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

Growth is the irreversible increase in dry mass, size and complexity of an organism brought about by the synthesis of new protoplasm.

Behavioural objectives

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

1. define growth and state the three phases involved in the basis of growth;
2. describe the five stages of mitosis;
3. state the importance of mitosis;
4. list the parameters used to measure growth;
5. describe the growth curve of man and of insects;
6. 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.

• Condition for growth: the rate of building up of materials (anabolism) must exceed the rate of breaking down (catabolism).
• Growth in plants: it is indefinite and apical, so it continues at the tips of roots and stems.
• Growth in animals: it is definite and uniform, so the whole body grows and then stops at maturity.

What is the basis of growth?

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

1. Cell division: life begins as a single fertilised cell, which divides into two daughter cells, then four and so on.
2. Cell enlargement: after division, the daughter cells increase in mass and size.
3. Cell differentiation: each cell develops into a special type of cell by changing its shape and structure to carry out a particular function. This is also called specialisation.

• Note: 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 and meiosis.

• Mitosis: division that gives two daughter cells identical to the parent cell, used for growth and repair.
• Meiosis: division that halves the chromosome number, used to produce gametes.

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.

• Where it happens: only in somatic cells, which are body cells not involved in the production of gametes.
• Examples: skin cells, bone marrow cells and meristematic tissues in plants.

What are the stages of mitosis?

Mitosis has five main stages: interphase, prophase, metaphase, anaphase and telophase.

1. Interphase: the stage between divisions, often called the resting stage.
   • The chromosomes become elongated.
   • They form a network of fine threads called chromatin.
2. Prophase: the chromosomes become visible and prepare to separate.
   • Early prophase: the chromosomes become visible and the nucleolus shrinks.
   • Early prophase: the centrioles move away from each other in opposite directions and spindle fibres begin to form.
   • Late prophase: the chromosomes become shorter and thicker.
   • Late prophase: each chromosome forms two distinct chromatids joined by a centromere.
   • Late prophase: the nucleolus and nuclear membrane disappear entirely.
3. Metaphase: paired chromatids arrange themselves along the equator of the spindle.
   • Each pair is attached to the spindle at the centromere.
4. Anaphase: sister chromatids separate.
   • They migrate to 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 reform in each daughter cell.
   • The spindle fibres degenerate.
   • The chromosomes regain their threadlike form.

What is the importance of mitosis?

Mitosis brings about growth and repair, and keeps the chromosome number constant from cell to cell.

1. Growth: it brings about growth, development and specialisation, especially in multicellular organisms.
2. Asexual reproduction: binary and multiple fissions in protozoa and budding in yeasts result from mitosis.
3. Constant chromosome number: it keeps the diploid condition of cells from generation to generation.
4. Repair: it replaces damaged cells.

Which life processes involve mitosis?

Five common life processes involve mitosis.

1. Skin renewal: new cells form in the Malpighian layer of the skin.
2. Blood formation: red and white blood cells are produced in the bone marrow.
3. Wound healing: new cells replace damaged ones.
4. Growth in meristems: plant tips add new cells.
5. Binary fission: a single-celled organism divides into two.

How is growth measured?

Growth is measured using parameters such as mass, size or length, and number of cells, and it varies from one organism to another.

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

Where is growth fastest in plants?

Growth is fastest at the root and stem apices, because plant growth is apical.

Each apex has three regions:

1. Region of cell division: called the apical meristem, it brings about the primary growth of a plant.
2. Region of elongation: the new cells increase in length.
3. Region of cell maturation: the cells become specialised for particular functions.

What is a growth curve?

A growth curve is a graph of size or mass against time, used to show the growth rate of a living organism.

What is the growth curve of man?

The growth pattern of man is a sigmoid (S-shaped) curve with three phases.

1. Lag phase: the initial stage of slow growth, when the cell is still accumulating the materials needed to begin.
2. Log or exponential phase: the phase of rapid growth.
3. Stationary phase: the stage where no observable growth is experienced.

What is the growth curve of insects?

The growth curve of insects is step-like, because each growth phase (instar) is interrupted by a period of moulting (ecdysis).

How do the growth curves of man and insects differ?

What factors affect growth?

Growth is affected by external factors from the environment and internal factors within the organism.

What are the external factors that affect growth?

• Nutrients: they supply the materials for making new protoplasm.
• Humidity: it affects water availability and water loss.
• Light: plants need it for photosynthesis.
• Temperature: it affects the rate of enzyme action in the cell.
• pH: it also affects enzyme action.
• Accumulation of metabolic products: a build-up of waste can slow growth.

What are the internal factors that affect growth?

The main internal factors are hormones.

• Plant hormones: the two major growth hormones are auxin and gibberellins.
• Animal hormones: they are secreted by the anterior pituitary gland, the thyroid gland and the gonads.
• Abnormal growth: over-secretion or under-secretion of any of these hormones leads to it.

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. In a tabular form, differentiate between growth in plants and growth in animals.