Chloroplasts are membrane structures in which photosynthesis takes place. This process in higher plants and cyanobacteria allowed the planet to maintain the ability to support life by utilizing carbon dioxide and replenishing oxygen concentration. Photosynthesis itself takes place in structures such as thylakoids. These are membrane "modules" of chloroplasts, in which proton transfer, water photolysis, glucose and ATP synthesis take place.
Structure of plant chloroplasts
Chloroplasts are called double-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 invaginations of the cytoplasm.
According to 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 the case ofcontact with ultraviolet light. Inside the chloroplast is its liquid stroma. In its composition, it resembles hyaloplasm and consists of 85% water, in which electrolytes are dissolved and proteins are suspended. The stroma of chloroplasts contains thylakoids, structures in which the light and dark phases of photosynthesis proceed directly.
Chloroplast hereditary apparatus
Next to the thylakoids there are granules with starch, which is a product of the polymerization of glucose obtained as a result of photosynthesis. Freely in the stroma are plastid DNA along with scattered ribosomes. There can 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 also makes it possible to judge the possibility of independent growth and reproduction of chloroplasts in the case of cell division. Therefore, chloroplasts, in some respects, do not depend on the nucleus of the cell and represent, as it were, a symbiotic underdeveloped organism.
Structure of thylakoids
Thylakoids are disc-shaped membrane structures located in the stroma of chloroplasts. In cyanobacteria, they are completely located on invaginations of the cytoplasmic membrane, since they do not have independent chloroplasts. There are two types of thylakoids: the first is a thylakoid with a lumen, and the second is a lamellar one. The thylakoid with a lumen is smaller in diameter and is a disc. Several thylakoids arranged vertically form a grana.
Lamellar thylakoids are wide plates that do not have a lumen. But they are a platform to which multiple grains are attached. In them, photosynthesis practically does not occur, since they are needed to form a strong structure that is resistant to mechanical damage to the cell. In total, chloroplasts can contain from 10 to 100 thylakoids with a lumen capable of photosynthesis. The thylakoids themselves are the elemental structures responsible for photosynthesis.
The role of thylakoids in photosynthesis
The most important reactions of photosynthesis take place in thylakoids. The first is the photolysis splitting of the water molecule and the synthesis of oxygen. The second is the transit of a proton through the membrane through the cytochrome b6f molecular complex and the electrotransport chain. Also in the thylakoids, the synthesis of the high-energy ATP molecule takes place. This process occurs with the use of a proton gradient that has developed between the thylakoid membrane and the chloroplast stroma. This means that the functions of the 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 greater than in mitochondrial membranes. It is more advantageous to take electrons and protons from water molecules to create an electrochemical potential in a cell. Under the action of an ultraviolet photon on the thylakoid membranes, this becomes available. An electron is knocked out of one water molecule, whichacquires a positive charge, and therefore, to neutralize it, it is necessary to drop one proton. As a result, 4 water molecules break down into electrons, protons and form oxygen.
The chain of photosynthesis processes
After the 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 that makes ATP synthesis possible. Adenosine triphosphate molecules will later be used for energy needs and the dark phase of photosynthesis. In particular, ATP is used by the cell to utilize carbon dioxide, which is achieved by its condensation and synthesis of glucose molecules based on them.
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 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 the energy of chemical bonds.
