Growth; Concept of growth and Development


Concept of growth

Growth is the irreversible process of increasing number of cells and size of an organism. The increase in number of cells is a result of the process called mitosis.

In this process, the nucleus and cytoplasm of a cell divide to form two identical daughter cells. In animals, growth occurs in all parts of the body whereas in plants it occurs in specific regions of the body such as at the tips of the shoot and root.

Usually, the growth rate of an organism increases with time. This type of growth is known as positive growth. At later stages of an organism’s growth, the rate of growth slows down.

This type of growth is called negative growth. Positive growth occurs when synthesis of materials (anabolism) exceeds the breakdown of materials (catabolism) while negative growth occurs when catabolism exceeds anabolism. The growth of an organism is accompanied by development. Development is the increase in complexity of the organism and ability of the organism’s structures to function.

Development involves differentiation of cells. Differentiation is the process by which cells and structures become specialised for performing specific functions. It involves changes in both, the shape and the physiology of the cells. Differentiated cells are capable of carrying out specific functions.

Importance of growth and development in living organisms

In multicellular organisms, the newly formed individuals have certain structures that are not well developed.

  • Growth and development enable the differentiation of cells in an organism, resulting in the specialisation of cells for different functions.  The specialisation of cells during growth enables an organism to adapt to its environment. An example of specialised cells is guard cell in the plant leaves which control the opening and closing of stomata and blood cells used to transport different materials in various parts of the animal body.

Factors affecting growth and development in organisms

The growth and development of an organism are affected by various factors.

These factors are categorised as

  • External and
  • internal factors.

External factors are environmental factors whereas internal factors are factors within an organism’s body.

External factors affecting growth and development in plants

Growth of plant and their spatial distribution are influenced by the environment. Any environmental factor that is not favourable to a plant becomes a limiting factor to plant growth. Limiting factors are also responsible for the geographical distribution of plants. For example, only plants adapted to limited amounts of water can survive in deserts. Therefore, it is important to understand the environmental factors that affect plant growth and distribution.

These External factors  factors include

  1. Light, temperature,
  2. Water,
  3. Nutrients, and
  4. Gases such as carbon dioxide and oxygen
Figure 1.1: Factors affecting plant growth
Figure 1.1: Factors affecting plant growth

Light: Plants need light to perform several functions including photosynthesis. The more sunlight a plant receives, the higher capacity it has to produce food through photosynthesis. As the intensity of sunlight decreases, the rate of the photosynthesis also decreases. This is because light supplies energy which plants use to synthesise food. When there is limited sunlight, stems of the plant become “leggy”, meaning that they become long and thin as they try to reach the source of light. The absence of light also makes plants undergo chlorosis. Chlorosis is characterised by the yellowing of leaf tissue due to lack of chlorophyll

Temperature: Environmental temperature influences most plant processes including photosynthesis, transpiration, reproduction, respiration, germination and flowering. The effect of temperature on plant growth and productivity is dependent on plant variety.

This is whether a plant is a warm-season or a cold season crop. The rate of all biological processes in plants are accelerated as the temperature rises to a plant optimum level and decelerates as the temperature deviates from the optimum level. Enzymes are responsible for metabolic activities that lead to growth. Enzymes work best at optimum temperature. At lower temperatures enzymes are inactive whereas at higher temperature enzymes are denatured. For example, if the environmental temperature is high for a long time, cool-season crops such as spinach will wilt. Similarly, high temperature for warm-season crops such as tomatoes can cause pollen to become unviable. Temperature also affects the flowering and pollination of flowers. Adverse temperature can also lead to stunted growth and poor-quality crops.

Nutrients: Plants need nutrients for growth and development. Normally, nutrients required by plants are found in the soil. Such nutrients include Nitrogen, Phosphorus, Potassium, Manganese, Magnesium, Caleium, and Sulphur. Other nutrients such as Carbon, Hydrogen, and Oxygen are obtained from the atmosphere and water. Nutrients required by plants in small amounts are known as micronutrients or trace elements. Examples of micronutrients include Zine, Manganese and Iron. Nutrients required by plants in large amounts are known as macronutrients. Examples of macronutrients include Nitrogen, Phosphorus, and Potassium.

Most of the nutrients that a plant needs are dissolved in water in the soil and then absorbed by its roots. Inadequate amount of nutrients affects the rate of growth and survival of plants.

Water: About 90% of the weight of a plant is made up of water. Water is one of the essential factors required for plant growth. It provides plants with nourishment and hydration. It plays a vital role in plant processes such as photosynthesis, respiration and transportation of minerals and other nutrients. Water is a medium through which biochemical reactions in a living organism take place. Water is also important in maintaining the turgor pressure of plant cells which maintains cell shape and ensures firmness of the plant.

Through turgor pressure and other changes in the cell, water regulates the opening and closing of stomata. Stomata opening and closing allow transpiration and exchange of gases like carbon dioxide and oxygen.

Evaporation of water from leaves during transpiration helps to cool the plant. The gradual evaporation of water from the leaf’s surface through the stomata helps plant to stabilise plants temperature. Water also dissolves minerals in the soil. When plant absorbs water through their roots, minerals are also picked up and transported along with the xylem tissue. Humidity or water vapour in the air also influences plant growth by affecting photosynthesis, transpiration and pollination.

Amount of carbon dioxide and oxygen gases: Both carbon dioxide and oxygen are required for plants growth. Carbon dioxide is one of the raw material in the manufacturing of sugar through the process of photosynthesis. The produced sugar is the source of energy to the plant for various physiological activities. Such physiological activities result in growth and development in plant structures and increase of leaf area and size of the roots, which is crucial for the plant to grow and survive. Carbon dioxide and oxygen gas which are dissolved in water are taken by the plants through roots. Plants also take oxygen gas at night through the stomata of the leaves. Plants use oxygen for aerobic respiration that releases the energy required for growth.

At low carbon dioxide gas concentrations in the environment, the rate of photosynthesis is lowered, hence decreasing the rate of plant growth. As the carbon dioxide gas concentration increases the rate of photosynthesis also increases and plant growth become rapid.

Internal factors affecting the growth and development of plants

Internal factors that affect the growth and development of plants include plant growth hormones and genetic factors.

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Plant growth hormones: These are chemical substances that regulate plant growth and development. Hormones help plants to respond to the stimuli such as sunlight, water and nutrients from the environment. Plant hormones include auxins, gibberellins, cytokinins, ethylene and abscisic acid.

Auxins: Auxins are highly concentrated in growing parts of the plant such as at the tips of the shoots and roots. Auxins hormone stimulates the growth and development of plants. This hormone is also transported downward from the top to the bottom of the plant where it causes several responses in plants such as phototropism and geotropism.

Auxins promote apical dominance, a process that prevents too many lateral buds and branches from growing on the side of the stem.

Gibberellins: These hormones cause similar effects as auxins in plants. They are present in young shoots, root and seeds. They stimulate cell division and elongation and promote elongation of the stem between the nodes. When the level of gibberellins increase, the internodes elongate and the stem length increases.

Similarly, deficiency of gibberellin reduces the length of the stem between internodes resulting in dwarfism in plants. Gibberellins initiate seed germination by breaking seed dormancy and speeding up the germination.

Hence, they contribute to plant growth and development.

Cytokinins: These hormones stimulate cell division and are often included in the sterile media used for growing plants from tissue culture. Cytokinins are produced in the root and transported upward to the stem through the xylem. They stimulate differentiation and growth of axillary buds and inhibit lateral root formation. Cytokinins also delay leaf ageing and death.

Ethylene: This hormone exists in gaseous form. It regulates the growth by inducing ripening of fruits and senescence of plants.

It also causes plant leaves to drop. Often, plants increase ethylene production in response to stress. A high concentration of ethylene is often found within cells at the end of plant’s life. An increase in ethylene in leaf tissue in the autumn is part of the reason why trees fall off their leaves during this period. Ethylene is also used to ripen fruits such as green bananas.

Abscisic acid: This hormone is important in seed development and maturation. It induces seed dormancy by preventing seed germination. It causes the abscission of leaves, fruits and flowers and induces the closing of stomata. A high concentration of abscisic acid in a plant cell during periods of drought plays a role in the closure of stomata. Closing of stomata reduces water loss, hence ensuring the survival of the plant.

Genetic factor: This is another internal factor to plant growth since their basis of expression is within the cells. Plant growth involves changing seeds into seedlings.

Other changes occurring during growth include an increase in height, width, development of branching pattern, flowers, fruits and seeds. All of these changes are coded in the genetic material of the plant which is found in the nucleus of the cell.

The genetic factors affect plant growth differently depending on whether the plants belong to the same species or different species. For example, a mango and baobab tree have different genetic materials hence their growth rates are different. Genetic factors also cause some crop varieties of the same species mature differently. For example, Pemba Red Dwarf coconuts (Kitamli), matured much earlier (at 4-5 years) compared to Local East African Tall coconuts (at 6-8 years). The genetic factors determine the characters of a plant.

However, environmental factors influence the extent to which such characters are expressed.

External factors affecting growth and development in animals

Like in plant, growth and development in animals are also affected by external factors. These factors include temperature, nutrition, availability of oxygen gas, mutagens and risk behaviour.

Temperature: Changes in environmental temperature affect the growth and development of animals. For example, eggs of organisms such as grasshoppers, snakes, lizards and turtles will not hatch if the temperature is not optimum. Some animals such as toads depend on a specific range of temperature to breed. This implies that low temperature limits the breeding of such animals. Sometimes the breeding season is delayed until the temperature is optimal. Similarly, the optimum temperature is required to activate many enzymatic reactions within the animal body

Nutrition: The growth and development of animals also depends on the availability of sufficient food. Some food substances are the raw materials for growth and others are the source of energy necessary for growth. For example, the development of a tadpole into a frog or toad depends on the availability of food and other nutrients.

Humans also grow normally when they get sufficient food. Therefore, the availability of quality and adequate food has a direct effect on growth. For instance, protein is very important in the growth of young mammals. Its insufficiency in the diet will lead to stunted growth

Oxygen gas: During respiration, oxygen gas is needed to release energy from the food. The energy released is used in different physiological processes which in turn bring about growth and development in an organism. Therefore, an inadequate supply of oxygen gas causes low metabolic rate which slows down the rate of growth.

For example, the high nutrient load leads to depletion of oxygen gas levels in the aquatic ecosystems. This is because the decomposition of it uses oxygen gas. This in turn affects the growth and development of a living organisms in that habitat.

Mutagens and risk behaviours: Mutagens are agents of mutation that alters the genetic materials of organisms resulting in altered gene expression or protein function. For example, mutations in genes can promote or inhibit growth and cellular replication.

Examples of mutagens include harmful radiations such as X-rays, gamma rays, and ultraviolet radiations. Some of the risk behaviours such as alcoholism, smoking marijuana and cigarettes also affect the growth of human being. Excessive alcohol consumption can lower the rate of growth in both adolescent boys and girls.

Internal factors affecting growth and development in animals

The growth and development in animals are also affected by internal factors such as growth hormones, genetic factors, and enzymes.

Growth hormones: Growth and development in animals are affected by hormones. In humans, growth is regulated by the growth hormone from the pituitary gland. This hormone promotes the growth of bones and cartilages and stimulates some metabolic activities necessary for animal growth. For example the gonads, ovaries and testes also produce estrogen and testosterone hormones which accelerate the development of secondary sexual characteristics during puberty.

Genetic factors: Animal growth and development are also influenced by genetic factors. This is more noticeable in livestock, where some animal breeds grow faster and become heavier or taller than others.

Genetic factors greatly influence the growth rate of animals. For example, human babies born on the same day can vary in some features such as their height and weight due to variations in their genetic make-up.

Enzymes: These are the chemical substances that catalyse the rate of chemical reactions in the body of a living organism but at the end of the reactions remain unchanged. For example, human digestive enzymes catalyse the breakdown of food substances into simpler forms that can be absorbed by the body. This in turn facilitates the growth and development of organisms. When there is under-secretion, over-secretion or absence of enzymes, growth is affected because the process of metabolism will also be affected.

Exercise 1.1

  1. Distinguish between “growth” and
  2. Give reason why a pruned single stemed plant develops many lateral buds and branches.
  3. Explain three (3) external factors affecting the rate of growth in plants.
  4. Describe how temperature affects the rate of growth and development in animals.
  5. Explain the role of hormones in the growth and development of plants.
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