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
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- state that some mineral ions and organic compounds can be transported within plants dissolved in water
- describe the transport of water from the soil to the xylem through the: • apoplast pathway, including reference to lignin and cellulose • symplast pathway, including reference to the endodermis, Casparian strip and suberin
- explain that transpiration involves the evaporation of water from the internal surfaces of leaves followed by diffusion of water vapour to the atmosphere
- explain how hydrogen bonding of water molecules is involved with movement of water in the xylem by cohesion-tension in transpiration pull and by adhesion to cellulose in cell walls
- make annotated drawings of transverse sections of leaves from xerophytic plants to explain how they are adapted to reduce water loss by transpiration
- state that assimilates dissolved in water, such as sucrose and amino acids, move from sources to sinks in phloem sieve tubes
- explain how companion cells transfer assimilates to phloem sieve tubes, with reference to proton pumps and cotransporter proteins
- explain mass flow in phloem sieve tubes down a hydrostatic pressure gradient from source to sink
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'.
- › Remember that while sieve tube elements lose their nucleus and most organelles, they must retain a thin layer of cytoplasm.
- › Always use 'water potential' (Ψ) when discussing the movement of water in biological systems; 'concentration' is for solutes.
- › 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.
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.
The apoplast pathway involves water moving through the non-living spaces in the cell walls (cellulose) and intercellular spaces. Lignin in xylem walls also contributes to this pathway, providing structural support.
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.
More 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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