Modern experimental studies have established that the cell is the most complex structural and functional unit of almost all living organisms, with the exception of viruses, which are non-cellular life forms. Cytology studies the structure, as well as the vital activity of the cell: respiration, nutrition, reproduction, growth. These processes will be considered in this paper.
Cell structure
By using a light and electron microscope, biologists have established that plant and animal cells contain a surface apparatus (supra-membrane and sub-membrane complexes), cytoplasm and organelles. In animal cells, a glycocalyx is located above the membrane, which contains enzymes and provides nutrition to the cell outside the cytoplasm. In plant cells, prokaryotes (bacteria and cyanobacteria), as well as fungi, a cell wall is formed above the membrane, which consists of cellulose, lignin or murein.
The nucleus is an essential organelleeukaryotes. It contains hereditary material - DNA, which looks like chromosomes. Bacteria and cyanobacteria contain a nucleoid that acts as a carrier of deoxyribonucleic acid. All of them perform strictly specific functions that determine the metabolic cellular processes.
What do we mean by cellular nutrition
The vital manifestations of a cell are nothing but the transfer of energy and its transformation from one form to another (according to the first law of thermodynamics). The energy found in nutrients in a latent, i.e., bound state, passes into ATP molecules. To the question of what is cell nutrition in biology, there is an answer that takes into account the following postulates:
- The cell, being an open biosystem, requires a constant supply of energy from the external environment.
- Organic substances needed for nutrition, the cell can get in two ways:
a) from the intercellular medium, in the form of ready-made compounds;
b) independently synthesizing proteins, carbohydrates and fats from carbon dioxide, ammonia, etc.
Therefore, all organisms are divided into heterotrophic and autotrophic, the metabolic features of which are studied by biochemistry.
Metabolism and energy
Organic substances entering the cell undergo splitting, as a result of which energy is released in the form of ATP or NADP-H2 molecules. The whole set of assimilation and dissimilation reactions is metabolism. Below we will consider the stages of energy metabolism that provide nutrition for heterotrophic cells. First proteins, carbohydrates and lipidsare broken down to their monomers: amino acids, glucose, glycerol and fatty acids. Then, during oxygen-free digestion, they undergo further breakdown (anaerobic digestion).
In this way, intracellular parasites are fed: rickettsia, chlamydia and pathogenic bacteria, such as clostridium. Unicellular yeast fungi break down glucose to ethyl alcohol, lactic acid bacteria to lactic acid. Thus, glycolysis, alcohol, butyric, lactic acid fermentation are examples of cell nutrition due to anaerobic digestion in heterotrophs.
Autotrophy and features of metabolic processes
For organisms living on Earth, the main source of energy is the Sun. Thanks to him, the needs of the inhabitants of our planet are met. Some of them synthesize nutrients due to light energy, they are called phototrophs. Others - with the help of the energy of redox reactions, they are called chemotrophs. In unicellular algae, the nutrition of the cell, the photo of which is presented below, is carried out photosynthetically.
Green plants contain chlorophyll, which is part of chloroplasts. It plays the role of an antenna that captures light quanta. In the light and dark phases of photosynthesis, enzymatic reactions occur (the Calvin cycle), which result in the formation of all organic substances used for nutrition from carbon dioxide. Therefore, the cell, which is nourisheddue to the use of light energy, is called autotrophic or phototrophic.
Single-celled organisms, called chemosynthetics, use the energy released as a result of chemical reactions to form organic substances, for example, iron bacteria oxidize ferrous compounds to ferric iron, and the released energy goes to the synthesis of glucose molecules.
Thus, photo-synthetic organisms capture light energy and convert it into the energy of covalent bonds of mono- and polysaccharides. Then, along the links of the food chains, energy is transferred to the cells of heterotrophic organisms. In other words, thanks to photosynthesis, all the structural elements of the biosphere exist. It can be said that a cell, the nutrition of which occurs in an autotrophic way, “feeds” not only itself, but also everything living on planet Earth.
How heterotrophic organisms eat
A cell whose nutrition depends on the intake of organic substances from the external environment is called heterotrophic. Organisms such as fungi, animals, humans, and parasitic bacteria break down carbohydrates, proteins, and fats using digestive enzymes.
Then, the resulting monomers are absorbed by the cell and used by it to build their organelles and life. Dissolved nutrients enter the cell by pinocytosis, while solid food particles enter the cell by phagocytosis. Heterotrophic organisms can be divided into saprotrophs and parasites. The former (for example, soil bacteria, fungi, some insects) feed on dead organic matter, the latter (pathogenic bacteria, helminths, parasitic fungi) feed on cells and tissues of living organisms.
Mixotrophs, their distribution in nature
Mixed type of nutrition in nature is quite rare and is a form of adaptation (idioadaptation) to various environmental factors. The main condition for mixotrophy is the presence in the cell of both organelles containing chlorophyll for photosynthesis, and a system of enzymes that break down ready-made nutrients coming from the environment. For example, the unicellular animal Euglena green contains chromatophores with chlorophyll in the hyaloplasm.
When the reservoir in which euglena lives is well lit, it feeds like a plant, i.e. autotrophically, through photosynthesis. As a result, glucose is synthesized from carbon dioxide, which the cell uses as food. Euglena feeds heterotrophically at night, breaking down organic matter with the help of enzymes located in the digestive vacuoles. Thus, scientists consider the mixotrophic nutrition of the cell to be proof of the unity of the origin of plants and animals.
Cell growth and its relationship with trophism
An increase in the length, mass, volume of both the whole organism and its individual organs and tissues is called growth. It is impossible without a constant supply of nutrients to the cells, which serve as a building material. To get an answer to the question of how a cell grows, the nutrition of whichoccurs autotrophically, it is necessary to clarify whether it is an independent organism or whether it is part of a multicellular individual as a structural unit. In the first case, growth will be carried out during the interphase of the cell cycle. The processes of plastic exchange intensively take place in it. The nutrition of heterotrophic organisms is correlated with the presence of food coming from the external environment. The growth of a multicellular organism occurs due to the activation of biosynthesis in educational tissues, as well as the predominance of anabolic reactions over catabolism processes.
The role of oxygen in the nutrition of heterotrophic cells
Aerobic organisms: Some bacteria, fungi, animals and humans use oxygen to completely break down nutrients like glucose into carbon dioxide and water (the Krebs cycle). It occurs in the matrix of mitochondria containing the enzymatic system H + -ATP-ase, which synthesizes ATP molecules from ADP. In prokaryotic organisms, such as aerobic bacteria and cyanobacteria, the oxygen dissimilation step occurs at the plasma membrane of the cells.
Specific nutrition of gametes
In molecular biology and cytology, cell nutrition can be briefly described as the process of nutrients entering it, their splitting and synthesis of a certain portion of energy in the form of ATP molecules. The trophism of gametes: eggs and spermatozoa has some features associated with the high specificity of their functions. This is especially true of the female germ cell, which is forced to accumulate a large supply of nutrients, mainly in the form ofyolk.
After fertilization, she will use them to crush and form an embryo. Spermatozoa in the process of maturation (spermatogenesis) receive organic substances from Sertoli cells located in the seminiferous tubules. Thus, both types of gametes have a high level of metabolism, which is possible due to active cellular trophism.
The role of mineral nutrition
Metabolic processes are impossible without the influx of cations and anions that are part of mineral s alts. For example, magnesium ions are necessary for photosynthesis, potassium and calcium ions are necessary for the operation of mitochondrial enzyme systems, and the presence of sodium ions, as well as carbonate anions, is necessary to maintain the buffer properties of hyaloplasm. Solutions of mineral s alts enter the cell by pinocytosis or diffusion through the cell membrane. Mineral nutrition is inherent in both autotrophic and heterotrophic cells.
Summing up, we are convinced that the importance of cell nutrition is really great, since this process leads to the formation of building material (carbohydrates, proteins and fats) from carbon dioxide in autotrophic organisms. Heterotrophic cells feed on organic substances formed as a result of the vital activity of autotrophs. They use the received energy for reproduction, growth, movement and other life processes.
