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 and vital functions of a cell: respiration, nutrition, reproduction, growth. These processes will be considered in this paper.
Cell structure
Using a light and electron microscope, biologists have established that plant and animal cells contain a surface apparatus (supmembrane and submembrane complexes), cytoplasm, and organelles. In animal cells, glycocalyx is located above the membrane, containing enzymes and providing cell nutrition outside the cytoplasm. In plant cells, prokaryotes (bacteria and cyanobacteria), as well as fungi, a cell wall forms over the membrane, which consists of cellulose, lignin or murein.
The nucleus is a mandatory organelle of eukaryotes. It contains hereditary material - DNA, which has the appearance of chromosomes. Bacteria and cyanobacteria contain a nucleoid that acts as a carrier for deoxyribonucleic acid. All of them perform strictly specific functions that determine metabolic cellular processes.
What do we mean by the term "cellular nutrition"
The vital manifestations of a cell are nothing more than the transfer of energy and its transformation from one species to another (according to the first law of thermodynamics). The energy that is in the nutrients in a hidden, i.e. bound state, passes into ATP molecules. To the question of what cell nutrition is in biology, there is an answer that takes into account the following postulates:
- A cell, being an open biosystem, requires a constant influx of energy from the environment.
- The 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 metabolism of which is studied by biochemistry.
Metabolism and energy
Organic matter entering the cell undergoes cleavage, resulting in the release of energy in the form of ATP or NADP-H2 molecules. The whole combination of assimilation and dissimilation reactions is metabolism. Below we consider the stages of energy metabolism that provide nutrition for heterotrophic cells. First, proteins, carbohydrates and lipids are broken down to their monomers: amino acids, glucose, glycerol and fatty acids. Then, during oxygen-free breakdown, they undergo further decomposition (anaerobic breakdown).
In this way, intracellular parasites are fed: rickettsia, chlamydia and pathogenic bacteria, for example, clostridia. Unicellular yeast fungi break down glucose to ethyl alcohol, lactic acid bacteria to lactic acid. Thus, glycolysis, alcoholic, butyric acid, 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 provided. Some of them synthesize nutrients through light energy, they are called phototrophs. Others - using 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 picks up light quanta. In the light and dark phases of photosynthesis, enzymatic reactions occur (Calvin cycle), the result of which is the formation of all organic substances used for nutrition from carbon dioxide. Therefore, a cell whose nutrition occurs due to the use of light energy is called autotrophic or phototrophic.
Unicellular 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, and the released energy is used to synthesize glucose molecules.
Thus, photo-synthetic organisms capture light energy and turn it into the energy of covalent bonds of mono- and polysaccharides. Then, through the links of food chains, energy is transferred to the cells of heterotrophic organisms. In other words, due to photosynthesis, all the structural elements of the biosphere exist. We can say that a cell whose nutrition occurs in an autotrophic way “feeds” not only itself, but also everything that lives on planet Earth.
How heterotrophic organisms feed
A cell whose nutrition depends on the intake of organic substances from the environment is called heterotrophic. Organisms such as fungi, animals, humans, and parasitic bacteria break down carbohydrates, proteins, and fats using digestive enzymes.
Then, the obtained monomers are absorbed by the cell and used by it to build their organelles and vital functions. Dissolved nutrients enter the cell through pinocytosis, and solid food particles through phagocytosis. Heterotrophic organisms can be divided into saprotrophs and parasites. The first (for example, soil bacteria, fungi, some insects) feed on dead organics, the second (pathogenic bacteria, helminths, parasitic fungi) - on the cells and tissues of living organisms.
Mixotrophs, their distribution in nature
A mixed type of nutrition in nature is quite rare and is a form of adaptation (idioadaptation) to various environmental factors. The main condition of 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 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 eats like a plant, i.e., autotrophically, through photosynthesis. As a result, glucose is synthesized from carbon dioxide, which the cell uses as food. At night, euglena feeds heterotrophically, breaking down organic substances with the help of enzymes located in the digestive vacuoles. Thus, scientists consider mixotrophic nutrition of cells to be evidence of the unity of the origin of plants and animals.
Cell growth and its relationship with trophic
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 the constant entry into the cells of nutrients that serve as building material. In order to get an answer to the question of how a cell grows whose nutrition occurs autotrophically, it is necessary to clarify whether it is an independent organism or is included as a structural unit in a multicellular individual. In the first case, growth will be carried out during the interphase of the cell cycle. The processes of plastic metabolism are intensively taking 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.
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, such as glucose, to carbon dioxide and water (Krebs cycle). It occurs in the matrix of mitochondria containing the enzyme system H + -ATP-ase, which synthesizes ATP molecules from ADP. In prokaryotic organisms, such as aerobic bacteria and cyanobacteria, the oxygen phase of dissimilation occurs on the plasma membrane of cells.
Gamete specificity
In molecular biology and cytology, the nutrition of a cell can be briefly characterized as the process of the ingress of nutrients into it, their splitting and synthesis of a certain portion of energy in the form of ATP molecules. Gamete trophism: ovum and sperm, has some features associated with the high specificity of their functions. This is especially true of the female reproductive cell, which is forced to accumulate a large supply of nutrients, mainly in the form of yolk.
After fertilization, she will use them to crush and form a fetus. Sperm cells during maturation (spermatogenesis) receive organic matter from Sertoli cells located in the seminiferous tubules. Thus, both types of gametes have a high metabolic rate, which is possible due to active cell trophism.
The role of mineral nutrition
Metabolic processes are impossible without the influx of cations and anions that make up the mineral salts. For example, magnesium ion is necessary for photosynthesis, potassium and calcium ions for the functioning of mitochondrial enzyme systems, and for the preservation of the buffer properties of hyaloplasm, the presence of sodium ions and carbonate acid anions. Mineral salt solutions enter the cell by pinocytosis or diffusion through the cell membrane. Mineral nutrition is inherent in both autotrophic and heterotrophic cells.
To summarize, we were convinced that the value 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 activity of autotrophs. They use the received energy for reproduction, growth, movement and other vital processes.