15.7 - Growth in Humans and Animals

Growth in Organisms

In living organisms, growth is an irreversible process which results in the increase in size, body mass, volume and number of cells. It also involves changes associated with differentiation and specialization, where unspecialized structures/cells are modified to perform specific functions.

Measuring Growth in Humans and Animals

Growth in humans and animals can be estimated by measuring certain parameters such as:

• Length or height to see whether there is any increase in size or volume
• Fresh weight , where the organism need not be killed. However, it is less accurate due to the presence of body fluid.
• Dry weight , where the organism needs to be killed to remove all body fluids. The body is dried in the oven at 100°C repeatedly until a constant weight is obtained. This method is suitable to be used in plants.

Growth in Insects

Insects undergo a different growth process compared to humans and animals. They can go through either complete metamorphosis like butterflies or incomplete metamorphosis such as grasshoppers during growth.

Figure 11 - Complete metamorphosis of a butterfly.

Figure 12 - Incomplete metamorphosis.

Growth Phases in Sigmoid Growth Curves of Humans and Animals

When a growth parameter such as the mass or height of an organism is plotted against time, a growth curve can be obtained. The growth curve is generally S-shaped and is known as a sigmoid curve. A sigmoid curve can be divided into 6 phases.

1. Lag phase

• Slow growth rate
• Little cell division and cell elongation
• Organism adapts to a new food source

2. Exponential phase

• Growth rate is the fastest
• Active cell division and elongation
• Size of organism increases rapidly

3. Stationary phase

• Growth rate is at a relatively constant rate
• Size of organism reaches its maximum
• Cells undergo differentiation to become specialized cells

4. Maturity phase

• The organism has reached its maturity
• Zero growth rate
• Rate of cell division is equal to rate of cell death
• Cell division occurs only to replace dead cells

5. Senescence phase

• Organism undergoes the process of ageing
• Negative growth rate

6. Death phase

• Organism eventually die due to old age, illness or lack of nutrients and space.

Intermittent Growth Curve of Animals with Exoskeletons

Insects have a hard, inflexible external skeleton made up of chitin which does not facilitate a smooth growth rate. In order to grow, they must undergo moulting process periodically. Moulting or ecdysis is the process of removing the old exoskeleton and replacing it with a new larger one. When the length of an insect is plotted against time, it gives an intermittent growth.

Figure 13 - The graph is shaped like a series of steps. Ecdysis happens periodically. Polar growth is only intermittent, not continuous. The horizontal parts indicate zero growth. This is the time where the insect stops growing. Each stage between ecdysis is called an instar. At this stage, the insect is known as a nymph. The vertical lines represent rapid growth. This is when the nymphs undergo ecdysis. The organism increases rapidly in size during ecdysis. The insect undergoes ecdysis multiple times before reaching adulthood.

How ecdysis occurs

1. A new exoskeleton forms below the old exoskeleton.
2. Before the new exoskeleton hardens, the insect enlarges its body volume by swallowing air.
3. This breaks the old exoskeleton and the new exoskeleton emerges.
4. The insect will enlarge its body volume once more before the new exoskeleton hardens.