The plant world is one of the main treasures 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 chemical compounds of an inorganic nature into organic substances.
They do this through photosynthesis. This most important process takes place in specific plant organoids,
chloroplasts. This smallest element actually ensures the existence of all life on the planet. By the way, what is chloroplast?
Basic definition
This is the name of specific structures in which photosynthesis processes take place, which are aimed at binding carbon dioxide and the formation of certain carbohydrates. A byproduct is oxygen. These are organoids elongated in length, reaching a width of 2-4 microns, their length reaches 5-10 microns. In some species of green algae , giant chloroplasts elongated by 50 microns are sometimes found!
The same algae may have another feature: for the whole cell they have only one organoid of this species. In the cells of higher plants, most often there are between 10-30 chloroplasts. However, in their case, there may be bright 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 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 chloroplast
So, what is chloroplast, we learned. Where did these organoids come from? How did it happen that plants have such a unique apparatus that turns carbon dioxide and water into complex organic compounds?
Currently, among scientists, the prevailing point of view is 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. At the same time, unicellular algae live inside the mushroom cell. Now scientists suggest that in time immemorial photosynthetic cyanobacteria penetrated into plant cells, and then partially lost their "independence", transferring most of the genome to the nucleus.
But the new organoid fully retained its main feature. This is just about the process of photosynthesis. However, the apparatus itself, which is necessary for performing this process, is formed under the control of both the cell nucleus and the chloroplast itself. So, the division of these organelles and other processes associated with the implementation of genetic information into DNA are controlled by the nucleus.
Proof of
Relatively recently, the hypothesis about the prokaryotic 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 nucleotide sequences in chloroplast DNA was carried out, 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 turned out to be extremely similar, as well as in the “apparatuses” of transcription and translation.
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 of bacterial ones. Of course, billions of years of evolutionary transformations have been able to make many changes to the chloroplast, but the sequences in the chloroplast genes have remained completely the same. And this is irrefutable, complete proof that chloroplasts actually once had a prokaryotic ancestor. Perhaps it was an organism from which modern cyanobacteria also originated.
Chloroplast development from proplastid
An "adult" organoid develops from proplastid. This is a small, completely colorless organelle, with only a few microns across. It is surrounded by a dense bilayer membrane that contains circular DNA specific for 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 extremely little data on their development.
It is known that several such protoplastids are present 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 associated with plastids are transmitted only through the maternal line.
The inner membrane of protoplastide during development develops inside the organoid. Thylakoid membranes grow from these structures, which are responsible for the formation of gran and lamellae of the organoid stroma. In complete darkness, protopastide begins to transform into a precursor to the chloroplast (etioplast). This primary organoid 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 this, the formation of the “traditional” internal structure of the chloroplast occurs, which is formed precisely by thylakoids and lamellae.
Distinctions of plants storing starch
Each meristemal cell contains several such 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, organoid precursors 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 in relation to those plants that are capable of starch accumulation.
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 clarify this question in more detail.
We found out what chloroplast is, simultaneously revealing the connection 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 kind of chloroplast.
How are amyloplasts formed?
An analogy can be drawn 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 conversion of chloroplasts from potato leaves to amyloplasts (and vice versa). A canonical example known to every student - potato tubers turn green in the light.
Other information on the ways of differentiation of these organoids
We know that in the process of ripening of the fruits of tomato, apples and some other plants (and in the leaves of trees, herbs and shrubs in the autumn period), the process of “degradation” occurs 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 there is a complete destruction of thylakoids, after which the organelle acquires a different internal organization. Here we are again returning to the issue that we began to discuss at the very beginning of the article: the effect 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 reorganization of the organoid.
The structure of chloroplast
After talking about the origin and development of chloroplasts, we should dwell 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 inside the organoid. In mature chloroplasts, there are two types of such “winding” membranes. The former form lamellae of the stroma, the latter form thylakoid membranes.
Lamellas and thylakoids
It should be noted that there is a clear connection that the chloroplast membrane has with similar structures 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 in any way.
Do not forget that inside the chloroplast there are also membrane thylakoids. These are closed "bags", which 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 total “dimensions” of such stacks can reach 0.5 μm, sometimes they can be detected using an ordinary light microscope.
The total number of granules that are contained in the chloroplasts of higher plants can reach 40-60. Each thylakoid is so tightly attached 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 similar structures, which are formed by adjacent thylakoids and lamellae, are quite common.
In 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 the state." In some aspects, the structure of these organoids is no less complicated than the entire cellular structure!
Granes communicate with each other precisely with the help of lamellae. But the thylakoid cavities that form the stacks are always closed and do not communicate with the intermembrane 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, various chloroplast pigments are found, but chlorophyll is the most common. It is divided into several types at once:
- Group A (blue-green). It occurs in 70% of cases, is found in 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. Available in chloroplasts of some species of lower algae and plants.
Red and brown algae in chloroplasts are not so rare to have completely different types of organic dyes. In some algae, almost all existing chloroplast pigments are contained in general.
Chloroplast Functions
Of course, their main function is the conversion of light energy into organic components. Photosynthesis itself occurs in grains with the direct participation of chlorophyll. It absorbs the energy of sunlight, translating it into the energy of excited electrons. The latter, having its excess supply, give up excess energy, which is used to decompose water and synthesize ATP. In the decay of water, 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 denoted by the abbreviation NADP. Accordingly, its reduced form is NADP-H2. Simply put, in the process of photosynthesis, the following substances are released: ATP, NADP-H2 and a by-product in the form of oxygen.
The energy role of ATP
The resulting ATP is extremely important, as it is the main "accumulator" of energy that goes to the various needs of the cell. NADP-H2 contains a reducing agent, hydrogen, and this compound is able to easily give it if necessary. Simply put, it is an effective chemical reducing agent: in the process of photosynthesis, there are many reactions that without it simply could not proceed.
Next, chloroplast enzymes come into play, which act in the dark and beyond the gran: hydrogen from the reducing agent and ATP energy are used by chloroplast in order to start the synthesis of a number of organic substances. Since photosynthesis occurs in conditions of good illumination, the accumulated compounds in the dark are used for the needs of the plants themselves.
You can rightly notice that this process in some aspects is suspiciously like breathing. What is the difference between photosynthesis? The table will help you understand this issue.
Comparison points | Photosynthesis | Breath |
When happens | Only in the afternoon, in the sunlight | Anytime |
Where does it flow | Cells containing chlorophyll | All living cells |
Oxygen | Selection | Absorption |
CO2 | Absorption | Selection |
Organic matter | Synthesis, Partial Cleavage | Splitting only |
Energy | Absorbed | Stands out |
This is the difference between photosynthesis and respiration. The table clearly shows their main differences.
Some "paradoxes"
Most of the further reactions proceed right there, in the stroma of the chloroplast. The further path of the synthesized substances is different. So, simple sugars immediately go beyond the limits of 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 take place, which are then removed to other areas of the cell.
It should be clearly understood that all synthesis reactions require a tremendous amount of energy. Its only source is the same photosynthesis. This is a process that often requires so much energy that you have to get it, destroying the substances formed as a result of the previous synthesis! Thus, most of the energy that is generated in its course is spent on conducting many chemical reactions inside the plant cell itself.
Only a fraction of it is used to directly obtain those organic substances that the plant takes for its own growth and development or stores 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 are described in detail by us), are located in exactly one place. This is not true. Chloroplasts can move around the cell. So, in low light, they tend to occupy a position near the most illuminated side of the cell, in conditions of moderate and low light they can choose some intermediate positions at which it is possible to "catch" most of the sunlight. This phenomenon is called “phototaxis”.
Like mitochondria, chloroplasts are quite autonomous organoids. 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 organoids, but it should be added that some scientists consider chloroplasts to be long-standing descendants of some parasitic organisms, which first became symbionts, and then completely turned into an integral part of the cell.
The value of chloroplasts
For plants, it is obvious - it is a 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 global scale. From carbon dioxide, water and sunlight, chloroplasts can synthesize a huge number of complex macromolecular compounds. This ability is characteristic only for them, and the person is far from repeating this process in artificial conditions.
All biomass on the surface of our planet owes its existence to these smallest organoids, which are located in the depths of plant cells. Without them, without the process of photosynthesis carried out by them on Earth, there would be no life in its modern manifestations.We hope you learned from this article about what chloroplast is and what is its role in the plant body.