15.7 Growth in Humans and Animals
Learn how growth is measured, how insects grow through metamorphosis and ecdysis, and how growth curves reveal the phases of life.
Learning Objectives
- Define the meaning and importance of growth in organisms.
- State and evaluate the parameters used to measure growth.
- Compare complete metamorphosis and incomplete metamorphosis in insects.
- Identify and explain the six phases of the sigmoid growth curve.
- Explain the intermittent growth curve of insects with exoskeletons and describe the process of ecdysis.
Comparison of Sigmoid and Intermittent Growth Curves
Continuous growth in mammals produces an S-shaped curve, while insects with exoskeletons grow in a staircase pattern through ecdysis.
Sigmoid Curve
Most organisms, including humans, show gradual and continuous growth. The curve rises smoothly through six phases from lag to death.
Intermittent Curve
Insects with a hard chitin exoskeleton grow in steps. Horizontal lines show zero increase in length during an instar; vertical lines show rapid expansion during ecdysis.
Short Explanation
Meaning of Growth
Growth is an irreversible, permanent process involving an increase in the number of cells, size, volume and weight. It also involves cell differentiation and cell specialisation to develop specific shapes and functions necessary for the maturation of bodily systems.
Parameters Used to Measure Growth
Growth can be measured through an increase in size or volume, such as height or length, and through changes in weight.
Dry Weight
Dry weight is measured after the organism is dried repeatedly in an oven at 100 °C until its weight remains constant. It is very accurate because it excludes fluctuations in water content, but the organism must be killed.
Fresh Weight
Fresh weight is measured on living organisms whenever needed without killing them. It is less accurate because body fluid volume fluctuates depending on fluid intake.
Growth in Insects
Insects with a hard outer exoskeleton undergo metamorphosis. Butterflies show complete metamorphosis, while grasshoppers show incomplete metamorphosis.
Complete Metamorphosis
Butterflies pass through four stages: Egg \(\rightarrow\) Larva (Caterpillar) \(\rightarrow\) Pupa \(\rightarrow\) Adult. The larva eats leaves actively to grow, the pupa encloses itself while rapid developmental changes occur, and the adult emerges with soft, folded wings.
Incomplete Metamorphosis
Grasshoppers pass through three stages: Egg \(\rightarrow\) Nymph \(\rightarrow\) Adult. The nymph resembles a miniature adult without fully developed wings and undergoes a series of ecdysis stages before becoming an adult.
Six Phases of the Sigmoid Growth Curve
When growth parameters are plotted against time, most organisms produce an S-shaped curve with six phases:
1. Lag Phase — low growth rate; little cell division and elongation; the organism adjusts to new environmental conditions.
2. Exponential Phase — highest growth rate; cell division and elongation occur actively; size increases rapidly.
3. Stationary Phase — growth rate slows to a constant rate; cells reach maximum size and undergo differentiation.
4. Maturity Phase — growth rate is zero; the rate of cell division equals the rate of cell death; cell division occurs only to repair or replace tissues.
5. Senescence Phase — negative growth rate; the organism ages and physiological systems deteriorate.
6. Death Phase — caused by ageing, illness, lack of nutrients or lack of space, ultimately terminating life.
Intermittent Growth and Ecdysis
Insects possess an external skeleton made of chitin. Chitin does not expand or grow proportionately with internal soft tissue growth. To allow growth, insects must periodically shed their hard exterior through a hormone-controlled process called ecdysis.
The stage between two consecutive ecdysis events is called an instar. During an instar, the insect actively builds body tissue and increases volume, but its external length cannot expand.
How ecdysis happens:
1. A new, soft exoskeleton forms underneath the old exoskeleton.
2. Before the new exoskeleton hardens, the insect sucks in air to expand its body volume.
3. This internal pressure causes the old, brittle exoskeleton to split and break open.
4. The insect with its soft new exoskeleton crawls out.
5. The insect expands its body once more before the new exoskeleton hardens completely.
Try to Answer First
Answer in your mind, then press "Check Answer".
Why is fresh weight considered less accurate than dry weight when measuring growth in an animal?
What occurs during the pupa stage of complete metamorphosis?
Why does the growth curve of an insect appear horizontal during an instar stage?
Common Mistakes
- Confusing the stationary phase with the maturity phase. In the stationary phase, the growth rate slows to a constant rate and cells differentiate; in the maturity phase, growth rate reaches zero because cell division equals cell death.
- Assuming that insects undergo no growth or biological activity during the instar stage. The insect is actively feeding, increasing body volume and building tissues internally; only the external body length cannot increase until ecdysis occurs.
- Thinking that dry weight can easily be applied to monitor human or mammalian growth. Measuring dry weight requires heating the organism to 100 °C until all water evaporates, which kills the organism; therefore, it is unfeasible for living animals or humans and is primarily used for plants.
Concept Misunderstandings
Cell division completely ceases during the maturity phase.
Cell division still occurs actively during the maturity phase, but its rate equals the rate of cell death, serving only to repair or replace dead and damaged tissues.
Insects suck in air after their new exoskeleton has hardened to expand their bodies.
Insects suck in air before the new exoskeleton hardens. Once the cuticle hardens, it becomes rigid and can no longer stretch.
Summary
- Growth is an irreversible, permanent increase in size, cell number, volume and mass, coupled with differentiation.
- Measurement parameters: size/volume (height/length), fresh weight (variable due to water intake), and dry weight (destructive, dried at 100 °C).
- Metamorphosis types: Complete: Egg \(\rightarrow\) Larva \(\rightarrow\) Pupa \(\rightarrow\) Adult (e.g., butterfly). Incomplete: Egg \(\rightarrow\) Nymph \(\rightarrow\) Adult (e.g., grasshopper).
- Sigmoid curve phases: Lag phase \(\rightarrow\) Exponential phase \(\rightarrow\) Stationary phase \(\rightarrow\) Maturity phase \(\rightarrow\) Senescence phase \(\rightarrow\) Death phase.
- Intermittent growth: Characterised by a staircase-shaped curve where horizontal lines represent instars (zero increase in length) and vertical lines represent ecdysis (rapid expansion after moulting).
Short Activity
Objective: test your understanding of growth phases, ecdysis and measurement parameters.
A. Objective Quiz
Which phase of the sigmoid growth curve has the highest growth rate, characterised by active cell division and rapid size increase?
What is the function of an insect sucking in air during the process of ecdysis?
B. Fill in the Blanks
The weight of an organism measured after all bodily fluid is completely removed by drying in an oven at 100 °C is known as .
The stage between two successive ecdysis events in insects is called an .
C. Matching / Drag and Drop
Match each growth phase of the sigmoid curve to its correct description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Growth Phase to Its Description
Write the correct growth phase next to each description.
| Characteristic Event | Growth Phase |
|---|---|
| 1. Low growth rate as the organism adjusts to environmental resources with minimal cell division. | |
| 2. Growth rate slows to a constant rate; cells reach maximum size and undergo differentiation. | |
| 3. Zero growth rate where the rate of cell division equals the rate of cell death. | |
| 4. Negative growth rate associated with the biological ageing of the organism. |
Answer choices: Lag Phase • Stationary Phase • Maturity Phase • Senescence Phase
