Plant transport

Question 1

Describe and explain structural features of xylem (4 points)

Answer 1

1. Long cells arranged in tubes with no end walls. This is to form a contiuous water coloumn.
2. No cytoplasm/organelles- hollow and free space. This allows easier water flow.
3. Thickening/lignin in walls to withstand tension.
4. Pits in walls to allow lateral movement and get around blocked vessels.

Question 2

Definition of transpiration

Answer 2

Loss of water vapour from the leaves of a plant due to evaporation.

Question 3

Describe the movement of water into the roots from the soil (7 points)

Answer 3

1. Water enters the root hair cell by osmosis.
2. Water then moves through roots by osmosis down a water potential gradient.
3. There is active tranpsort of ions into the base of the xylem.
4. This decreaeses the water potential.
5. Therefore, water enters xylem bottom by osmosis.
6. Forcing water up the stem.
7. Pushing force from the bottom of xylem vessel.

Question 4

What is cohesion?

Answer 4

In water, hydrogen atoms have a slight positive charge and the oxygen atoms have a slight negative charge. This causes hydrogen bonds to form between neighbouring water molecules. This make water molecules cohere (stick together). Individually, hydrogen bonds are weak but lots of them together are strong.

Question 5

Describe the cohesion tension theory (7 points)

Answer 5

1. Water lost from leaf due to evaporation.
2. Water vapour diffuses out of leaves into atmosphere through stomata.
3. Water evaporates from cell surface into air spaces in the leaves.
4. This descreases water potential in leaf cell.
5. Water is drawn out the xylem, by osmosis.
6. Departure of water causes a tension/negative pressure inside the xylem (compared to atmopheric pressure).
7. Due to cohesion between water molecules due to hydrogen bonds, water moves up as a column.

Question 6

Why do plants lose so much water?

Answer 6

Stomata are open during the day the allow gaseous exchange for photosynthesis. Therefore, plants will always lose water while stomata are open- transpiration.

Question 7

Explain why transpiration steam to plants is useful. (4 points)

Answer 7

1. Water keeps cells turgid so the cell can grow and elongate.
2. Contains dissolved minerals which are useful to the plant.
3. Evaporation of water keeps the plant cool.
4. Water is required for photosynthesis.

Question 8

Why does the potometer not actually measure transpiration?

Answer 8

Potometer measures water uptake, not loss.
Water loss (transpiration) does not precisely eual water uptake because some water is used in photosynthesis/released in respiration, some water is used to maintain turgif=dity in the plant, some water could be lost through evaporation/leaks from apparatus, not leaf.

Question 9

Describe how to set up a potometer. (11 points)

Answer 9

1. Use a healthy shoot, taking care not to get the leaves wet.
2. Cut shoot under water.
3. Cut shoort at a slant.
4. Insert into the apparatus under water.
5. Apparatus should be full of water with no extra bubbles.
6. Ensure apparatus is air/watertight.
7. Introduce an air bubble into capillary tube (meniscus).
8. Using the scale, note where the bubble is at the start.
9. Measure the distance moved per unit time.
10. Using mean values, the volume of water lost is calculated.
11. Air bubble near junction of reservoir and capillary tube, the tap on the reservoir is opened and syringe is pushed down until air bubble (meniscus) is oushed back to start position on the scale.

Question 10

Explain how light can increase rate of transpiration.

Answer 10

During the day, stomata are open to allow CO2 in for photosynthesis. This means that water vapour can evaporate and diffuse out of leaf.

Question 11

Explain how light decreases the rate of trnapsiration.

Answer 11

At night, stomata are closed as photosynthesis can’t take place and CO2 isn’t required. Water vapour is unable to diffuse out of the leaf, therefore decreased transpiration rate.

Question 12

Explain how temperature can increase the rate of transpiration.

Answer 12

A higher temeprature increases kinetic energy of water molecules in cells. This increases evaporation of water from cells and increases rate of diffusion of water vapour. Therefore, rate of transpiration increases.

Question 13

Explain how temperature decreases the rate of transpiration.

Answer 13

A lower temperature decreases the kinetic energy of water molecules in the cells. Reduced evaporation rate, so reduced diffusion of water vapour. Theefore, a reduced transpiration rate.

Question 14

Explain how humidity increases the rate of transpiration

Answer 14

Surrounding arid (dry) air with low water potential increases water potential gradient between atmosphere and inside cells, which increases rate of evaporation and diffusion of water vapour. Therefore, the rate of transpiration is increased.

Question 15

Explain how humidity decreases the rate of transpiration.

Answer 15

Surrounding humid (wet) air with a high water potential reduces the water potential gradient, so decreased rate of evaporation and diffusionof water vapour from cells. Therefore, decreased transpiration rate.

Question 16

Explain how air movement increases the rate of transpiration.

Answer 16

High wind speed moves water vapour away from the stomata. This increases water potential gradient. Water vapour diffuses out of leaf at a faster rate. Therefore, transpiration rate is increased.

Question 17

Explain how air movement decreases transpiration rate.

Answer 17

When air is still, water vapour builds up around the stomata. Descreases water potential gradient. Water vapour diffuses out of leaf at slower rate.

Question 18

Explain why the shoot has to be cut under the water and at a slant in the potometer experiment.

Answer 18

This is to prevent air bubbles entering the xylem, breaking the transpiration pull.

Question 19

Explain why the leaves must be dry in the potometer experiment

Answer 19

To prevent excess water on leaf increasing humidity of surrounding air.

Question 20

Definition of translocation.

Answer 20

The movement of organic compounds.

Question 21

Definition of source.

Answer 21

Area of plant where organic compounds, for example: sucrose, enters the phloem sieve tube elements.

Question 22

Definition of sink.

Answer 22

Area of plant where organic compounds, for example: sucrose, are moved towards during translocation.

Question 23

Describe the process of loading from source (3 points)

Answer 23

1. In source, sugars are actively transported into the phloem.
2. By companion cells.
3. Lowers water potential of sieve cell tube and water enters by osmosis.

Question 24

Describe the process of the pressure flow from source to sink (2 points)

Answer 24

1. Water eneters the sieve tube elements from the surrounding xylem vessels by osmosis.
2. This increases the pressure within the sieve tube elements, pushing their contents through the pores in the sieve plates.

Question 25

Describe the process of uploading to a sink (4 points)

Answer 25

1. As solutes are pushed down a pressure gradient, they are surrounded by cells with a lower solute concentration.
2. Solutes leave the phloem into these cells, lowering water potential.
3. Water leaves the phloem by osmosis, returned to the surrounding xylem and is re-circulated.
4. Cells in the sink use, or store, the solutes.

Question 26

Describe the model of mass flow (5 points)

Answer 26

1. Water enters the ‘cell A’ by osmosis because it has the more concentrated solution (lower/more negative water potential).
2. This increases the volume within ‘cell A’ and hence, the hydrostatic pressure within it.
3. Increase in pressure pushes the solution from ‘cell A’ to ‘cell B’ through tube C.
4. Pressure increases in ‘cell B,’ water is forced through the partially permeable membrane surrounding it.
5. Water flows back to the container holding ‘cell A’ as water finds its own level.

Question 27

Describe and explain how organic substances are transported in plants (8 points)

Answer 27

1. Phloem companion cells will load phloem sieve tube elements with organic molecules, for example: sucrose from source.
2. Happens via active transport between the source and companion cells.
3. This lowers the water potential in the sieve tube element cells.
4. Water moves from xylem to sieve tube element cells by osmosis downa water potenial gradient.
5. Causes an increase in hydrostatic pressure within sieve tube element cells.
6. The pressure gradient forces flow of organic substances to sink.
7. Organic molecules unloaded at sink.
8. Water moves from the sieve tube elemnt cells by osmosis back into xylem at sink.

Question 28

Outline the tracer experiment (6 points)

Answer 28

1. The radioactive tracer used is 14C.
2. This can be supplied via placing the leaf in a container and pumping it in.
3. The radioactive carbon incorporates with the organic substances produced by the leaf.
4. The movement of these sugars can be tracked using a technique called “autoradiography.”
5. The plant is killed using liquid nitrogen, and placed on X-ray film.
6. Where the radioactive tracer has moved within the plant is seen as black on the X-ray film.

Question 29

Outline the ringing experiment (4 points).

Answer 29

1. In a ringing experiment, the outer layers of bark and phloem are removed.
2. The section of the stem above the cut part swells.
3. Samples from the swollen part are rich in sigars.
4. Some non-photosynthetic tissue below the ring will die while those above continue to grow.

Question 30

Describe the evidence for the tracer experiment.

Answer 30

Other tissues don’t blacken the film- phloem alone is responsible for translocation of sugars.

Question 31

Describe the evidence of the ringing experiment.

Answer 31

Shows organic molecules can’t pass below or above region of removal ring. Shows the direction in phloem varies in time of year. Summer: large swelling above the ring, sucrose made in leaf (source) being transported to roots. Spring: large swelling below the ring as sucrose transferred upwards from root stored (source) to leaves (sink).

Question 32

Explain how studying aphids can be used to show the dialy variations of the contents of the leaves.

Answer 32

Aphids have needle-like mouth parts which penetrate the phloem and extract contents of sieve tubes. Their sucrose contents show daily variations in that kirror the contents of leaves. Sucrose leaks out faster at source (highest pressure).
When aphids feed, they pierce the surface. The sap can flow out of these holes. Sap can be recorded flowing quicker at the leaves than the stem. This is due to higher hydrostatic pressure at the source end, than at the sink.