A green pigment found in the grana of chloroplasts. Chloroplasts, their composition, structure, properties and functions

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The entire process of photosynthesis takes place in green plastids - chloroplasts. There are three types of plastids: leucoplasts - colorless, chromoplasts - orange, chloroplasts - green. It is in chloroplasts that the green pigment chlorophyll is concentrated. Non-green plants, such as mushrooms, lack plastids. These plants do not have the ability to photosynthesize. In the process of evolution, plastid differentiation occurred very early. True, photosynthetic bacteria and blue-green algae do not yet have plastids; their role is played by the colored part of the protoplasm adjacent to the shell. This is the most primitive organization of the photosynthetic apparatus. However, algae already have special formations (chromatophores) in which pigments are concentrated; they are varied in shape (spiral, ribbon, in the form of plates or stars). Higher plants are characterized by a fully formed type of plastid in the shape of a disk or biconvex lens. Having taken the form of a disk, chloroplasts become a universal photosynthetic apparatus.

The chemical composition of chloroplasts is quite complex and is characterized by a high (75%) water content. About 75-80% of the total amount of dry matter comes from various organic compounds, 20-25% from mineral substances. The structural basis of chloroplasts are proteins, the content of which reaches 50-55% of dry weight, approximately half of them are water-soluble. Such a high protein content is explained by their diverse functions in chloroplasts. These are structural proteins that are the basis of membranes, enzyme proteins, transport proteins that maintain a certain ionic composition that differs from the cytosol, contractile proteins, similar to muscle actomyosin, which ensure the motor activity of chloroplasts. Proteins also perform a receptor function, taking part in the regulation of the intensity of photosynthesis under changing conditions of the internal and external environment.

The most important component of chloroplasts are lipids, the content of which ranges from 30 to 40% of dry weight. Chloroplast lipids are represented by three groups of compounds.

Carbohydrates are not constitutional substances of the chloroplast. In very small quantities, phosphorus esters of sugars participate in the carbon reduction cycle; these are mainly products of photosynthesis. Therefore, the carbohydrate content in chloroplasts varies significantly (from 5 to 50%). In actively functioning chloroplasts, carbohydrates usually do not accumulate; their rapid outflow occurs. With a decrease in the need for photosynthetic products, large starch grains are formed in the chloroplasts. In this case, the starch content can increase to 50% of the dry weight and the activity of chloroplasts will decrease.

Chloroplasts have a high mineral content. The chloroplasts themselves make up 25-30% of the leaf mass, but they contain up to 80% of iron, 70-72% of magnesium and zinc, about 50% of copper, 60% of calcium contained in the leaf tissues. These data are in good agreement with the high and diverse enzymatic activity of chloroplasts. Mineral elements act as prosthetic groups and cofactors for enzyme activity. Magnesium is part of chlorophyll. The important role of calcium is to stabilize the membrane structures of chloroplasts.

The structure of the chloroplast, observed using an electron microscope, is very complex. Like the nucleus and mitochondria, the chloroplast is surrounded by shell, consisting of two lipoprotein membranes. The internal environment is represented by a relatively homogeneous substance - matrix, or stroma, which the membranes penetrate - lamellae. Lamelae connected to each other form bubbles - thylakoids. Tightly adjacent to each other, thylakoids form grains, which can be distinguished even under a light microscope. In turn, the grains in one or several places are united with each other using intergranular strands - stromal thylakoids. Chloroplast pigments involved in capturing light energy, as well as enzymes necessary for the light phase of photosynthesis, are embedded in thylakoid membranes.

Fig.1. Chloroplast structure

1 - outer membrane; 2 - internal membrane; 3 - starch grain; 4 - DNA; 5 - stromal thylakoids (frets); 6 - thylakoid grana; 7 - matrix (stroma)

The structure of mature chloroplasts is the same in all higher plants, as well as in the cells of different organs of the same plant (leaves, green roots, bark, fruits). Depending on the functional load of cells, the physiological state of chloroplasts, and their age, the degree of their internal structure is distinguished: size, number of grains, connection between them. Thus, in the guard cells of stomata, the main function of chloroplasts is photoregulation of stomatal movements. This process is provided with energy by highly structured mitochondria. Chloroplasts contain large starch grains, swollen thylakoids, and lipophilic globules, which indicates their low energy load.

With age, the structure of chloroplasts changes significantly. Young chloroplasts are characterized by a lamellar structure; in this state, chloroplasts are able to reproduce by division. In mature chloroplasts, the gran system is well expressed. In aging chloroplasts, stromal thylakoids rupture, the connection between grana decreases, and subsequently the breakdown of chlorophyll and destruction of grana are observed. In autumn foliage, degradation of chloroplasts leads to the formation of chromoplasts, in which carotenoids are concentrated in plastoglobules.

Physiological features of chloroplasts

An important property of chloroplasts is their ability to move. Chloroplasts not only move together with the cytoplasm, but are also capable of spontaneously changing their position in the cell. The speed of movement of chlorollasts is about 0.12 µm/s. Chloroplasts can be distributed evenly throughout the cell, but more often they accumulate near the nucleus and near the cell walls. The direction and intensity of illumination are of great importance for the location of chloroplasts in the cell. At low light intensity, chloroplasts become perpendicular to the incident rays, which is an adaptation to better capture them. Under high illumination, chloroplasts move to the side walls and turn edge-on towards the incident rays. Depending on the lighting, the shape of chloroplasts can also change. At higher light intensity their shape becomes closer to spherical.

The main function of chloroplasts is the process of photosynthesis. In 1955, D. Arnon showed that the entire process of photosynthesis can be carried out in isolated chloron plasts. It is important to note that chloroplasts are not only found in leaf cells. They are found in cells of organs that are not specialized in photosynthesis: in stems, glumes and awns of ears, roots, potato tubers, etc. In some cases, green plastids are found in tissues located not in the outer, illuminated parts of plants, but in layers remote from light, in the tissues of the central cylinder of the stem, in the middle part of the lily bulb, as well as in the embryonic cells of the seeds of many angiosperms. The latter phenomenon (chlorophyll-bearing embryo) attracts the attention of plant taxonomists. There are proposals to divide all angiosperms into two large groups: chloroombryophytes and leukoembryophytes, i.e. those containing and not containing chloroplasts in the embryo (Yakovlev). Studies have shown that the structure of chloroplasts located in other plant organs, as well as the composition of pigments, are similar to leaf chloroplasts. This suggests that they are capable of photosynthesis.

If they are exposed to light, photosynthesis appears to actually occur. Thus, photosynthesis of chloroplasts located in the awns of the ear can account for about 30% of the total photosynthesis of the plant. Roots that turn green in the light are capable of photosynthesis. In chloroplasts, located in the peel of the fruit until a certain stage of its development, photosynthesis can also occur. According to the assumption of A.L. Kursanov, chloroplasts located near the conducting pathways, releasing oxygen, contribute to an increase in the intensity of metabolism of the sieve tubes. However, the role of chloroplasts is not limited to their ability to photosynthesize. In certain cases, they can serve as a source of nutrients (E.R. Gübbenet). Chloroplasts contain more vitamins, enzymes and even phytohormones (in particular, gibberellin). Under conditions in which assimilation is excluded, green plastids can play an active role in metabolic processes.

The flora is one of the main riches of our planet. It is thanks to the flora on Earth that there is oxygen, which we all breathe, and there is a huge food base on which all living things depend. Plants are unique in that they can convert inorganic chemical compounds into organic substances.

They do this through photosynthesis. This important process takes place in specific plant organelles; the smallest element actually ensures the existence of all life on the planet. By the way, what is a chloroplast?

Basic definition

This is the name given to specific structures in which photosynthesis processes occur, which are aimed at binding carbon dioxide and the formation of certain carbohydrates. The byproduct is oxygen. These are elongated organelles, reaching a width of 2-4 microns, their length reaches 5-10 microns. In some species, giant chloroplasts are sometimes found, elongated by 50 microns!

These same algae may have another feature: they have only one organelle of this species for the entire cell. Cells most often contain between 10 and 30 chloroplasts. However, in their case there may be striking exceptions. Thus, in the palisade tissue of a common shag there are 1000 chloroplasts per cell. What are these chloroplasts for? Photosynthesis is their main, but far from the only role. To clearly understand their significance in the life of a plant, it is important to know many aspects of their origin and development. All this is described in the further part of the article.

Origin of the chloroplast

So, we learned what a chloroplast is. Where did these organelles come from? How did it happen that plants developed such a unique apparatus that converts carbon dioxide and water into complex

Currently, the prevailing view among scientists is that these organelles have an endosymbiotic origin, since their independent occurrence in plant cells is rather doubtful. It is well known that lichen is a symbiosis of algae and fungus. while living inside. Now scientists suggest that in ancient times, photosynthetic cyanobacteria penetrated inside and then partially lost their “independence,” transferring most of the genome to the nucleus.

But the new organoid fully retained its main feature. We are talking about the process of photosynthesis. However, the apparatus itself, necessary to perform this process, is formed under the control of both the cell nucleus and the chloroplast itself. Thus, the division of these organelles and other processes associated with the implementation of genetic information on DNA are controlled by the nucleus.

Proof

Relatively recently, the hypothesis about the prokaryotic origin of these elements was not very popular in the scientific community; many considered it “the fabrications of amateurs.” But after an in-depth analysis of the nucleotide sequences in the DNA of chloroplasts was carried out, this assumption received brilliant confirmation. It turned out that these structures are extremely similar, even related, to the DNA of bacterial cells. Thus, a similar sequence was found in free-living cyanobacteria. In particular, the genes of the ATP-synthesizing complex, as well as in the “apparatuses” of transcription and translation, turned out to be extremely similar.

Promoters, which determine the beginning of reading genetic information from DNA, as well as terminal nucleotide sequences that are responsible for its termination, are also organized in the same way as bacterial ones. Of course, billions of years of evolutionary transformations were able to introduce many changes into the chloroplast, but the sequences in the chloroplast genes remained absolutely the same. And this is irrefutable, complete proof that chloroplasts actually once had a prokaryotic ancestor. It may have been the organism from which modern cyanobacteria also evolved.

Development of chloroplast from proplastid

The “adult” organelle develops from the proplastid. It is a small, completely colorless organelle, only a few microns across. It is surrounded by a dense bilayer membrane that contains circular DNA specific to the chloroplast. These “ancestors” of organelles do not have an internal membrane system. Due to their extremely small size, their study is extremely difficult, and therefore there is extremely little data on their development.

It is known that several such protoplastids are present in the nucleus of each egg cell of animals and plants. During the development of the embryo, they are divided and transmitted to other cells. This is easy to check: genetic traits that are somehow related to plastids are transmitted only through the maternal line.

The inner membrane of the protoplastid protrudes into the organelle during development. From these structures grow thylakoid membranes, which are responsible for the formation of grana and lamellae of the organoid stroma. In complete darkness, the protopastid begins to transform into a chloroplast precursor (ethioplast). This primary organelle is characterized by the fact that a rather complex crystalline structure is located inside it. As soon as light hits a plant leaf, it is completely destroyed. After this, the “traditional” internal structure of the chloroplast is formed, which is formed precisely by thylakoids and lamellae.

Differences between plants that store starch

Each meristem cell contains several of these proplastids (their number varies depending on the type of plant and other factors). Once this primary tissue begins to transform into a leaf, the organelle precursors develop into chloroplasts. Thus, young wheat leaves that have completed their growth have chloroplasts in the amount of 100-150 pieces. Things are a little more complicated with regard to those plants that are capable of accumulating starch.

They accumulate a supply of this carbohydrate in plastids, which are called amyloplasts. But what do these organelles have to do with the topic of our article? After all, potato tubers do not participate in photosynthesis! Let me explain this issue in more detail.

We found out what a chloroplast is, simultaneously revealing the connection of this organelle with the structures of prokaryotic organisms. Here the situation is similar: scientists have long found out that amyloplasts, like chloroplasts, contain exactly the same DNA and are formed from exactly the same protoplastids. Therefore, they should be considered in the same aspect. In fact, amyloplasts should be considered a special type of chloroplast.

How are amyloplasts formed?

An analogy can be drawn between protoplastids and stem cells. Simply put, amyloplasts at some point begin to develop along a slightly different path. Scientists, however, learned something interesting: they managed to achieve the mutual conversion of chloroplasts from potato leaves into amyloplasts (and vice versa). A canonical example known to every schoolchild - potato tubers turn green in the light.

Other information about the ways of differentiation of these organelles

We know that during the ripening of tomato fruits, apples and some other plants (and in the leaves of trees, grasses and shrubs in the autumn), a process of “degradation” occurs when chloroplasts in a plant cell turn into chromoplasts. These organelles contain coloring pigments and carotenoids.

This transformation is due to the fact that under certain conditions the thylakoids are completely destroyed, after which the organelle acquires a different internal organization. This is where we return again to the issue that we began to discuss at the very beginning of the article: the influence of the nucleus on the development of chloroplasts. It is this that, through special proteins that are synthesized in the cytoplasm of cells, initiates the process of restructuring the organelle.

Chloroplast structure

Having talked about the origin and development of chloroplasts, we should dwell in more detail on their structure. Moreover, it is very interesting and deserves a separate discussion.

The basic structure of chloroplasts consists of two lipoprotein membranes, internal and external. The thickness of each is about 7 nm, the distance between them is 20-30 nm. As in the case of other plastids, the inner layer forms special structures that protrude into the organelle. In mature chloroplasts, there are two types of such “winding” membranes. The former form stromal lamellae, the latter - thylakoid membranes.

Lamella and thylakoids

It should be noted that there is a clear connection that the chloroplast membrane has with similar formations located inside the organelle. The fact is that some of its folds can extend from one wall to another (like mitochondria). So the lamellae can form either a kind of “bag” or a branched network. However, most often these structures are located parallel to each other and are in no way connected with each other.

Do not forget that inside the chloroplast there are also membrane thylakoids. These are closed “bags” that are arranged in a stack. As in the previous case, there is a distance of 20-30 nm between the two walls of the cavity. Columns of these “bags” are called grana. Each column can contain up to 50 thylakoids, and in some cases there are even more. Since the overall “dimensions” of such stacks can reach 0.5 microns, they can sometimes be detected using an ordinary light microscope.

The total number of grains contained in the chloroplasts of higher plants can reach 40-60. Each thylakoid fits so tightly against the other that their outer membranes form a single plane. The thickness of the layer at the junction can reach up to 2 nm. Note that such structures, which are formed by thylakoids and lamellae adjacent to each other, are not uncommon.

At the places of their contact there is also a layer, sometimes reaching the same 2 nm. Thus, chloroplasts (the structure and functions of which are very complex) are not a single monolithic structure, but a kind of “state within a state.” In some aspects, the structure of these organelles is no less complex than the entire cellular structure!

The granae are connected to each other precisely with the help of lamellae. But the thylakoid cavities that form stacks are always closed and do not communicate in any way with the intermembrane space. As you can see, the structure of chloroplasts is quite complex.

What pigments can be contained in chloroplasts?

What can be contained in the stroma of each chloroplast? There are individual DNA molecules and quite a few ribosomes. In amyloplasts, it is in the stroma that starch grains are deposited. Accordingly, chromoplasts have coloring pigments there. Of course, there are various chloroplast pigments, but the most common is chlorophyll. It is divided into several types:

Group A (blue-green). It is found in 70% of cases and is found in the chloroplasts of all higher plants and algae.
Group B (yellow-green). The remaining 30% is also found in plants and algae of higher species.
Groups C, D and E are much less common. Found in the chloroplasts of some species of lower algae and plants.

It is not uncommon for red and brown seaweeds to contain completely different types of organic dyes in their chloroplasts. Some algae generally contain almost all existing chloroplast pigments.

Functions of chloroplasts

Of course, their main function is to convert light energy into organic components. Photosynthesis itself occurs in grana with the direct participation of chlorophyll. It absorbs the energy of sunlight, converting it into the energy of excited electrons. The latter, having an excess supply of it, give up excess energy, which is used for the decomposition of water and the synthesis of ATP. When water breaks down, oxygen and hydrogen are formed. The first, as we wrote above, is a by-product and is released into the surrounding space, and hydrogen binds to a special protein, ferredoxin.

It oxidizes again, transferring hydrogen to a reducing agent, which in biochemistry is abbreviated NADP. Accordingly, its reduced form is NADP-H2. Simply put, the process of photosynthesis releases the following substances: ATP, NADP-H2 and a byproduct in the form of oxygen.

The energetic role of ATP

The resulting ATP is extremely important, as it is the main “accumulator” of energy that goes to various needs of the cell. NADP-H2 contains a reducing agent, hydrogen, and this compound can easily give it away if necessary. Simply put, it is an effective chemical reducing agent: during the process of photosynthesis, many reactions occur that simply cannot occur without it.

Next, chloroplast enzymes come into play, which act in the dark and outside the grana: hydrogen from the reducing agent and ATP energy are used by the chloroplast to begin the synthesis of a number of organic substances. Since photosynthesis occurs under good light conditions, the accumulated compounds in the dark are used for the needs of the plants themselves.

You may rightly note that this process is in some respects suspiciously similar to breathing. How is photosynthesis different from it? The table will help you understand this issue.

Comparison points	Photosynthesis	Breath
When it happens	Only during the day, in sunlight	Anytime
Where does it leak		All living cells
Oxygen	Selection	Absorption
	Absorption	Selection
Organic matter	Synthesis, partial cleavage	Only splitting
Energy	Absorbed	Stands out

This is how photosynthesis differs from respiration. The table clearly shows their main differences.

Some "paradoxes"

Most of the further reactions take place right there, in the stroma of the chloroplast. The further path of the synthesized substances is different. Thus, simple sugars immediately leave the organelle, accumulating in other parts of the cell in the form of polysaccharides, primarily starch. In chloroplasts, both the deposition of fats and the preliminary accumulation of their precursors occur, which are then transported to other areas of the cell.

It should be clearly understood that all fusion reactions require enormous amounts of energy. Its only source is the same photosynthesis. This is a process that often requires so much energy that it must be obtained by destroying substances formed as a result of previous synthesis! Thus, most of the energy that is obtained during its course is spent on carrying out many chemical reactions inside the plant cell itself.

Only a certain proportion of it is used to directly obtain those organic substances that the plant takes for its own growth and development or deposits in the form of fats or carbohydrates.

Are chloroplasts static?

It is generally accepted that cellular organelles, including chloroplasts (the structure and functions of which we have described in detail), are located strictly in one place. This is wrong. Chloroplasts can move around the cell. Thus, in low light they tend to take a position near the most illuminated side of the cell; in conditions of medium and low illumination they can choose some intermediate positions in which they manage to “catch” the most sunlight. This phenomenon is called “phototaxis”.

For plants it is obvious - this is the synthesis of energy and substances that are used by plant cells. But photosynthesis is a process that ensures the constant accumulation of organic matter on a planetary scale. From carbon dioxide, water and sunlight, chloroplasts can synthesize a huge number of complex high-molecular compounds. This ability is characteristic only of them, and humans are still far from repeating this process under artificial conditions.

All biomass on the surface of our planet owes its existence to these tiny organelles, which are located in the depths of plant cells. Without them, without the photosynthesis process they carry out, there would be no life on Earth in its modern manifestations.

We hope you learned from this article about what a chloroplast is and what its role is in the plant body.

Chloroplasts are plastids of higher plants in which the process of photosynthesis occurs, i.e., the use of the energy of light rays to form organic substances from inorganic substances (carbon dioxide and water) with the simultaneous release of oxygen into the atmosphere. Chloroplasts have the shape of a biconvex lens, their size is about 4-6 microns. They are found in the parenchyma cells of leaves and other green parts of higher plants. Their number in a cell varies between 25-50.

The structure of the chloroplast, observed using an electron microscope, is very complex. Like the nucleus and mitochondria, the chloroplast is surrounded by a shell consisting of two lipoprotein membranes. The internal environment is represented by a relatively homogeneous substance - the matrix, or stroma, which is penetrated by membranes - lamellae. The lamellae connected to each other form vesicles - thylakoids. Closely adjacent to each other, thylakoids form grana, which can be distinguished even under a light microscope. In turn, the grana in one or several places are united with each other using intergranal strands - stromal thylakoids. Chloroplast pigments involved in capturing light energy, as well as enzymes necessary for the light phase of photosynthesis, are embedded in thylakoid membranes.

Chemical composition of chloroplasts: water - 75%; 75-80% of the total amount of dry matter is organic. compounds, 20-25% mineral.

The structural basis of chloroplasts are proteins (50-55% of dry weight), half of them are water-soluble proteins. Such a high protein content is explained by their diverse functions within chloroplasts (structural membrane proteins, enzyme proteins, transport proteins, contractile proteins, receptor proteins). The most important component of chloroplasts are lipids (30-40% dry weight).

Chloroplasts contain various pigments. Depending on the type of plant it is:

chlorophyll:
- chlorophyll A (blue-green) - 70% (in higher plants and green algae);
- chlorophyll B (yellow-green) - 30% (ibid.);
- chlorophyll C, D and E are less common - in other groups of algae;

carotenoids:
- orange-red carotenes (hydrocarbons);
- yellow (less often red) xanthophylls (oxidized carotenes). Thanks to the xanthophyll phycoxanthin, the chloroplasts of brown algae (pheoplasts) are colored brown;

· phycobiliproteins contained in rhodoplasts (chloroplasts of red and blue-green algae):
- blue phycocyanin;
- red phycoerythrin.

The chloroplast has its own DNA, that is, its own genome and its own apparatus for realizing genetic information through the synthesis of RNA and protein.

The main function of chloroplasts is to capture and convert light energy.

The membranes that form grana contain a green pigment - chlorophyll. It is here that the light reactions of photosynthesis occur - the absorption of light rays by chlorophyll and the conversion of light energy into the energy of excited electrons. Electrons excited by light, i.e., having excess energy, give up their energy to the decomposition of water and the synthesis of ATP. When water decomposes, oxygen and hydrogen are formed. Oxygen is released into the atmosphere, and hydrogen is bound by the protein ferredoxin.

Ferredoxin then oxidizes again, donating this hydrogen to a reducing agent called NADP. NADP goes into its reduced form - NADP-H2. Thus, the result of the light reactions of photosynthesis is the formation of ATP, NADP-H2 and oxygen, and water and light energy are consumed.

A lot of energy is accumulated in ATP - it is then used for synthesis, as well as for other needs of the cell. NADP-H2 is a hydrogen accumulator, and then easily releases it. Therefore, NADP-H2 is a chemical reducing agent. A large number of biosyntheses are associated precisely with reduction, and NADP-H2 acts as a supplier of hydrogen in these reactions.

Further, with the help of enzymes in the stroma of chloroplasts, i.e., outside the grana, dark reactions occur: hydrogen and the energy contained in ATP are used to reduce atmospheric carbon dioxide (CO2) and include it in the composition of organic substances. The first organic substance formed as a result of photosynthesis undergoes a large number of rearrangements and gives rise to the entire variety of organic substances synthesized in the plant and making up its body. A number of these transformations occur right there, in the stroma of the chloroplast, where there are enzymes for the formation of sugars, fats, as well as everything necessary for protein synthesis. The sugars can then either move from the chloroplast to other cell structures, and from there to other plant cells, or form starch, the grains of which are often seen in the chloroplasts. Fats are also deposited in chloroplasts, either in the form of drops, or in the form of simpler substances, precursors of fats, and exit the chloroplast.

Chloroplasts have a certain autonomy in the cell system. They have their own ribosomes and a set of substances that determine the synthesis of a number of their own proteins of the chloroplast. There are also enzymes, the work of which leads to the formation of lipids that make up the lamellae and chlorophyll. As we have seen, the chloroplast also has an autonomous system for producing energy. Thanks to all this, chloroplasts are able to independently build their own structures. There is even a view that chloroplasts (like mitochondria) originated from some lower organisms that settled in a plant cell and first entered into symbiosis with it, and then became its integral part, an organelle.

Federal Agency for Science and Education.

Siberian Federal University.

Institute of Fundamental Biology and Biotechnology.

Department of Biotechnology.

On the topic: Structure and functions of chloroplasts.

Plastid genome. Proplastids.

Completed by: student

31gr. Osipova I.V.

Checked:

associate professor of the department

biotechnology

Doctor of Biological Sciences Golovanova T.I.

Krasnoyarsk, 2008

Introduction. 3

Chloroplasts... 4

Functions of chloroplasts. 6

Plastid genome… 9

Proplastids… 13

Conclusion. 15

Literature. 16

Introduction.

Plastids are membrane organelles found in photosynthetic eukaryotic organisms (higher plants, lower algae, some unicellular organisms). Plastids are surrounded by two membranes; their matrix has its own genomic system; the functions of plastids are related to the energy supply of the cell, which is used for the needs of photosynthesis.

All plastids share a number of common features. They have their own genome, the same in all representatives of the same plant species, their own protein synthesizing system; Plastids are separated from the cytosol by two membranes - outer and inner. For some phototrophic organisms, the number of plastid membranes may be greater. For example, the plastids of euglena and dinflagellates are surrounded by three, and in golden, brown, yellow-green and diatom algae they have four membranes. This is due to the origin of plastids. It is believed that the symbiotic process, which resulted in the formation of plastids, occurred several times during evolution (at least three times).

A whole set of different plastids (chloroplast, leucoplast, amyloplast, chromoplast) have been found in higher plants, representing a series of mutual transformations of one type of plastid into another. The main structure that carries out photosynthetic processes is the chloroplast.

Chloroplasts.

Chloroplasts are structures in which photosynthetic processes occur, ultimately leading to the binding of carbon dioxide, the release of oxygen and the synthesis of sugars. They are elongated structures with a width of 2-4 microns and a length of 5-10 microns. Green algae have giant chloroplasts (chromatophores) reaching a length of 50 microns.

green algae may have one chloroplast per cell. Typically, there are an average of 10-30 chloroplasts per cell of higher plants. There are cells with a huge number of chloroplasts. For example, about 1000 chloroplasts were found in the giant cells of the palisade tissue of shag.

Chloroplasts are structures bounded by two membranes - internal and external. The outer membrane, like the inner one, has a thickness of about 7 microns; they are separated from each other by an intermembrane space of about 20-30 nm. The inner membrane of chloroplasts separates the plastid stroma, which is similar to the mitochondrial matrix. In the stroma of the mature chloroplast of higher plants, two types of internal membranes are visible. These are membranes that form flat, extended stromal lamellae, and membranes of thylakoids, flat disc-shaped vacuoles or sacs.

The stromal lamellae (about 20 µm thick) are flat hollow sacs or have the appearance of a network of branched and interconnected channels located in the same plane. Typically, the stromal lamellae inside the chloroplast lie parallel to each other and do not form connections with each other.

In addition to stromal membranes, membrane thylakoids are found in chloroplasts. These are flat, closed, disc-shaped membrane bags. The size of their intermembrane space is also about 20-30 nm. These thylakoids form coin-like stacks called grana.

The number of thylakoids per grana varies greatly: from a few to 50 or more. The size of such stacks can reach 0.5 microns, so the grains are visible in some objects in a light microscope. The number of grains in the chloroplasts of higher plants can reach 40-60. The thylakoids in the grana are close to each other so that the outer layers of their membranes are closely connected; at the junction of the thylakoid membranes, a dense layer about 2 nm thick is formed. In addition to the closed chambers of the thylakoids, the grana usually also includes sections of lamellae, which also form dense 2-nm layers at the points of contact of their membranes with the thylakoid membranes. The stromal lamellae thus seem to connect the individual grana of the chloroplast with each other. However, the cavities of the thylakoid chambers are always closed and do not pass into the chambers of the intermembrane space of the stromal lamellae. The stromal lamellae and thylakoid membranes are formed by separation from the inner membrane during the initial stages of plastid development.

DNA molecules and ribosomes are found in the matrix (stroma) of chloroplasts; This is also where the primary deposition of the reserve polysaccharide, starch, occurs in the form of starch grains.

A characteristic feature of chloroplasts is the presence of pigments, chlorophylls, which give color to green plants. With the help of chlorophyll, green plants absorb energy from sunlight and convert it into chemical energy.

Chloroplasts contain various pigments. Depending on the type of plant it is:

chlorophyll:

Chlorophyll A (blue-green) - 70% (in higher plants and green algae);

Chlorophyll B (yellow-green) - 30% (ibid.);

Chlorophyll C, D and E are less common in other groups of algae;

carotenoids:

Orange-red carotenes (hydrocarbons);

Yellow (less often red) xanthophylls (oxidized carotenes). Thanks to the xanthophyll phycoxanthin, the chloroplasts of brown algae (pheoplasts) are colored brown;

phycobiliproteins contained in rhodoplasts (chloroplasts of red and blue-green algae):

Blue phycocyanin;

Red phycoerythrin.

Functions of chloroplasts.

Chloroplasts are structures in which photosynthetic processes are carried out, ultimately leading to the binding of carbon dioxide, the release of oxygen and the synthesis of sugars.

A characteristic feature of chloroplasts is the presence of chlorophyll pigments, which give color to green plants. With the help of chlorophyll, green plants absorb the energy of sunlight and convert it into chemical energy. The absorption of light with a certain wavelength leads to a change in the structure of the chlorophyll molecule, and it passes into an excited, activated state. The released energy of activated chlorophyll is transferred through a series of intermediate stages to certain synthetic processes leading to the synthesis of ATP and the reduction of the electron acceptor NADPH (nicotinamide adenine dinucleotide phosphate) to NADP*H, which are spent in the reaction of CO2 binding and the synthesis of sugars.

The overall reaction of photosynthesis can be expressed as follows:

nCO2 + nH2 O-(CH2 O)n+nO2

Thus, the main final process here is the binding of carbon dioxide using water to form various carbohydrates and to release oxygen. The oxygen molecule, which is released during photosynthesis in plants, is formed due to the hydrolysis of a water molecule. Therefore, the process involves the process of hydrolysis of water, which serves as one of the sources of electrons or hydrogen atoms. Biochemical studies have shown that the process of photosynthesis is a complex chain of events consisting of 2 stages: light and dark. The first, which occurs only in light, is associated with the absorption of light by chlorophylls and the conduct of a photochemical reaction (Hill reaction). In the second phase, which can occur in the dark, CO2 fixation and reduction occur, leading to the synthesis of carbohydrates.

As a result of the light phase, photophosphorylation occurs, the synthesis of ATP from ADP and phosphate using the electron transport chain, as well as the reduction of the coenzyme NADP to NADPH, which occurs during the hydrolysis and ionization of water. During this phase of photosynthesis, the energy from sunlight excites electrons in chlorophyll molecules, which are located in the thylakoid membranes. These excited electrons are transported along the oxidative chain components in the thylakoid membrane, just as electrons are transported along the respiratory chain in the mitochondrial membrane. The energy released by this electron transfer is used to pump protons across the thylakoid membrane into the thylakoids, resulting in an increase in the potential difference between the stroma and the space within the thylakoid. As in the membranes of the mitochondrial cristae, the thylakoid membranes contain molecular complexes of ATP synthetase, which then begin to transport protons back into the chloroplast matrix, or stroma, and in parallel phosphorylate ADP, i.e., synthesize ATP.

Thus, as a result of the light phase, ATP synthesis and NADP reduction occur, which are then used in the reduction of CO2, in the synthesis of carbohydrates already in the dark phase of photosynthesis.

In the dark (independent of the photon flux) stage of photosynthesis, due to reduced NADP and ATP energy, atmospheric CO2 is bound, which leads to the formation of carbohydrates. The process of CO2 fixation and carbohydrate formation consists of many stages in which a large number of enzymes are involved (Calvin cycle). Biochemical studies have shown that enzymes involved in dark reactions are contained in the water-soluble fraction of chloroplasts, which contains components of the matrix-stroma of these plastids.

The process of CO2 reduction begins with its addition to ribulose diphosphate, a carbohydrate consisting of five carbon atoms, to form a short-lived C6 compound, which immediately breaks down into two C3 compounds, two molecules of glyceride-3-phosphate.

It is at this stage, during the carboxylation of ribulose diphosphate, that CO2 binding occurs. Further reactions of glyceride-3-phosphate conversion lead to the synthesis of various hesoses and pentoses, to the regeneration of ribulose diphosphate and to its new involvement in the cycle of CO2 binding reactions. Ultimately, in the chloroplast, one hexose molecule is formed from six CO2 molecules. This process requires 12 molecules of NADPH and 18 molecules of ATP, coming from the light reactions of photosynthesis. Fructose-6-phosphate, formed as a result of the dark reaction, gives rise to sugars, polysaccharides (starch) and galactolipids. In the stroma of chloroplasts, in addition, fatty acids, amino acids and starch are formed from part of the glyceride-3-phosphate. Sucrose synthesis is completed in the cytoplasm.

In the stroma of chloroplasts, nitrates are reduced to ammonia due to the energy of electrons activated by light; in plants, this ammonia serves as a source of nitrogen during the synthesis of amino acids and nucleotides.

Plastid genome.

Like mitochondria, chloroplasts have their own genetic system that ensures the synthesis of a number of proteins within the plastids themselves. DNA, various RNAs and ribosomes are found in the chloroplast matrix. It turned out that the DNA of chloroplasts differs sharply from the DNA of the nucleus. It is represented by cyclic molecules up to 40-60 microns in length, with a molecular weight of 0.8-1.3x108 daltons. There can be many copies of DNA in one chloroplast. Thus, in an individual corn chloroplast there are 20-40 copies of DNA molecules. The duration of the cycle and the rate of replication of nuclear and chloroplast DNA, as has been shown in green algae cells, do not coincide. Chloroplast DNA is not complexed with histones. All these characteristics of chloroplast DNA are close to the characteristics of the DNA of prokaryotic cells. Moreover, the similarity of the DNA of chloroplasts and bacteria is also reinforced by the fact that the main transcriptional regulatory sequences (promoters, terminators) are the same. All types of RNA (messenger, transfer, ribosomal) are synthesized on chloroplast DNA. Chloroplast DNA encodes rRNA, which is part of the ribosomes of these plastids, which belong to the prokaryotic 70S type (contain 16S and 23S rRNA). Chloroplast ribosomes are sensitive to the antibiotic chloramphenicol, which inhibits protein synthesis in prokaryotic cells.

Just as in the case of chloroplasts, we are again faced with the existence of a special protein synthesis system, different from that in the cell.

These discoveries renewed interest in the theory of the symbiotic origin of chloroplasts. The idea that chloroplasts arose by combining heterotrophic cells with prokaryotic blue-green algae, expressed at the turn of the 19th and 20th centuries. (A.S. Fomintsin, K.S. Merezhkovsky) again finds its confirmation. This theory is supported by the amazing similarity in the structure of chloroplasts and blue-green algae, the similarity with their main functional features, and primarily with the ability for photosynthetic processes.

There are numerous known facts of true endosymbiosis of blue-green algae with cells of lower plants and protozoa, where they function and supply the host cell with photosynthetic products. It turned out that isolated chloroplasts can also be selected by some cells and used by them as endosymbionts. In many invertebrates (rotifers, mollusks) that feed on higher algae, which they digest, intact chloroplasts end up inside the cells of the digestive glands. Thus, in some herbivorous mollusks, intact chloroplasts with functioning photosynthetic systems were found in the cells, the activity of which was monitored by the incorporation of C14 O2.

As it turned out, chloroplasts can be introduced into the cytoplasm of mouse fibroblast culture cells by pinocytosis. However, they were not attacked by hydrolases. Such cells, which included green chloroplasts, could divide for five generations, while the chloroplasts remained intact and carried out photosynthetic reactions. Attempts were made to cultivate chloroplasts in artificial media: chloroplasts could photosynthesize, RNA synthesis took place in them, they remained intact for 100 hours, and divisions were observed even within 24 hours. But then there was a drop in the activity of chloroplasts, and they died.

These observations and a number of biochemical works showed that those features of autonomy that chloroplasts possess are still insufficient for long-term maintenance of their functions, much less for their reproduction.

Recently, it was possible to completely decipher the entire sequence of nucleotides in the cyclic DNA molecule of chloroplasts of higher plants. This DNA can encode up to 120 genes, among them: genes of 4 ribosomal RNAs, 20 ribosomal proteins of chloroplasts, genes of some subunits of chloroplast RNA polymerase, several proteins of photosystems I and II, 9 of 12 subunits of ATP synthetase, parts of proteins of the electron transport chain complexes , one of the subunits of ribulose diphosphate carboxylase (the key enzyme for CO2 binding), 30 tRNA molecules and another 40 as yet unknown proteins. Interestingly, a similar set of genes in chloroplast DNA was found in such distant representatives of higher plants as tobacco and liver moss.

The bulk of chloroplast proteins is controlled by the nuclear genome. It turned out that a number of the most important proteins, enzymes, and, accordingly, the metabolic processes of chloroplasts are under the genetic control of the nucleus. Thus, the cell nucleus controls individual stages of the synthesis of chlorophyll, carotenoids, lipids, and starch. Many dark stage enzymes and other enzymes, including some components of the electron transport chain, are under nuclear control. Nuclear genes encode DNA polymerase and aminoacyl-tRNA synthetase of chloroplasts. Most of the ribosomal proteins are under the control of nuclear genes. All these data make us talk about chloroplasts, as well as mitochondria, as structures with limited autonomy.

The transport of proteins from the cytoplasm to plastids occurs in principle similar to that of mitochondria. Here, too, at the points of convergence of the outer and inner membranes of the chloroplast, channel-forming integral proteins are located, which recognize the signal sequences of chloroplast proteins synthesized in the cytoplasm and transport them to the matrix-stroma. From the stroma, imported proteins, according to additional signal sequences, can be included in plastid membranes (thylakoids, stromal lamellae, outer and inner membranes) or localized in the stroma, being part of ribosomes, enzyme complexes of the Calvin cycle, etc.

The amazing similarity of the structure and energy processes in bacteria and mitochondria, on the one hand, and in blue-green algae and chloroplasts, on the other, serves as a strong argument in favor of the theory of the symbiotic origin of these organelles. According to this theory, the emergence of a eukaryotic cell went through several stages of symbiosis with other cells. At the first stage, cells such as anaerobic heterotrophic bacteria included aerobic bacteria, which turned into mitochondria. In parallel, in the host cell, the prokaryotic genophore is formed into a nucleus isolated from the cytoplasm. This is how heterotrophic eukaryotic cells could arise. Repeated endosymbiotic relationships between primary eukaryotic cells and blue-green algae led to the appearance of chloroplast-type structures in them, allowing the cells to carry out autosynthetic processes and not depend on the presence of organic substrates. During the formation of such a composite living system, part of the genetic information of mitochondria and plastids could change and be transferred to the nucleus. For example, two thirds of the 60 ribosomal proteins of chloroplasts are encoded in the nucleus and synthesized in the cytoplasm, and then integrated into chloroplast ribosomes, which have all the properties of prokaryotic ribosomes. This movement of a large part of prokaryotic genes into the nucleus led to the fact that these cellular organelles, retaining part of their former autonomy, came under the control of the cell nucleus, which largely determines all the main cellular functions.

Proplastids.

Under normal lighting, proplastids turn into chloroplasts. First, they grow, with the formation of longitudinally located membrane folds from the inner membrane. Some of them extend along the entire length of the plastid and form stromal lamellae; others form thylakoid lamellae, which are stacked to form the grana of mature chloroplasts. Plastid development occurs somewhat differently in the dark. In etiolated seedlings, the volume of plastids, etioplasts, initially increases, but the system of internal membranes does not build lamellar structures, but forms a mass of small vesicles that accumulate in separate zones and can even form complex lattice structures (prolamellar bodies). The membranes of etioplasts contain protochlorophyll, a yellow precursor of chlorophyll. Under the influence of light, chloroplasts are formed from etioplasts, protochlorophyll is converted into chlorophyll, new membranes, photosynthetic enzymes and components of the electron transport chain are synthesized.

When cells are illuminated, membrane vesicles and tubes quickly reorganize, and from them a complete system of lamellae and thylakoids, characteristic of a normal chloroplast, develops.

Leukoplasts differ from chloroplasts in the absence of a developed lamellar system. They are found in the cells of storage tissues. Due to their indeterminate morphology, leucoplasts are difficult to distinguish from proplastids and sometimes from mitochondria. They, like proplastids, are poor in lamellae, but nevertheless are capable of forming normal thylakoid structures under the influence of light and acquiring a green color. In the dark, leucoplasts can accumulate various reserve substances in the prolamellar bodies, and grains of secondary starch are deposited in the stroma of leucoplasts. If the so-called transient starch is deposited in chloroplasts, which is present here only during CO2 assimilation, then true starch storage can occur in leucoplasts. In some tissues (endosperm of cereals, rhizomes and tubers), the accumulation of starch in leucoplasts leads to the formation of amyloplasts, completely filled with reserve starch granules located in the stroma of the plastid.

Another form of plastid in higher plants is the chromoplast, which usually turns yellow as a result of the accumulation of carotenoids in it. Chromoplasts are formed from chloroplasts and much less frequently from their leucoplasts (for example, in carrot roots). The process of bleaching and changes in chloroplasts is easily observed during the development of petals or when fruits ripen. In this case, yellow-colored droplets (globules) may accumulate in the plastids, or bodies in the form of crystals may appear in them. These processes are associated with a gradual decrease in the number of membranes in the plastid, with the disappearance of chlorophyll and starch. The process of formation of colored globules is explained by the fact that when the lamellae of chloroplasts are destroyed, lipid droplets are released in which various pigments (for example, carotenoids) are well dissolved. Thus, chromoplasts are degenerating forms of plastids, subject to lipophanerosis - the disintegration of lipoprotein complexes.

Conclusion.

Plastids. Plastids are special organelles of plant cells in which

synthesis of various substances is carried out, and primarily photosynthesis.

There are three main types of plastids in the cytoplasm of higher plant cells:

1) green plastids - chloroplasts; 2) painted red, orange and

other colors of chromoplasts; 3) colorless plastids - leucoplasts. All these types of plastids can transform into one another. In lower plants, for example algae, one type of plastid is known - chromatophores. The process of photosynthesis in

of higher plants occurs in chloroplasts, which, as a rule, develop only in the light.

Externally, chloroplasts are bounded by two membranes: outer and inner. According to electron microscopy, the chloroplasts of higher plants contain a large number of grana arranged in groups. Each

The grana consists of numerous round plates, shaped like flat bags, formed by a double membrane and stacked with each other like a column of coins. The granae are connected to each other by means of special plates or tubes located in the stroma of the chloroplast and forming

unified system. Only grana contain green pigment in chloroplasts; their stroma is colorless.

The chloroplasts of some plants contain only a few grains, while others contain up to fifty or more.

In green algae, photosynthesis occurs in chromatophores that do not contain grana, and the products of primary synthesis - various carbohydrates - are often deposited around special cellular structures called pyrenoids.

The color of chloroplasts depends not only on chlorophyll; they may also contain other pigments, such as carotene and carotenoids, colored in different colors - from yellow to red and brown, as well as phycobilins. The latter include phycocyanin and phycoerythrin from red and blue-green algae. Plastids develop from special cellular structures called proplastids. Proplastids are colorless formations that are similar in appearance to mitochondria, but differ from them in their larger size and the fact that they always have an elongated shape. On the outside, plastids are bounded by a double membrane; a small number of membranes are also located in their internal part. Plastids reproduce by fission, and control over this process is apparently exercised by the DNA contained in them. During division, the plastid is constricted, but the division of plastids can also occur through the formation of a septum. The ability of plastids to divide ensures their continuity in a series of cell generations. During sexual and asexual reproduction of plants, plastids are transferred to daughter organisms.

Literature.

1) Yu.S.Chentsov. Introduction to cell biology./Yu.S. Chentsov.-M.: ICC “Akademkniga”, 2005-495 pp.: ill.

2) Plant Physiology: Textbook for students / N.D. Alyokhina, Yu.V. Balnokin, V.F. Gavrilenko, T.V. Zhigalova, N.R. Meichik, A.M. Nosov, O.G. Polesskaya, E.V. Kharitonashvili; Ed. I.P.Ermakova.-M.: publishing center “Academy”, 2005.-640 p.

Its shell consists of two membranes - external and internal, between which there is an intermembrane space. Inside the chloroplast, by detaching from the inner membrane, a complex thylakoid structure is formed. The gel-like contents of the chloroplast are called stroma.

Each thylakoid is separated from the stroma by a single membrane. The interior space of the thylakoid is called the lumen. Thylakoids in the chloroplast they are combined into stacks - grains. The number of grains varies. They are connected to each other by special elongated thylakoids - lamellae. An ordinary thylakoid looks like a rounded disk.

The stroma contains the chloroplast's own DNA in the form of a circular molecule, RNA and prokaryotic-type ribosomes. Thus, it is a semi-autonomous organelle capable of independently synthesizing some of its proteins. It is believed that in the process of evolution, chloroplasts originated from cyanobacteria that began living inside another cell.

The structure of the chloroplast is determined by the function of photosynthesis. Reactions associated with it occur in the stroma and on thylakoid membranes. In the stroma - the reactions of the dark phase of photosynthesis, on the membranes - the light phase. Therefore, they contain different enzymatic systems. The stroma contains soluble enzymes involved in the Calvin cycle.

Thylakoid membranes contain pigments chlorophylls and carotenoids. All of them are involved in capturing solar radiation. However, they catch different spectra. The predominance of one or another type of chlorophyll in a certain group of plants determines their shade - from green to brown and red (in a number of algae). Most plants contain chlorophyll a.

The structure of the chlorophyll molecule consists of a head and a tail. The carbohydrate tail is immersed in the thylakoid membrane, and the head faces the stroma and is located in it. The energy of sunlight is absorbed by the head, leading to the excitation of an electron, which is picked up by carriers. A chain of redox reactions is started, ultimately leading to the synthesis of a glucose molecule. Thus, the energy of light radiation is converted into the energy of chemical bonds of organic compounds.

Synthesized organic substances can accumulate in chloroplasts in the form of starch grains, and are also removed from it through the membrane. There are also fat droplets in the stroma. However, they are formed from lipids of destroyed thylakoid membranes.

In the cells of autumn leaves, chloroplasts lose their typical structure, turning into chromoplasts, in which the internal membrane system is simpler. In addition, chlorophyll is destroyed, causing carotenoids to become noticeable, giving the foliage yellow-red hues.

The green cells of most plants usually contain many chloroplasts, shaped like a ball slightly elongated in one direction (volume ellipse). However, a number of algae cells may contain one huge chloroplast of a bizarre shape: ribbon-shaped, star-shaped, etc.

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