About Transport in plants in A Level Biology
Transport in plants is one of the 11 main topics in our A Level Biology (Cambridge (CAIE) 9700) question bank, and about 8% of its real questions come from it. Tayyari Karo has 53 real questions on this topic from 2021 to 2026, from Paper 1, Paper 2, Paper 4; 41 are multiple choice and marked instantly, and the rest are structured questions with a model answer. By difficulty: 30 medium, 20 hard, 3 easy.
Work through the questions below, then open the explanation for each one. Redo the ones you missed a few days later.
Try 7 real questions
These are real past-paper questions. Think of your answer first, then open the explanation.
May/June 2026 · Paper 1 · Transport in plants
Which combination of changes for loading of sucrose into the phloem by companion cells is correct?
- The water potential inside companion cells is increased, because protons are actively transported from the companion cell cytoplasm into the cell wall.
- The water potential inside the companion cell is increased, because protons and sucrose are co-transported into the companion cell cytoplasm from the cell wall.
- The water potential inside the companion cells is decreased, because protons are actively transported from the companion cell cytoplasm into the cell wall.
- The water potential inside the companion cells is decreased, because protons and sucrose are co-transported into the companion cell cytoplasm from the cell wall.
Show answer and explanation
Answer: D. The water potential inside the companion cells is decreased, because protons and sucrose are co-transported into the companion cell cytoplasm from the cell wall.
Co-transport of H+ and sucrose into the companion cell lowers its water potential.
October/November 2025 · Paper 1 · Transport in plants
What causes water to move from the root hair cells to the endodermis?
- diffusion through cell walls and osmosis down a water potential gradient in the cytoplasm
- diffusion through the symplast and osmosis and root pressure through the apoplast
- osmosis from cell vacuole to cell vacuole and active transport into the endodermis
- osmosis through the intercellular spaces and diffusion in cell walls and cytoplasm
Show answer and explanation
Answer: A. diffusion through cell walls and osmosis down a water potential gradient in the cytoplasm
Water moves via the apoplast pathway (diffusion through cell walls) and via the symplast/vacuolar pathway (osmosis down a water potential gradient through the cytoplasm).
October/November 2024 · Paper 1 · Transport in plants
Which statement correctly identifies the movement of sucrose and amino acids in plant vascular tissue?
- They are carried from source to sink by mass flow in upward and downward directions.
- They are carried from sink to source by cohesion-tension in upward and downward directions.
- They are carried from sink to source by mass flow in one direction.
- They are carried from source to sink by cohesion-tension in one direction.
Show answer and explanation
Answer: A. They are carried from source to sink by mass flow in upward and downward directions.
Sucrose and amino acids are translocated in phloem from source to sink by mass flow, and this can occur both upwards and downwards in the plant depending on the position of sources and sinks.
May/June 2023 · Paper 1 · Transport in plants
How are companion cells involved in loading sucrose into phloem sieve tube elements?
- actively through cotransporter proteins and passively through plasmodesmata
- actively through cotransporter proteins and plasmodesmata
- passively through cotransporter proteins only
- actively through plasmodesmata only
Show answer and explanation
Answer: A. actively through cotransporter proteins and passively through plasmodesmata
In the apoplastic route, companion cells actively load sucrose using H+/sucrose cotransporter proteins in the cell surface membrane; sucrose then moves passively into sieve tube elements down its concentration gradient through plasmodesmata.
May/June 2022 · Paper 1 · Transport in plants
Which row is correct for the movement of water in a root?
- apoplast pathway through intercellular spaces; suberin in Casparian strip
- apoplast pathway through plasmodesmata; lignin in Casparian strip
- symplast pathway through plasmodesmata; lignin in Casparian strip
- symplast pathway through intercellular spaces; suberin in Casparian strip
Show answer and explanation
Answer: A. apoplast pathway through intercellular spaces; suberin in Casparian strip
Water moving via the apoplast pathway travels through cell walls and intercellular spaces; the Casparian strip contains suberin (a waterproofing substance) which blocks this pathway at the endodermis.
October/November 2022 · Paper 1 · Transport in plants
Which properties of lignin are important for the function of xylem vessels in the stem of a tall plant, such as a tree?
1 It is inflexible so does not bend easily.
2 It is not permeable to water.
3 It is strong to resist collapse under pressure.
4 It has weaker adhesion to water molecules than cellulose.
- 1, 2 and 3
- 1, 2 and 4
- 1, 3 and 4
- 2, 3 and 4
Show answer and explanation
Answer: A. 1, 2 and 3
Lignin is rigid/inflexible (1), waterproof preventing lateral loss of water (2), and mechanically strong, resisting collapse under the negative pressures generated during transpiration pull (3); its adhesion to water is not the key relevant property here — 1, 2 and 3.
October/November 2021 · Paper 4 · Transport in plants
Section B (answer one of Question 9 or Question 10).
(a) Describe and explain the mechanism by which guard cells open stomata. [7]
(b) Explain how the anatomy and physiology of the leaves of maize or sorghum are able to maximise carbon dioxide fixation at high temperatures. [8]
Show model answer
(a) Any seven: an H+ carrier/pump protein is present in the cell surface membrane of the guard cell; hydrogen ions/protons (H+) leave/exit the guard cell, using energy/ATP (active transport); this leaves a low H+ concentration inside the cell, giving it a more negative charge than outside; this causes K+ channel proteins to open; K+ ions then move into/enter the cell by facilitated diffusion (down the electrochemical gradient created); Cl– ions also move into/enter the cell; the water/solute potential of the guard cell decreases (becomes more negative) as solute concentration rises; water then moves into the cell by osmosis; the cell/vacuole volume increases; the guard cells become turgid; b …
The full model answer is in the app.
