The following points highlight the top eight constituents of an animal cell. The constituents are: 1. Cell membrane 2. Nucleus 3. Mitochondria 4. Endoplasmic Reticulum 5. Ribosomes 6. Centrioles 7. Lysosomes 8. Cytosol.
Constituents # 1. Cell Membrane:
(i) The boundary of every cell has a thin membrane of thickness 75A units (although the thickness varies from70A to 100A).
This is also known as unit membrane or plasma membrane which is formed by the ingredients of the plasma (cytoplasm).
(ii) This membrane consists of three layers:
(a) Inner layer (20Å) of proteins,
(b) Middle layer (35Å) consisting of phospholipids and cholesterol, and
(c) The outer layer (20Å) consisting of protein and polysaccharide.
These are arranged continuously to form pores of about 8Å. The phospholipid molecules in the light band are arranged in two rows that the phosphate containing ends (the polar or hydrophobic ends) point to the outside while the non-polar or hydrophobic ends point to the inside.
(iii) The orientation and nature of the lipid-protein components in the cell membrane determines the charge in the membrane and movement of molecules across the membrane.
(iv) The surface of the cell membrane bears pores, of internal diameter 3Å and length about 75Å, which lead from cytoplasm to the exterior.
(v) The most important function of biologic membranes is to restrict the exchange of substances among various compartments. Thus different types of body fluids are separated from each other by different membranes, e.g., R.B.C. membrane separates plasma from R.B.C.
This is performed by the selective permeability of the various membranes for which certain substances are passed through different rates —namely high, moderate or low; while others are prevented from passing through.
Constituents # 2. Nucleus:
(i) Nucleus is the heaviest component of the cell and is separated from the cytoplasm by a double membrane. The outer membrane is continuous with the endoplasmic reticulum. The two membranes are separated from each other by the perinuclear cisterns (150Å) and join at intervals to form pores which allow the passage of materials from the cytoplasm to the nucleus, and vice versa.
(ii) The pores on nuclear membranes are of higher diameter than those on the cell walls and allow the passage of larger molecules such as protein, nucleic acids, etc.
(iii) The nucleolus within the nucleus contains ribonucleic acid (RNA) in granular form.
(iv) “Chromatin”, the regular clumps of thread-like material, distributed throughout the nucleus, is the most important component of the nucleus. This consists of deoxyribonucleic acid (DNA).
(v) The chromatin, immediately before cell division, organises into simple small thread-like structures known as “chromosomes”, which consist of nucleoprotein molecules. Most of the DNA present in the nucleus occur exclusively in chromosomes.
(vi) The gene or cistron, the unit of genetic information, forms part of the chromosomes and is a segment of the DNA molecule (about 600 bases). The genes are, therefore, arranged in an orderly manner along the length of the DNA molecule in the chromosomes.
(vii) The DNA molecules form the template for the synthesis of RNA which is later on transported into the cytoplasm and the ribosomes. The RNA molecules then synthesize different types of proteins (hormones and enzymes). Since the enzymes regulate the metabolic processes of the body nucleus is, therefore, said to be governing the metabolic processes in the true sense.
(viii) In a nutshell, the nucleus contains proteins (about 15 per cent of the cell), RNA (about 30 per cent of the cell of which 20 per cent in nucleolus and 10 per cent in chromosomes), and DNA.
Constituents # 3. Mitochondria:
(i) Mitochondria are the largest components in cytoplasm.
(ii) They contain about 35 per cent of the total protein of the liver. In addition to proteins, they also contain lipids (25 per cent) most of which (about two-thirds) are phospholipids, and a small amount of nucleic acids.
(iii) They are the “power house of the cell” and each cell may contain 50 to 2,500 mitochondria depending on the respiratory activity of the cell. The cells of skeletal muscle, kidney, and liver contain large number of mitochondria while those of heart muscle contain less.
(iv) They vary in shape (spherical, filamental or sausage shaped) and in size (0.5 to 3µ. long and 0.1 to 0.6µ wide).
(v) They have two membranes – the outer membrane is smooth while the inner membrane is folded to form ridges or cristae which extend into the matrix of the mitochondrion.
Two spaces – the intracristae space and the matrix space are thereby developed. The matrix space is rich in enzymes; while the intracristae space contains substances of low molecular weight in solution.
The outer surface of the outer membrane and inner surface of the inner membrane possess indefinite protuberances or saucer-like structures which are rich in proteins, most of which are the enzymes in biologic oxidation namely NAD- and NADP- dependent dehydrogenases, flavin-linked enzymes, and cytochromes.
(vi) Mitochondria swell extensively in a hypotonic medium and contract again in a hypertonic medium faster with the addition of ATP. This mechanism facilitates the exchange of metabolites.
(vii) The mitochondria contain a large number of enzyme systems known as “cyclophorase” which are involved in: (1) oxidation of pyruvic acid, acetyl-CoA through Krebs’s cycle, fatty acids, amino acids; (b) electron transport and oxidative phosphorylation; (c) synthesis of fatty acids.
They also contain enzymes concerned with the biosynthesis of amino acids, porphyrin, phosphatides, heme, hippuric-acid, and urea. They also contain trans-hydrogenases, adenylate kinase, glutaminase, and the enzymes of phosphorylation of nucleoside di-phosphatase and carboxylation’s.
Constituents # 4. Endoplasmic Reticulum:
(i) This is the complicated and organised system of membranes in the cytoplasm of the cell. This membrane is constituted of protein lipid double layers and is very well-developed in tissues with active protein synthesis.
(ii) The Golgi bodies serve as a means of producing and maintaining the internal membrane. The interior of this membrane is connected with perinuclear space and sometimes also with the extracellular space.
The cisternae (enclosed spaces) of this endoplasmic reticulum play a role in the exchange of material between the cell and the extracellular fluid.
This exchange of material takes place by the processes of diffusion, active transport and pmocytosis.
(iii) This and the surrounding matrix are not only meant for specific metabolic activity, but also provide the means of communication between the nucleus, mitochondria and ribosomes.
(iv) The important functions of this membrane and cytoplasm matrix are:
(a) Transport of nutrients and metabolites from in and out of the cell;
(b) Regulation of protein synthesis,
(c) Glycolysis;
(d) Glycogenesis;
(e) Pentose phosphate pathway;
(f) Fatty acid synthesis.
Constituents # 5. Ribosomes:
(i) Ribosomes or microsomes are small granules on the outsides of the membranes of the endoplasmic reticulum.
(ii) The size of the ribosomes ranges from 15 to 20 mill microns (10-6 mm); the diameter being 150Å.
(iii) They are composed of ribonucleic acid (RNA) protein complex. They also contain lipids, mainly phospholipids.
(iv) They occur separately or in a polysomal cluster. They consist mainly of particles of two sizes—one with a sedimentation constant of 50S (S=Svedberg unit) and the other with 30S. The combination of the two particles is 70S. They are the main site of protein synthesis.
(v) They also possess steroid reductase (responsible for metabolism of cholesterol and steroid hormones), phosphatases, hydrolases, hydroxylases, glucuronyl transferase, and ATPase.
(vi) The Golgi apparatus acts as the store house of various hormones and secretory enzymes which are released at the time of requirement by the process of emeiocytosis.
Constituents # 6. Centrioles:
(i) Centrioles are two short cylindrical structures known to exist on either side of the nucleus at right angles to each other.
(ii) They are not bound by any membrane.
(iii) They help in the equal division of the chromosomes by taking them apart and they are, thereby, responsible for equal distribution of the characteristics in the offspring’s.
Constituents # 7. Lysosomes:
(i) The size (mean diameter 0.4µ) of the lysosomes is in-between that of microsomes and mitochondria. They are surrounded by a lipoprotein membrane.
(ii) Since they contain hydrolytic enzymes they are named lysosomes.
(iii) They contain digestive enzymes such as cathepsin, acid phosphatase, ribonuclease, de-oxy-ribonuclease, catalase, collagenase, α-glucosidase, β-galactosidase, α-man-nosidase, phosphoprotein phosphatase.
(iv) Unless the lipoprotein membrane of the lysosomes are ruptured, the enzymes are not released into the cytoplasm. Hydrolases in lysosomes are separated from their substrates by means of lipoprotein membrane—only when this membrane is ruptured, the hydrolases enter the cytoplasm and destroy the bacteria or foreign particles when the latter enter the cells.
The rupture of the lipoprotein membrane takes place under conditions of cytolysis which include fat solvents, detergents, protease, lecithinase, acid pH, and high temperature.
Constituents # 8. Cytosol:
(i) Lardy introduced the term ‘cytosol’ for the cell sap. It is the unstructured “soluble” portion of the cell. It is the ‘supernatant’ that remains after centrifugation of mitochondria, microsomes, lysosomes, and nuclei. The supernatant is a complex mixture of proteins (35-40 per cent of the total cell), RNA, and other organic or inorganic compounds of low molecular weight.
(ii) It mainly contains the enzyme systems involved in:
(a) Glycolysis,
(b) Fatty acid synthesis,
(c) Pentose phosphate pathway,
(d) Activation of amino acids in protein synthesis, and
(e) Catabolism of purines and pyrimidine’s.