CIE A Level Biology Topic 7: Transport in Plants Exam Questions

About CIE A Level Biology Topic 7: Transport in Plants

Transport in plants covers how water, mineral ions and assimilates move through a plant. In the Cambridge 9700 syllabus you need to draw and label transverse sections of stems, roots and leaves, describe the structure of xylem vessel elements, phloem sieve tube elements and companion cells, and relate each to its function. Water movement is explained through the apoplast and symplast pathways, the Casparian strip, transpiration and cohesion-tension. You also need to explain adaptations of xerophytes and how sucrose is loaded into phloem and moved by mass flow. Questions often involve potometer data, drawing from micrographs and comparing xylem with phloem.

What you need to know

• Tissues Recognise xylem, phloem, endodermis, cortex and epidermis in sections of stems, roots and leaves. Xylem vessel elements are dead, lignified, with no end walls and pits; sieve tube elements are living with sieve plates and depend on companion cells.
• Water uptake Water enters root hair cells by osmosis and crosses the cortex by the apoplast (cell walls) and symplast (cytoplasm and plasmodesmata) pathways. The suberin Casparian strip forces water into the symplast at the endodermis.
• Transpiration Water evaporates from mesophyll cell walls and diffuses out through stomata. Rate increases with temperature, light intensity and wind speed, and decreases with humidity.
• Cohesion-tension Water molecules are held together by hydrogen bonds (cohesion) and to xylem walls (adhesion). Loss of water from leaves creates tension that pulls a continuous column up the xylem.
• Xerophytes Adaptations such as sunken stomata, rolled leaves, hairs, thick waxy cuticles and reduced leaf area trap humid air or reduce surface area to cut water loss.
• Translocation Sucrose is actively loaded into companion cells using a proton pump and cotransporter proteins, then moves into sieve tubes. This lowers water potential, water enters by osmosis and the high hydrostatic pressure drives mass flow from source to sink.

Common mistakes in Topic 7: Transport in Plants

• Saying transpiration is the movement of water up the stem; it is the loss of water vapour from the leaves.
• Writing that the Casparian strip blocks water entirely; it blocks the apoplast pathway so water must pass through the cytoplasm of endodermal cells.
• Assuming a potometer measures transpiration directly; it measures water uptake, which is close to but not the same as water loss.
• Describing phloem transport as one way only; assimilates move from source to sink, which can be up or down the plant.
• Forgetting that sucrose loading needs ATP for the proton pump; it is active, not passive.

Try three Topic 7: Transport in Plants exam-style questions