Modifications of plastids are a common phenomenon in the plant world. Plastids: structure, functions

One of the main differences between a plant and an animal cell is the presence in the cytoplasm of the first such organelles, as plastids. The structure, features of the processes of their vital activity, as well as the importance of chloroplasts, chromoplasts and leukoplasts will be considered in this article.

Structure of chloroplast

Green plastids, the structure of which we now study, refer to the compulsory organelles of cells of higher spore and seed plants. They are two-membrane cellular organelles and have an oval shape. Their number in the cytoplasm may 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, which are part of the organoid, have a different structure: the outer membrane is smooth, three-layered, similar to the membrane of the plant cell itself. The inner contains many folds, called lamellas. They are joined by flat sacs - thylakoids. Lamellae form a network consisting of parallel tubules. Between the lamellas there are calf-thylakoids. They are collected in stacks - granules, which can be connected together. Their quantity in one chloroplast is 60-150. The entire internal cavity of the chloroplast is filled with a matrix.

The organelle has signs of autonomy: its own hereditary material is the ring DNA, through which chloroplasts can multiply. There is also a closed outer membrane, which limits the organelle from the processes taking place in the cytoplasm of the cell. Chloroplasts have their own ribosomes, molecules and-RNA and t-RNA, and therefore are capable of protein synthesis.

Functions of thylakoids

As it was said earlier, plastids of a plant cell - chloroplasts, contain in their composition 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 ensures the reaction of the light and dark phases of photosynthesis. The thylakoids function as antennas: they focus light quanta and direct them to chlorophyll molecules.

Photosynthesis is the main process of chloroplasts

Autotrophic cells are able to independently synthesize organic substances, in particular glucose, using carbon dioxide and light energy. The green plastids, whose functions we are now studying, are an integral part of phototrophs - multicellular organisms such as:

• Higher spore plants (mosses, horsetails, mosses, ferns);
• Seed (gymnosperms - gingovye, conifers, ephedra and angiosperms or flowering plants).

Photosynthesis is a system of oxidation-reduction reactions, based on the process of transfer of electrons from donor substances to compounds that "perceive" them, 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 action of light energy. Absorbed quanta of light excite the electrons of the magnesium atoms that make up the green pigment - chlorophyll.

The energy of electrons is used to synthesize energy-intensive substances: ATP and NADP-H2. They are cleaved by the cell for the reactions of the dark phase occurring in the matrix of chloroplasts. 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

The green plastids, the structure and functions of which we considered earlier, are in leaves, green stems and are not the only species. So, in the skin of fruit, in the petals of flowering plants, in the outer covers of underground shoots - tubers and bulbs, there are other plastids. They are called chromoplastics or leukoplasts.

Colorless organelles (leukoplasts) have a different shape and differ from chloroplasts in that their internal cavity does not have thin 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 enzymes - synthetases, involved in the formation of starch molecules from glucose. As a result, colorless plastids of plant cells accumulate spare nutrients: protein granules and starch grains. These plastids, whose functions consist in the accumulation of organic substances, can turn into chromoplasts, for example, during the maturation of tomatoes in the stage of milk ripeness.

Under a scanning microscope having a high resolving power, the differences in the structure of all three kinds of plastids are clearly visible. This, first of all, concerns chloroplasts, which have the most complex structure, connected with the function of photosynthesis.

Chromoplast - colored plastids

Along with green and colorless in plant cells 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 lamellas and an insignificant 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

Leukoplasts, chromoplasts, chloroplasts are plastids (the structure and functions of which we study) having a single origin. They are derived from meristematic (educational) tissues, from which protoplastides are formed - two-membrane sack-shaped organelles up to 1 μm in size. In the light, they complicate their structure: an inner membrane containing lamellae is formed, and a green pigment chlorophyll is synthesized. Protoplastides become chloroplasts. Leukoplasts can also transform under the action of light energy into green plastids, and then into chromoplasts. Modifications of plastids are 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, molecules of which are located on the internal outgrowths of the cytoplasmic membrane. Microbiologists consider chromatophores of bacteria as precursors of plastids.

This is confirmed by their structure similar to chloroplasts, namely, the presence of reaction centers and light capture 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 explained by the fact that the chlorophyll-containing formations - chromatophores, took upon themselves their function - photosynthesis.

How did chloroplasts originate?

Among the many hypotheses of the origin of plastids, let us dwell on symbiogenesis. According to his ideas, plastids are cells (chloroplasts), which arose in the Archean era due to 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 the two-membrane organelles of plant cells: leukoplasts, chloroplasts and chromoplasts. And also found out their significance in cellular life.