Plant transport, stomata, guard cells, transpiration and auxins — the plant topics students most often under-revise and most often lose marks on.
Plant biology is one of the most under-revised areas of GCSE Biology. Students focus heavily on human biology and tend to leave plant topics until last — then run out of time. Yet plant questions appear on every paper and often include 4 or 6 mark extended questions that are relatively straightforward if you know the content well. This guide covers everything.
Leaves are highly adapted organs — their entire structure is optimised for photosynthesis. Understanding the structural adaptations helps you answer questions about both photosynthesis and transpiration.
Guard cells are kidney-shaped cells that flank each stoma. They control the opening and closing of the stoma in response to conditions.
When guard cells take up water by osmosis, they swell. Because their inner wall (facing the stoma) is thicker and less elastic than the outer wall, they bend outward as they swell — pulling the stoma open. When guard cells lose water, they become flaccid and the stoma closes.
Stomata open in daylight (when CO₂ is needed for photosynthesis) and close in darkness. They also close when the plant is water-stressed — this reduces water loss through transpiration but also slows photosynthesis because CO₂ can no longer enter.
Guard cells open stomata by becoming turgid (taking up water by osmosis). They close stomata by losing water and becoming flaccid. The unequal thickness of the cell wall is what causes the bending that opens the pore. This mechanism is tested in detail in 4-mark questions.
Xylem transports water and dissolved mineral ions from the roots upward through the stem to the leaves. It is a one-way system — water moves upward only.
Xylem vessels are dead cells with thick, lignified cell walls. The cells have no end walls — they form continuous hollow tubes. The lignin strengthens the vessels to withstand the tension created by the water column moving upward.
Water moves through xylem by transpiration pull. As water evaporates from leaves through stomata, it creates a tension that pulls the water column upward. This is cohesion-tension theory — water molecules cohere to each other (cohesion) and adhere to the xylem walls (adhesion), allowing a continuous column to be pulled upward without breaking.
Phloem transports dissolved sugars (mainly sucrose) produced by photosynthesis from the leaves to other parts of the plant. Unlike xylem, phloem can transport in both directions — upward to growing shoots or downward to roots and storage organs.
This movement of sugars in phloem is called translocation. Phloem cells are alive and use energy (ATP) to actively load sugars into the phloem at the source (leaves) and unload them at the sink (growing regions, storage organs, fruits).
❌ Xylem and phloem are frequently confused. Remember: Xylem = water (and minerals), one-way upward, dead cells. Phloem = food/sugars, two-way, living cells. A helpful memory: phlOEm transports fOOd. Xylem carries water — like an X marks the spot for H₂O.
Transpiration is the evaporation of water from plant leaves, mainly through stomata. It is a consequence of leaves being open to allow gas exchange — water vapour diffuses out whenever stomata are open.
Factors that increase transpiration rate:
Transpiration is not entirely wasteful — it drives the uptake of water and minerals from the soil through the xylem, and the evaporation of water helps cool the leaf.
Plants respond to their environment through hormones. Auxins are the key plant hormones at GCSE. They are produced in shoot tips and control growth by causing cells to elongate.
When a shoot is illuminated from one side, auxin moves to the shaded side of the shoot tip. Higher auxin concentration on the shaded side causes those cells to elongate more than the cells on the lit side. The shoot therefore bends toward the light. This is positive phototropism.
Roots grow downward (positive gravitropism) and shoots grow upward (negative gravitropism). In roots, auxin moves to the lower side. At the concentrations found in roots, higher auxin actually inhibits growth — so the lower side grows more slowly and the root bends downward. This is a key distinction: the same auxin concentration that promotes elongation in shoots inhibits it in roots.
High auxin concentration → promotes growth in shoots (cells elongate more). High auxin concentration → inhibits growth in roots (cells elongate less). The same hormone has opposite effects depending on which tissue it is in. Exam questions specifically test this distinction — make sure you specify whether you're talking about roots or shoots when describing auxin effects.
The AQA Biology plant biology specification is at the AQA GCSE Biology specification page.
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