The basic unit of all systems, organs and tissues is the cell. The cell consists of a number of parts including the nucleus, with not all cells containing all of the various parts. The contents of a cell are enclosed in a cell membrane or wall that will permit the various nutrients and compounds required for its normal functioning to move into and out of it. While there are many structures in the cell, the important ones for this topic are the chromosomes carrying the genes that control those features that are passed from parent to offspring.
The metabolic processes that are the basis of life are carried out in the cell by many different enzymes. Cells reproduce by mitosis or simple division where one cell divides to become two – this is normal growth. The sex cells are the exception to this – these are formed in the gonads (ovary or testes) by meiosis or reduction division and carry one instead of two of each chromosome so that when two sex cells join i.e. one each from the male and female at mating, the resultant cell will carry the normal cell chromosome number. This is the start of the embryo.
Not all cells are exactly alike, but depend on the function of the tissue, organ or system to which they belong. Some are glandular, others are protective, others provide support, others provide a means of communication and others have a connecting function holding tissues, organs and systems together or in place. Animals consist of systems, organs, tissues and cells that sever certain functions.
Cells have a number of special features that differentiate them from non-living things:
1. They provide cooperative support of own structures i.e. the cells, tissues, organs and systems support each other.
2. They carry out the metabolic processes i.e. the chemical changes, both anabolic (constructive) and catabolic (destructive) that occur in living organisms to provide for the normal functioning of the organism or animal.
3. They respond to irritation. Irritation is defined as that ability inherent in living organisms to receive external impressions or stimuli and reacting to them.
4. They can reproduce their own kind as a normal function.
5. They provide their own protection or defence to the many potentially harmful situations in their environment.
6. They provide for the coordinated growth and development of their own body.
Virus particles are very similar to the cell nucleic material and can enter the cell and become part of the cell nucleic material. This results in change and many of these changes give rise to the symptoms of diseases caused by viruses.
