3.6 Mass Transport in Plants Flashcards
What vessel transports water and mineral ions in plants?
Xylem
What vessel transports sugars in plants?
Phloem
Name and explain adaptations of the xylem.
- Thick cell walls → Withstand tension/negative pressure
- Lignin in cell walls → Walls waterproof / withstand tension/negative pressure
- Xylem cells have no end walls → So a continuous column of water
- Xylem vessels are stacked on top of each other → So a continuous column of water
- Have no cytoplasm/hollow → Reduces resistance to flow of water, so a continuous column of water
- Xylem cells have pores → Enable sideways flow, allows entry or exit of water
- Narrow tubes → Increased surface area for adhesion
- Molecules in cell walls → Allows adhesion
What is cohesion?
Attraction of water molecules to each other
What is adhesion?
Attraction of water molecules to other substances
Explain the cohesion-tension theory.
- Transpiration of water through stomata/from leaves
- Lowers water potential of mesophyll
- Water pulled up xylem (creating tension)
- Water molecules cohere by hydrogen bonds forming continuous water column
- Adhesion of water molecules to walls of xylem
What is transpiration?
The absorption of water via the roots which then travels up the plant through the xylem followed by the evaporation of water through the stomata.
What factors effect transpiration? (how do they effect transpiration)
- Light intensity → high = guard cells responsive to LI, guard cells turgid so stomata open allowing water loss, MORE transpiration
- Humidity → high = when air is saturated with water the water potential gradient is weaker so less water lost, LESS transpiration
- Temperature → high = more KE of particles so water molecules evaporate from mesophyll and diffuse away faster, MORE transpiration
- Air movement → high = removes water vapour from the environment so sets up a water potential gradient so increases water loss, MORE transpiration
What is the mass flow hypothesis?
- In source sugars actively transported into phloem by companion cells
- Lowers water potential of sieve cell and water enters by osmosis
- Increase in hydrostatic pressure causes mass movement (towards sink) or to storage tissue or organs/respiring cells
- Sugars converted in root for respiration for storage (unloaded from phloem by AT)
Evidence for mass flow: If a ring of bark (including phloem but not xylem) is removed from a woody stem, then a bulge forms above it. The fluid from the bulge has a higher concentration of sugars than the fluid below the ring.
This shows that movement of sucrose can occur downwards as sugars can’t move past the area where the bark has been removed as there is no phloem.
Evidence for mass flow: An aphid’s mouth can pierce the phloem, allowing it to feed. Scientists can snap off the aphid’s body leaving their mouth parts in the plant. If this is done near the leaves, the phloem sap flows out quicker than if it was further down the stem
This shows that there is a pressure gradient (higher pressure at source and lower pressure at sink) due to movement of water into and out of the phloem by osmosis.
Evidence for mass flow: Carbon-14 can be used as a radioactive tracer. Radioactive CO2 can be supplied to a plant which is used in photosynthesis to form sucrose, which will then be moved around the plant in translocation. The radioactivity can be traced using autoradiography , where the plant is placed onto photographic film. Radioactivity is observed more at the leaves at first, then further away from the leaves over time.
Tis shows that the leaves photosynthesise, forming radioactive sucrose at the source. The radioactive sucrose is then actively transported into the phloem and translocated towards the sink via mass flow.
