Organisms hold their shape in different ways, and a comparison question needs the same criteria on both sides. Use the same criteria for every organism: what gives the support, where it sits, and how the organism grows with it.
This lesson follows explaining antagonistic muscle action in the support and movement chapter.
How do five organisms compare?
| Organism | Support provided by | Position | Notes |
|---|---|---|---|
| Herbaceous plant | Turgor pressure of water-filled cells, with some fibres | Inside the cells and stem | Wilts when water is lost |
| Tree | Lignified xylem (wood) and sclerenchyma | Inside the trunk | Thicker with secondary growth |
| Earthworm | Fluid in the body cavity (hydrostatic skeleton) | Inside the body | Muscles squeeze against the fluid |
| Grasshopper | Hard chitin exoskeleton | Outside the body | Shed in stages to allow growth |
| Human | Bone and cartilage (endoskeleton) | Inside the body | Grows with the body |
Worked example: grasshopper and human
Question: compare the support of a grasshopper and a human. [4 marks]
A full answer:
- A grasshopper has an exoskeleton outside the body, but a human has an endoskeleton inside the body.
- The grasshopper’s skeleton is made of chitin, but the human skeleton is made of bone and cartilage.
- The grasshopper must shed its exoskeleton to grow, but the human skeleton grows with the body.
- Both have muscles attached to the skeleton that pull to cause movement.
The last point is a similarity. Comparison questions reward one shared feature along with the differences.
The mistake that costs marks
The common slip is to write “the earthworm has no skeleton” or “a plant has no support”. An earthworm has a hydrostatic skeleton, and plants have turgor, xylem and other tissues.
| Wrong | Right | |
|---|---|---|
| Earthworm | No skeleton | Hydrostatic skeleton of body fluid |
| Herbaceous plant | Stiff because of wood | Firm because of turgor pressure |
| Grasshopper growth | Exoskeleton grows with the insect | Exoskeleton is shed in stages |
A second slip is to describe each organism in its own paragraph. Put each point side by side and use “but” or “whereas”.
Check yourself
Explain why a herbaceous plant wilts on a hot day but a tree trunk does not.
Answer
On a hot day, the herbaceous plant loses water faster than it takes it up, so its cells lose turgor pressure and become flaccid. The soft stem cannot hold itself up, so it wilts.
A tree trunk is supported by lignified xylem and fibres, which are rigid even when the cells lose some water, so the trunk stays upright.
What to study next
For the plant structures behind the comparison, read relating plant tissues to function. Then test the chapter with the support and movement practice set.
The biological data interpretation trainer gives more tables to compare, and the mistake log and paper-error review records your slips. For a teacher to check your comparisons, see online one-to-one Biology tuition.