The cell consist of protoplasm, the living material that exist as “heterogeneous aqueous phase” but the consistency of which varies form fluid to a more viscous state.
In primitive cells such as bacteria, the hereditary & metabolic components are mixed and these cells are called Prokaryotic Cells.
In all higher plants and animals, the bulk of hereditary material is isolated in a membrane bound nucleus, lying in the remainder of cell or cytoplasm. These cells are called Eukaryotes.
Protoplasm is the basic material of life. It contains proteins, nucleic acids, carbohydrates, lipids and inorganic material. Proteins form the main structural elements of the cell & intercellular material, existing as pure proteins or combined with lipid as lipoproteins or with carbohydrates as glycoproteins, proteoglycans or mucoprotein. Many secretory products of cells are proteins such as enzymes or some hormones.
Two types of nucleic acids are found in cell. DNA (Deoxyribonucleic acid) the genetic material is found only in nucleus, while RNA (Ribonucleic acid) is located both in nucleus and cytoplasm. It is concerned with protein synthesis by cell and carries messages from the nucleus to the cytoplasm.
The energy source of most cells is carbohydrates in the form of glucose which is often stored in cells as glycogen. Lipid also serves as energy source for the cell and is an important component of cell membrane.
Inorganic material is found both as free radicals and in combination with proteins and lipid. Its important functions are maintenance of osmotic pressure and gradient between cell and its environment, muscle contraction and relaxation, tissues rigidity, cell adhesion and enzyme activation.
Approximately 75% of protoplasm is water, partly free and available as a solvent for metabolic processes and partly bound to protein as a structural component.
Properties of protoplasm are numerous and these indicate the functions of a cell. Function of any cell is direct expression of one or more of these properties of protoplasm. These properties are:
It is the capacity of protoplasm to respond to stimuli and is an expression of life itself which disappears with cell death.
It is the transmission of waves of excitation throughout the cell from the point of stimulus. This property is highly developed in nerve cell & to a lesser extent in muscle cells.
Contraction is a property of change in size, usually in the sense of shortening. It is highly developed in muscles.
Respiration is a process whereby oxygen and food substances within the cell interact chemically to produce energy, carbon dioxide and water.
It involves imbibition of materials that may be assimilated by the cells in metabolism or otherwise utilized.
It is a process by which a cell extrudes materials. These materials may be a useful (secretion) or useless product (excretion) e.g. digestive enzymes and hormones or urine and sweat etc.
Growth means an increase in the size of the cell resulting in an increase in the amount of cytoplasm. The maximum size of a cell is limited basically by its surface area. Beyond the maximum size, if further growth of tissue is necessary, the increase in number of cells occurs. This is accomplished by cell division.