One of the main differences between plant and animal cells is the presence in the cytoplasm of the first organelles such as plastids. The structure, features of their vital processes, as well as the significance of chloroplasts, chromoplasts and leukoplasts will be discussed in this article.
Chloroplast structure
Green plastids, the structure of which we will now study, belong to the obligatory organelles of cells of higher spore and seed plants. They are double-membrane cellular organelles and have an oval shape. Their number in the cytoplasm can be different. For example, the cells of the columnar parenchyma of a leaf blade of tobacco contain up to a thousand chloroplasts, in the stems of plants of the cereal family from 30 to 50.
Both membranes that make up the organoid have a different structure: the outer one is smooth, three-layered, similar to the membrane of the plant cell itself. The inner one contains many folds called lamellae. Adjacent to them are flat sacs - thylakoids. The lamellae form a network ofparallel tubules. Between the lamellae are thylakoid bodies. They are collected in stacks - grains that can be connected to each other. Their number in one chloroplast is 60–150. The entire internal cavity of the chloroplast is filled with matrix.
Organella has signs of autonomy: its own hereditary material - circular DNA, thanks to which chloroplasts can multiply. There is also a closed outer membrane that limits the organelle from the processes occurring in the cytoplasm of the cell. Chloroplasts have their own ribosomes, i-RNA and t-RNA molecules, which means they are capable of protein synthesis.
Thylakoid functions
As mentioned earlier, plant cell plastids - chloroplasts - contain special flattened sacs called thylakoids. Pigments were found in them - chlorophylls (participating in photosynthesis) and carotenoids (performing supporting and trophic functions). There is also an enzymatic system that provides the reactions of the light and dark phases of photosynthesis. Thylakoids act as antennas: they focus light quanta and direct them to chlorophyll molecules.
Photosynthesis is the main process of chloroplasts
Autotrophic cells are capable of independently synthesizing organic substances, in particular glucose, using carbon dioxide and light energy. Green plastids, whose functions we are currently studying, are an integral part of phototrophs - multicellular organisms, such as:
- higher spore plants (mosses, horsetails, club mosses,ferns);
- seeds (gymnosperms - ginga, conifers, ephedra and angiosperms or flowering plants).
Photosynthesis is a system of redox reactions, which are based on the process of electron transfer from donor substances to compounds that “receive” them, the so-called acceptors.
These reactions lead to the synthesis of organic substances, in particular glucose, and the release of molecular oxygen. The light phase of photosynthesis occurs on thylakoid membranes under the action of light energy. The absorbed light quanta excite the electrons of the magnesium atoms that make up the green pigment - chlorophyll.
Energy of electrons is used for the synthesis of energy-intensive substances: ATP and NADP-H2. They are cleaved by the cell for dark phase reactions occurring in the chloroplast matrix. The combination of these synthetic reactions leads to the formation of molecules of glucose, amino acids, glycerol and fatty acids, which serve as the building and trophic material of the cell.
Plastid types
Green plastids, the structure and functions of which we discussed earlier, are found in leaves, green stems and are not the only species. So, in the skin of fruits, in the petals of flowering plants, in the outer covers of underground shoots - tubers and bulbs, there are other plastids. They are called chromoplasts or leucoplasts.
Colorless organelles (leucoplasts) have a different shape and differ from chloroplasts in that theythe inner cavity does not have thin plates - lamellae, and the number of thylakoids immersed in the matrix is small. The matrix itself contains deoxyribonucleic acid, protein-synthesizing organelles - ribosomes and proteolytic enzymes that break down proteins and carbohydrates.
Leucoplasts also have enzymes - synthetases involved in the formation of starch molecules from glucose. As a result, colorless plant cell plastids accumulate reserve nutrients: protein granules and starch grains. These plastids, whose function is to accumulate organic substances, can turn into chromoplasts, for example, during the ripening of tomatoes that are in the stage of milky ripeness.
Under a high-resolution scanning microscope, differences in the structure of all three types of plastids are clearly visible. This, first of all, concerns chloroplasts, which have the most complex structure associated with the function of photosynthesis.
Chromoplasts - colored plastids
Along with green and colorless plant cells, there is a third type of organelle called chromoplasts. They have a variety of colors: yellow, purple, red. Their structure is similar to leukoplasts: the inner membrane has a small number of lamellae and a small number of thylakoids. Chromoplasts contain various pigments: xanthophylls, carotenes, carotenoids, which are auxiliary photosynthetic substances. It is these plastids that provide the color of the roots of beets, carrots, fruits of fruit trees and berries.
How do they ariseand mutually transform plastids
Leukoplasts, chromoplasts, chloroplasts are plastids (the structure and functions of which we are studying) that have a common origin. They are derivatives of meristematic (educational) tissues, from which protoplastids are formed - two-membrane sac-like organelles up to 1 micron in size. In the light, they complicate their structure: an inner membrane containing lamellae is formed, and the green pigment chlorophyll is synthesized. Protoplastids become chloroplasts. Leukoplasts can also be transformed by light energy into green plastids and then into chromoplasts. Plastid modification is a widespread phenomenon in the plant world.
Chromatophores as precursors of chloroplasts
Prokaryotic phototrophic organisms - green and purple bacteria, carry out the process of photosynthesis with the help of bacteriochlorophyll A, the molecules of which are located on the inner outgrowths of the cytoplasmic membrane. Microbiologists consider bacterial chromatophores to be precursors of plastids.
This is confirmed by their similar structure to chloroplasts, namely the presence of reaction centers and light-trapping systems, as well as the general results of photosynthesis, leading to the formation of organic compounds. It should be noted that lower plants - green algae, like prokaryotes, do not have plastids. This is due to the fact that chlorophyll-containing formations - chromatophores, have taken over their function - photosynthesis.
How chloroplasts originated
Among many hypothesesorigin of plastids, let us dwell on symbiogenesis. According to his ideas, plastids are cells (chloroplasts) that arose in the Archean era as a result of the penetration of phototrophic bacteria into the primary heterotrophic cell. It was they who later led to the formation of green plastids.
In this article, we studied the structure and functions of two-membrane organelles of a plant cell: leukoplasts, chloroplasts and chromoplasts. And also found out their significance in cellular life.
