A comparison answer needs both items in the same sentence, measured against the same feature. Describing each one separately leaves the comparing to the marker.
This lesson belongs to from biological recall to causal explanation. It pairs well with separating an adaptation from the advantage it can provide.
How do I build a two-sided sentence?
Pick the criterion first, then fill both sides, then add the reason. The order stops you from writing about one item only.
- Choose a feature, such as wall thickness or blood pressure.
- Write what item A has for it.
- Write “whereas” or “but”, then what item B has.
- Add “so” and the reason the difference exists.
Worked example: arteries and veins
Question. Compare an artery and a vein, and explain the differences in wall thickness and valves. (4 marks)
One-sided answer: “An artery has a thick wall. A vein has valves.”
The two sentences use different criteria, so nothing is compared. A marker cannot award the comparison marks.
Two-sided answer: “An artery has a thick, muscular wall, whereas a vein has a thin wall, because the wall must withstand high-pressure blood without bursting. A vein has valves, whereas an artery has none, because blood in veins is at low pressure and valves stop backflow.”
Each sentence compares the two on one criterion, then gives a reason. The criteria are wall thickness and valves, and both items appear in both.
The mistake that costs marks
The slip is to write “a vein has valves” and leave the artery unmentioned. The marker has to assume that the artery has none.
Always name the other side: “whereas an artery has no valves”. A few extra words complete the comparison and secure the mark.
Check yourself
Compare a plant cell and an animal cell on rigidity, and give a reason for the difference. (2 marks)
Answer
A plant cell is rigid because it has a cellulose cell wall that resists pressure, whereas an animal cell is flexible because it has only a thin cell membrane of lipids and proteins.
The criterion is rigidity, both sides are named, and a reason is given.
What to study next
Move to avoiding a purpose-only explanation when a mechanism is requested, then test yourself with the integrated practice. For a teacher to rebuild your comparisons, see online one-to-one Biology tuition.