The plant world is one of the main we alth of our planet. It is thanks to the flora on Earth that there is oxygen that we all breathe, 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 most important process takes place in specific plant organelles, chloroplasts. This 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 of the specific structures in which the processes of photosynthesis take place, which are aimed at the binding of carbon dioxide and the formation of certain carbohydrates. The by-product is oxygen. These are elongated organelles, reaching a width of 2-4 microns, their length reaches 5-10 microns. Some species of green algae sometimes have giant chloroplasts, elongated by 50 microns!
The same algae can haveanother feature: for the whole cell they have only one organelle of this species. In the cells of higher plants, most often there are within 10-30 chloroplasts. However, in their case, there may be striking exceptions. So, in the palisade tissue of ordinary 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 plant life, it is important to know many aspects of their origin and development. All this is described in the rest of the article.
The origin of the chloroplast
So, what is a chloroplast, we learned. 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 organic compounds?
Currently, among scientists, the point of view of the endosymbiotic origin of these organelles prevails, since their independent occurrence in plant cells is rather doubtful. It is well known that lichen is a symbiosis of algae and fungus. Unicellular algae live inside the mushroom cell. Now scientists suggest that in ancient times, photosynthetic cyanobacteria penetrated into plant cells, and then partially lost their “independence”, transferring most of the genome to the nucleus.
But the new organoid retained its main feature in full. It's just about the process of photosynthesis. However, the apparatus itself, necessary to perform this process, is formed undercontrol 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 into DNA are controlled by the nucleus.
Evidence
Relatively recently, the hypothesis of the prokaryotic origin of these elements was not very popular in the scientific community, many considered it "inventions of amateurs." But after an in-depth analysis of the nucleotide sequences in the DNA of chloroplasts, this assumption was brilliantly confirmed. It turned out that these structures are extremely similar, even related, to the DNA of bacterial cells. So, a similar sequence was found in free-living cyanobacteria. In particular, the genes of the ATP-synthesizing complex, as well as in the "machines" of transcription and translation, turned out to be extremely similar.
Promoters that determine the start of reading genetic information from DNA, as well as terminal nucleotide sequences that are responsible for its termination, are also organized in the image and likeness of bacterial ones. Of course, billions of years of evolutionary transformations could make many changes to the chloroplast, but the sequences in the chloroplast genes remained absolutely the same. And this is irrefutable, complete proof that chloroplasts indeed once had a prokaryotic ancestor. It may have been the organism from which modern cyanobacteria also evolved.
Chloroplast development from proplastids
"Adult" organoid develops from proplastids. This is a small, completely colorlessan organelle that is only a few microns across. It is surrounded by a dense bilayer membrane that contains chloroplast-specific circular DNA. 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 of these protoplastids are present in the nucleus of each egg cell of animals and plants. During the development of the embryo, they divide and are transferred to other cells. This is easy to verify: genetic traits that are somehow associated with plastids are transmitted only through the maternal line.
The inner membrane of the protoplastid protrudes into the organoid during development. From these structures, thylakoid membranes grow, which are responsible for the formation of granules and lamellae of the stroma of the organoid. In complete darkness, the protopastid begins to transform into the precursor of the chloroplast (etioplast). This primary organelle is characterized by the fact that a rather complex crystalline structure is located inside it. As soon as light hits the leaf of the plant, it is completely destroyed. After that, the formation of the "traditional" internal structure of the chloroplast occurs, which is formed just by thylakoids and lamellae.
Differences in starch storage plants
Each meristem cell contains several of these proplastids (their number varies depending on the type of plant and other factors). As soon as this primary tissue begins to transform into a leaf, the precursor organelles turn into chloroplasts. So,young wheat leaves that have completed their growth have chloroplasts in the amount of 100-150 pieces. Things are a little more complicated for those plants that are capable of accumulating starch.
They store this carbohydrate in plastids called amyloplasts. But what do these organelles have to do with the topic of our article? After all, potato tubers are not involved in photosynthesis! Let me clarify this issue in more detail.
We found out what a chloroplast is, along the way revealing the connection of this organoid 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 as a special kind of chloroplast.
How are amyloplasts formed?
One can draw an analogy between protoplastids and stem cells. Simply put, amyloplasts from some point begin to develop along a slightly different path. Scientists, however, learned something interesting: they managed to achieve the mutual transformation of chloroplasts from potato leaves into amyloplasts (and vice versa). The canonical example, known to every schoolchild, is that potato tubers turn green in the light.
Other information about the ways of differentiation of these organelles
We know that in the process of ripening the fruits of tomatoes, apples and some other plants (and in the leaves of trees, grasses and shrubs in the autumn)"degradation", when chloroplasts in a plant cell turn into chromoplasts. These organelles contain coloring pigments, 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. Here we again return to the issue that we started discussing at the very beginning of the article: the influence of the nucleus on the development of chloroplasts. It is it, through special proteins that are synthesized in the cytoplasm of cells, that initiates the process of restructuring the organoid.
Chloroplast structure
Having talked about the origin and development of chloroplasts, we should dwell on their structure in more detail. Moreover, it is very interesting and deserves a separate discussion.
The basic structure of chloroplasts consists of two lipoprotein membranes, inner and outer. 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 organoid. In mature chloroplasts, there are two types of such "tortuous" membranes at once. The former form stromal lamellae, the latter form 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 organoid. The fact is that some of its folds can extend from one wall to another (as in mitochondria). So the lamellae can form either a kind of "bag" or a branchednetwork. However, most often these structures are located parallel to each other and are not connected in any way.
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. The columns of these "bags" are called grains. 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 adheres so tightly to the other that their outer membranes form a single plane. The layer thickness at the junction can be up to 2 nm. Note that such structures, which are formed by adjacent thylakoids and lamellae, are not uncommon.
In 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!
Granas are interconnected precisely with the help of lamellae. But the cavities of thylakoids, which form stacks, are always closed and do not communicate with the intermembrane in any way.space. As you can see, the structure of chloroplasts is quite complex.
What pigments can be found in chloroplasts?
What can be contained in the stroma of each chloroplast? There are individual DNA molecules and many 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 at once:
- Group A (blue-green). It occurs in 70% of cases, is contained in the chloroplasts of all higher plants and algae.
- Group B (yellow-green). The remaining 30% is also found in higher species of plants and algae.
- Groups C, D and E are much rarer. Found in the chloroplasts of some species of lower algae and plants.
It is not uncommon for red and brown seaweeds to have completely different types of organic dyes in their chloroplasts. Some algae generally contain almost all existing chloroplast pigments.
Chloroplast functions
Of course, their main function is to convert light energy into organic components. Photosynthesis itself occurs in grains with the direct participation of chlorophyll. It absorbs the energy of sunlight, converting it into the energy of excited electrons. The latter, having its excess supply, give off 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 as NADP. Accordingly, its reduced form is NADP-H2. Simply put, photosynthesis produces the following substances: ATP, NADP-H2, and a by-product in the form of oxygen.
The energy role of ATP
The formed 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 is able to easily give it away if necessary. Simply put, it is an effective chemical reducing agent: in the process of photosynthesis, many reactions take place that simply cannot proceed without it.
Next, chloroplast enzymes come into play, which act in the dark and outside the gran: hydrogen from the reducing agent and the energy of ATP are used by the chloroplast in order to start the synthesis of a number of organic substances. Since photosynthesis occurs in conditions of good light, the accumulated compounds are used for the needs of the plants themselves during the dark time of the day.
You can rightly notice that this process is suspiciously similar to breathing in some aspects. How is photosynthesis different from it? The table will help you understand this issue.
| Comparison items | Photosynthesis | Breathing |
| When it happens | Daytime only, in sunlight | Anytime |
| Where it leaks | Chlorophyll containing cells | All living cells |
| Oxygen | Highlight | Absorption |
| CO2 | Absorption | Highlight |
| Organic matter | Synthesis, partial splitting | Split only |
| Energy | Swallowing up | 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. So, simple sugars immediately go beyond the organoid, 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 excreted to other areas of the cell.
It should be clearly understood that all fusion reactions require an enormous amount of energy. Its only source is the same photosynthesis. This is a process that often requires so much energy that it has to be obtained,destroying the substances formed as a result of the previous synthesis! Thus, most of the energy that is obtained in its course is spent on carrying out many chemical reactions within the plant cell itself.
Only some 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 not true. Chloroplasts can move around the cell. So, in low light, they tend to take a position near the most illuminated side of the cell, in conditions of medium and low light, they can choose some intermediate positions in which they manage to “catch” the most sunlight. This phenomenon is called "phototaxis".
Like mitochondria, chloroplasts are fairly autonomous organelles. They have their own ribosomes, they synthesize a number of highly specific proteins that are used only by them. There are even specific enzyme complexes, during the work of which special lipids are produced, which are required for the construction of lamella shells. We have already talked about the prokaryotic origin of these organelles, but it should be added that some scientists consider chloroplasts to be ancient descendants of some parasitic organisms that first became symbionts, and then completelyhave become an integral part of the cell.
The importance of chloroplasts
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 for them, and a person is still far from repeating this process in artificial conditions.
All biomass on the surface of our planet owes its existence to these smallest organelles, which are located in the depths of plant cells. Without them, without the process of photosynthesis carried out by them, there would be no life on Earth in its modern manifestations.
We hope you have learned from this article what a chloroplast is and what its role is in a plant organism.
