BiologyCode 9700
Transport mechanisms
What this topic asks you to be able to do
- state that some organic compounds and mineral ions may be transported through plants in solution in water
- describe how water moves from the soil into the xylem by the apoplast pathway, including mention of lignin and cellulose; and by the symplast pathway, including mention of the endodermis, Casparian strip and suberin
- explain that transpiration involves water evaporating off the internal surfaces of leaves, with the resulting vapour then diffusing into the atmosphere
- account for the part played by hydrogen bonds between water molecules in water's upward journey through the xylem, through cohesion-tension and the transpiration pull, and through adhesion to the cellulose of cell walls
- make annotated drawings of transverse sections through the leaves of xerophytic plants, and use them to explain how these leaves are adapted to lose less water by transpiration
- state that assimilates such as sucrose and amino acids are moved in solution in water along phloem sieve tubes, from sources to sinks
- account for how companion cells move assimilates into phloem sieve tubes, drawing on proton pumps and cotransporter proteins
- account for mass flow inside phloem sieve tubes as it proceeds down a hydrostatic pressure gradient that runs from the source end to the sink end