Some Properties and Functions of the Cell III – SS1 Biology Lesson Note (First Term)

Behavioural objectives

By the end of the lesson, students should be able to:

• define growth and state the three phases involved in the basis of growth;
• describe the five stages of mitosis;
• state the importance of mitosis;
• list the parameters used to measure growth;
• describe the growth curve of man and of insects;
• state the external and internal factors that affect growth.

What is growth?

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, the rate of 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.

What is the basis of growth?

The basis of growth involves three major phases: cell division (mitosis), cell enlargement and cell differentiation.

Life begins as a single fertilised cell, which 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 (specialisation) by changing its shape and structure to carry out a particular function. Most specialised cells, at maturity, lose their ability to divide.

What are the types of cell division?

There are two types of cell division:

• Mitosis
• Meiosis

What is mitosis?

Mitosis is a cell division that produces two daughter cells with the same number of chromosomes and characteristics as those of the parent cell. It takes place only in somatic cells (body cells that are not involved in the production of gametes), such as skin, bone marrow and meristematic tissues in plants.

What are the stages of mitosis?

Mitotic cell division involves five main stages:

1. Interphase: This is referred to as the resting stage. The chromosomes become elongated and form a network of fine threads called chromatin.
2. Prophase: In early prophase, the chromosomes become visible, the nucleolus shrinks, the centrioles start moving away from each other in opposite directions and the formation of spindle fibres begins. During late prophase, the chromosomes become shorter and thicker. Each chromosome now forms two distinct chromatids joined by a centromere. The nucleolus and nuclear membrane disappear entirely.
3. Metaphase: Paired chromatids arrange themselves along the equator of the spindle and are attached to the spindle at the centromere.
4. Anaphase: Sister chromatids separate, start migrating to the opposite poles of the cell and eventually reach the poles.
5. Telophase: The cell starts dividing into two by constricting at the equator. The nucleolus and nuclear membrane are reformed in each daughter cell, the spindle fibres degenerate and the chromosomes eventually regain their threadlike form.

What is the importance of mitosis?

1. It brings about growth, development and specialisation, especially in multicellular organisms.
2. Asexual reproduction, such as binary and multiple fissions in protozoa and budding in yeasts, is the result of mitosis.
3. It ensures that the diploid condition of the cells is retained from generation to generation.
4. It aids in the repair of damaged cells.

Which life processes involve mitosis?

1. Formation of new cells in the Malpighian layer of the skin
2. Production of red and white blood cells in the bone marrow
3. Healing of wounds
4. Growth in meristem
5. Binary fission

How is growth measured?

Growth varies from one organism to another. To measure growth, the following parameters are used:

• Mass: the dry mass is more accurate and reliable than the wet mass.
• Size and length, for example the height of a man.
• Increase in the number of cells, for example budding in yeast cells.

Where is growth fastest 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:

1. The region of cell division (called the apical meristem), which brings about the primary growth of a plant
2. The region of elongation
3. The region of cell maturation or specialisation

What is a growth curve?

The growth rate of a living organism is measured using a growth curve. The growth pattern of man shows a sigmoid curve and consists of three phases:

1. 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.
2. Log or exponential phase: This is the phase of rapid growth.
3. Stationary phase: This is a stage where no observable growth is experienced.

The growth curve of insects is like a step. Each growth phase (instar) is interrupted by a period of moulting (ecdysis).

What factors affect growth?

External factors

These include availability of nutrients, humidity, light, temperature, pH and accumulation of metabolic products.

Internal factors

Internal factors are mainly hormones. The two major growth hormones in plants are auxin and gibberellins. In animals, the hormones concerned with growth are secreted by the anterior pituitary gland, the thyroid gland and the gonads. Over-secretion or under-secretion of any of these hormones leads to abnormal growth.

Evaluation

1. Define growth.
2. Name the three major phases involved in the basis of growth.
3. List the five stages of mitosis in order.
4. State two external factors and two internal factors that affect growth.
5. Name the three phases of the sigmoid growth curve.

Assignment

1. Growth includes the following processes except:
   A. differentiation
   B. meiosis
   C. mitosis
   D. elongation
2. When the zygote of an organism continues to grow, it produces new cells by:
   A. meiosis
   B. mitosis
   C. crossing-over
   D. gestation
3. The aspects of growth in living organisms include all the following except:
   A. increase in dry weight
   B. irreversible increase in length
   C. reversible increase in size
   D. increase in number of cells
4. The organelle involved in tissue respiration is the:
   A. endoplasmic reticulum
   B. Golgi body
   C. mitochondrion
   D. ribosome
5. In the absence of oxygen, the pyruvic acid produced during glycolysis is converted to CO₂ and:
   A. water
   B. glycerol
   C. ethanol
   D. citric acid
6. In a tabular form, differentiate between growth in plants and animals.
7. State three points on the importance of alcoholic fermentation.