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Flashcards · 9700 · AS Topic 7

Transport in plants (AS Level) flashcards

12 cards on AS Topic 7, subtopics 7.1 to 7.2. Pick a subtopic or study them all.

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7.1 Structure of transport tissues

Position of xylem and phloem in a dicot stem, root and leaf
Stem: bundles in a ring; xylem on the inner side, phloem on the outer side. Root: xylem in a central star or cross; phloem between its arms. Leaf midrib: xylem towards the upper surface, phloem towards the lower.
How is a xylem vessel element adapted to its function?
Dead, empty lumen: low resistance to flow. No end walls: continuous water column. Lignified walls: resist collapse under tension, waterproof. Pits: water moves sideways.
How is a sieve tube element adapted to its function?
No nucleus, few organelles and peripheral cytoplasm give a clear lumen for mass flow. Sieve plates let sap flow between elements.
Role of the companion cell
It has a nucleus and dense cytoplasm and carries out metabolism for the sieve tube element. Many mitochondria provide ATP for proton pumps; plasmodesmata link it to the sieve tube.
Xylem vs phloem
Xylem: dead, lignified; carries water and mineral ions; upwards only. Phloem: living, with sieve plates; carries assimilates; from source to sink, in either direction.

7.2 Transport mechanisms

Transpiration
Evaporation of water from the internal surfaces of leaves, followed by diffusion of water vapour to the atmosphere (through the stomata).
Apoplast vs symplast pathway
Apoplast: water moves through cellulose cell walls and intercellular spaces. Symplast: water moves through cytoplasm, cell to cell via plasmodesmata. Only the apoplast is blocked by the Casparian strip.
Casparian strip
A band of waterproof suberin in the walls of endodermal cells. It blocks the apoplast, so water must enter the symplast (cross a membrane) to reach the xylem. It is not lignin.
Cohesion vs adhesion in the xylem
Cohesion: hydrogen bonding between water molecules, so the column is pulled up as a whole under tension. Adhesion: hydrogen bonding between water and cellulose in the walls.
How is sucrose loaded into phloem?
Proton pumps in companion cells use ATP to pump H⁺ out. H⁺ diffuse back in through cotransporter proteins, carrying sucrose. Sucrose passes into the sieve tube through plasmodesmata.
Mass flow in phloem
Loading sucrose at the source lowers water potential; water enters by osmosis, raising hydrostatic pressure. At the sink sucrose is removed, water leaves and pressure falls. Sap moves down the hydrostatic pressure gradient.
Xerophyte adaptations and how each reduces water loss
Rolled leaves, sunken stomata and hairs trap humid air, lowering the water vapour potential gradient. A thick waxy cuticle reduces evaporation through the epidermis. Few stomata reduce the area for diffusion.

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