9700 · 7.2
Transport mechanisms flashcards
Revision flashcards for Cambridge 9700 Transport mechanisms (syllabus 7.2). Flip, recall, then mark a real past-paper question.
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Define the apoplast pathway.
Movement of water through the cellulose cell walls and intercellular spaces, without crossing a cell surface membrane.
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Define the symplast pathway.
Movement of water through the cytoplasm and vacuoles of cells and from cell to cell through plasmodesmata, by osmosis across membranes.
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What is the Casparian strip and what does it do?
A band of suberin in the walls of endodermal cells; it is impermeable to water, so it blocks the apoplast pathway and forces water and ions through a cell surface membrane into the symplast.
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Define transpiration.
The loss of water vapour from the aerial parts of a plant: evaporation from the internal surfaces of the leaf followed by diffusion of water vapour through the stomata to the atmosphere.
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What is cohesion, and why does it matter in the xylem?
The attraction between water molecules due to hydrogen bonding; it transmits the tension created by evaporation down the whole water column so the column moves as one.
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What is adhesion in the context of water transport?
The attraction of water molecules to the cellulose in xylem cell walls; it stops the water column pulling away from the walls and helps it rise.
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State the cohesion-tension theory in one sentence.
Evaporation from the leaf creates a tension that is transmitted down a cohesive column of water, which adheres to the xylem walls, so water is pulled up from the roots to replace what is lost.
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Give three leaf adaptations of a xerophyte and their effect.
Thick waxy cuticle (less evaporation through the epidermis); sunken stomata or hairs (trap humid air, smaller water potential gradient); rolled leaves or reduced leaf area (less surface and stomata exposed to moving air).
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What are a source and a sink?
A source releases assimilates into the phloem (photosynthesising leaves, storage organs unloading, germinating seeds); a sink removes them (meristems, growing roots, developing seeds, fruits and storage organs).
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How do companion cells load sucrose into sieve tubes?
Proton pumps use ATP to move H⁺ out of the companion cell; H⁺ diffuses back in through a cotransporter protein that carries sucrose with it against its gradient; sucrose then passes into the sieve tube through plasmodesmata.
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Define mass flow in the phloem.
Movement of the whole sap solution along a sieve tube down a hydrostatic pressure gradient, from a source (high pressure, water entering by osmosis) to a sink (low pressure, water leaving).
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Why is glucose transported as sucrose?
Sucrose has a smaller osmotic effect than the same mass of glucose and is less reactive, so it can be carried at high concentration without disturbing cells or being used on the way.