mass transport in plants Flashcards

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1
Q

Describe the function of xylem tissue

A

Transports water (and mineral ions) through the stem, up the plant to leaves of plants

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2
Q

Suggest how xylem tissue is adapted for its function

A

● 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

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3
Q

Explain the cohesion-tension theory of water transport in the xylem

A

Leaf
1. Water lost from leaf by transpiration - water evaporates from mesophyll cells into air spaces and water vapour diffuses through (open) stomata
2. Reducing water potential of mesophyll cells
3. So water drawn out of xylem down a water potential gradient

Xylem
4. Creating tension (‘negative pressure’ or ‘pull’) in xylem
5. Hydrogen bonds result in cohesion between water molecules (stick
together) so water is pulled up as a continuous column
6. Water also adheres (sticks to) to walls of xylem

Root
7. Water enters roots via osmosis

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4
Q

Describe how to set up a potometer

A
  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
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5
Q

Describe how a potometer can be used to measure the rate of transpiration

A

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

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6
Q

Describe how a potometer can be used to investigate the effect of a named
environmental variable on the rate of transpiration

A

● Carry out the above, 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

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7
Q

Suggest limitations in using a potometer to measure rate of transpiration

A

● 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

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8
Q

Suggest how different environmental variables affect transpiration rate

A

Light
intensity
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

Temperature
Increases rate of
transpiration
● Water molecules gain kinetic energy as temperature increases
● So water evaporates faster

Wind intensity
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

Humidity
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

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9
Q

Describe the function of phloem tissue

A

Transports organic substances eg. sucrose in plants

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10
Q

Suggest how phloem tissue is adapted for its function

A
  1. Sieve tube elements
    ● No nucleus / few organelles → maximise space for / easier flow of organic substances
    ● End walls between cells perforated (sieve plate)
  2. Companion cells
    ● Many mitochondria → high rate of respiration to make ATP for active transport of solutes
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11
Q

What is translocation?

A

● 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

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12
Q

Explain the mass flow hypothesis for translocation in plants

A
  1. At source, sucrose is actively transported into phloem sieve tubes / cells
  2. By companion cells
  3. This lowers water potential in sieve tubes so water enters (from xylem) by osmosis
  4. This increases hydrostatic pressure in sieve tubes (at source) / creates a hydrostatic pressure gradient
  5. So mass flow occurs - movement from source to sink
  6. At sink, sucrose is removed by active transport to be used by respiring cells or stored in storage organs
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13
Q

Describe the use of tracer experiments to investigate transport in plants

A
  1. Leaf supplied with a radioactive tracer eg. CO2 containing radioactive isotope 14C
  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
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14
Q

Describe the use of ringing experiments to investigate transport in plants

A
  1. Remove / kill phloem eg. remove a ring of bark
  2. Bulge forms on source side of ring
  3. Fluid from bulge has higher conc. of sugars than below - shows sugar is transported in phloem
  4. Tissues below ring die as cannot get organic substances
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15
Q

Suggest some points to consider when interpreting evidence from tracer &
ringing experiments and evaluating evidence for / against the mass flow
hypothesis

A

● 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?

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