The science that studies the structure and function of cells is called cytology.
A cell is an elementary structural and functional unit of living things.
Cells, despite their small size, are very complex. The internal semi-liquid content of the cell is called the cytoplasm.
The cytoplasm is the internal environment of the cell, where various processes take place and the components of the cell – organelles (organelles) – are located.
The cytoplasm of eukaryotic cells is laced with a three-dimensional network of protein filaments called the cytoskeleton. It consists of three elements: microtubules, intermediate filaments, and microfilaments.
The cytoskeleton is the mechanical skeleton of the cell to maintain its shape. The components of the cytoskeleton determine cell division, the movement of organelles within the cell, and the movement of the cytoplasm.
The cell nucleus is the most important part of the cell.
The nucleus is separated from the cytoplasm by a membrane consisting of two membranes. There are numerous pores in the shell of the nucleus, they are needed so that various substances can get from the cytoplasm into the nucleus and vice versa.
The inner contents of the nucleus are called karyoplasm, or nuclear juice. The nuclear juice contains chromatin and nucleolus.
Chromatin is a strand of DNA. If the cell begins to divide, then the chromatin threads are tightly wound in a spiral onto special proteins, like threads on a spool. Such dense formations are clearly visible under a microscope and are called chromosomes.
The nucleus contains genetic information and controls the life of the cell.
The nucleolus is a dense, rounded body inside the nucleus. Usually, there are from one to seven nucleoli in the cell nucleus. They are clearly visible between cell divisions, and during division they are destroyed.
The function of the nucleoli is the synthesis of RNA and proteins, from which special organelles are formed – ribosomes.
Ribosomes are involved in protein biosynthesis. In the cytoplasm, ribosomes are most often located on a rough endoplasmic reticulum. Less commonly, they are freely suspended in the cytoplasm of the cell.
The endoplasmic reticulum (EPS) is involved in the synthesis of cell proteins and the transport of substances inside the cell.
A significant part of the substances synthesized by the cell (proteins, fats, carbohydrates) is not consumed immediately, but through the EPS channels it enters for storage into special cavities, stacked in peculiar piles, “cisterns”, and separated from the cytoplasm by a membrane. These cavities are called the Golgi apparatus (complex). Most often, the cisterns of the Golgi apparatus are located near the cell nucleus.
The Golgi apparatus takes part in the transformation of cell proteins and synthesizes lysosomes – the digestive organelles of the cell.
Lysosomes are digestive enzymes, “packed” into membrane vesicles, budding and being carried along the cytoplasm.
The Golgi complex also accumulates substances that the cell synthesizes for the needs of the whole organism and which are removed from the cell to the outside.
Mitochondria are energy organelles of cells. They convert nutrients into energy (ATP) and take part in cell respiration.
Mitochondria are covered with two membranes: the outer membrane is smooth, and the inner one has numerous folds and protrusions – cristae.
Enzymes are built into the membrane of the cristae, which synthesize adenosine triphosphate (ATP) molecules at the expense of the energy of nutrients absorbed by the cell.
ATP is a universal source of energy for all processes in the cell.
The number of mitochondria in the cells of various living things and tissues is not the same.
For example, sperm may contain only one mitochondrion. On the other hand, in tissue cells where energy expenditures are high (in the cells of flight muscles in birds, in liver cells), there are up to several thousand of these organelles.
Mitochondria have their own DNA and can multiply independently (before cell division, the number of mitochondria in it increases so that there are enough of them for two cells).
Mitochondria are found in all eukaryotic cells, but they are not in prokaryotic cells. This fact, as well as the presence of DNA in mitochondria, allowed scientists to hypothesize that the ancestors of mitochondria were once free-living creatures resembling bacteria. Over time, they settled in the cells of other organisms, possibly parasitizing in them. And then over many millions of years they turned into the most important organelles, without which no eukaryotic cell can exist.
For a cell to be a single system, it is necessary that all its parts (cytoplasm, nucleus, organelles) are held together. For this, in the process of evolution, a plasma membrane has developed, which, surrounding each cell, separates it from the external environment. The outer membrane protects the inner contents of the cell – the cytoplasm and the nucleus – from damage, maintains the constant shape of the cell, ensures the communication of cells with each other, selectively passes the necessary substances into the cell and removes metabolic products from the cell.
The structure of the membrane is the same for all cells. The membrane is based on a double layer of lipid molecules, in which numerous protein molecules are located. Some proteins are located on the surface of the lipid layer, while others penetrate both lipid layers through and through.
Special proteins form the thinnest channels through which potassium, sodium, calcium and some other ions with a small diameter can pass into or out of the cell. However, larger particles (molecules of food substances – proteins, carbohydrates, lipids) cannot pass through the membrane channels and enter the cell using phagocytosis or pinocytosis:
- In the place where the food particle touches the outer membrane of the cell, an invagination is formed, and the particle enters the cell, surrounded by the membrane. This process is called phagocytosis (plant cells on top of the outer cell membrane are covered with a dense layer of fiber (cell membrane) and cannot capture substances through phagocytosis).
- Pinocytosis differs from phagocytosis only in that in this case the invagination of the outer membrane captures not solid particles, but droplets of liquid with substances dissolved in it. This is one of the main mechanisms for the penetration of substances into the cell.
When various nutrients enter the cell by phagocytosis or pinocytosis, they must be digested (i.e., proteins must be broken down to individual amino acids, polysaccharides to glucose or fructose molecules, lipids to glycerol and fatty acids). For intracellular digestion to become possible, the phagocytic or pinocytic vesicle must fuse with the lysosome.