Chloroplasts are membrane structures in which photosynthesis occurs. This process in higher plants and cyanobacteria has allowed the planet to maintain the ability to maintain life by utilizing carbon dioxide and replenishing oxygen concentration. Photosynthesis itself occurs in structures such as thylakoids. These are membrane “modules” of chloroplasts in which proton transfer, photolysis of water, synthesis of glucose and ATP proceed.
The structure of plant chloroplasts
Chloroplasts are called two-membrane structures that are located in the cytoplasm of plant cells and chlamydomonas. In contrast, cyanobacterial cells carry out photosynthesis in thylakoids, and not in chloroplasts. This is an example of an underdeveloped organism that is able to provide its nutrition through photosynthesis enzymes located on the invaginations of the cytoplasm.
By its structure, the chloroplast is a two-membrane organelle in the form of a bubble. They are located in large numbers in the cells of photosynthetic plants and develop only in case of contact with ultraviolet light. Inside the chloroplast is its liquid stroma. In its composition, it resembles a hyaloplasm and for 85% consists of water in which electrolytes are dissolved and proteins are weighed. The stroma of chloroplasts contains thylakoids, structures in which the light and dark phase of photosynthesis directly proceeds.
Chloroplast Hereditary Apparatus
Next to thylakoids are granules with starch, which is a product of the polymerization of glucose resulting from photosynthesis. Loosely located in the stroma are DNA plastids along with scattered ribosomes. There may be several DNA molecules. Together with the biosynthetic apparatus, they are responsible for restoring the structure of chloroplasts. This happens without using the hereditary information of the cell nucleus. This phenomenon allows us to judge the possibility of independent growth and reproduction of chloroplasts in the case of cell division. Therefore, chloroplasts in some respects are independent of the cell nucleus and represent, as it were, a symbiotic underdeveloped organism.
Thylakoid structure
Thylakoids are disk-shaped membrane structures located in the stroma of chloroplasts. In cyanobacteria, they are completely located on the protrusions of the cytoplasmic membrane, since they do not have independent chloroplasts. There are two types of thylakoids: the first is a thylakoid with lumen, and the second is lamellar. The thylakoid with lumen is smaller in diameter and is a disk. Several thylakoids arranged vertically form a granule.
Lamellar thylakoids are wide plates that do not have lumens. But they are a platform to which multiple grains are attached. Photosynthesis practically does not proceed in them, since they are necessary for the formation of a strong structure resistant to mechanical damage to the cell. In total, chloroplasts can contain from 10 to 100 thylakoids with lumen, capable of photosynthesis. Thylakoids themselves are the elementary structures responsible for photosynthesis.
The role of thylakoids in photosynthesis
The most important photosynthesis reactions occur in thylakoids. The first is the photolysis of a water molecule and the synthesis of oxygen. The second is the transit of the proton through the membrane through the cytochrome b6f molecular complex and the electric transport chain. Also, synthesis of the macroergic ATP molecule proceeds in thylakoids. This process occurs using the proton gradient formed between the thylakoid membrane and the chloroplast stroma. This means that the functions of thylakoids make it possible to realize the entire light phase of photosynthesis.
Light phase of photosynthesis
A necessary condition for the existence of photosynthesis is the ability to create a membrane potential. It is achieved through the transfer of electrons and protons, due to which an H + gradient is created, which is 1000 times larger than in mitochondrial membranes. It is more profitable to take electrons and protons to create an electrochemical potential in a cell from water molecules. Under the influence of an ultraviolet photon on the membranes of thylakoid, this becomes available. An electron is knocked out of one water molecule, which acquires a positive charge, and therefore, to neutralize it, it is necessary to drop one proton. As a result, 4 water molecules decompose into electrons, protons and form oxygen.
Photosynthesis process chain
After photolysis of water, the membrane is recharged. Thylakoids are structures that can have an acidic pH during proton transfer. At this time, the pH in the stroma of the chloroplast is slightly alkaline. This generates an electrochemical potential, due to which ATP synthesis becomes possible. Adenosine triphosphate molecules will later be used for energy needs and the dark phase of photosynthesis. In particular, ATP is used by a cell to utilize carbon dioxide, which is achieved by its condensation and synthesis of glucose molecules on their basis.
Along the way, in the dark phase, NADP-H + is reduced to NADP. In total, the synthesis of one glucose molecule requires 18 ATP molecules, 6 carbon dioxide molecules and 24 hydrogen protons. This requires the photolysis of 24 water molecules to utilize 6 carbon dioxide molecules. This process allows you to release 6 oxygen molecules, which will later be used by other organisms for their energy needs. At the same time, thylakoids are (in biology) an example of a membrane structure that allows the use of solar energy and a transmembrane potential with a pH gradient to convert them into energy of chemical bonds.