Paper 1 - Mass Transport In Plants Flashcards
Xylem vessel
- Transports water and ions from roots to leaves
- Formed from dead cells
- No end walls/cytoplasm → continuous column - less resistance to flow of water
- Walls strengthened and waterproofed with lignin
- Cohesive
Transpiration
Evaporation of water from plant’s surface - stomata’s open and water moves out of the leaf down the water potential gradient
Cohesion - tension theory
1) Water evaporates from leaves at top of xylem - transpiration
2) Water potential is reduced in leaf cells = water drawn out by osmosis
3) This creates low pressure at the top of the xylem - water is under tension and is pulled up towards the leaves
4) Cohesion - column of water moves upwards
5) Water enters the stem through the roots by osmosis
Factors affecting transpiration - light intensity
- Lighter = faster rate
- More stomata open in light to allow CO2 to enter for photosynthesis
- Dark - stomata closed = less transpiration
Factors affecting transpiration - Temperature
- Higher temp = faster rate
- Molecules have more kinetic energy - evaporates from cells inside leaf quicker
- Increase in water potential gradient between inside and outside leaf = water diffuses out faster
Factors affecting transpiration - Wind speed
- Windier = faster rate
- Air movement blows away water molecules from around stomata
- Increases water potential gradient
- Water diffuses out faster
Factors affecting transpiration - Humidity
- Lower humidity = faster rate (negative correlation)
- If air around plant dry - bigger water potential gradient between leaf and air
- Water diffuses out faster
Estimating rate of transpiration practical
- Cut a shoot underwater and at a slant - prevents air entering xylem and increases SA available for water uptake
- Assemble photometer underwater and insert shoot with apparatus
- Remove apparatus from the water but keep capillary tube submerged in beaker of water
- Check apparatus is watertight and airtight
- Dry the leaves and allow time for shoot to acclimatise
- Remove end of capillary tube from beaker of water until one air bubble has formed
- Record starting position of air bubble
- Start stopwatch, record distance moved by bubble per unit time e.g. per hour. Rate of air bubble movement = estimation of transpiration rate
To work out rate of water uptake in mm3 per min - measure the distance moved by the bubble per min and the diameter of the capillary tube.
Structure of xylem vessels
Lignin - strengthens xylem walls and makes them waterproof
Pits - if vessel becomes blocked/damaged, the water can be diverted laterally so upwards movement of water can continue
Structure of phloem vessels
Little cytoplasm
Companion cells
Many mitochondria
Sieve plates (perforations)
Mass flow movement
- Sucrose is actively transported into the sieve tube
- Sieve tubes have a lower water potential
- Water moves from the xylem into sieve tubes by osmosis, creating a high hydrostatic pressure
- Sucrose is actively transported from sieve tube elements through companion cells into sink cells
- Water leaves the sieve tubes by osmosis
- The hydrostatic pressure of the sieve tubes is lowered
- Mass flow of sucrose solution down this hydrostatic pressure gradient in the sieve tubes
Evidence supporting the mass flow theory
- There’s hydrostatic pressure in the phloem as shown by the release of sap when it is cut
- The concentration of sucrose is higher in the leaves (source) than in the roots (sink)
- Flow in the phloem occurs in daylight but stops when leaves are shade (night)
- Companion cells have many mitochondria and readily produce ATP
Evidence against the mass flow theory
- Sucrose is delivered at more or less the same rate to all sinks
- Not all solutes move at the same speed
