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Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next. (2026)
Question
A diagram of a leaf cross-section has arrows labelled A, B, C and D pointing to: a single layer of clear cells at the top, a column of cells full of chloroplasts just below, loosely arranged cells with air spaces, and a stiff hollow tube in a vein. Name each tissue.
Solution
A = upper epidermis — transparent (1).
B = palisade mesophyll — column of cells packed with chloroplasts (1).
C = spongy mesophyll — loose cells, air spaces for gas exchange (1).
D = xylem — dead, hollow, carries water UP (1).
Answer
A = upper epidermis; B = palisade mesophyll; C = spongy mesophyll; D = xylem.
Question
Compare the structure and function of xylem and phloem.
Solution
Xylem is dead and hollow; phloem is living with sieve plates (1).
Xylem walls strengthened with lignin; phloem walls are not (1).
Xylem carries water + minerals UP only; phloem carries sucrose in BOTH directions (1).
Xylem driven by transpiration; phloem (translocation) uses energy from respiration (1).
Answer
Xylem is dead, hollow, lignified and carries water and minerals up the plant via the transpiration stream. Phloem is made of living cells with sieve plates and carries dissolved sucrose in both directions (translocation), powered by active transport.
Question
Explain why a plant left in direct sunlight on a hot, windy day may wilt, even when its roots are in moist soil.
Solution
High temperature and wind increase the rate of transpiration (evaporation from leaves) (1).
Water is lost from leaves faster than roots can absorb it (1).
Leaf and stem cells lose turgor pressure (1).
Without turgor pressure, the plant wilts (1).
Answer
Heat and wind raise the rate of transpiration. Water leaves the leaves faster than the roots can absorb it, so cells lose turgor pressure and the plant wilts.
Question
A student measures water uptake with a potometer. The air bubble moves 1.5 cm in 5 minutes in still air, then 4.5 cm in 5 minutes after switching on a fan. Explain the change.
Solution
The fan increases air movement around the leaf (1).
This removes the water vapour from the leaf surface, creating a steeper concentration gradient (1).
So water evaporates faster and more water is drawn up through the xylem — transpiration rate increases (1).
Answer
The fan steepens the concentration gradient of water vapour between leaf and air. Evaporation speeds up, so transpiration — and water uptake measured by the bubble — increases.
Examiner note
Mention CONCENTRATION GRADIENT explicitly; mark schemes often require it.
The middle layers of a leaf where photosynthesis (palisade) and gas exchange (spongy) take place.
Plant tissue made of dead, hollow, lignified cells that transports water and minerals from roots to leaves.
Plant tissue made of living cells with sieve plates that transports dissolved sucrose around the plant by translocation.
Tissue at growing tips of roots and shoots whose cells can divide and differentiate into any plant cell type.
Pores in the lower epidermis of a leaf, opened and closed by guard cells, that allow gas exchange and water loss.
The loss of water vapour from the leaves of a plant, mainly through stomata.
The continuous movement of water from the roots, up through the xylem, to the leaves, driven by evaporation from the leaf surface.
The movement of dissolved sucrose (and amino acids) through the phloem from source to sink.
Apparatus used to measure the rate of water uptake by a leafy shoot, used as a proxy for the rate of transpiration.
A specialised plant cell with a long extension that increases surface area for the absorption of water and mineral ions from the soil.
Mistake
Writing 'xylem is alive'.
Why it happens
Confusion with phloem.
How to avoid it
Xylem cells are DEAD and HOLLOW — efficient for water transport.
Source: AQA examiner report 2023
Mistake
Writing 'phloem only moves sugar downwards'.
Why it happens
Common textbook oversimplification.
How to avoid it
Phloem transports in BOTH directions — up to fruit, down to roots, wherever needed.
Mistake
Forgetting the waxy cuticle in a leaf diagram.
Why it happens
Cuticle is thin and easy to miss.
How to avoid it
Always label the waxy cuticle on top of the upper epidermis.
Mistake
Writing 'translocation moves glucose'.
Why it happens
Glucose is the most familiar sugar.
How to avoid it
Phloem transports SUCROSE, not glucose. AQA accepts sugars/dissolved sugars but always prefer SUCROSE.
Source: AQA examiner report 2024
Mistake
Describing transpiration as 'waste'.
Why it happens
Transpiration loses water.
How to avoid it
It's a side-effect of stomata being open for photosynthesis; it also cools the leaf and pulls minerals up.
Mistake
Saying high humidity INCREASES transpiration because the air has water in it.
Why it happens
Confusion about gradients.
How to avoid it
Humid air has lots of water vapour, so the gradient between leaf and air is SMALL. Less evaporation, less transpiration.