To compare organisms, choose the criteria first and fill each row in the same order. The surface, the moisture, the route to the cells and the gradient are the four that work every time.
This lesson follows explaining ventilation in humans and belongs to the gas exchange section.
What does the comparison look like?
Here is an original table for four organisms. Check it against your own syllabus notes.
| Criterion | Earthworm | Insect | Fish | Human |
|---|---|---|---|---|
| Exchange surface | Moist skin | Ends of fine tracheal tubes | Gill lamellae | Alveoli |
| How it stays moist | Mucus and damp soil | Fluid at tube ends | Surrounded by water | Moisture lining the alveolus |
| How gases reach cells | Blood in vessels under the skin | Air carried by tracheae directly to tissues | Blood in the lamellae | Blood in capillaries |
| What keeps the gradient steep | Blood flow | Use of oxygen by the cells | Water flow over gills and blood flow | Breathing and blood flow |
The four organisms differ in structure but answer the same four questions.
Worked example: writing a comparison
Question: compare gas exchange in a fish and a human.
- Similarity: both have thin, moist surfaces with a rich blood supply, the lamellae and the alveoli.
- Difference in medium: a fish takes oxygen dissolved in water, whereas a human takes oxygen from air.
- Difference in ventilation: a fish passes water over its gills, whereas a human moves air in and out of the lungs with the diaphragm and ribs.
Each difference names both organisms in one sentence. That is what turns two descriptions into a comparison.
The mistake that costs marks
The common slip is to describe each organism in its own paragraph and never link them. A second slip is to give an insect blood-borne oxygen transport.
| Claim | Why it fails |
|---|---|
| Two separate paragraphs with no link words | The question asks for a comparison |
| “The insect’s blood carries oxygen to its cells” | Tracheae bring air directly to tissues |
| Correct: “In an insect, air reaches cells through tracheae, whereas in a human, blood carries oxygen from the alveoli” | Uses a linking word and both organisms |
Check yourself
State one similarity and one difference between gas exchange in an earthworm and in an insect.
Answer
Similarity: both exchange gases by diffusion across a moist surface.
Difference: an earthworm exchanges gases across its skin and uses blood to carry them, whereas an insect has tracheae that carry air directly to its tissues.
What to study next
The next lesson applies comparison to numbers: interpreting oxygen and carbon-dioxide data. Then test the section in the gas exchange practice. Log any mixed-up organisms in the mistake log and paper-error review.
For a teacher to go through your comparisons, see online one-to-one Biology tuition.