Growth is defined as an irreversible increase in volume, size, and number of parts, length and weight of an organism It is an organic process which takes sometimes to accomplish.
The three processes involve in growth are
- Cell division:- is a process by which cell increases in number and is achieved by cell division called mitosis. The synthesis of new protoplasm leading to the doubling of the chromosomes number in a process called replication before cell actually divide into two, with each daughter cell having the same chromosome as parent cell.
- Cell enlargement:- This is a process after cell division in which the daughter cell absorb nutrients from their surrounding which it uses to increase in mass and size. Part of the nutrient is used to generate energy while the remaining is assimilated resulting in enlargement of the cells.
- Cell differentiation– This process takes place after cell enlargement in which each cell develop into a special type of cell by changing its shape and structure in order to carryout growth, a specialized or particular function.
Mitosis is a division of cell which produces two identical cells with the same number of chromosomes and characteristics as those of the parent cell. Mitosis is a cell division that lead to growth and it occurs in somatic cell(body cells) such as skin, bone marrow, lymph nodes and injured places and meristimatic tissues of plant. Mitosis occurs in five stages namely interphase, prophase, metaphase, anaphase and telephase.
- Interphase- This is a resting stage of the cell: At this stage, the cell has normal appearance of non-dividing cell condition. Chromosomes are not clearly visible.
- Prophase- The chromosomes become visible as chromatin threads. The chromosomes become shorter, thicker and clearly visible. Each chromosome now forms two district chromatids joined by a centromeres.
Nucleolus is gradually disappearing and formation of spindle fibres commence.
- Metaphase– At metaphase the chromosomes (now parried) called chromatids arranged themselves along the equator or middle of the spindle. The chromatids are attached to the spindle by centrometre.
- Anaphase- The chromatids of each chromosome separate. The start migrating to the poles of the cell by elongation of the spindle axis. The chromatids eventually reach the pole.
- Telophase (cytokinesis)- The cell starts dividing into two by line of division at the equator. The chromosome loses their thick appearance and the nuclear material, nucleus and nuclear membrane reformed. The spindle structure disappears and the cell split completely into two daughter cells having the same condition as interphase. The
- division of the cell at telophase into daughter cells is called cytokinesis
Aspect of Growth
Growth does not occur any how in all parts of the plant. It takes place in certain tissue and places in a plant bodies. The tissue where growth takes place plant is called meristem or meristematic tissue. A meristem is a tissue that retains their ability to divide by mitosis and turn out new cells. Such meristematic tissues include root apex, stem or shoot apex base or internodes and vascular cambium.
Growth which occurs in the apical stem and root meristem is called apical growth while growth which occurs at meristem base of internodes is called auxiliary growth
Regulation of growth by Hormones
Hormones or chemical or biochemical substances produced in small quantity by cell of the body of plant and animal that has profound effect on other part of their body where they are needed for some form of growth
They are produced only at the growing parts such as apical meristem of shoot or root i.e. shoot tip or root tip and transported to other part by diffusion.
Examples of plant hormones are auxin, gibberellin, cytokinin, abscissic acid and florigens. Plant hormones help plant in the following ways.
- It promotes stem elongation
- It makes plant to respond to tropism
iii. It causes root and stem apical division
- It causes fruit growth and ripening
- It causes flower bud and lateral root initiation
In animal, hormones are produced in ductless glands called endocrine system and release straight to the blood stream which transport them to the target organ. Animal hormones include the following
GLANDS SITE HORMONE
- Pituitary gland base of head Pituitrin
- Thyroid gland Neck Thyroxine
- Parathyroid Neck Parathormone
- Pancreas Loop of duodenum Insulin
- Adrenal gland Top of each kidney Adrenaline
- Testes Testes Testosterone
- Ovaries Ovaries Progesterone
FUNCTION OF ANIMAL HORMONE
- It controls body metabolism
- It promotes growth of the animal
- It induces or stimulates reproduction
Irritability is the ability of an organism to perceive and respond to changes in internal and external environment or stimuli. Stimulus is a change in condition which produces a change in the activities of the organism or part of its body. A stimulus can either be external or internal. External stimuli are those environmental factors that evoke response. Organisms respond to stimulus in three ways namely tactic, nastic and trophic response.
Tactic Response or taxis is a directional type of response or movement in which the whole organism moves from one place to another in response to external stimulus such as light, temperature, water or certain chemical. The response is thus said to directional and positive if the organism moves towards the stimulus or it is negative if it moves away from the stimulus.
– Phototaxis is the response to variation in light intensity
– Chemotaxis is the response to variation in concentration of chemical substance
Nastic Response or Nastism: This is a type of response in which a part of a plant moves in response to non-directional stimulus such as changes in light intensity, temperature and humidity. Nastic responses are usually described according to the stimulus evoking them.
– Nyctinasty is a response to changing in day and night condition (temperature) or light e.g. the opening of the petals of sun flower in the light and closes in the dark
– The closing of the morning glory flower when light intensity is low
– Haptonasty is the response due to contact or touch
– The infolding of the leaflet of mimosa plant when touched.
The closing of leaflet of flamboyant tree.
Tropic response or tropism: This is a type of response in which a part of a plant moves in response to directional stimulus.
Tropism are growth movement named according to the stimulus e.g. phototropism, hydrotropism, chemotropism, haptotropism or thigmotropism, geotropism.
Phototropism is the growth movement in response to light, thus the direction of growth movement depends on the direction of light. The shoot of plant is positively phototropic.
Geotropism: This is a response to gravitational force the shoot of plant is negatively geotropic while the root is positively geotropic.
Chemotropism is the response of plant to concentration of chemical substance e.g. roots respond positively to the presence of salt, particularly salt of calcium but negatively to alkaline or acid.
Haptrotropism /(Thigmotropism) is a response of plant to contact and is characteristic of tendrils and other organs by which the plant secures support. Tendrils show positive response to touch by twinkling around a support while roots on the other hand show a negative response to touch by growing away from stones.
Movement is the ability of living organisms to move from one place to another.
Reasons for Movement
- To search for food
ii To escape from danger
iii. To respond to stimulus either positively or negatively
- For the sake of reproduction
Cyclosis or cytoplasmic streaming: This is the mass rotational streaming or movement of the cytoplasm and its contents in cells with the vacuole. In some cases, the streaming may be restricted to a particular region or the cell while at times the whole cytoplasm is subjected to cyclical movement of the cell.
Organisms Organelles for movement Mechanism of movement
Amoeba Pseudopodia Cytoplasmic streaming (cyclosis)
Paramecium Cilia Beating the cilia against water
Euglena Flagellum Lashing movement of flagellum
Hydra Tentacles Swimming, swaying, hoping and
Earthworm Chaetae Crawling
Fishes Fins Swimming
Toads and frog Limbs Hopping
- Explain (i) Cell division
(ii) Cell enlargement
(iii) Cell differentiation
- State the organelles or locomotion in the following organisms
- (iv) Earthworm