Mechanisms of transport in plants
How does a tall tree lift water many metres into the sky without a single pump, and how does the sugar made in its leaves reach its roots? In this lesson we follow water on its whole journey. We start with transpiration — water evaporating from the moist internal surfaces of the leaf and then diffusing out through the stomata — and we explain every movement in terms of water potential. We investigate the factors that change the transpiration rate using a potometer, epidermal peels and grids. We see how hydrogen bonding gives water cohesion and adhesion, powering the cohesion-tension theory that pulls an unbroken water column up the xylem. We trace water from soil to xylem by the apoplast and symplast pathways, meet the Casparian strip that forces water through the symplast, and look at how xerophyte leaves are adapted to save water. Finally we turn to translocation: how sucrose and amino acids move from sources to sinks in the phloem, loaded by companion cells using proton pumping and co-transport, and driven along by mass flow down a hydrostatic pressure gradient.
By the end you should be able to (NSSCAS Biology (AS) 2.6.2):
- Explain that transpiration involves the evaporation of water from the internal surfaces of leaves followed by diffusion of water vapour to the atmosphere
- Describe, in terms of water potential, the movement of water: between plant cells; between the plant and its environment (no calculations involving water potential will be set)
- Investigate experimentally and explain the factors that affect transpiration rate using simple potometer, epidermal peels and grids for determining surface area
- Explain how hydrogen bonding of water molecules is involved with movement in the xylem by cohesion-tension in transpiration pull and adhesion to cellulose cell walls
- Describe the pathways and explain the mechanisms by which water and mineral ions are transported from soil to xylem and from roots to leaves (include reference to the symplastic pathway, apoplastic pathway and Casparian strip)
- Make annotated drawings of transverse sections of leaves from xerophytic plants to explain how they are adapted to reduce water loss by transpiration
- State how assimilates, such as sucrose and amino acids, move between sources (e.g. leaves and storage organs) and sinks (e.g. buds, flowers, fruits, roots and storage organs) in phloem sieve tubes
- Explain how sucrose is loaded into phloem sieve tubes by companion cells using proton pumping and the co-transporter mechanism in their cell surface membranes
- Explain mass flow in phloem sap down a hydrostatic pressure gradient from source to sink
Miss Rachel and Mike talk through the whole topic — with the figure and working drawn live.