Transport mechanisms
Cambridge A-Level Biology (9700) · Unit 7: Transport in plants · 9 flashcards
Transport mechanisms is topic 7.2 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 7 — Transport in plants , alongside Structure of transport tissues. In one line: Mineral ions and organic compounds are transported within plants dissolved in water. These are essential for plant growth and various metabolic processes.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 9 flashcards — 2 definitions and 7 key concepts — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
What substances are transported within plants dissolved in water
Mineral ions and organic compounds are transported within plants dissolved in water. These are essential for plant growth and various metabolic processes.
What the Cambridge 9700 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- State that, within plants, some organic compounds and mineral ions can be carried in solution in water.
- Describe how water moves from the soil into the xylem by the apoplast pathway (referring to cellulose and lignin) and by the symplast pathway (referring to the endodermis, suberin and the Casparian strip).
- Explain that transpiration is water evaporating from internal leaf surfaces, then water vapour diffusing out into the atmosphere.
- Explain how hydrogen bonding helps move water up the xylem: bonds between water molecules give cohesion-tension in the transpiration pull, and bonds between water and the cellulose of cell walls give adhesion.
- Draw and annotate transverse sections through leaves of xerophytic plants, using them to explain the adaptations that cut down water loss by transpiration.
- State that assimilates, for example sucrose and amino acids, are carried in solution in phloem sieve tubes from sources to sinks.
- Explain how companion cells load assimilates into phloem sieve tubes, referring to cotransporter proteins and proton pumps.
- Explain how mass flow carries phloem contents through sieve tubes from source to sink, down a hydrostatic pressure gradient.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Transport mechanisms
- › Avoid 'concentration of water'. Always use 'water potential' and describe movement from a 'higher water potential' to a 'lower water potential'.
- › Transpiration is specifically the evaporation of water from mesophyll cell walls followed by the diffusion of water vapour through stomata.
- › Define the 'apoplast pathway' as movement through cell walls and 'symplast pathway' as movement through the cytoplasm and plasmodesmata.
- › Describe adhesion of water molecules to the walls of the xylem vessels, and cohesion between water molecules.
- › Distinguish between the physical change of state (evaporation) at the cell wall and the subsequent movement of water vapour (diffusion).
What substances are transported within plants dissolved in water?
Mineral ions and organic compounds are transported within plants dissolved in water. These are essential for plant growth and various metabolic processes.
Describe the apoplast pathway of water transport in plants.
In the apoplast pathway water moves through the non-living parts of the plant: through the spaces between the cellulose fibres in the cell walls and the spaces between cells, without crossing any cell surface membrane. It is pulled along by cohesion and tension. In the xylem it flows through the hollow vessels, whose walls are strengthened and waterproofed by lignin so they do not collapse under tension; water passes between vessels through pits. In the root the apoplast pathway is blocked at the endodermis by the Casparian strip (suberin), so water must enter the symplast.
Explain the function of the Casparian strip in the endodermis.
The Casparian strip, made of suberin, is an impermeable layer in the endodermis cell walls. It forces water and ions to enter the symplast pathway, allowing the plant to control the uptake of minerals.
Explain how transpiration contributes to water movement in plants.
Transpiration is the evaporation of water from leaves followed by diffusion of water vapor into the atmosphere. This creates a tension, pulling water up the xylem in a continuous stream from the roots.
Describe the role of hydrogen bonding in water transport within the xylem.
Hydrogen bonding between water molecules causes cohesion (water molecules sticking together) and adhesion (water molecules sticking to xylem walls). Cohesion-tension from transpiration pulls water upwards, while adhesion helps counter gravity.
Describe two adaptations of xerophytic leaves to reduce water loss.
Xerophytes often have features such as a thick waxy cuticle to reduce evaporation and sunken stomata to trap moist air and reduce the water potential gradient.
What are the main assimilates transported in the phloem?
The main assimilates transported in the phloem are sucrose and amino acids, dissolved in water. These are transported from sources (
Explain the role of proton pumps in the transfer of assimilates into phloem sieve tubes.
Proton pumps in companion cells actively transport H+ ions out of the cell, creating a proton gradient. This gradient drives the cotransport of sucrose into the companion cells, and then into the phloem, against its concentration gradient.
Explain the mechanism of mass flow in phloem sieve tubes.
Mass flow is the movement of assimilates down a hydrostatic pressure gradient in phloem. High concentration of assimilates at the source lowers water potential, causing water to enter and increase pressure. Lower concentration at the sink results in water leaving, decreasing pressure.
Review the material
Read full revision notes on Transport mechanisms — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 7 — Transport in plants
Transport mechanisms sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Transport mechanisms deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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