15.7 - Growth in Humans and Animals

Growth in Organisms

• Growth is an irreversible, permanent increase in cell number, size, volume and body weight.
• Growth also involves cell differentiation and specialisation, including increased complexity of cell shapes and functions.
• Growth enables the development and maturation of body systems.

Measuring Growth in Humans and Animals

• Size or volume: Measure changes in height or length.
• Dry weight: Weight after all body fluid is removed by repeated drying at $100{}^{\circ }\mathrm{C}$ until constant. The organism must be killed; this method is suitable for plants.
• Fresh weight: Can be measured over a period without killing the organism, but is less accurate because body fluid depends on fluid intake.

Growth of Insects

Complete Metamorphosis

• Four stages: egg $\to$ larva $\to$ pupa $\to$ adult.
• Example: butterfly.

Incomplete Metamorphosis

• Three stages: egg $\to$ nymph $\to$ adult.
• Example: grasshopper.
• The nymph undergoes several stages of ecdysis before becoming an adult.

Phases in a Sigmoid Growth Curve

• A growth curve plots a growth parameter against time.
• Human and animal growth is gradual and continuous, producing a sigmoid (S-shaped) curve.

Lag Phase

• Low growth rate; little cell division and elongation.
• Adjustment to new environmental resources.

Exponential Phase

• Highest growth rate.
• Active cell division and elongation; size increases rapidly.

Stationary Phase

• Growth slows and continues at a constant rate.
• Cells reach maximum size and differentiate into specialised cells.

Maturity Phase

• The organism reaches maturity; growth rate is zero.
• Cell division equals cell death and replaces damaged or dead tissues.

Senescence Phase

• Negative growth rate; the organism ages.

Death Phase

• Ageing, illness, lack of nutrients or lack of space eventually causes death.

Intermittent Growth Curve of Animals with Exoskeletons

• The insect exoskeleton is made of chitin and does not grow proportionately with the insect.
• Ecdysis is the periodic shedding of the exoskeleton; it is controlled by hormones.
• The staircase-shaped curve shows intermittent growth.
• Horizontal sections show zero increase in length during an instar; the insect builds tissue and increases body volume.
• Vertical sections show rapid growth during ecdysis.
• A new exoskeleton forms beneath the old one. The insect sucks in air, expands and breaks the old exoskeleton; it expands again before the new exoskeleton hardens.
• Ecdysis repeats until adulthood.