Concept of Growth and Development - K.C.S.E Biology Notes

​Growth is a characteristic feature of all living organisms.
Most multi cellular organisms start life as a single cell and gradually grow into complex organisms with many cells. This involves multiplication of cells through the process of cell division.
This quantitative permanent increase in size of an organism is referred to as growth.

For growth to take place the following aspects occur

• Cells of organisms assimilate nutrients hence increase in mass.
• Cell division (mitosis) that lead to increase in the number of cells.
• Cell   expansion   that   leads   to enlargement an increase in the volume and size of the organism. It is therefore possible to measure growth using such parameters as mass, volume, length, height, surface area.
• On the other hand development is the qualitative aspect of growth which involves differentiation of cells and formation of various tissues in the body of the organism in order for tissues to be able to perform special functions.
• It is not possible to measure all aspects of development quantitative.
• Therefore development can be assessed in terms of increase in complexity of organism e.g. development of leaves, flowers and roots.
• A mature human being has millions of cells in the body yet he or she started from; single cell, that is, a fertilised egg.
• During sexual reproduction mammals an ovum fuses with a sperm to form a zygote.
• The zygote divides rapidly without increasing in size, first into 2, 4, 8, 16,32, 64 and so on, till it forms a mass cells called morula.
• These first cell division is called cleavages.
• The morula develops a hollow part, resulting into a structure known as a blastula (blastocyst).
• Later, blastocyst cells differentiate into an inner layer (endoderm) and the outer layer (ectoderm).
• The two-layered embryo implants into the uterine wall and, by obtaining nutrients from the maternal blood, starts to grow and develop.

EMBRYONIC DEVELOPMENT

• As the embryo grows and develops, changes occur in cell sizes and cell -types.
• Such changes are referred to as growth and development respectively.
• These processes lead to morphological and physiological changes in the developing young organism resulting into an adult that is more complex and efficient.
• In the early stages, all the cells of the embryo look alike, but as the development process continues the cells begin to differentiate and become specialized into different tissues to perform different functions.
• Growth involves the synthesis of new material and protoplasm.
• This requires a continuous supply of food, oxygen, water, warmth and means of removing waste products.
• In animals, growth takes place all over the body but the rates differ in the various parts of the body and at different times.
• In plants however, growth and cell division mostly take place at the root tip just behind the root cap and stem apex.
• This is referred to as apical growth which leads to the lengthening of the plant.
• However, plants do not only grow upwards and downwards but sideways as well.
• This growth leads to an increase in width (girth) by the activity of cambium cells.
• The increase in girth is termed as secondary growth.

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