The below mentioned article provides notes on animal cells.
According to the cell theory animals are composed of cells and of cell products. The cell is the structural and functional unit of the animal body and the branch of Zoology which deals with cells is known as cytology.
In unicellular animals the cell and the organism are one, whereas in multicellular animals the cells are integrated for proper functioning and arranged in the form of tissues and the tissues are aggregated into tissue-systems or organs.
The study of tissues and tissue systems is known as histology. The protoplasm and the cell theory have already been dealt with. The consideration of animal cell, methods of cell division and the various tissues and tissue systems of animals.
The protoplasm of an animal cell is normally differentiated into two main parts, the cytoplasm and the nucleus. The cytoplasm is covered by a delicate cell membrane which is a living part of the cell. The membrane is 7.5-9 nm thick and is composed of a double layer of lipid with a layer of protein on either side of the lipid bilayer.
The membrane is perforated by small pores 0.8 nm in diameter through which water and water soluble molecules may pass. In addition to its limiting function, the cell membrane serves to regulate the entrance into and exit of substances out of the cell. It is said to be semi-permeable, because it allows certain dissolved substances and liquids to pass through it by diffusion and osmosis, but serves as barriers to others.
The nucleus is the headquarter of all cellular activities. It is usually spherical or oval in shape and lies in the centre of the cell, enclosed in a distinct nuclear membrane. The nuclear membrane is about 40 nm thick and is made up of Lipoprotein arranged in the same fashion as that of the cell membrane.
The nuclear membrane bears large pores (50-100 nm in diameter). Some of the pores remain attached to the endoplasmic reticulum while other are free and through them the nucleus maintains a direct communication with the surrounding cytoplasm.
The nucleus is composed of a rather specialised kind of protoplasm known as nucleoplasm, which consists of a fluid nuclear sap and solid threads of varying shape—the chromosomes. In most cases, the nucleoplasm of the living cell is perfectly transparent and the only visible structures are one or two rounded bodies called nucleoli (singular—nucleolus).
This is explained by the fact that the chromosomes in the nucleus of a resting cell often remain invisible or only partially visible but they can easily be seen in a dividing nucleus (Fig. 168). In any case, the chromosomes always retain their identity and can be demonstrated in the living cell by micro dissection.
The chromosomes are mainly composed of deoxyribonucleic acid (DNA) and a protein and are chiefly concerned with heredity. The protoplasm of the cell, excluding the nucleoplasm, is known as the cytoplasm. It constitutes the cell body and is often divided into outer, clear ectoplasm and inner, granular endoplasm.
Small fluid-containing spaces called vacuoles may be detected in the cytoplasm. A distinct central body or centrosome, including one or two minute centrioles, is often seen close to the nucleus. The centriole is usually surrounded by a zone of clear cytoplasm Fig. 168.
The central body plays an important part during the division of the parent cell into two daughter cells. Embedded in the cytoplasm are rod-like or granular bodies called mitochondria, (Fig. 169A) occurring either in groups or in a scattered state. They are of diverse size and shape.
Average length and breadth of mitochondria is about 2 5 µm and 5 µm respectively. It consists of two mitochondrial membranes surrounding a central fluid matrix. Structurally the membranes are similar to that of cell membrane. The two membranes remain separate. The inner membrane forms a number of folds towards the central matrix. These folds are known as cristae.
The outer mitochondrial membrane contains ribosomes, some DNA and RNA. The central matrix is rich in protein and Ca++ binding granules. These bodies are responsible for cellular oxidation and energy liberation. For this reason they are called the ‘power-house’ of the cell. A Golgi body, in the form of a network, often is present surrounding the central body (Fig. 169B).
It consists of a system of flat smooth surfaced and single membrane vesicles and cisternae packed close together in the form of a stack. The single membrane forming the wall of the vesicles cisternae is 6-8 nm in thickness and structurally similar to that the cell membrane. They are most abundant in secretary cells. The Golgi body is concerned with the secretion, synthesis and storage of certain cell substances.
Between the cell and nuclear membranes there exists an extensive network of minute tubes, sacs and cisterns. This network is known as Endoplasmic Reticulum (Fig. 169C). The wall of these tubes is about 6-7 nm thick. The endoplasmic reticulum serves functions like protein synthesis, and transporting secretory materials to the Golgi body.
Many small granular aggregates are found in the cytoplasm. These are called Ribosomes. They remain free in the cytosol or remain attached to the wall of the endoplasmic reticulum. The overall diameter of a single ribosome is about 20-25 nm and it is made up of two unequal subunits.
Both free and endoplasmic reticular bound ribosomes participate in protein synthesis good number of mitochondria like membrane enclosed organelle called Lysosomes are found in the cells. These are particularly abundant in phagocytic cells like WBC.
A newly formed lysosome is a spherical vesicle having a diameter of 20-80 nm. The wall of the vesicle is 7-8 nm thick and is made up of lipoprotein. The lysosomes contain hydrolytic enzymes. In case of damage or death of a cell the lysosomes undergo dissolution releasing the enzymes. As a result autolysis occurs.
The various cytoplasmic structures just mentioned are detected only by special cytological technique and differential staining Otherwise they are not visible. Apart from the living inclusions there are non-living cytoplasmic inclusions consisting of granules scattered throughout the cytoplasm. These include substances such as yolk granules, glycogen, globules of fat, secretions and excretions.
Such then is a brief sketch of an animal cell. We are already aware of what a cell can do. It feeds, grows, breathes and reproduces its kind. We can grow cells in the laboratory apart from the parent-body by special methods of tissue-culture.
We can alter ceils considerably by treating them with X-rays, or kill them with heat and cold, or dissect their parts by the recently discovered micro-dissecting apparatus. In recent years considerable amount of knowledge has accumulated regarding the structure and function of the cell.
It is now well-understood that cells are the units of the living body and all the working of a living system are the manifestations of the cellular function.
It is the activity of the cell which determines different complicated life processes—development, growth, maintenance, heredity, diseases and death. Still there are many more unsolved riddles in Biology, the answers of which are expected only to come from the future studies of cell structure and cell function.