Plant Transport

Question 1

Describe the arrangement of xylem and phloem tissue in the roots of a dicotyledonous plant.

Answer 1

In roots, the xylem is arranged in an X shape, with the phloem found between the arms of the X.

Question 2

Describe the arrangement of xylem and phloem tissue in the stem of a dicotyledonous plant.

Answer 2

In the stem, the vascular bundles are found around the outside of the stem in a ring shape. The xylem is on the inside, with the phloem on the outside and they are separated by a layer of cambium (a layer of meristem cells which can divide to produce new xylem and phloem).

Question 3

Describe the arrangement of xylem and phloem tissue in the leaves of a dicotyledonous plant.

Answer 3

The xylem is on top of the phloem in the ‘veins’ of a leaf.

Question 4

Describe the structure of a xylem vessel.

Answer 4

Long thick walls with lignin deposits which waterproof the cell walls and kill it’s contents; a xylem vessel is long column of dead cells. The lignin deposit forms in a pattern keeping the vessel flexible and in some places there are pores where the lignification isn’t complete to allow water to move in/out.

Question 5

Describe the structure of a sieve tube element.

Answer 5

Very little cytoplasm and no nucleus arranged in a tube (phloem) with perforated cross walls at intervals. Sieve tube usually have 5 or 6 sides.

Question 6

Describe the structure of a companion cell.

Answer 6

Dense cytoplasm with a large nucleus and many mitochondria to produce ATP (for loading sucrose). These are between sieve tubes and there are many plasmodesmata between them allowing communication/ flow of minerals between cells.

Question 7

Define transpiration.

Answer 7

The loss of water vapour from the aerial parts of a plant due to evaporation via stomata.

Question 8

Explain why transpiration is a consequence of gas exchange.

Answer 8

The stomata are open during photosynthesis to allow for gas exhchange down a water potential gradient.

Question 9

What structural and behavioural adaptations do plants have to reduce water loss?

Answer 9

A waxy cuticle waterproofs the leaf preventing water loss through the epidermis.
Stomata are on the underside of leaves to prevent water loss through direct heating and evaporation.
Most stomata close when no photosynthesis is occuring i.e. at night.

Question 10

Explain how the number of leaves affects transpiration.

Answer 10

More leaves means a larger surface which water can be lost through.

Question 11

Explain how the location, number and size of stomata affects transpiration.

Answer 11

If the stomata are on the underside of a leaf water loss is slower.
If a plant has many large stomata water vapour is lost more quickly.

Question 12

Explain how light intensity affects transpiration.

Answer 12

Low light intensity (night) means photosynthesis can’t be carried out so the stomata close.

Question 13

Explain how temperature affects transpiration.

Answer 13

Higher temperature will increase the rate of water loss by: increasing the rate of evaporation, increasing the rate of diffusion (kinetic energy of molecules), decreasing the relative water vapour potential in the air.

Question 14

Explain how relative humidity affects transpiration.

Answer 14

High relative humidity causes a low water potential gradient so slow diffusion of water vapour.

Question 15

Explain how wind affects transpiration.

Answer 15

Air movement around the leaf carries water vapour away from the leaf causing a high water potential gradient.

Question 16

Explain how water availability affects transpiration.

Answer 16

When water isn’t available water can’t be lost so is retained by closing stomata or losing leaves in winter.

Question 17

Outline how a potometer is used to estimate transpiration rates.

Answer 17

1. Cut healthy shoot underwater to stop air entering xylem.
2. Cut shoot at a slant to increase surface area.
3. Ensure apparatus is full of water and that there is only the desired air bubble.
4. Insert shoot into apparatus underwater.
5. Remove potometer form water and ensure it is airtight around the shoot.
6. Dry leaves.
7. Keep conditions constant to allow shoot to acclimatise
8. Shut screw clip.
9. Keep scale fixed and record position of air bubble.
10. Start timing and measure distance moved per unit of time.

Question 18

Why does water move up the root hair cell toward the xylem?

Answer 18

Because minerals are actively pumped from the root cortex into the xylem which maintains a water potential gradient.

Question 19

What is the symplast pathway?

Answer 19

Where water enters the cytoplasm and travels via the plasmodesmata (gaps in the cytoplasm).

Question 20

What is the apoplast pathway?

Answer 20

Where water travels between cell walls (this pathway is blocked by the Casparian strip betwenn the zylem and cortex meaning water has to take the symplast pathway there).

Question 21

Explain how water travels from the root cortex to the air surrounding the leaves (mass flow).

Answer 21

Water moves into the xylem down water potential gradient as there is high hydrostatic pressure at the bottom of the xylem and transpiration at the top of the plant creating low hydrostatic pressure at top of the xylem.
Water is under tension due to cohesion thus pulled up in a continuous column aided by adhesion to the xylem (capillary action).

Question 22

Explain how the leaves of some xerophytes are adapted to reduce water loss.

Answer 22

Hairy leaves trap water vapour outside the leaf reducing the water potential gradient.
Stomata are found in pits which trap water vapour and reduce the water potential gradient.
Rolled leaves reduce surface area for evaporation and trap water vapour reducing the water potential gradient.
High solute concentrations in cells increase the water potential inside leaf cells.
Thicker cuticles reduce evaporation.
Small leaves or needles reduce surface area for evaporation.
Stomata close during the day.
More densely packed spongy mesophyll reducing evaporation.

Question 23

What is a source and what is a sink in relation to the phloem.

Answer 23

The source is where the sugars are loaded (via active transport) into the phloem and the sink is where they are unloaded (via active transport).

Question 24

Describe the how sucrose is moved.

Answer 24

Hydrogen ions are pumped out via active transport and this large excess of ions causes the hydrogen ions to move back in through a cotransporter protein that also transports sucrose. Sucrose moves into the sieve tube via the plasmodesmata.

Question 25

Describe the mechanism of movement through the phloem.

Answer 25

Dissolved solutes are actively loaded at the source which lowers the water potential and so water enters (sieve tube).
Solutes are actively removed from the sink which increases water potential and so water leaves (sieve tube).