One of the main differences between a plant and an animal cell is the presence of such organelles as plastids in the cytoplasm of the first. The structure, features of their life processes, as well as the importance of chloroplasts, chromoplasts and leukoplasts will be considered in this article.
The structure of chloroplast
Green plastids, the structure of which we will now study, are obligatory organelles of cells of higher spore and seed plants. They are two-membrane cell organoids and have an oval shape. Their number in the cytoplasm may be different. For example, the cells of the columnar parenchyma of the tobacco leaf blade 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 contains many folds called lamellae. They are adjoined by flat bags - thylakoids. The lamellae form a network consisting of parallel tubules. Between the lamellae are thylakoid bodies. They are collected in piles - grains that can be interconnected. Their number in one chloroplast is 60–150. The entire internal cavity of the chloroplast is filled with a matrix.
Organella has signs of autonomy: its own hereditary material is circular DNA, due to which chloroplasts can multiply. There is also a closed external 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 that they are capable of protein synthesis.
Thylakoid functions
As mentioned earlier, plant cell plastids, the chloroplasts, contain special flattened sacs called thylakoids. They found pigments - chlorophylls (taking part in photosynthesis) and carotenoids (performing supporting and trophic functions). There is also an enzymatic system that provides reactions of the light and dark phases of photosynthesis. Thylakoids act as antennas: they focus light quanta and direct them to chlorophyll molecules.
Photosynthesis - the main process of chloroplasts
Autotrophic cells are able to independently synthesize organic substances, in particular glucose, using carbon dioxide and light energy. Green plastids, the functions of which we are currently studying, are an integral part of phototrophs - multicellular organisms, such as:
- higher spore plants (mosses, horsetails, crowns, ferns);
- seed (gymnosperms - gingival, coniferous, 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 "perceive" 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 the membranes of thylakoids under the influence of light energy. The absorbed light quanta excite the electrons of the magnesium atoms that make up the green pigment - chlorophyll.
Electron energy 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 glucose molecules, amino acids, glycerol and fatty acids, which serve as the building and trophic material of the cell.
Types of Plastids
Green plastids, the structure and functions of which we examined earlier, are 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 leukoplasts.
Colorless organelles (leukoplasts) have a different shape and differ from chloroplasts in that their internal 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.
Leukoplasts also have synthetase enzymes involved in the formation of starch molecules from glucose. As a result, colorless plastids of the plant cell accumulate reserve nutrients: protein granules and starch grains. These plastids, whose functions are to accumulate organic substances, can turn into chromoplasts, for example, in the process of ripening of tomatoes that are in the stage of milk ripeness.
Under a scanning microscope with a high resolution, differences in the structure of all three types of plastids are clearly visible. This primarily concerns chloroplasts, which have the most complex structure associated with the function of photosynthesis.
Chromoplasts - Colored Plastids
Along with green and colorless in the cells of plants there is a third kind of organelles, called chromoplasts. They have a variety of colors: yellow, purple, red. Their structure is similar to leukoplasts: the inner membrane has a small amount 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 coloring of root crops of beets, carrots, fruits of fruit trees and berries.
How do plastids arise and mutually transform?
Leukoplasts, chromoplasts, chloroplasts - plastids (the structure and functions of which we are studying), having a common origin. They are derivatives of meristematic (educational) tissues, from which protoplastids are formed - two-membered saccular organelles up to 1 μm in size. In the light, they complicate their structure: an inner membrane containing lamellae is formed, and the green pigment chlorophyll is synthesized. Protoplastides become chloroplasts. Leukoplasts can also turn under the influence of light energy into green plastids, and then into chromoplasts. Modification of plastids 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 using bacteriochlorophyll A, whose molecules are located on the internal outgrowths of the cytoplasmic membrane. Microbiologists consider the chromatophores of bacteria to be the precursors of plastids.
This is confirmed by their similar structure to chloroplasts, namely, the presence of reaction centers and light-collecting systems, as well as the general results of photosynthesis, leading to the formation of organic compounds. It should be noted that the lower plants - green algae, like prokaryotes, do not have plastids. This is due to the fact that chlorophyll-containing formations - chromatophores, took over their function - photosynthesis.
How did chloroplasts originate?
Among the many hypotheses of the origin of plastids, we 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. They subsequently 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 importance in cellular life.