(a) Individual growth is a permanent increase in the biomass of a cell or organism. (Biomass refers to the mass of living material).
(b) Population growth is an increase in the number of individual in a population.
If a process is described simply as growth, then this will indicate that it refers to individual growth.
Growth occurs when the anabolic activities exceed the catabolic activities.
At the cellular level, the growth involves:
(i) Increase in size of cells due to synthesis of protoplasmic structures (hypertropy)
(ii) Increase in the number of cells by cell division (hyperplasia).
(iii) Increase in the amount of apoplasmic structures like the fibres, matrix, etc.
At molecular level, the growth involves synthesis of new molecules and their aggregation into organelles and storage products in the cell.
During fasting, the catabolic activities exceed the anabolic activities. Due to this, the reserve food and living material decrease in amount and is called degrowth.
I. Types of Growth in Multicellular Organisms
The volume of body increases due to the growth of cells without any increase in the number of cells. Such kind of growth is seen only in nematodes, rotifers and tunicates (early chordates i.e. urochordates).
The growth of the body is due to increase in the number of cells. The cells divide mitotically to increase their number but their size remains the same. The prenatal growth or embryonic development of higher vertebrates is an example of multiplicative growth.
In postembryonic period of organism’s life, the cells of the tissues and organs get differentiated for specific functions. These cells lose the capacity of division and perform physiological functions for the survival of the organisms.
At certain locations, special cells remain in an undifferentiated state as reserve cells. In case of necessity, these reserve cells divide by mitosis to form new cells that replace the worn out differentiated cells. E.g. germ cells of germinal epithelium of the gonads; cells of stratum germinativum of epidermis of the vertebrate skin, etc.
It involves the addition of new layers on the previously formed layers. For example, the addition of lamellae in the formation of bone. It is the characteristic mode of growth in rigid materials.
II. Pattern of Growth:
On the basis of body proportions, pattern of growth are of two types –
In this an organ grows at the same rate as the rest of the body. As the organisms grows, the external form of body does not change. E.g. Fish and certain insects (Locusts)
In this, an organ grows at a rate different from that at which the body grows. As the organisms grows, the external form of body changes. E.g. Mammal.
III. Growth Curve:
Growth curve is the graphic representation of growth against time. The sigmoid curve i.e. S-shaped growth curve is the characteristic growth curve of all higher animals including man. The sigmoid growth curve consists of the following stages-
(i) Lag phase – Little growth
(ii) Exponential/acceleration phase/log phase – Maximum growth. The point where the exponential growth begins to slow down is known as inflexion point.
(iii) Senescent/decelerating phase – Declining growth i.e. growth with decreasing rate.
(iv) Steady phase – No growth. The study of the growth curve reveals that
i. The curve rises at a very slow rate of growth initially.
ii. Following initial slow rate of growth, it then rises steeply indicating very fast rate of growth in acceleration phase
iii. Finally rate of growth again slows down till the curve becomes horizontal. It signifies that no further growth is taking place and the individualsimply is maintaining itself. The growth curve thus attains S-shape and is called sigmoid curve.
Rapid growth occurs in the prenatal and puberty periods. Growth is retarded in the juvenile and post-adolescent periods. There is little or no growth after the post-adolescent period.
(i) In man, the rate of growth is very slow during the first 10-13 years (childhood). During this age growth is controlled by thymosin (secreted by thymus gland).
(ii) During puberty the rate of growth increases with the increased secretion of thyroxine hormone from the thyroid gland and somatotrophic hormone (STH) from the anterior lobe of pituitary gland.
(iii) This increased rate of growth persists during 14-18 years of age. At this phase of life, the secretion of sex hormones, i.e., testosterone in the male and estrogens and progesterone in the female starts which leads to the development of secondary sex organs and characters.
(iv) At around 18 years of age, the individual becomes sexually mature. After this period, the growth rate starts declining and it almost stops at about 22-23 years of age.