Growth is the irreversible increase in dry mass, size and complexity of an organism brought about by the synthesis of new protoplasm.
For growth to be effective, building up of materials (anabolism) must exceed the rate of breaking down (catabolism).
In plants, growth is indefinite and apical while it is definite and uniform in animals.
BASIS OF GROWTH
The basis of growth involves three major phases i.e. cell division (mitosis), cell enlargement and cell differentiation.
Life begins as a single fertilized cell, continuous as the cell divides into two daughter cells then into four and so on. After cell division, the daughter cells increase in mass and size (enlargement). Eventually, each cell develops into a special type of cell (specialization) by changing its shape and structure to carry out a particular function. Most specialized cells, at maturity lose their ability to divide.
TYPES OF CELL DIVISION
There are two types of cell division: (a) Mitosis (b) Meiosis
Mitosis is a cell division to produce two daughter cells with the same number of chromosomes and characteristics as those of the parent cell. This cell division takes place only in somatic cells (i.e. body cells that are not involved in the production of gametes) such as skin, bone marrow and meristematic tissues in plants.
Mitotic cell division involve five main stages
- Interphase: This is referred to as the resting stage where the chromosomes becomes elongated and form a network of fine threads called chromatids
- Prophase: in early prophase chromosomes become visible, nucleolus shrinks, centrioles start moving away from each other in opposite direction and the formation of spindle fibres begins. During the late prophase, chromosomes become shorter, thicker and visible. Each chromosome now forms two distinct chromatids joined by a centromere. Nucleolus and nuclear membrane disappear entirely.
- Metaphase: paired chromatids arrange themselves along the equator of the spindle and are attached to the spindle at the centromere.
- Anaphase: sister chromatids separate and start migrating to the opposite poles of the cell and eventually reach the poles.
- Telophase: the cell starts dividing into two by constricting at the equator, nucleolus and nuclear membrane are reform in each daughter cell, spindle fibres degenerates and chromosomes eventually regain their threadlike form
Importance of mitosis
- It brings about growth, development and specialization especially in multicellular organisms.
- Asexual reproduction binary and multiple fissions in protozoa, budding in yeasts are result of mitosis.
- It ensures the diploid condition of the cells is retained from generation to generation
- It aids the repair of damaged cells
Life processes involved in mitosis
- Formation of new cells in Malpighian layer of the skin
- Production of red and white blood cells in the bone marrow
- Healing of wounds
- Growth in meristem
- Binary fission
ASPECTS OF GROWTH
Growth varies from one organism to another. To measure growth, the following parameters are used:
- Man: the dry mass is more accurate and reliable than the wet mass
- size and length e.g. height of man
- increase in number of cells e.g. budding in yeast cell
REGIONS OF FASTEST GROWTH IN PLANTS
The regions of fastest growth in plants are the root and stem apices. Since growth in plants is apical, the root and stem apices can be divided into
- Region of cell division (called the apical meritstem) brings about primary growth of a plant
- Region of elongation
- Region of cell maturation/specialization
The growth rate of a living organism is measured using a growth a curve. The growth pattern of man shows a sigmoid curve. The growth pattern consists of three phase:
- Lag phase: This is the initial stage where slow growth is experienced. At this stage, the cell is still accumulating the necessary material to begin the process.
- Log or exponential phase: This is the phase of rapid of rapid growth.
- Stationary phase: This is a stage where no observable growth is experienced.
The growth curve of insect is like a step. Each growth phase (instar) is interrupted by a period of molting (ecdysis)
FACTORS AFFECTING GROWTH
- External factors it include availability of nutrients, humidity, light, temperature, PH and accumulation of metabolic products.
- Internal factors are mainly hormones. The two major growth hormones in plant are auxin and gibberellins. In animals, the hormones concerned with growth are secreted by the anterior pituitary gland, thyroid gland and the gonads. Over or under secretion of any of these hormones leads to abnormal growth.