Home News BIOLOGY FORM FOUR TOPIC 1: GROWTH

BIOLOGY FORM FOUR TOPIC 1: GROWTH

Concept of Growth
The concept of Growth
Explain the concept of growth
Growth
is an increase in size/mass or growth. It is the progressive
development of living thing, 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, growth
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
Investigate 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:
  1. 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.
  2. 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:
  1. 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.
  2. 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 estrogens in small
    quantities from the time of birth onwards. 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.
  3. Testosterone:Testosterone,
    produced by the testicle, is important in stimulating growth and it is
    responsible for the greater growth of muscles.
  4. 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:
  1. 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; Bone 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 control the activities of
    osteoblasts. In vitamin C deficiency the intercellular 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.
  2. 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.
  3. Cultural factors:The
    physical growth of human beings is definitely affected by cultural
    factors. Culture differs from ethnic group to ethnic group. 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.
  4. 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.Size of family exerts an indirect influence on the rate of
    growth. In a large family with limited income the children do not get
    proper nutrition. As a result the growth is affected. The number of
    children in the family exerts an effect on the children’s rate of
    growth. Children in large families are usually smaller and lighter than
    children in small families. Possibly this is because in large families
    children tend to get less individual care and attention.
Internal factors affecting growth in plants
The internal factors that influence plant growth include following:
Hereditary factors
Heredity
factors are internal factors that affect the growth of plants. They
affect the physical appearance and the size of a plant
Hereditary
units called genes are found in chromosomes inside the nucleus of all
plant cells. These units control the various characteristics of plants
such as flower colour number of floral parts, growth pattern and so on.
Genes are passed from parents to off spring. For example, tall plants
produce tall offspring and short plants produce short offspring.
Growth hormones
Certain
hormones such as growth hormones are known to affect growth. Hormones
are chemical substances that influence physiological processes. Drastic
changes in their concentrations in the body will, therefore, affect
growth.
There
are several known growth hormones. Some of them, like auxins,
cytokinins are growth-promoting while others like abscissic acid and
ethylene are growth inhibitors. Most of the growth regulators are
synthesized by plants while a few are synthetic in nature.The table
below summarizes the role of certain plant hormones on growth of plants
and seeds.
Hormone Role in plant growth
Indoleacetic
acid(IAA)—the main auxin. Other three auxins seem to have rather
marginal importance for plants in natural environments.
  • Promotes cell division
  • Promotes cell enlargement
  • Promotes response of shots and roots to stimuli such as light, water and gravity
  • Promote growth of adventitious roots
  • Induces parthenocarpy (formation of fruits without fertilization)
  • Causes formation of the abscission layer at the base of the leaf stalk, leading to falling of leaves (abscission).
  • Inhibits development of lateral buds, thus promoting apical dominance
  • Causes formation of callus tissue. Callus tissue forms at the site of an injury to bring about healing in the plant.
  • Controls division in the vascular cambium and xylem differentiation.
  • Used as the rooting hormones in stem cuttings.
  • 2-4 D is used as an herbicide to kill broadleaf, dicotyledonous weeds.
  • Promotes flowering in pineapples.
Gibberellins
  • Promote cell division and elongation of internodes in dwarf plants.
  • Induce parthenocarpy by initiating formation of indoleacetic acid (IAA)
  • Promote lateral bud development
  • Inhibit development of adventitious roots
  • Inhibit formation of the abscission layer on the leaf petiole
  • Promote germination of seeds
  • It
    helps in inducing seed germination by breaking seed dormancy and
    initiating the synthesis of hydrolases enzymes for digesting reserve
    food.
Cytokinins
  • Stimulate cell division
  • Stimulate formation of callus tissue
  • Promote flowering
  • Break seed dormancy
  • Promote formation of adventitious roots
  • Promote development of lateral buds by inhibiting apical dominance.
  • Low concentration of clytokinin induces cell elongation and causes ageing of leaves
  • Help in the production of new leaves, chloroplasts, and adventitious shoots.
  • Help in delaying senescence by promoting nutrient mobilisation.
Ethylene (ethane)
  • Promotes ripening of fruits
  • Causes formation of callus tissue, leading to falling of fruit and leaves
  • Stimulates thickening of the stem while inhibiting stem elongation
  • Helps in breaking seed and bud dormancy.
  • Promotes root-growth and formation of root hairs.
Abscisic acid (ABA)
  • induces seed dormancy by inhibiting seed germination, growth of stems, and sprouting of buds
  • Causes fruits and leaves to fall (abscission)
  • Promotes flowering
  • Stimulate apical dominance by suppressing development of lateral buds
  • Stimulates stomatal closure during water stress
Indolebutyric acid Synthetic plant hormone that promotes elongation of stems and roots
Apical dominance
An
apical bud is found at the top of the plant. Apical buds are
responsible for increase in plant’s height (apical growth). Lateral buds
are found on the sides of the plant. Lateral buds are responsible for
the formation of branches. Apical dominance is the inhibition of the
growth of lateral buds by the presence of the growing apical bud. Apical
dominance causes plant shoots to have a conical shape.
The
apical bud produces auxins that diffuse to the lower parts of the
plant. These auxins retard the development of lateral buds. The lateral
branches of such a plant are short. A plant that has strong apical
dominance gains more height in comparison to its width. Thus the plant
assumes a conical outline.
Cutting
the apex of the shoot causes the lateral buds to sprout. The dominance
is overcome since the source of auxins at the apex is removed. The
lateral buds sprout, branches develop, and the plant assumes an umbrella
shape. Tea bushes are pruned so that they can develop many side
branches. Rose plant, cypress and bougainvillea plants are pruned so
that they can make a good hedge.
External factors that affect growth in plants
The
external factors that affect plant growth include light, nutrients,
temperature, relative humidity, water, carbon dioxide and oxygen, soil
condition, biotic factors, and pollutants. Each of these factors is
explained in detail below:
  1. Light:The effect of light on growth can be studied under three headings: light intensity, light quality and duration of light.
    Growth is generally favoured by darkness, but light is necessary
    because of its role in the manufacture of food. Young plants growing in
    the absence of light develop elongated thin stems with narrow leaves and
    poorly developed shoot system. Such plants are said to be etiolated.In
    weak intensity of light the internodes are short and the leaves are
    expanded. In strong intensity of light, the plant assumes a normal
    height. Very low light intensity reduces the rate of overall growth of
    the plant, by lowering the rate of photosynthesis.Growth in full
    spectrum of visible light is found to be better than the growth in any
    one of the different colours of light. Red colour seems to be the most
    favourable for growth.The duration of light has a pronounced effect on
    the growth of vegetative as well as reproductive structures. The
    influence of duration of light is most marked in inducing or suppressing
    flowering. This phenomenon is termed as photoperiodism.
  2. Nutrients:Availability,
    quality and quantity of food substances will automatically affect
    growth. For growth to occur in living things, food must be broken down
    to release energy. In areas where nutrients and water are adequate,
    competition is reduced and population increases. In case of shortage of
    nutrients and water, competition sets in and most individuals die.There
    are different mineral nutrients required for optimum plant growth. These
    nutrients are classified as either macronutrients or micronutrients.
    Macronutrients are those nutrients required by plants in high doses
    while micronutrients are the nutrients required in small quantities.
    Examples of macronutrients include nitrogen, potassium, magnesium,
    calcium, phosphorous and sulphur. Micronutrients include iron, zinc,
    molybdenum, manganese, boron, copper, cobalt and chlorine.
  3. Temperature:Atmospheric
    and soil temperatures are very crucial for plant growth as it affects
    many plant processes such as photosynthesis, metabolism, respiration,
    transpiration, breaking of seed dormancy, seed germination, protein
    synthesis, translocation, and flowering. At high temperatures the
    translocation of manufactured food is faster so that plants tend to
    mature earlier.Growth can take place between 0°C and 50°C. But the
    optimum temperature for the growth is between 20° and 30°C. Low
    temperature, however, is necessary for many plants to flower. Different
    physiological processes such as photosynthesis and respiration are
    controlled by enzymes. The enzymes are affected by temperature and pH.
    Enzyme activity and the rate of most chemical reactions generally
    increase with rise in temperature. Up to a certain point, there is
    doubling of enzymatic reaction with every 10°C temperature increase. But
    at excessively high temperatures, denaturation of enzymes and other
    proteins occur.It follows, therefore, that drastic changes in
    temperature and pH will affect growth.
  4. Relative humidity:Relative
    humidity (RH) is the amount of water vapour in the air, expressed as
    the proportion (in percent) of the maximum amount of water vapour it can
    hold at certain temperature. For example, an air having a relative
    humidity of 60% at 27°C temperature means that every kilogram of the air
    contains 60% of the maximum amount of water that it can hold at that
    temperature.The relative humidity affects the opening and closing of the
    stomata which regulates loss of water from the plant through
    transpiration as well as photosynthesis. Transpiration is slower in
    humid conditions. This is because diffusion of water vapour out of the
    leaf slows down if the leaf is already surrounded by moist air.
  5. Water:As
    mentioned earlier, water is a primary component of photosynthesis. It
    maintains the turgor pressure or firmness of tissue and transports
    nutrients throughout the plant. In maintaining turgor pressure, water is
    the major constituent of the protoplasm of a cell. By means of turgor
    pressure and other changes in the cell, water regulates the opening and
    closing of the stomata, thus regulating transpiration. Water also
    provides the pressure to move a root through the soil. Among water’s
    most critical roles is that of a solvent for minerals moving into the
    plant and for carbohydrates moving to their site of use or storage.
    Gradual evaporation of water from the surface of the leaf near the
    stomata helps stabilize plant temperature.
  6. Carbon dioxide and oxygen:The
    oxygen and carbon dioxide in the air are of particular importance to
    the physiology of plants. Oxygen is essential in respiration for the
    production of energy that is utilized in various growth and development
    processes. Carbon dioxide is a raw material in photosynthesis. However, a
    high concentration of carbon dioxide reduces growth because of its
    effect on the closing of stomata, and maintenance of dormancy. If the
    concentration of carbon dioxide in the plant leaf is higher than the
    surrounding air, the stomata will open to let in more of the gas from
    the surrounding air so as to balance the equilibrium of the gas between
    the two media (air and leaf air spaces). The opposite is the case if the
    concentration of the gas is higher in the air than in the leaf.
  7. Soil condition:The
    characteristics of soil play a big part in the plant’s ability to
    extract water and nutrients. If plants are to grow to their potential,
    the soil must provide a satisfactory environment for plant growth.Plant
    growth is influenced by the soil properties such as texture or
    structure, salinity, acidity, waterlogging, or compaction.
  8. Biotic factors:Diseases,
    plant pests, weeds and harmful substances released by roots
    (allelopathy) affect plant growth drastically. Weeds compete with plants
    for moisture, nutrients, and light. Root knot nematodes reduce
    absorption, so more fertilizer is necessary. All of these have negative
    impacts on plant growth and development.
  9. Pollutants:Pollutants
    can hamper plant growth. Many pollutants composed of poisonous gasses
    (such as carbon monoxide, sulphur dioxide, hydrogen fluoride, hydrogen
    sulphide) are capable of restraining growth, even bringing plants to
    death. Pollutants from household or industrial wastes are also able to
    restrain plant growth.
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