3.3.4.2 Mass transport in plants (3.3.4 Mass transport) Flashcards
describe the function of xylem tissue
transports water and mineral ions through the stem up the plant to the leaves of the plant
suggest how xylem tissue is adapted for its function
• cells joined with no end walls forming a long continuous tube - water flows as a continuous column
• cells contain no cytoplasm - easier water flow
• thick cell walls with lignin - provides support and prevents water loss
• Pits in side walls - allow lateral water movements
explain the cohesion - tension theory of water transport in the xylem
1) leaf :
• water lost from leaf by transpiration - water evaporates from mesophyll cells into air spaces and water vapour diffuses through stomata
• reducing ¥ of mesophyll cells
• so water drawn out of xylem down a ¥ gradient
2) Xylem :
• creating tension in xylem
• hydrogen bonds result in cohesion between water molecules so water is pulled up as a continuous columns
• water also adheres to walls of xylem
3) roots :
• water enters roots via osmosis
describe how to set up a potometer
1) cut a shoot underwater at a slant - prevent air entering xylem
2) assemble potometer with capillary tube end submerged in a beaker of water
3) insert shoot underwater
4) ensure apparatus is watertight / airtight
5) dry leaves and allow time for shoot to acclimatise
6) shut tap to reservoir
7) form an air bubble - quickly remove end of capillary tube from water
describe how a potometer can be used to measure the rate of transpiration
potometer estimates transpiration rate by measuring water uptake :
1) record position of air bubble
2) record distance moves in a certain amount of time
3) calculate volume of water uptake in a given time :
• use radius of capillary tube to calculate cross-sectional area of water
• multiply this by distance moved by bubble
4) calculate rate of water uptake - divide volume by time taken
describe how a potometer can be used to measure the rate of transpiration
potometer estimates transpiration rate by measuring water uptake :
1) record position of air bubble
2) record distance moves in a certain amount of time
3) calculate volume of water uptake in a given time :
• use radius of capillary tube to calculate cross-sectional area of water
• multiply this by distance moved by bubble
4) calculate rate of water uptake - divide volume by time taken
describe how a potometer can be used to investigate the effect of a named environmental variable on the rate of transpiration
• carry out the above , change one variable at a time
e.g wind , humidity , light , temp
• keep all other variables constant
suggest limitations in using a potometer to measure rate of transpiration
•rate of water uptake might not be the same as rate of transpiration
-water used for support
- water used in photosynthesis and produced during respiration
• Rate of movement through shoot in potometer layout be the same as rate of movies through shoot of whole plant
- shoot in potometer has no roots whereas a plant does
- xylem cells very narrow
suggest how different environmental variables affect transpiration rate
Light intensity increases rate of transpiration :
• stomata open in light to let CO2 for photosynthesis
• Allowing more water to evaporate faster
• stomata close when it’s dark so there’s a low transpiration rate
Temperature increases rate of transpiration
• Water molecules gain kinetic energy as temperature increases
• so water evaporates faster
Wind intensity increasing rate of transpiration
• Wind blows away water molecules from around stomata
• Decreasing ¥ other air around stomata
• Increasing ¥ gradient so water evaporates faster
Humidity decreases rate of transpiration
• more water in air so it has a higher ¥
• decreasing ¥ gradient from leaf to air
• Water evaporates slower
describe the function of phloem tissue
transports organic substances for example sucrose in plants
suggest how phloem tissue is adapted for its function
1) sieve tube elements
• no nucleus / few organelles - maximise space for flow of organic substances
- end walls between cells perforated
2) companion cells
• many mitochondria so high rate of respiration to make ATP for active transport of solutes
what is translocation
- movement of assimilates / solutes such as sucrose
- from source cells to sink cells by mass flow
explain the mass flow hypothesis for translocation in plants
1) at source , sucrose is actively transported into phloem sieve tubes
2) by companion cells
3) this lowers ¥ in sieve tubes so water enters from xylem by osmosis
4) this increases hydrostatic pressure in sieve tubes , creating a hydrostatic pressure gradient
5) so mass flow occurs - movement from source to sink
6) at sink , sucrose is remove by active transport to be used by respiring cells or stored in storage organs
describe the use of tracer experiments to investigate transport in plants
1) leaf supplied with a radioactive tracer e.g CO2 containing radioactive isotopes C14
2( radioactive carbon incorporated into organic substances during photosynthesis
3) these move around plant by translocation
4) movement tracked using autoradiography or a geiger counter
describe the use of ringing experiments to investigate transport in plants
1) remove or kill phloem
2) bulge forms on source side of ring
3) fluid from bulge has a higher conc of sugars than below - shows sugar is transported in phloem
4) tissues below ring due as cannot get organic substances
