exchange and transport (plants)

Question 1

what is transpiration?

Answer 1

loss of water vapour from the stomata by evaporation

Question 2

how does light intensity affect transpiration?

Answer 2

more light=faster rate
more stomata open
larger surface area for evaporation

Question 3

how does temperature affect transpiration?

Answer 3

higher temperature=faster rate
more kinetic energy
faster movement so more evaporation

Question 4

how does humidity affect transpiration?

Answer 4

more water vapour in the air decreases the water potential gradient
water less likely to evaporate out

Question 5

how does wind affect transpiration?

Answer 5

more wind=faster rate
wind moves water vapour
which increases the water potential gradient

Question 6

where does the cohesion tension theory occur?

Answer 6

xylem

Question 7

how does water travel as a column?

Answer 7

water is polar

hydrogen bonds between different water molecules results in cohesion

Question 8

what is adhesion?

Answer 8

when water molecules stick to other molecules

Question 9

how does root pressure force water upwards?

Answer 9

water moves into the roots
which increases the volume of liquid
root pressure increases
which forces water upwards

Question 10

what are the stages of the cohesion tension theory?

Answer 10

1) water evaporates out of cells due to the sun’s energy
2) this lowers the water potential of leaf cells
3) water moves down the water potential gradient by osmosis
4) lower pressure at the top of the xylem
5) water pulled up the xylem vessels down a pressure gradient
6) cohesive forces mean that water travels as a continuous column
7) water molecules adhere to xylem walls

Question 11

what is translocation?

Answer 11

movement of sucrose around a plant

Question 12

what are sieve tube elements?

Answer 12

living cells with no nucleus and few organelles

Question 13

what are companion cells?

Answer 13

cells that provide ATP for active transport

Question 14

what is the source?

Answer 14

where plants provide food using photosynthesis

Question 15

what is the sink?

Answer 15

where plants store food produced by photosynthesis

Question 16

where is the mass flow hypothesis based?

Answer 16

in the phloem

Question 17

what are the stages of the mass flow hypothesis?

Answer 17

1) sucrose made in photosynthesis moves into companion cells by diffusion
2) hydrogen ions actively transported out of companion cells into sieve tube elements
3) sucrose is co transported with hydrogen
4) sucrose in the sieve tube elements lowers the water potential
5) water leaves the xylem and enters the phloem down the water potential gradient
6) which increases the hydrostatic pressure so substances move down a pressure gradient
7) sucrose used/stored at the sink
8) sucrose is actively transported from the sieve tubes to cells down the water potential gradient
9) water moves into cells which reduces the hydrostatic pressure
10) this results in a pressure gradient down the phloem

Question 18

what evidence supports the mass flow hypothesis?

Answer 18

• if a ring of bark is removed from a woody stem, a bulge forms above the ring- the fluid from the bulge has a higher concentration of sugars than the fluid from below the ring which supports the idea of a downwards flow of sugars
• radioactive tracer such as radioactive carbon can be used to track the movement of organic substances in a plant
• pressure in the phloem can be investigated using aphids (which pierce the phloem allowing sap to flow out), sap flows out quicker nearer the leaves than further down the stem- supports the idea of a pressure gradient
• if a metabolic inhibitor (stopping ATP production) is put into the phloem, translocation stops which suggests that active transport is involved

Question 19

what evidence objects the mass flow hypothesis?

Answer 19

• sugar travels to many different sinks- not just the one with the highest water potential
• sieve plates would create a barrier to mass flow- lots of pressure is needed for the solutes to get through

Question 20

how can radioactive carbon be used to model the mass flow hypothesis?

Answer 20

supply part of a plant with radioactive isotope 14C
this is then incorporated into organic substances produced by the leaf, which will be moved around the plant by translocation
tracked using autoradiography
plant is killed, then placed on photographic film- where the radioactive substance is present the film turns black
demonstrates movement from source to sink over time