3.4.2 - Mass Transport in Plants Flashcards
describe the function of xylem tissue
-transports water (and mineral ions) through the stem, up the plant to leaves of plants
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 / nucleus → easier water flow / no obstructions
● thick cell walls with lignin → provides support / withstand tension / prevents water loss
● pits in side walls → allow lateral water movements
explain the cohesion-tension theory of water transport in the xylem
- in the leaf
- water lost from leaf by transpiration - water evaporates from mesophyll
cells into air spaces and water vapour diffuses through (open) stomata
- reducing water potential of mesophyll cells
- so water drawn out of xylem down a water potential gradient - in the xylem
- creating tension (‘negative pressure’ or ‘pull’) in xylem
- hydrogen bonds result in cohesion between water molecules (stick together) so water is pulled up as a continuous column
- water also adheres (sticks to) to walls of xylem - in the root
- water enters roots via osmosis
describe how to set up a potometer
- cut a shoot underwater at a slant → prevent air entering xylem
- assemble potometer with capillary tube end submerged in a beaker of water
- insert shoot underwater
- ensure apparatus is watertight / airtight
- dry leaves and allow time for shoot to acclimatise
- shut tap to reservoir
- 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 moved in a certain amount of time (eg. 1 minute)
3. calculate volume of water uptake in a given time:
○ use radius of capillary tube to calculate cross-sectional area of water (πr2)
○ 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
● use potometer method, change one variable at a time (wind, humidity, light or temperature)
○ eg. set up a fan OR spray water in a plastic bag and wrap around the plant OR change
distance of a light source OR change temperature of room
● keep all other variables constant
suggest limitations in using a potometer to measure rate of transpiration
● rate of water uptake might not be same as rate of transpiration
○ water used for support / turgidity
○ water used in photosynthesis and produced during respiration
● rate of movement through shoot in potometer may not be same as rate of movement through shoot of whole plant
○ shoot in potometer has no roots whereas a plant does
○ xylem cells very narrow
suggest and explain how light intensity affects transpiration rate
-increases rate of transpiration:
● stomata open in light to let in CO2 for photosynthesis
● allowing more water to evaporate faster
● stomata close when it’s dark so there is a low transpiration rate
suggest and explain how temperature affects transpiration rate
-increases rate of transpiration:
● water molecules gain kinetic energy as temperature increases
● so water evaporates faster
suggest and explain how wind intensity affects transpiration rate
-increases rate of transpiration:
● wind blows away water molecules from around stomata
● decreasing water potential of air around stomata
● increasing water potential gradient so water evaporates faster
suggest and explain how humidity affects transpiration rate
-decreases rate of transpiration
● more water in air so it has a higher water potential
● decreasing water potential gradient from leaf to air
● water evaporates slower
describe the function of phloem tissue
-transports organic substances eg. sucrose in plants
suggest how phloem tissue is adapted for its function
- sieve tube elements
○ no nucleus / few organelles → maximise space for / easier flow of organic substances
○ end walls between cells perforated (sieve plate) - companion cells
○ many mitochondria → 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 (where made, eg. leaves) to sink cells (where used / stored, eg. roots) by mass flow
explain the mass flow hypothesis for translocation in plants
- at source, sucrose is actively transported into phloem sieve tubes / cells
- by companion cells
- this lowers water potential in sieve tubes so water enters (from xylem) by osmosis
- this increases hydrostatic pressure in sieve tubes (at source) / creates a hydrostatic pressure gradient
- so mass flow occurs movement from source to sink
- at sink, sucrose is removed 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
- leaf supplied with a radioactive tracer eg. CO2 containing radioactive isotope 14C
- radioactive carbon incorporated into organic substances during photosynthesis
- these move around plant by translocation
- movement tracked using autoradiography or a Geiger counter
describe the use of ringing experiments to investigate transport in plants
- remove / kill phloem eg. remove a ring of bark
- bulge forms on source side of ring
- fluid from bulge has higher conc. of sugars than below - shows sugar is transported in phloem
- tissues below ring die as cannot get organic substances
suggest some points to consider when interpreting evidence from tracer &
ringing experiments and evaluating evidence for / against the mass flow
hypothesis
● is there evidence to suggest the phloem (as opposed to the xylem) is involved ?
● is there evidence to suggest respiration / active transport is involved?
● is there evidence to show movement is from source to sink? what are these in the experiment?
● is there evidence to suggest movement is from high to low hydrostatic pressure?
● could movement be due to another factor eg. gravity?
