The flora is one of the main wealth of ourthe planets. It is thanks to the flora on Earth that there is oxygen, which we all breathe, there is a huge food base, on which all living things depend. Plants are unique in that they can convert chemical compounds of inorganic nature into organic matter.
So called specific structures in whichprocesses of photosynthesis, which are aimed at the binding of carbon dioxide and the formation of certain carbohydrates. The byproduct is oxygen. These are organelles elongated in length, reaching 2-4 microns in width, their length reaching 5-10 microns. In some species of green algae, chloroplasts-giants are sometimes found elongated by 50 microns!
These algae may have differentpeculiarity: they have only one organoid of this species for the whole cell. In the cells of higher plants most often there are within 10-30 chloroplasts. However, in their case, there may be bright exceptions. Thus, in the palisade tissue of the usual shag, there are 1000 chloroplasts per cell. What are these chloroplasts for? Photosynthesis is their main, but far from the only role. In order 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 rest of the article.
Итак, что такое хлоропласт, мы узнали.And where did these organelles come from? How did it come about that plants had such a unique apparatus that turns carbon dioxide and water into complex organic compounds?
Currently, among the scientists prevailing pointof the endosymbiotic origin of these organoids, since their independent occurrence in plant cells is rather doubtful. It is well known that lichen is a symbiosis of algae and fungus. In this case, unicellular algae live inside the fungal cell. Now scientists assume that in time immemorial photosynthetic cyanobacteria penetrated inside plant cells and then partially lost their “independence”, transferring most of the genome to the nucleus.
Relatively recent hypothesis about prokaryoticthe origin of these elements was not very popular in the scientific community, many considered it to be “fabrications of amateurs”. But after an in-depth analysis of the nucleotide sequences in chloroplast DNA was carried out, this assumption was brilliantly confirmed. It turned out that these structures are extremely similar, even related, DNA of bacterial cells. Thus, a similar sequence was found in free-living cyanobacteria. In particular, the genes of the ATP-synthesizing complex turned out to be extremely similar, as well as in the “devices” of transcription and translation.
Promoters that determine the start of readinggenetic information from DNA, as well as terminal nucleotide sequences, which are responsible for its termination, are also organized in a manner similar to bacterial. Of course, billions of years of evolutionary changes have been able to make many changes to the chloroplast, but the sequences in the chloroplast genes remain completely the same. And this is irrefutable, complete proof that chloroplasts did indeed once have a prokaryotic ancestor. Perhaps it was an organism from which modern cyanobacteria also originated.
«Взрослый» органоид развивается из пропластиды.It is a small, completely colorless organelle, having only a few microns in diameter. It is surrounded by a dense bilayer membrane that contains a circular DNA specific to the chloroplast. These "ancestors" of organoids do not have an internal membrane system. Due to their extremely small size, their study is extremely difficult, and therefore there is very little data on their development.
It is known that several such protoplastidsavailable in the nucleus of each egg of animals and plants. During the development of the embryo, they divide and are transmitted to other cells. This is easy to verify: genetic traits that are somehow related to plastids are transmitted only along the maternal line.
The inner membrane of the protoplastids in timedevelopment bulges inside the organoid. Thylakoid membranes grow from these structures, which are responsible for the formation of granules and lamellae of the organelle stroma. In complete darkness, protopastide begins to transform into a precursor of a chloroplast (etioplast). This primary organoid is characterized by the fact that a rather complex crystal structure is located inside it. As soon as light gets on a leaf of a plant, it completely collapses. After this, the formation of the "traditional" internal structure of the chloroplast, which is formed by the very same thylakoids and lamellae, occurs.
Each meristem cell containsseveral such proplastids (their number varies depending on the type of plant and other factors). As this primary tissue begins to transform into a leaf, the precursors of the organoids turn into chloroplasts. Thus, the young wheat leaves that have completed their growth have chloroplasts in the amount of 100-150 pieces. Slightly more complicated is the situation with respect to those plants that are capable of starch accumulation.
We found out what chloroplast is by identifyingconnection of this organoid with the structures of prokaryotic organisms. Here the situation is similar: scientists have long found 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 type of chloroplast.
An analogy can be drawn between protoplastids andstem cells. Simply put, amyloplasts from some point begin to develop in a slightly different way. Scientists, however, learned something curious: they managed to achieve the mutual transformation of chloroplasts from potato leaves into amyloplasts (and vice versa). A canonical example, known to every schoolchild - potato tubers turn green in the light.
We know that in the process of fruit ripeningTomatoes, apples and some other plants (and in the leaves of trees, grasses and bushes in the autumn) the process of "degradation" occurs, when the chloroplasts in the plant cell are transformed into chromoplasts. These organelles contain coloring pigments and carotenoids.
The transformation is due to the fact that in certainunder conditions of complete destruction of the thylakoids, after which the organelle acquires a different internal organization. It is here that we again return to the question that we began to discuss at the very beginning of the article: the influence of the nucleus on the development of chloroplasts. It is through special proteins that are synthesized in the cytoplasm of cells that initiates the process of reorganization of the organoid.
After talking about issues of the origin and development of chloroplasts, it is necessary to elaborate on their structure. Especially since it is very interesting and deserves a separate discussion.
The main structure of chloroplasts consists of twolipoprotein 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 stick out inside the organoid. In mature chloroplasts, there are two types of such "tortuous" membranes. The former form the stromal lamellae, the latter - the thylakoid membranes.
It should be noted that there is a clear link,which has a chloroplast membrane 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 lamellae can form either a kind of "bag" or an extensive network. However, most often these structures are located parallel to each other and are not connected with each other.
The total number of grains contained inchloroplasts of higher plants, can reach up to 40-60. Each thilacoid fits so tightly to 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 adjacent to each other thylakoids and lamellae, are quite often.
In the places of their contact there is also a layersometimes 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 the state”. In some aspects, the structure of these organoids is no less complicated than the entire cellular structure!
Grana communicate with each other precisely bylamella But the thylakoid cavities, which form stacks, are always closed and in no way connected with the intermembrane space. As you can see, the structure of chloroplasts is quite complex.
What may be contained in the stroma of eachchloroplast? There are separate DNA molecules and a lot of ribosomes. In amyloplasts, it is in the stroma that starch grains are deposited. Accordingly, chromoplastics have colored pigments there. Of course, there are various chloroplast pigments, but the most common is chlorophyll. It is divided into several types at once:
In red and brown algae inChloroplasts do not rarely have completely different types of organic dyes. In some algae, almost all existing chloroplast pigments are generally contained.
Of course, their main function isconversion of light energy into organic components. Photosynthesis itself takes place in grana with the direct participation of chlorophyll. It absorbs the energy of sunlight, converting it into the energy of excited electrons. The latter, possessing an excess of its stock, give off excess energy, which is used for the decomposition of water and the synthesis of ATP. The decomposition of water produces oxygen and hydrogen. The first, as we wrote above, is a by-product and is released into the surrounding space, and hydrogen is bound to a special protein, ferredoxin.
The resulting ATP is extremely important as it isthe main "battery" of energy that goes to the 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: in the process of photosynthesis, there are many reactions that can not proceed without it.
Then chloroplast enzymes come into play,which act in the dark and outside the gran: 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 in good light, the accumulated compounds in the dark are used for the needs of the plants themselves.
You may rightly note that this process is suspiciously like breathing in some aspects. What is different from it photosynthesis? The table will help you understand this question.
Comparison items | Photosynthesis | Breath |
When happens | Only in the afternoon, in sunlight | Anytime |
Where is flowing | Chlorophyll Containing Cells | All living cells |
Oxygen | Allocation | Absorption |
CO2 | Absorption | Allocation |
Organic matter | Synthesis, partial cleavage | Splitting only |
Energy | Is absorbed | Stand out |
This is how photosynthesis differs from respiration. The table clearly shows their main differences.
Most of the further reactions take place here.same, in the stroma of the chloroplast. The further route of the synthesized substances is different. Thus, simple sugars immediately go beyond the boundaries of the organoid, accumulating in other parts of the cell in the form of polysaccharides, first of all - starch. In chloroplasts, both fat deposition and pre-accumulation of their precursors occur, which are then transported to other areas of the cell.
It should be clearly understood that all synthesis reactionsrequire an enormous amount of energy. Its only source is still the same photosynthesis. This is a process that often requires so much energy that it has to be obtained by destroying substances formed as a result of the previous synthesis! Thus, most of the energy that is obtained in its course is spent on conducting a variety of chemical reactions inside the plant cell itself.
It is believed that cellular organelles, includingThe number and chloroplasts (the structure and functions of which are described in detail by us) are strictly in one place. This is not true. Chloroplasts can move through the cell. So, in low light, they tend to occupy a position near the most illuminated side of the cell, in conditions of medium and low light, they can choose some intermediate positions at which they can “catch” most of the sunlight. This phenomenon is called "phototaxis".
Like mitochondria, chloroplasts arepretty 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 that are required for the construction of lamella shells. We have already talked about the prokaryotic origin of these organoids, but it should be added that some scientists consider chloroplasts to be long-time descendants of some parasitic organisms, which first became symbionts, and then turned into an integral part of the cell.
For plants, it is obvious - it is a synthesis of energy andsubstances that are used by plant cells. But photosynthesis is a process that ensures the continuous accumulation of organic matter on a global scale. From carbon dioxide, water and sunlight, chloroplasts can synthesize a huge amount of complex high-molecular compounds. This ability is characteristic only for them, and man is still far from repeating this process in artificial conditions.
We hope you learned from this article what chloroplast is and what its role is in the plant body.
