3.3.4.2 mass transport in plants

Question 1

Xylem

Answer 1

Tissue that transports water in the stem and leaves of plants

Question 2

Cohesion-tension theory of water transport in the xylem

Answer 2

Theory: how water moves up the xylem against gravity via transpiration stream

Water evaporates from the leaves via the stomata due to transpiration

Reducing water potential in the cell and increasing water potential gradient

Water drawn out of xylem, creating tension

Cohesive forces between water molecules pull water up as a column

Water lost enters roots via osmosis

Water is moving up, against gravity

Water is also cohesive so sticks to edges of the column

Question 3

Adaptations of xylem

Answer 3

Waterproof preventing water loss

Rigid so less likely to collapse under low pressure

Question 4

Phloem

Answer 4

Tissue that transports organic substances in plants

Question 5

Translocation

Answer 5

Movement of solutes from source to sink

Question 6

Translocation at source

Answer 6

High concentration of solute

Active transport loads solutes from companion cells to sieve tubes of the phloem

Lowering water potential inside sieve tubes

Water enters sieve tubes by osmosis from xylem and companion cells

Increasing pressure inside sieve tubes at the source end

Question 7

Translocation at sink

Answer 7

Low concentration of solute

Solutes removed to be used up

Increasing water potential inside sieve tubes

Water leaves tubes via osmosis

Lowering pressure inside sieve tubes

Question 8

Mass flow

Answer 8

Pressure gradient from source to sink

Pushes solutes from source to sink

Solutes used or stored at sink

Question 9

Adaptations of phloem

Answer 9

Sieve tube elements have no nucleus and few organelles

Companion cell for each sieve tube element to carry out the living functions for the sieve cells

Question 10

Use of tracers

Answer 10

Supply plant with radioactive tracer (e.g. 14C) in CO2 to a photosynthesising lead by pumping the radioactive CO2 into a container surrounding the leaf

14C is incorporated into the organic substances produced by the leaf

Organic substances undergo translocation

Autoradiography - plant killed and placed in a photographic film, film turns black where the radioactive substance is present

Identifies where radioactive substance has moved to and thus where the organic substances have moved to via translocation from source to sink

Can show this over time by taking autoradiographs at different times

Question 11

Aphid

Answer 11

Aphids pierce the phloem using mouthpiece

Releasing sap from plants

Flow of sap higher at source than at sink

Evidence of a pressure gradient, higher pressure near source

Question 12

Metabolic inhibitor

Answer 12

Add a metabolic inhibitor to phloem

Translocation stops

Proves active transport is involved

As it requires ATP to move against a concentration gradient

Question 13

Potometer to investigate effect of named environmental variable on rate of transpiration

Answer 13

Potometer estimates the transpiration rate by measuring water uptake

• assume water uptake is directly related to water loss of leaves

Question 14

Method w/ potometer

Answer 14

Cut a shoot underwater
- to prevent air entering xylem > interrupt water flowing in column, stopping transpiration

Assemble potometer with capillary tube and submerged in a beaker of water

Insert shoot underwater

Ensures apparatus is watertight and airtight

Dry leaves and allow time for shoot to acclimatise

Shut off tap to reservoir

Remove end of capillary tube from the water beaker until one hair bubble has formed, then put the tube back into the water

Record position of the air bubble

Use a stopwatch to record time

Record distance moved per unit time

Rate of air movement = estimate of transpiration rate

Change one variable at a time and keep all other variables constant

Question 15

Variables

Answer 15

Wind

Humidity

Light

Temperature

Question 16

How light affects transpiration rate

Answer 16

The higher the light intensity, the faster the transpiration rate

Because stomata open in light to let CO2 for photosynthesis

Allowing more water to evaporate faster

Stomata close when it’s dark so there is a low transpiration rate

Question 17

How temperature affects transpiration rate

Answer 17

The higher the temperature, the faster the transpiration rate

Water molecules gain kinetic energy as temperature increases

Move faster

Water evaporates faster

Question 18

How humidity affects transpiration rate

Answer 18

The lower the humidity, the faster the transpiration rate

Because as humidity increases, more water is in the air so it has a higher water potential

Decreasing the water potential gradient from leaf to air

Water evaporates slower

Question 19

How wind affects transpiration rate

Answer 19

The windier, the faster the transpiration rate

Wind blows away water molecules from around the stomata

Decreasing the water potential of the air around the stomata

Increasing the water potential gradient

Water evaporates faster

Question 20

Describe the mass flow hypothesis (5)

Answer 20

At source, sucrose actively transported to phloem sieve cells

From companion cells

Sieve cell wp lowered at source so water enters from xylem by osmosis

HP increases causing mass movement of sucrose towards sink

Sucrose actively transported into sink and converted to starch or glucose