukaryotes with chloroplasts (although within those taxa, there are many groups where the chloroplast has been lost).

One of the latest versions of the phylogenetic tree of life, which shows one of the current hypothesis of the times when chloroplasts were acquired by endosymbiosis in the different eukaryotic groups. Perhaps the acquisition of a green algae and the acquisition of a red algae have occurred more than once among the chromists. The eukaryotic supergroups (Archaeplastida, Rhizaria, Excavata, Chromalveolata and Unikonta, sometimes divided into Opisthokonta and Amoebozoa) are quite consensual, what is in research are the relationships between them. Drawing from Cavalier-Smith (2013, 2010a, 2010b, 2009).

The Chloroplast

Chloroplasts are the organelles of the plant cell responsible for plants having their main characteristic: that they are autotrophic organisms (they produce "their own food" from inorganic substances), since it is within the chloroplasts where the process of photosynthesis takes place, which uses the energy of sunlight to store it in the form of chemical energy in organic molecules. Organic molecules are made from smaller, inorganic molecules found in air and water (water itself is an inorganic molecule). To "join" Inorganic molecules need energy to each other, which is stored in that union (a union is represented by a stick, as in C-C, the union between two carbons). That is why it is said that plants "store chemical energy" from the energy of sunlight, and for this reason it is said that they are autotrophic organisms, "that make their own food".

Chloroplasts are also responsible for making plants green, since chlorophyll a, the pigment responsible for capturing the energy of sunlight to start photosynthesis, cannot take advantage of all sunlight as a source of energy, can only use red and blue light, being mainly reflected, visible light, green light. By reflecting green light, that is the color that reaches our eyes and the one we observe. Plants that have other colors in their photosynthetic parts also have other pigments that give them color, but if they did not have them they would also be green. (Models for the nature of light and the explanation for why this is so are found in quantum physics.)

Structure

The structure of the chloroplast can vary somewhat depending on which group of plants it is. Below is a schematic of the structure of a chloroplast of green plants (land plants and "green algae"), which are the most common plants for us.

A chloroplast schema.

The structure of these chloroplasts consists of two membranes one inside the other with an intermembrane space between them, and within the innermost membrane is the stroma, which is a liquid environment. A series of stacked coin-like sacs called thylakoids invaginate from the innermost membrane of the chloroplast (each stack of thylakoids is called a grana). As they are invaginations, the space within the thylakoids (the intrathylakoid space, or thylakoid lumen) is initially continuous with the intermembrane space of the chloroplast; this communication is severed in mature chloroplasts. In mature chloroplasts, the thylakoids are a third membrane, and the intrathylakoid space has a different chemical composition than that found in the intermembrane space. Within the stroma are a number of objects that are expected to be found in the cytoplasm of bacteria, such as circular DNA, which contains, for example, the orders for the chloroplast to synthesize its own ribosomes.

Function: plant metabolism

Chemistry of photosynthesis: reactants and products. Photosynthesis occurs within the chloroplasts of the plant's cells.

The main function of chloroplasts inside the cell is to carry out the metabolism of the plant. This metabolism is photosynthetic, or more precisely oxygenic photolithoautotrophic, that is, phototrophic due to the capture of solar energy through the absorption of light, autotrophic or synthetic due to the ability to synthesize its own organic molecules from simpler inorganic molecules (fixing carbon dioxide), lithotrophic by the use of inorganic substances as reducing agents (dissociation of water) and oxygenic by the final release of oxygen.

Photosynthesis stores light energy from sunlight as chemical energy in the organic molecules that are formed, both in "carbon fixation" as in the formation of ATP. Photosynthesis is the set of chemical reactions that, with the energy of sunlight, converts carbon dioxide (an atmospheric gas) and water (which it acquired, for example, by absorbing it through the roots), into glucose (an organic molecule) and oxygen. (another gas that is released into the atmosphere). The entire process of photosynthesis is carried out in two phases:

Simplification of photochemical reactions (light phase) and biosynthetics (dark phase) of plant photosynthesis.

• Light phase: It is also called a photochemical phase, because by grasping the light of the Sun as a source of energy, it drives the entire chemical process in the complex. Chlorophyll is the pigment that absorbs light energy, gives the green color to the plants and forms part of a complex (photosystem) in the membrane of the chloroplast thylacoids. The captured light photons (light energy) raise the level of electrons in the electron transport chain (electrochemical gradient), which produces that the water molecules (dissociation or photolysis of the water) are "red" in an oxygen atom, 2 hydrogens (H+TP protons) and two electrons; so that a part that is not used will be released (the oxygen atmos) Summarizing the reaction of water photolysis:

2H2OΔ Δ 4H++4e− − +O2{displaystyle mathrm {2 H_{2}Olongrightarrow 4 H^{+}4 e^{-}+O_{2}}}} }

• Dark phase: This phase of photosynthesis is performed in the chloroplast stroma, producing the fixing of carbon dioxide through the Calvin cycle. It is called a dark phase because it is independent of the light; therefore it is done both day and night. In this phase the CO₂ Atmospheric is captured by the RuBisCO enzyme, and in conjunction with the result of water dissociation (in the form of ATP and NADPH), organic molecules are constructed. Each carbon dioxide molecule contains a carbon atom (C) and then the "carbon fixation" comes to a compound of 3 C-C-C atoms (the glyceraldehyde-3-phosphate or G3P). These reactions are summarized as follows:

3CO2+5H2O+9ATP+6NADPH/H+Δ Δ C3H7O6P+9ADP+8Pi+6NADP+{displaystyle mathrm {3 CO_{2}+5 H_{2}O+9 ATP+6 NADPH/H^{+}longrightarrow C_{3}H_{7}O_{6}P+9 ADP+8 P_{i}+6 NADP^{+}}

Actually, photosynthesis is considered complete at this point, but then the biosynthesis process continues through chemical reactions until glucose is synthesized, a sugar-like organic molecule that contains a 6-carbon skeleton. Glucose and other intermediate products are subsequently modified to build all other organic molecules such as carbohydrates, lipids and proteins. The chemical reactions described in both phases and that go from the primary reagents to the synthesis of glucose, are summarized in the following table:

Scheme of the chemical reaction of oxygenic photosynthesis, typical in plants and ranging from simple inorganic substances, such as water and carbon dioxide, to glucose biosynthesis. Note the equality of the number of atoms in the equation.

Pigments

A chromophore is a material that absorbs light from certain colors, reflecting the light of others. The light absorbed by the chromosophores of the chloroplastic membrane is used as a source of energy that drives photosynthesis.

Absorption of light by chlorophiles a and b, typical of green plants.

Pigments in plants are chromophores found in the thylakoids of chloroplasts and their main function is to absorb sunlight for photosynthesis. The main pigment is chlorophyll a, which absorbs more energy in the wavelengths of blue-violet and orange-red light (0.43 and 0.66 μm), consequently it reflects the green light giving the typical color to the plants. Its distribution is universal, since it is found in higher plants, all types of algae and in cyanobacteria. It is also the most abundant, masking the other pigments and giving the vegetation in general its characteristic predominant green tone.

Chlorophylls are essential for photosynthesis due to their role as the main donor of electrons in the electron transport chain. In addition to chlorophyll a, plants have xanthophylls, which are colored yellow and are also photosynthetic, and carotene, which is an orange accessory pigment. When the leaves dry out and oxidize, they turn orange or yellowish, because chlorophyll a is the one that degrades the fastest, masking tones appearing. Evolutionarily, the main divergence is in Viridiplantae on the one hand, where chlorophyll b (light absorption 0.45 and 0.65 μm) of greenish yellow color develops, and in Biliphyta on the other hand (Glaucophyta and Rhodophyta) where phycobilins develop..

The accessory pigments, in addition to helping to capture energy for photosynthesis, many of them have various functions that have been added with evolution. For example, there are fabrics that accumulate some accessory pigment in order to reflect their color, which is what can be found in flower petals and fruit shells. They may also have other functions in the cell that are not related to absorbing or reflecting colors. Accessory pigments are widely used as characters to classify groups of plants, since they provide information about the evolutionary history of each taxon.

• 

  Green Alga. Its color is mainly given by the chlorophylls they possess in the thylacoids of their chloroplasts.

• 

  Red Alga. Its color is given by several pigment accessories (xantophiles, carotene β) that mainly capture the blue colors.

Chloroplast Reproduction

The chloroplast chromosome is responsible for its duplication, inside the plant cell. It consists of a single continuous (sometimes called circular) strand of DNA. DNA duplicates itself so that there are several per chloroplast, which duplicates by binary fission. This process is sometimes repeated, which is why in many cells there are more chloroplasts when they are older. When it is the plant cell that divides, it distributes the chloroplasts among its daughter cells, in a process that is still under investigation.

The chloroplast is born in the young tissues of the plant in the form of a proplast, which later differentiates. The proplast is the precursor of a whole family of plastids, with varied functions (see plant cell).

All chloroplasts are descendants of a single ancestral chloroplast, which was formed by an integration process after a eukaryote engulfed a cyanobacterium, a process called "primary endosymbiosis". In some groups of plants, the chloroplast was taken by swallowing the alga that already possessed it, becoming part of the host cell, in subsequent endosymbiosis events, called "secondary endosymbiosis" (see below in "Origin of all plants").

Taxonomic groups evolve, and the chloroplast evolved together with the cells of which they are a part, so there are variations in the structure of the chloroplast described here that correspond to different evolutionary events, each variation and its place in the evolution will be pointed out in their corresponding places in the Plant Diversity section.

Photosynthetic Organs

In plants organized into organs with work specialization (terrestrial plants), there are specialized organs for carrying out photosynthesis. Organs called leaves are only found in the diploid phase (what we commonly call 'plant') of vascular plants (mainly ferns, gymnosperms, and angiosperms) and are often flattened to increase the surface area exposed to light. In other groups of plants there are structures that have a similar appearance because they fulfill the same function, but they have a different evolutionary origin, which is why they have other names. As the name is given by its evolutionary origin and not by its function, sometimes the structures evolve in order to change their function, but they continue to maintain the name. For example, the leaves are still called leaves ("modified leaves") in cacti, in which they became thorns and the photosynthetic function is fulfilled by the stem, which is green. The spines derived from leaves are called leaf spines.

The plant cell

The "plant cell" (of terrestrial plants) has variations according to the taxonomic groups that are treated and according to the tissue in which it is found in each taxonomic group, for example, wood is different from what is described here; it may also be different from that described in this section for algae. When it is described in relation to some tissue, it usually refers to the sporophyte of vascular plants (ferns, gymnosperms and angiosperms). In the Diversity section, the differences with the cell described in this section will be pointed out when necessary.

The following is a diagram of the plant cell (here the differences with animal cells will be highlighted, for an explanation of all its components see cell).

Scheme of a plant cell.

Two organelles that are worth mentioning, in addition to the chloroplasts already explained, are the mitochondria that are in charge of cellular respiration.

Breathing

In the same way as any other eukaryotic organism, plants have aerobic respiration, with consumption of O2 and expulsion of CO2. It is especially important at night, given the lack of energy from the Sun. It is a vital process, but the reverse of photosynthesis. It is produced mainly in the mitochondria, metabolizing substances such as glucose and fructose, and producing energy. The plant respires through its roots and leaves, and in the absence of oxygen it can resort to fermentation to obtain energy necessary for its subsistence. In certain cases, photorespiration (breathing during the day) can occur.

Cell wall

Many organisms, especially those called plants, have cells with a cell wall, a more or less rigid structure that the cell secretes outside its cell membrane, which limits its shape and volume. The cell wall appeared several times in the course of evolution, so there are different groups of organisms that have cell walls, which can be distinguished from each other by their architecture and chemical composition. In studied plants and algae, the machinery responsible for synthesizing the cell wall may possess some elements common to some groups, but our understanding of cell wall evolution is still limited and under investigation, as well as its structure and function. Its chemical composition varies depending on the developmental stage of the cell, the cell type, and the season of the year. In plants such as land plants, "green algae", red algae, brown algae, diatoms and dinoflagellates, the cell walls chemically consist mainly of polysaccharides. The cell wall fulfills multiple and varied functions: granting rigidity, determining cell shape, resisting cell expansion, acting as a defensive barrier, and acting as a filter, allowing the passage of certain substances and not allowing others. In multicellular organisms with cell walls, the cell walls provide structural support and form an important part of the texture of the plant body. For example, in plants with woody parts, it is the thickened cell wall that gives them support and the woody appearance. In multicellular plants with cell walls, all the space outside the cell membranes, including all cell walls, is called the apoplast, and the movement of substances through it is called the apoplast pathway.

Intercellular communication

In land plants and some closely related algae, the cytoplasm of cells communicates with that of other cells through small channels in the cell membrane that pass through the cell walls through pores in them. These structures are called plasmodesmata. The space inside the plasma membranes of all plant cells is called the symplast, the movement of substances through it is called the symplast pathway.

Apoplastic absorption diagram and simplistic water by the roots of a plant.

The cytoplasm of eukaryotic cells contains a large number of vesicles, which are generally small organelles, with functions of temporary storage and transport of materials. A particular type of vesicle is the vacuole, present in most plant cells. The vacuole is a large vesicle, which can occupy from 30% to 90% of the cell size. It is born in the form of small provacuoles in the young cell, in the meristematic tissue, which as the cell matures merge into a single large vacula, which then becomes a central support element for the cell. The vacuole consists of a membrane (the vacuolar membrane or tonoplast) with liquid inside, and can function as a storage organ for a wide variety of substances, which the cytoplasm takes or deposits according to the needs of the cell. For example, pigments, secondary metabolites that function as a chemical defense for the plant, or substances found in the cytoplasm that can be harmful to the cell can be deposited in the vacuole. The size of the vacuole means that the cytoplasm is in intimate contact with the cell membrane, in which all kinds of exchange of chemical substances between the cytoplasm and the environment occur. The vacuole also enters "salts" (solutes) into it from the cytoplasm, so that the concentration of solutes is higher inside the vacuole than in the cytoplasm. In a physical process called osmosis, water crosses the vacuolar membrane and is attracted where there is a higher concentration of solutes. This expands the vacuole, which along with it expands the entire cell, which is thus kept pressed against its cell wall. This pressure is called turgor pressure, and it is what keeps the herbaceous parts of the plant erect. This pressure is also responsible for expanding the cell during cell growth.

The vacuola of the plant cell is the main responsible for keeping it turgent, when it has the possibility to absorb water.

Other plastids

Finally, one more difference with animals is that plants have other types of plastids besides chloroplasts, they are called plastids because they are generated from the same organelle as the chloroplast: the proplast, but then they differ in other ways. functions. Its function can be, for example, the production and storage of different chemical substances that the cell needs (for example, amyloplasts store starch, chromoplasts synthesize and store pigments that give color, for example, to flowers and fruits). Although plastids are traditionally classified based on their function and internal structures, they are sometimes difficult to assign to any category, and their biological significance is not always unique or obvious. Members of the plastid family play important roles in the regarding photosynthesis, the synthesis of amino acids and lipids, the storage of starch and oils, the coloration of flowers and fruits, the sensation of gravity, the functioning of the stomata, and the perception of the environment.

Genetics

Plant cells have three different sets of DNA:

• on the one hand the cell has its own genome in its core,
• mitochondria have their own genome (1 continuous chromosome)
• chloroplasts have their own genome (1 continuous chromosome)

The nucleus of plant cells contains a eukaryotic-like genome: as in animals, DNA is arranged in chromosomes consisting of a linear DNA strand, more or less packed with its associated proteins. In contrast, mitochondria and chloroplasts have a bacterial-like genome: they have a single continuous DNA chromosome per organelle, just like their bacterial ancestors. Mitochondria and chloroplasts divide within the cell, and when the cell that hosts them divides, it is thought to have mechanisms for these organelles to be distributed among the daughter cells, so that there is never a cell without mitochondria and chloroplasts.

Reproduction and life cycles

To understand what we see when we look at a plant, we must first have an understanding of what its life cycle can be like.

A life cycle includes all the stages that take place from the moment you have an individual until you get another descendant individual with the same amount of DNA, starting the cycle over again. The offspring may be identical in its DNA content to its only parent, then the offspring is said to have been obtained by asexual reproduction, or the offspring's DNA may be a combination of the DNA content of two different parents, then it is said to be says that the offspring were obtained by sexual reproduction. In plants in the broadest sense there is a wide variety of life cycles, which can often include both asexual and sexual reproduction, to understand them here 3 different types of life cycles will be exposed, the 3 include multicellularity and sexual reproduction, to From them you can understand the others.

Eukaryotic cells divide into two daughters, but the daughters do not necessarily inherit the same amount of DNA as the parent cell. As we remember, the DNA of eukaryotes is in the form of packed linear strands of DNA (each strand called a chromosome). In general terms we can say that in eukaryotes, the amount of DNA in a cell can be in the form of a single set of chromosomes (n) or two sets of chromosomes (2n). Sometimes the mother cell contains the same amount of DNA as its daughter cells (both mother and daughters are 2n, or both mother and daughters are n), so cell division is called mitosis. Sometimes the mother cell has twice as much DNA as its 4 daughter cells (the mother is 2n but the 4 granddaughters are n, the daughters being an intermediate stage between mother and granddaughters), this type of cell division is called meiosis.

The 3 life cycles outlined here exemplify 3 examples of sexual reproduction. In sexual reproduction, the organism alternates between an n phase and a 2n phase: the DNA content is split (n) and then that of two different parents is combined (2n). At one point in the life cycle of all the plants outlined here, the DNA is in the form of a single set of each chromosome (n), when this is the case, it is said that the plant is in the haploid phase of its life cycle.. In some plants the haploid phase becomes multicellular by mitosis, when this is the case the multicellular adult is also haploid. Later in the life cycle, two haploid cells from two different parents (whether or not they have become multicellular) fuse (during fertilization) to form a diploid (2n) cell, entering the diploid phase of their life cycle. This 2n cell can also become multicellular by mitosis, or not, if it becomes multicellular the adult multicellular individual is also diploid (2n). Subsequently, whether or not it has become multicellular, meiosis occurs in some of these diploid cells giving haploid cells (n), starting the cycle over again. Regardless of whether any of the phases has become multicellular or not, the fact of dividing their DNA content during meiosis (from 2n to n) and then recombining that of different parents during fertilization (from n to 2n) means that there have been sexual reproduction. Note that if there is sexual reproduction there will always be alternate haploid and diploid phases, which do not necessarily imply multicellularity.

When only the haploid cell becomes multicellular, giving only haploid adults, the life cycle is said to be haplontic. When only the diploid cell becomes multicellular, giving only diploid adults, the life cycle is said to be diplontic. When both the haploid and diploid cell become multicellular, giving alternately haploid and diploid adult individuals, the life cycle is said to be haplo-diplont.

These explanations are summarized in the 3 tables shown below:

Huge life cycle. M!: meiosis F!: fecundación m!: mitosis circle: first stage of life cycle, square unicelular: following stages of life cycle, multicellular

Cycle of diploent life M!: meiosis F!: fecundación m!: mitosis circle: first stage of life cycle, square unicelular: following stages of life cycle, multicellular

Haplo-diploent life cycle M!: meiosis F!: fecundación m!: mitosis circle: first stage of life cycle, square unicelular: following stages of life cycle, multicellular

As seen in the chart, in the three life cycles, the two haploid cells that fuse during fertilization are called gametes. In all sexually reproducing plants, the gametes are divided into two sexes, and one gamete of each sex is needed for fertilization to occur. Normally the sexes evolve in such a way that one of the gametes is mobile and actively seeks the other, and the other gamete is immobile but larger and with reserve substances ("food") in the cytoplasm. When this is the case, the mobile gamete is called male, the gamete with reserve substances is called female. The diploid cell that forms during fertilization is called a zygote.

Finally, there are life cycles in which the amount of DNA in the cells is conserved throughout the cycle, there is no division into haploid and diploid phases, there is no meiosis or fertilization and, to give offspring, new cells are generated by mitosis. Therefore reproduction is asexual. Many times it is found that the same species is capable of giving offspring both sexually and asexually, when this is the case, asexual reproduction can be integrated into sexual reproduction schemes such as those described, adding an asexual reproduction cycle where it occurs. In general, species reproduce sexually (although in many cases asexual reproduction is more common).

In plants in the broadest sense (that of eukaryotes with chloroplasts) we can find any of these 3 life cycles, more complex variations of them, and also life cycles that do not imply multicellularity in any phase, in the so-called unicellular organisms.

Two specific examples will be mentioned, which serve to understand the evolution of terrestrial plants (embryophytes): the life cycle of those "green algae" from which land plants evolved is haplont, with only haploid multicellular individuals, like its ancestors. On the other hand, those descendants that we call terrestrial plants have a haplo-diplonte life cycle, because the diploid phase became multicellular by mitosis before giving the gametes, appearing two alternate generations of individuals: the sporophyte 2n and the gametophyte n, that in the terrestrial plants that exist at the present time they are not the same morphologically (see later).

In the Diversity section, tables will be shown illustrating some life cycles with specific cases.

Terrestrial plants or embryophytes

Algae predated land plants. Land plants are divided into groups that are very familiar to us: bryophytes, ferns, gymnosperms, and flowering plants.

Plants called embryophytes (scientific name Embryophyta) are known as "land plants" for they are practically the only group to have colonized the surface of the earth, and the most successful. As colonizing plants of a different environment, compared to the algae from which they originated, they have characters that allow them to adapt to new conditions, mainly they have adaptations to lack of water, to greater exposure to ultraviolet rays from the Sun in comparison to that in the water, and the greater exposure to oxygen compared to that in the water. They are "land plants" or embryophytes the plants that are best known to us, in particular mosses, ferns, gymnosperms (the best known group of gymnosperms are conifers) and angiosperms (wrongly called "flowering plants", they are almost all embryophytes). Virtually all other eukaryotes with chloroplasts are found in water and are called "algae".

To understand what we see when we observe an embryophyte we must first understand what its life cycle is like. Embryophytes have a haplo-diplonte life cycle, that is, they have two alternate generations of individuals: the 2n sporophyte and the n gametophyte. In current land plants, the sporophyte and the gametophyte are very different from each other (although there are scientists who believe that they were morphologically similar at first), this situation is called "heteromorphic alternation of generations" (heteromorphic: "with different morphs"). The gametophyte and the sporophyte can be incredibly different depending on the group in question, we can differentiate between mosses in a broad sense, pteridophytes and spermatophytes. In mosses, the photosynthetic body is the haplontic part of their life cycle, while the diplontic stage is limited to a small non-photosynthetic foot, nutritionally dependent on the haplontic stage. In pteridophytes (lycopods, ferns and the like) what we normally call "fern" it is the diplontic stage of its life cycle, and the haplontic stage is represented by a small photosynthetic body without tissue organization ("tallus") that grows in the soil. In seed plants or spermatophytes (gymnosperms and angiosperms), what we normally recognize as the body of the plant is only the diplonte stage of its life cycle, with the haplonte "masked" inside the pollen grain and ovule.

The multicellular stages of embryophytes have a way of growing to be multicellular that is different from what we are used to seeing in animals. In plants, some region is always preserved whose cells have the ability to divide and give all kinds of tissues, these regions are called meristems. There are usually meristems, for example, at the tip of the stem and the tip of the roots. Only in the meristems is the growth of new cells that will later differentiate.

Plants, unlike animals, are modular organisms, this means that their body is structured in the form of modules that are repeated indefinitely: for example, each branch of a tree, with its stem and leaves, is a module. The modules can be produced and lost without major risk to the plant, as long as enough of them are kept so that the organs can fulfill all their functions efficiently.

More features:

• They don't move.
• Nutrition: photosynthesis (so carbon dioxide is necessary), and breathing (so oxygen is necessary).
• With "vegetable cell" with cell wall, plasmodesmos, vacuola.
• They contain flavonoids, which help them survive under the ultraviolet rays of the Sun, more intense on the terrestrial surface than under water.
• They contain a metabolism different from the algae from which they originated to survive under the high oxygen pressure present in the Earth's atmosphere.

Plant embryophytes evolved from green algae of the group Chlorophyta (scientific name Chlorophyta) during the Paleozoic.

• Briophytes (Bryophyta): mosses, cravings and livers.

Bryophytes are small plants confined to humid environments, they also need liquid water for fertilization. They are haplo-diplont, like all embryophytes, and both the haploid and diploid phases of the life cycle are visible, although it is the adult haploid (the gametophyte) that photosynthesizes. This group presents in its gametophyte cuticle resistant to desiccation and tube-like tissues that transport water through the organism (but do not have the same evolutionary origin as the transporting tissues of vascular plants).

• Cormophytes or vascular plants.

Vascular plants (also called tracheophytes) are haplo-diplontes with an adult gametophyte and sporophyte, but the most developed individual is the sporophyte, which is organized into tissues and organs. It is the sporophyte that has the tubes that carry water and sap (vascular bundles) that give the group its name. Vascular plants are subdivided into pteridophytes and spermatophytes.

• Pteridophytes (Pteridophyta division).

Pteridophytes have a gametophyte that is a thallus (unorganized body in tissues and organs) that is photosynthetic and free-living, unlike what happens in spermatophytes. The best known pteridophytes are ferns.

• Spermatophytes (Spermatophyta Division).

Vascular plants include, as a subgroup, spermatophytes, or seed plants, which diversified at the end of the Paleozoic. In these organisms the gametophyte is more reduced than in pteridophytes, it lives confined within the ovule (the female gametophyte) or within the pollen grain (the male gametophyte) in a new structure called a flower, and the sporophyte begins its life by mitosis then entering in dormancy until the conditions to continue growing are given: this period of latency is called a seed. Among the spermatophytes we find gymnosperms and angiosperms. Gymnosperms include conifers and cycads, gnetids, and Ginkgo biloba. Angiosperms (misnamed 'flowering plants') have different flowers from gymnosperms, and the seed is contained within a new structure called the fruit.

• Angiospermas (Angiospermae).

This is the most numerous group of plants, they appeared during the Jurassic and have become the dominant flora of almost the entire planet.

Vascular plants or tracheophytes

There are specialized tissue systems in the roots, stems, and leaves of vascular plants. Vascular plants consist of three main tissue systems: epidermal, vascular, and fundamental. Epidermal tissue is like the “skin” of the plant in that it is the outer layer of cells. Vascular tissue is like your "bloodstream" as it carries water and nutrients throughout the plant; and the fundamental fabric is everything else.

• Epidermal tissue: The outer cover of a plant consists of epidermal tissue, which consists of a single layer of epidermal cells. The external surface of these is usually covered by a thick zero layer that protects the plant from water loss and injuries. The thick zero layer of epidermal cells is known as a cuticle. Some epidermal cells have small projections called tricomas, which help protect the blade and sometimes give it a hairy look. In the roots, the epidermal tissue includes cells with radical hairs that increase the surface exposed to the earth and contribute to the absorption of water. Depending on the species in particular, the leaves have in the epidermis of small pore leaves called stomata, surrounded by occlusive cells, which regulate the loss of water and the exchange of gases.

• Vascular tissue: Vascular tissue forms a transport system that moves water and nutrients throughout the plant. Vascular tissue consists of xylene, a tissue that leads water, and the floem, a tissue that leads food. Vascular tissue contains several types of specialized cells. Xilema consists of tracheids and glasses. The floem consists of cribous tubes and accompanying cells.

• Basic tissue: The cells that are found between the epidermal and vascular tissues form the fundamental tissue. In most plants, the fundamental tissue consists mainly of parenchyma. Parish cells have thin cell walls and large central vacuolas surrounded by a thin layer of cytoplasm. In the leaves, the cells of the fundamental tissue are filled with chloroplasts and are the site where most of the plant's photosynthesis occurs. The fundamental tissue can also contain two types of tissues called bra tissues, with thickened, flexible and strong cell walls that help support the plant. They're colhenchima and sclerénquima. The cells that form the colhenchima are usually found just below the epidermis of herbal stems and leaves, and their thick cell walls contain a lot of pectin. They're living cells. The cells that form the sclerénquima also support the plant and have thickened cell walls, but are dead to maturity and stiffness is given the lignine present on their cell walls.

These tissues are grouped into organs. The organs of tracheophytes are:

• Raíz
• Tallo
• Sheet
• Flower (present only in spermatophytes)
• Fruit (present only in angiospermas)

Angiosperms

Gymnosperms and ferns tend to grow indefinitely. Angiosperms, however, can be annual (grow for only one year or one growing season), biennial (grow for only two years or two growing seasons), or perennial. An annual plant is a plant that lives one year or less, typically living for one growing season within the year. Annual plants are herbs (although not all herbs are annuals), and can usually be detected by lacking an underground stem and showing no evidence of growth from the previous season (i.e., no thickened stems or other grass structure). storage, nor dormant buds, nor ancient fruits). Biennial plants are those that live for two years (or for two growing seasons), usually flowering in the second year. Biennial plants typically form a basal rosette of leaves during the first year and form an inflorescence in the second year. Biennial plants can be difficult to spot without observing the plants for two years. A perennial plant is one that lives more than two years. Perennials include herbs with underground stems, shrubs, lianas, and trees. Some perennials can live for millennia.

Among the annual species are

• CentennialSecale cereale)
• Mijo (Panicum miliaceum)
• Trigo (Triticum aestivum)

Among the biennial species are

• Acelgas (Beta vulgaris var. cicla)
• Break us up.Raphanus sativus)
• ZanahoriasDaucus carota)

Among the perennial species are

• Abeto (Abis alba), and practically all other gymnastics.
• Encina (Quercus ilex)
• MelisaMelissa officinalis)
• Romero (Rosmarinus officinalis)

Monocots and Dicots

See more detailed in Dicotyledoneae and Monocotyledoneae.

Traditionally, angiosperms have been divided into monocots and dicots, although today, the dicot group, which was paraphyletic, is subdivided into several groups, each with its own common ancestor. Many textbooks continue to study how to differentiate monocots from the rest of the angiosperms or dicots:

• The most outstanding feature is that monocotyledons possess only one cotton in their seed, while most of the rest of the angiospermas possesses 2 cots in their seed.
• Dicothylenes have root root root of root origin (originating from the embryo radicle) persisting in adult form (can be recognized at first sight a primary root of secondary), but monocothylenes have only a root of adventic origin (originating in other parts of the plant).
• monocotyleddons have a stem with atactostele, dicotyleons with eustela in this way can own logs with wood (secondary growth).
• Monocotyleons have flowers whose verticils usually occur in 3 pieces, in dicotyleons verticils usually have 4, 5 or many pieces.
• the leaves of the monocothylenes in general have parallel venation, unlike the reticulada of the dicothydoneas

Seaweed

Algae are all protist eukaryotes that acquired chloroplasts by endosymbiosis and that do not belong to the group of land plants or embryophytes. The name alga is put in lower case to emphasize that it does not correspond to a monophyletic group nor is it in any formal classification system. Here the groups of algae will be exposed, since they are studied by Botany.

Almost all algae are aquatic, descendants of the first eukaryotes, which appeared in the sea. Some of them are multicellular with formation of tissues with division of labor, do not move and are exclusively autotrophic: some green algae, some red algae and brown algae (all 3 were considered within Plantae in Whittaker's 1969 classification of 5 kingdoms).). The other algae can be unicellular sessile autotrophs (chlorarachneas, haptophytes); but there are taxa (euglenoids, golden algae in a broad sense, diatoms, dinoflagellates, cryptomonas, Bolidomonas) that have organisms that, in addition to photosynthesizing and having chloroplasts, are mobile and can feed independently. heterotrophic (they are mixotrophic), so in addition to being considered algae they are considered protozoa.

Glaucophytes

Glaucophytes are a small group of microscopic algae. The only chlorophyll they contain is chlorophyll a, and they are distinguished by the presence of a relict peptidoglycan wall that may have existed outside the symbiont cyanobacterial membrane, and fell between the two. chloroplast membranes.

Plant systematics

Main article: Systematic Botanical

Systematic botany is the theory and practice of grouping individuals into species, grouping those species into larger groups, and giving those groups names, thus producing a classification. Classifications are used to organize information about plants.

There are many ways to build a classification. For example, plants may be classified on the basis of their medicinal properties (as they are in some herbal medicine systems), or on the basis of their preferred ecological niches (as they are in some classifications used in ecology). A phylogeny-based classification, such as the one used here, attempts to sort organisms into groups on the basis of their evolutionary relationships. This is what is currently considered a "natural" classification.

A classification such as the one used here occurs in two steps. The first is the reconstruction of the phylogeny of a group of organisms, the second is the construction of the classification system from the phylogenetic tree. The science that deals with the latter is called Taxonomy.

Botanical taxonomy

Main article: Classification of plant organisms

Taxonomy is the branch of science that deals, on the one hand, with naming and describing all species for science, and on the other hand, with building a classification system with its phylogenetic tree, converting the clades in taxa. A taxon is a clade that has been assigned a name, a description if it is a species, and a "type" (which in the case of plant species, is a specimen of the species kept in a herbarium). In the prevailing classification today (the "Linnean"), taxa are also placed in taxonomic categories such as those shown in the table:

Taxonomy at present: the taxon is built from the edges of the phylogenetic tree. Each node of the tree is an ancestor, a nail is that ancestor plus all his descendants. In addition, each taxon has its taxonomic category (although for some scientists this is not necessary).

The following will explain the rules for naming plants (in "Nomenclature") and what is the classification system used today (in "Classification").

Botanical Taxonomy: Nomenclature

Main article: Botanical Name

This section needs to be revised

Given the need to give a clear name to each species of plants, the use of common names is not feasible, which does not mean that they should be forgotten. Common names have the disadvantage of varying considerably from one region to another or of different botanical species having the same designation. On the other hand, there are many species that are not known by any common name.

For this reason, when naming plants, a series of rules agreed upon by the scientific community in the International Code of Botanical Nomenclature must be followed, which also regulates the nomenclature of other living beings previously considered plants, such as algae and mushrooms.

Here are the most important rules:

1. The names prior to 1753 are not valid, from which the botanist Carlos Linneo began the scientific nomenclature of the plants currently used. In some specific groups, this start date is different.
2. It is considered valid that name given to the taxon for the first time after being published in a scientific journal under certain rules. The subsequent names of the same taxon will not be valid, as they are considered synonyms.
3. The names must be latinized since Latin is the language used for the nomenclature in the sciences.
4. The scientific name of a plant is binominal, that is, it contains two words (names) (for example, Cupressus sempervirens(c):
   1. The name of the genus to which the plant belongs goes in capital, in front of the specific name. When the species is renamed in a publication, the name of the genus can be shortened if there is no ambiguity, in the previous example, C. sempervirens.
   2. The specific name given to the species goes in tiny, which, in general, will be an epithet that characterizes the species in question (e.g. Sibbaldia procumbens, for being a prostrated plant. It can also be dedicated to a person (e.g. Rubus castroviejoiwhich is dedicated to the Spanish botanist Santiago Castroviejo Bolíbar) or place (e.g. Crataegus granatensis, granadino, Granada), or transfer a vernacular name, as in the case of Prunus mahaleb (from Arabic).
5. Following the scientific name you must write the initial, initial or full name of the author or authors who for the first time described the plant (e.g. Thymus vulgaris L.). This list is official and other abbreviations cannot be used. The dates may be added if deemed appropriate, although there is no tradition of doing so.

Sometimes, after the scientific name, the particles ex or in appear between the abbreviation of two authors (eg Rosa micrantha Borrer former Sm.). In the first case, it means that the second author grants the authorship of the name to the first, but that the true botanical authorship corresponds to the second, that is, the first suggested the name and the second validly published it. In the second case, the true author is the first, but he does so in a work or magazine article that corresponds to the second, so it is convenient that it be cited as a reminder.

When it is necessary to transfer a species from one genus to another, the name of the first author should be cited in parentheses before the author who transferred the species. Thus, for example, the species Valeriana rubra described by Carlos Linnaeus (L.) was transferred to the genus Centranthus by Augustin Pyrame de Candolle (DC.), therefore its name remained as Centranthus ruber (L.) DC.

It is also common to use a series of signs and abbreviations in names, including the following:

• sp. / spp.: species / species.
• subsp. / subspp.: subspecies / subspecies.
• var. / wander.: variety / varieties.
• × × {displaystyle times }: hybrid.
• fl.: Latin floruit (flowered), it is placed next to the abbreviation of author, followed by one or more years and indicates that it is only known as a botanist (e.g. Andrews fl. 1975).
• aff.: abbreviation affinis, 'similar', and it is used to indicate in a work that the studied specimens have most of the characters of a taxon, but differ in others (e.g. Sempervivum aff. tectorum).

For cultivars, use the abbreviation cv. or single quotes (eg Citrullus lanatus cv. Crimson Sweet or Citrullus lanatus & #39;Crimson Sweet').

Botanical Taxonomy: Classification

The search for a "natural" Ranking is a discipline with a lot of historical background, because each new ranking is based on a previous ranking that users have become accustomed to. Since the beginning of science as we know it today, many classification systems have transcended, and often several of them were used at the same time by different users, although some were more successful than others. For example, a particularly successful book was Species Plantarum by Linnaeus.

At one time, classification systems were a reflection of the conception that each naturalist had of what relationships between species were like. Since the appearance of Darwin's theory of evolution, scientists have agreed that classification should be based on the phylogenetic tree of species. The problem at the time was that it was not known what the "true" phylogeny was; of plants, but there were various hypotheses that still resulted in different classification systems. In plants, this scenario has been changing in recent decades, with the advent of new traits, such as cell ultrastructure and molecular DNA analysis. In the higher taxa of plants (those closest to the kingdom Plantae), for the first time a single phylogenetic tree hypothesis with broad consensus began to emerge, resulting in the classification shown here.

The prevailing classification today in the scientific environment (that of the cladist school) considers that only groups that comprise a common ancestor plus all its descendants ("monophyletic") can be taxa. For example, dicots, which are paraphyletic, are not considered a taxon by many scientists, who divide them into taxa that correspond to their monophyletic groups.

Therefore, due to what has already been stated, the classification of higher plant taxa presented here is currently the only one used in the scientific environment, it is based on phylogeny, it has only groups as taxa monophyletic, and have names taken from earlier classification systems, which are still used for historical reasons. It must be taken into account that although this part of the phylogenetic tree is well resolved, most scientists have not yet decided in which taxonomic categories to place these taxa, for this reason they are called here with informal names or in general with names ending in -phyta ("plant", derived from ancient Greek) without going into detail about the taxonomic category in which they should be placed.

Today it is widely agreed among scientists that the Archaeplastida or Primoplantae taxon (for many, the new Plantae) should be started to build the monophyletic groups shown below, some important paraphyletic groups and a group extinct polyphyletic, enclosed in quotation marks to distinguish them from monophyletic groups.

Plantae (clade of primary chloroplast acquisition) also called Primoplantae or Archaeplastida

• Rhodelphis
• Glaucophytes (Glaucophyta)
• Red seaweed (Rhodophyta)
• Green plants (Viridophyta, Viridiplantae or Chloroplastide)
  • "Green algae" (paraphylical group, all Viridiplantae except Embryophyta)
  • Clorophy (Chlorophyta)
  • Estreptophytes (Streptophyta)
    • "Carophytes" ("Charophyta") (paraphytic, all green algae not chlorophytes, the small groups that make up it will not be shown here, but we will mention the Charales, which is the group of which the earth plants originated)
    • Land plants or embryophic (Embryophyta)
      • "Bryophyta in broad sense" ****
      • Antoceros (Anthocerophyta)
      • Musgos (Bryophyta in the strict sense)
      • Vascular plants or tracheofitas (Tracheophyta, Cormophyta)
        • "Pteridophyta" (paraphytic group comprising Lycopodiophyta and Monilophyta)
        • Lycopodiophyta o Lycophyta
        • Plants with megaphiles or eufilofitas (Euphyllophyta)
          • Goodies and related or monilofitas (Monilophyta)
            • Psilotopsida
            • Equisetopsida
            • Marattiopsida
            • Polypodiopsy
          • Plants with seed or spermatophy (Spermatophyta, fans)
            • †"Seed ferns" ("Pteridospermatophyta", polyphiletic group, extinct)
            • Gimnospermas (Gymnospermae)
              • Conifers (Pinophyta)
              • Cycadophyta
              • Ginkgophyta
              • Gnetophyta
            • Plants with flowers (Angiospermae, Magnoliophyta)
              • Monocotlers (Monocotyledoneae)
              • "Dicotyledoms" (Dicotyledoneae, paraphylaxis, all non-monocotyledon angiosperms)

Los análisis de filogenia hechos en las últimas décadas también lograron una gran resolución por debajo de las categorías mencionadas en este esquema. A continuación los últimos sistemas de clasificación publicados, que son los que se utilizan hoy en día:

• For the classification of angiospermas, see APG III classification system (APG III 2009, see also LAPG III 2009).
• For gymnastics, see Christenhusz et al. Living gymnastics classification system. 2011 (Christenhusz et al. 2011a, c).
• For ferns and lycophytes, see Christenhusz et al Pteridophyte Sorting System. 2011 (Christenhusz et al. 2011b, c and d).

All these systems consider only monophyletic groups as taxa and, at least in the categories closest to species, name the taxa according to the principles of Nomenclature and place them in Linnaean taxonomic categories.

For algae outside the taxon Archaeplastida, see Adl et al. (2005)'s classification of eukaryotes.

Communication

Plants are integrated into complex communities that are allowed to communicate with each other in order to defend themselves, survive, or secure a number of means for their growth. In order to compensate for the fact that they are fixed to the soil and the lack of specialized organs, plants have obtained by natural selection communication systems based on positive allelopathic phenomena, that is, they have created mechanisms with which they can produce biochemical compounds that can have a great weight in the survival, growth and reproduction of other neighboring living beings, allowing them to survive the adversities they suffer.

By air

Through volatile organic components (VOCs), when the plant is attacked, these components will be released into the air, a transport method to reach the surrounding plants and notify them, creating communication between them. The molecules used are secondary metabolites that the plant synthesizes for its physiological functioning and that are stored in cellular reservoirs.

These VOCs can be in different forms, and they differ by their composition and structure. The diversity of VOCs is due to the fact that for each type of attack, there is a specific type of VOC. There are also some specific ones that can only release certain species of plants, meaning that the others around them, if they are not from the same family, will not have the appropriate receptors to be able to get and understand them.

Underground

This method takes place in the rhizosphere, that is, the area where the roots do their work. For this communication, the use of biological mediators, fungi, is necessary.

Plants and fungi create an underground cooperation called mycorrhizal network, allowing all the plants in the same forest to communicate with each other, covering distances of square kilometers. Then, using this phenomenon, plants can recognize and perceive each other using the movement of signals and compounds between the hyphae and the roots. Thus, the compounds act as neurotransmitters that move across the "synapses" between the cell walls of the roots and the fungal hyphae.

Ultrasounds

When plants are in a drought situation or their stems are cut, a sound arises, produced by cavitations when small air bubbles are created and create an explosion in the xylem.

Plants usually communicate when they are in danger, and for this reason, the information that is transmitted the most is related to diseases or insects that may be attacking them, since they tend to be the most common threats to them.

When plants receive this information, one of their tactics is to release more VOCs to warn the other plants, making a larger number of them prepared for danger. They can also release another type of VOC that attracts carnivorous predators and parasites, which will defend the plants against the herbivores that threatened them.

When a plant is attacked and releases VOCs, it's not really warning the others, but the unaffected parts of its body because they prepare, and then the others find out by chance.

These means of communication are also used to fight for subsoil terrain. The roots of the plants synthesize the VOC and release it into the rhizosphere, which have the mission of exploring the area to detect the presence of the surrounding plants, with the aim of attacking or avoiding them.

Importance of plants for humans

The importance of plants to humans cannot be underestimated. Without them, neither we nor most animal species could exist. Photosynthesis in plants and other groups of smaller photosynthetic organisms has changed the Earth in two ways. The first is the fixation of carbon dioxide and the release of oxygen molecules that have directly altered the planet's atmosphere in these last few billion years. What used to be an oxygen-deficient atmosphere underwent a gradual change. As a mass of oxygen accumulated in the atmosphere, selection for oxygen-dependent respiration occurred (mainly via mitochondria), which must have been a precursor to the emergence of many multicellular organisms, including all animals. In addition, the oxygen-rich atmosphere allows the accumulation of an ozone layer on top, which does not allow access to the surface of excess UV radiation. This allowed organisms to occupy ecological niches exposed to radiation that had previously been inaccessible.

Second, the compounds produced by photosynthetic species are used, directly or indirectly, by non-photosynthetic, heterotrophic organisms. For virtually all creatures that live on the land surface, and for many aquatic ones, land plants are what are called the primary producer in the food chain, the source of compounds that store energy like carbohydrates, the source of compounds that build structures like amino acids, and other essential compounds for the metabolism of some heterotrophs. So most of the species on the earth's surface today are absolutely dependent on plants for their survival. As primary producers, plants are the main components of many communities and ecosystems. Plant survival is essential to maintaining the health of these ecosystems, the disruption of which would result in the disappearance of species and disastrous changes in erosion, water flow, and ultimately climate.

For humans, plants are monumentally important in a direct way:

Plants of agricultural importance, most of which are angiosperms, are our main source of food. We use all parts of plants as food products: roots (like sweet potatoes and carrots), stems (like potatoes, cassava), leaves (like cabbage, lettuce), flowers (like broccoli), and fruits and seeds, including grains such as rice, wheat, corn, peas and beans, and an important set of fruits such as bananas, tomatoes, chili peppers, pineapples, kiwis, citrus, olives, and others too numerous to mention. Other plants are used as flavorings, among them there are herbs (such as parsley, sage, rosemary, thyme) and non-herb spices (such as cinnamon, vanilla, pepper), others are used as stimulating drinks, such as coffee, tea, chocolate, and cola, or as alcoholic beverages, such as beer, wine, distilled spirits, and sweet liqueurs.

Woody coniferous and angiosperm trees are used for lumber and to make pulp products such as paper. In tropical regions, bamboos, palm trees, and a variety of other species serve in the construction of human dwellings. Plant fibers are used to make rope such as sisal, bags such as burlap, and textiles, mainly cotton but also linen and hemp.

Fossil fuel deposits such as oil are derived from accumulated plant biomass.

In many cultures, plants or their products are used as euphorics or hallucinogens (legally or illegally), like marijuana, opium, cocaine, and a wide variety of other species that have been used by indigenous people for centuries.

Plants are important for their aesthetic beauty, and the cultivation of plants as ornamentals is an important industry.

Finally, plants have great importance in medicine, to treat a variety of diseases or to maintain good health.

Plant products are important in the pharmacological industry, their compounds are extracted, semi-synthesized, or used as a template to synthesize new drugs. Many "modern" drugs, from aspirin (originally extracted from willow bark) to vincristine and vinblastine (derived from Madagascar periwinkle and used to treat childhood leukemia) are ultimately derived from plants. In addition, various plant parts of a large number of species are used whole or are processed as so-called herbal supplements, which have become tremendously popular recently.

Plants that modify the composition of an ecosystem are also important to man, such as plants introduced in places from which they do not originate ("exotic plants") and those that are harmful For the economy of an agricultural system, they are considered pests or weeds.

Sciences that study plants

Botany is the science that studies most of the organisms that were traditionally treated as plants, including virtually all photosynthetic eukaryotic organisms (land plants and algae) plus other eukaryotic organisms that do not photosynthesize but possess cell walls and spores (fungi and some groups that were previously considered fungi, such as Oomycota), although the latter are increasingly studied in their own department of Mycology. Algae may also be in its own Phycology department.

Normally the other sciences that study plants take into account only terrestrial plants. Some have a purely practical orientation: agriculture deals with increasing the yield or resistance to diseases of food products, and horticulture deals with doing so in plants grown for ornamentals. For example, in these two sciences, hybridization studies are carried out and new cultivars are identified. Forestry (silviculture) deals with the cultivation and harvesting of trees used for their wood and pulp. Pharmacognosy is the branch of pharmacology that deals with natural drugs in their raw state, and they are usually of plant origin (although not necessarily).

In contrast to those more practical fields of plant science, the "pure" have as their objective the advancement of scientific knowledge, both "applied" like the "basics". Among the pure sciences are the anatomy of plants, which deals with the structure of cells and tissues and their structure, the phytochemistry and physiology of plants, which deals with biochemical and biophysical processes and their products, the molecular biology of plants, which deals with the structure and function of genetic material; plant ecology, which deals with their interactions with the environment; and plant systematics, which deals with plant taxonomy and phylogeny.

Annex: Plantae according to Whittaker, 1969

This author grouped organisms into kingdoms primarily on the basis of their modes of nutrition and cellular characteristics. In its classification Plantae contains, mainly, photosynthetic multicellular eukaryotic organisms, with cells with chloroplasts and with a cell wall (what some call plant cell, defined as the cell type of plants), organized in such a way that the cells possess at least certain degree of functional specialization. This definition corresponds to constituencies of that time of what are now red algae, brown algae, and green plants (containing "green algae" and land plants). The author included in these groups some organisms that did not correspond to this description, for example because they were unicellular or because their mode of nutrition was absorption, which were already known to be related to them. Plants thus defined, for the most part are autotrophic organisms: they "manufacture" their own "food", in this case photosynthetic: they use sunlight as a source of energy and store it in the form of chemical energy in the organic molecules they synthesize (see Chloroplast). They also explore the environment around them (usually through specialized organs such as roots) to absorb other essential nutrients, such as minerals and compounds with nitrogen and phosphorus, used to build proteins and other molecules they need to survive. Plants are also usually anchored to a substrate (for example through the roots themselves) and have specialized tissues to support them.

These groups have primarily sexual reproduction, with alternate haploid and diploid generation, each of which may or may not be multicellular (see under "Sexual reproduction and life cycles").

This classification includes three subkingdoms:

• Rhodophycophyta to red algae
• Phaeophycophyta algae pardas
• Euchlorophyta to green plants, among which are the "green algae" and land plants

Estimated number of species

The following table lists the estimated number of described plant species by IUCN class. The kingdom Plantae constitutes the second with the largest number of species after the kingdom Animalia.

Plant group	Image	Estimated number
Bryophyta (musgos)		16,236
Pteridophyta (facts and related)		12,000
Angiospermae (plants with flowers)		280,000
Gymnospermae (gimnospermas)		1,052
Chlorophyta (green algae)		4,242
Rhodophyta (red algae)		6,144
Other		10,000.
	Total	~323,674

References Cited