Form Four Biology Notes All Topics
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List Of Form Four Biology Topics
TOPIC 3 – CLASSIFICATION OF LIVING THINGS
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Concept of Growth
Growth is an increase in size/mass. It is the progressive development of living things, especially the process by which the body reaches its point of complete physical development
The growth process is not a steady one; sometimes growth occurs rapidly, at other times slowly. Individual patterns of growth vary widely because of differences in heredity and environment.
When the rate of cell increase is higher than the rate of cell loss,this is referred to as positive growth. When the rate of cell increase is lower than the rate at which cells are lost from the body, the organism decreases in size and weight. This is also referred to as negative growth. Several factors are known to affect growth, example nutrients, temperature, light and hormones.
Internal and External Factors Affecting Growth in Plants and Animals
Growth in plants and animals is influenced by a number of factors, which can be grouped into two categories: internal and external.
Internal factors affecting growth in humans
These are the factors which are associated with genetic make up of an organism plus all the other processes which take place in the organism’s body. These factors include the following:
Heredity: A person’s physical development is strongly affected by their genes inherited from their parents. Parent’s genes predetermine the limits of an individual’s height and other characteristics including the variability in eye colour, hair colour, body composition, and skin tone.With physical attributes such as height, parents’ genes dictate the range of height their offspring can obtain. The variability in height is a result of many external factors in the environment including nutrition and events during the child’s growth.
Hormones: Human growth is affected by biochemical products such as hormones. Hormones are regarded as growth-promoting substances. Probably all the endocrine glands influence growth. Most of the hormones are secreted by the endocrine glands and play a significant role in regulating the pattern of growth and development as per instructions of the genes. Examples of these hormones and their actions are as follows:
Somatotrophin: The most important hormone controlling growth from birth up to adolescence is growth hormone or somatotrophin. This is a polypeptide secreted by the pituitary. It helps in growth of bones and thereby increases the height of persons. It also causes an overall growth rate of most of tissues including brain.
Thyroid hormone: This hormone plays a vital role throughout the whole period of growth. The activity of the thyroid decreases gradually from birth to adolescence. In low secretion of the hormone, skeletal maturity, dental maturity and growth of the brain are all affected. During adolescence a new phase of growth occurs under the control of steroid hormones secreted by the adrenals and gonads.
The gonads of both sexes secrete oestrogens in small quantities from the time of birth on wards. At puberty the oestrogen level rise sharply in girls and to a much more limited extent in boys; the sex differences is possibly due to an inhibitory hormone secreted by the seminiferous tubules of the testicle.
Testosterone: Testosterone, produced by the testicle, is important in stimulating growth and it is responsible for the greater growth of muscles.
Gonadotrophins: Gonadotrophins are responsible for the growth of the ovaries and testis, and later on, the secretion of estrogens and testosterone responsible for the growth and development of secondary sex characters.
External factors affecting growth in humans
Growth is also affected by external factors which include the following:
Nutrients: Growth is closely related to nutrition. A sufficiency of food is essential for normal growth. An adequate supply of nutrients is naturally essential for the normal growth of humans and the need varies with the phase of development. For example: Zinc plays a part in protein synthesis and is a constituent of certain enzymes.
A deficiency of zinc causes stunting, interference with sexual development and falling out of hair; Iodine is needed for the manufacture of the thyroid hormones; Bones will not grow properly without an adequate supply of calcium, phosphorus and other inorganic constituents such as magnesium and manganese; Iron is required for the production of haemoglobin; Vitamins play an important part in growth. Vitamin A is thought to be a control of the activities of osteoblasts. In vitamin C deficiency the inter cellular substance of bone is inadequately formed.
Vitamin D deficiency is the cause of rickets. Malnutrition during childhood delays growth and malnutrition in the years proceeding adolescence delays the onset of the adolescence. Malnutrition may also result to diseases which decrease the appetite or interfere with digestion and assimilation. A majority of malnourished children fail to achieve their full genetic potential of body growth and are thus stunted or wasted or both.
Diseases: Diseases are alteration of the normal body functions, disorders or morbid conditions of the mind. Diseases slow down growth in humans and other animals. A child that suffers from diseases very often is likely to have his growth stunted or retarded. Such a child may end up having a small body or deformed body parts.
Cultural factors: The physical growth of human beings is definitely affected by cultural factors. Culture differs from one ethnic group to another. The body growth differences correlate with varied cultural groups. The physical growth of the body follows some adaptations in different geographical areas of distribution of the groups.
Socioeconomic factors: Socioeconomic influence on human growth is also a well known factor. Children from different socioeconomic levels differ in average body size at all ages. It is clear that growth of the children and adults in those families with good financial status is always good compared to the case in poor families. However, growth differences are more closely related to the home conditions than to the strictly economic status of the families.
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