The cell biology. The scheme of animal cell structure

All life matter is represented by monocellular organisms and multicellular organisms.

A cell is a smallest structure, which has all properties of life matter and can maintain all this properties by itself and also give these properties to next generations. A cell is elementary structural functional genetic unit of all live organisms, providing exchange of energy and substances, reproduction, growth and development, irritability and movement, heredity and diversity, homeostasis. The structural elements of eukaryotic cell are cell membrane, cytoplasm and nucleus.

The cell membrane separates protoplasm of a cell from outside environment and at the same time, it regulates ions and substance passing inside and outside of the cell. According to contemporary findings plasma membrane consist of phospholipids bilayer. The hydrophobic nonpolar surfaces look toward each other, and polar hydrophilic surfaces look outside of membrane.

The scheme of animal cell structure: 1 - pinocytosis canals; 2 - desmosome; 3 - intercellular gap; 4 - rough endoplasmic reticulum; 5 - cell membrane; 6 - tight junction; 7 - mitochondrion; 8 - basement membrane; 9 - basal lacunas; 10 - lysosomes; 11 - centrioles; 12-Golgi complex; 13-chromatin; 14-nucleolus; 15-nuclear envelope with pores; 16 - ribosomes; 17 - smooth endoplasmic reticulum; 18 - vilia (by E. Hadom, R. Vener, 1989)

There are proteins incorporated into membrane. The hydrophilic parts of the proteins binds with hydrophilic parts of phospholipids, hydrophobic regions of protein binds with hydrophobic parts of phospholipids. Beside that, an animal cell has glycocalyx outside of phospholipids bilayer with width 10-20 nm, presented by glycolipids and glycoproteins.

A plant cell has cell wall, which is made of cellulose. The inner cell membranes, which form organelles, have a same structural principle, without glycocalyx (pic 3.2). The cortical layer of cytoplasm lies close to inner cell membrane surface. It has a lot of microtubules and microfilaments, containing contractive proteins.

Pic. 3.2. The electronic microscope photo (A) and scheme (B) of plasmalemm: A : 1 - three layer elementary membranes, 2 - intercellular gap. В: 1 protein molecule, 2 - phospholipids molecule, 3 - - intercellular gap, 4 - phospholipids bilayer, 5 - intracellular space (by E. Hadom, R. Vener, 1989)

The plasmalemm carry out the following functions: separation, defense, transportation, regulation of chemical balance inside of the cell. In the plasmolemm are receptors, which are able to recognize biological active substances. With help of receptors a cell can percept outside signals and react to changes in environment or in organism state.

The cytoplasm is presented by semifluid matrix with several organelles and inclusions. The matrix is a main substance of the cell. The colloid features, viscosity, elastic properties, internal movement depends on it. The cytoplasm matrix is a very complex colloid system, which is able to change fluid condition to gel condition and back. The compounds of cytoplasm are soluble proteins, such as glycolisis enzyme, ATPases etc, amino acids, lipids carbohydrates. Microtubules are made from matrix proteins. Functionally, cytoplasm is internal cell environment - the place for intracellular metabolism performing.

The organelles are stable, highly differentiated cytoplasm bodies, carrying out certain function. It can be distinguished organelles of special and general purpose. Organelles of general purpose (endoplasmic reticulum, ribosomes, complex Golgi, lysosomes, mitochondria and centrosome) are in an all cell types. The organelles of special purpose (myofibrils, neurofilaments, vilia, cilia, flagella, microtubules and microfilaments) are in certain cell types.

According to its structure, organelles are divided to organelles derived from membranes (lysosomes, complex Golgi, endoplasmic reticulum) and non-membrane organelles (ribosomes, centrosome, microtubules and microfilaments).

The endoplasmic reticulum. The endoplasmic reticulum, weaving sheets through the interior of the cell, creates the serious of channels and interconnections between its membranes that isolates some spaces as membrane-enclosed sacs called vesicles. The membranes may be rough and smooth. The rough endoplasmic reticulum has ribosomes attached to its membrane. The rough endoplasmic reticulum produces proteins for external use, as a secretion of secretary cells. The most active regions of protein synthesis are called ergastoplasm.

The channels of smooth endoplasmic reticulum contain enzymes that provide carbohydrate, steroids and lipid synthesis. When the synthesis is complete, substances travels to vesicle forming system, called complex Golgi. In the endoplasmic reticulum of liver cells occur detoxications of harmful and toxic substances. In the channels and vesicles of smooth endoplasmic reticulum of striated muscle, the calcium ions participating in muscle contraction are stored.

The ribosomes are round shape ribonucleoprotein structures with diameter 15-35 nm. Each ribosome consists of small subunit and large subunit. They merge in the presence of mRNA. If there are several ribosomes merged by one mRNA, such structure called polysome. The polysomes may stay free in cytoplasm or attach to endoplasmic reticulum membranes. They are the place of active protein synthesis. They allow making proteins in large amounts. If they spread in cytoplasm, they make proteins for internal use. If they attached to endoplasmic reticulum membranes, they make proteins for external use (examples are milk synthesis and digestion enzymes synthesis).

The Golgi complex, named in honor of Italian scientist who described it first, is visible in light microscope as differentiated region of cytoplasm situated near the nucleus. It is formed from flattened stacks of membranes. On a side of stacks there are folds called cistemae. In the plant cells complex Golgi are made from small bodies named dictiosome. Dictiosome is a deck of small disk shaped vesicles. Vesicles are separated from sides of dictiosome.

It is believed that the main complex Golgi function is concentration and condensing of internally produced substances for further excretion from a cell. It is stated that in complex Golgi glycolipids, glycoproteins, yolk granules and lysosomes are formed.

The lysosomes (from Greek - lysis - dissolving) are sphere shaped vesicles with diameter 0.2 - 0.4 mcm, containing set of acid hydrolases enzymes. They help to catalyze reaction of nucleic acids, proteins, lipids and carbohydrates splitting. A lysosome is surrounded by one layer biological membrane, sometimes it may be a protein fibrils over its surface. Lysosome enzymes also help to digest aged cell structures or even completely died cell. The lysosome damage and its enzyme liberation lead to total cytoplasm dissolving.

Digestive vacuoles in protests bodies and phagocytes are probably made of merged lysosomes. There are primary (inactive) lysosomes and secondary lysosomes. Secondary lysosomes are activated primary lysosome, and in these lysosomes, a process of digestion takes place. Secondary lysosomes may be subdivided to heterolysosomes (phagolysosomes) and autolysosomes (cytolysosomes). Heterolysosomes digest substances obtained by phagocytosis and pinocytosis. Autolysosomes digest internal cell structures, which are not able to perform its functions any more.

The microbodies - are group of vesicle shaped cell organelles with diameter 0.1 - 1.5 mcm. They are surrounded by one layer biological membrane. Peroxisomes are referred to this group. They contain catalase enzyme, which catalyze hydrogen peroxide degradation. There are around 70 to 100 peroxisomes in one liver cell.

The mitocondria (from Greek - mitos - thread, chondros - corpuscule) - are round shape or stab shape structures of 5 -10 mcm. long and 0.5 mcm. width. The mitochondria number is varied from 150 to 1500 per cell or even several hundreds thousands in female sex cells. The mitochondrion coat is consists of two biological membranes. The inner membrane makes an internal leaf shaped invaginations that is called cristae, or tubular shaped invaginations, which is called tubules.

The inner membrane surround internal mitochondrion matrix. There are apparatus of protein biosynthesis in it. It is presented by circular, closed DNA molecule without histons, ribosomes, tRNA, enzymes of DNA replication, transcription and translation. The main function of mitochondria is to obtain energy by oxidative phosphorilation of chemical substances and to store it in ATP form. Mitochondria take part in a steroid hormone and some amino acid synthesis.

The plastids - are the group of organelles existing in plants. The plastid body is bounded by two membranes that resemble those in mitochondria. An internal plastid membrane lies in close association with one another; by fusing their peripheries, two adjacent membranes form a disk shaped close compartment, called tilakoid.Plastids contain stacks of such tilakoids. Each stack, called granum, may contain several dozen tilakoids and a plastid may contain hundreds or more grana. According to tilakoid pigment plastids may be divided to chloroplast, chromoplasts and leucoplasts.

Green plant cells usually contain chloroplasts. They perform photosynthesis. The photosynthesis provides formation of mono-, di-, and polysugars. Having its own circle DNA and ribosomes chloroplasts may perform a protein biosynthesis. Like mitochondria, all plastids come from division of existing plastids. In early stages of development, plastids look like mitochondria. It may be due to their similar function. Mitochondria perform energy transformation from dissimilation processes, and chloroplasts perform photosynthesis to transform sun energy to energy of chemical bond. Mitochondria apparently originated as endosymbiotic aerobic bacteria, whereas chloroplasts seem to be originated as endosimbiotic aerobic photosynthetic bacteria.

The cell center - is a good visible organelle, consisting of one or two small centrioles and radiated sphere surrounding them. With help of electronic microscope it was revealed that each centurion is small cylindrical body of 0.3 - 0.5 mcm. long and with a diameter 0.15 mcm (pic 3.3).The walls of cylinder are made of nine parallel tubules. Cell center works actively during mitosis. Centrioles come to a cell poles. Spindles are attached to them. During mitosis, chromosomes move, by spindle, toward centrioles on different poles. General purpose organelles also include microtubules and microfilaments.

Pic. 3.3. The electronic microscope photo of mitochondrion (a), rough EPR (b), and centrosome (c) (in Biology Science an inquiry into life, 1980)

Microtubules are organelles of different length with diameter 24nm. They are structural elements of flagella, cilia, centriole, spindle. Also they may sty free in cytoplasm carrying out a support function and providing cell shape. Microfilaments are long thin organelles spread through out all cell cytoplasm. They provide cell movement and form a cell frame, take part in intracellular organelles movement.

The inclusions - are temporal cytoplasmic structures, related with cellular metabolism. The cell’s functional state provides their appearance or dissolving.

There are these following types of inclusions: trophical (carbohydrates, proteins, lipids), secretoral (secrete granules in glands), pigment (melanin, lipofuscin, hemoglobin etc.) and excretory (uric acid etc.).