Unit 1: Section 6 - Transport in Plants

Question 1

What does the xylem tissue and the phloem tissue do?

Answer 1

Xylem - Transports water and mineral ions, also provides support
Phloem - Transports dissolved substances like sugars

Question 2

Where are the xylem and phloem positioned in a root?

Answer 2

At the centre to provide support for the root as it pushes through the soil

Question 3

Where are the xylem and phloem positioned in the stems?

Answer 3

Near the outside to reduce bending. The xylem is the ‘most inner’ of the two.

Question 4

Where are the xylem and phloem positioned in the leaf?

Answer 4

Make up a network of veins which support the thin leaves.

Question 5

How are xylem vessels adapted for transporting water and mineral ions?

Answer 5

1) Xylem vessels are very long, tube like structures formed from cells called vessel elements joined end to end
2) There are no end walls on these cells, making an uninterrupted tube that allows water to pass up through the middle easily
3) The cells are dead, so they contain no cytoplasm
4) Walls are thickened with a woody substance called lignin which helps to support the xylem vessels and stops them collapsing inwards
5) Amount of lignin increases as cell gets older
6) Water and ions move in and out of the vessels through small pits in the walls where there’s no lignin

Question 6

How are phloem tissues adapted for transporting solutes?

Answer 6

1) Formed from cells arranged in tubes
2) Purely a transport tissue - not used for support as well
3) Phloem tissues contain phloem fibres, phloem parenchyma, sieve tube elements and companion cells
4) Sieve tube elements and companion cells are the most important cell types in phloem for transport

Question 7

What is the structure of the sieve tube elements?

Answer 7

1) These are living cells that form the tube for transporting solutes through the plant
2) They are joined end to end to form sieve tubes
3) The sieve plates are the end walls, which have holes in them to allow solutes to pass through.
4) Unusually for living cells, sieve tube elements have no nucleus, a very thin layer of cytoplasm and few organelles
5) The cytoplasm of adjacent cells is connected through the holes in the sieve plates

Question 8

What is the structure of companion cells?

Answer 8

1) No nucleus and no other organelles

2) One companion cells for every sieve tube element

Question 9

What is the function of companion cells?

Answer 9

Carry out the living functions for both themselves and their sieve cells.

Question 10

How does water get from the soil to the xylem?

Answer 10

Water enters through the root hair cells, passes through the root cortex, including the endodermis, to reach the xylem.

Question 11

What is the symplast pathway?

Answer 11

Goes through the living parts of the cell - the cytoplasm

Question 12

What is the apoplast pathway?

Answer 12

Goes through the non-living part of the cell - the cell walls. The walls are very absorbent so water can diffuse through them, as well as passing through the spaces between them.

Question 13

What is the casparian strip and what is it’s job?

Answer 13

A waxy strip in the cell walls, when water in the apoplast pathway gets to the endodermis cells in the root, its path is blocked by the casparian strip. The water then has to take the symplast pathway. This is useful as it means that the water has to go through a cell membrane which can control which substances in the water get through.

Question 14

Why does transpiration occur?

Answer 14

1) A plant needs to open it’s stomata to let in carbon dioxide
2) But this also lets water out - there’s a higher concentration of water inside the leaf than in the air outside, so water moves out of the leaf.

Question 15

What are the 4 main factors that affect rate of transpiration?

Answer 15

Light
Temperature
Humidity
Wind

Question 16

How does light affect the rate of transpiration?

Answer 16

The lighter it is the more transpiration, this is because when there’s more light the stomata open.

Question 17

How does temperature affect the rate of transpiration?

Answer 17

Higher the temperature, higher the rate of transpiration, warmer molecules evaporate quicker, so increases rate of transpiration

Question 18

How does humidity affect the rate of transpiration?

Answer 18

The lower the humidity, the faster the rate of transpiration. If air around the plant is dry, the concentration gradient will be steeper, increasing the rate of transpiration.

Question 19

How does wind affect the rate of transpiration?

Answer 19

Windier it is, faster transpiration. Lots of wind blows away water from around the stoma, which increases the concentration gradient.

Question 20

How do stomata sunk in pits help xerophytes reduce water loss?

Answer 20

They’re sheltered from the wind, which helps to slow transpiration down.

Question 21

How does a layer of hairs on the epidermis help xerophytes reduce water loss?

Answer 21

Traps moist air round the stomata, which reduces the water potential gradient between the leaf and the air, slowing transpiration down.

Question 22

How does a reduced number of stomata help xerophytes reduce water loss?

Answer 22

Fewer places where water can be lost.

Question 23

How do curled leaves help xerophytes reduce water loss?

Answer 23

Traps moist air, slowing down transpiration. This also lowers the exposed surface area for losing water and protects stomata from wind.

Question 24

How does a waxy cuticle help xerophytes reduce water loss?

Answer 24

Reduces water loss by evaporation as the layer is waterproof.

Question 25

How doe spines help xerophytes reduce water loss?

Answer 25

Reduces surface area for water loss.

Question 26

What is translocation?

Answer 26

The movement of dissolved substances to where they are needed in the plant

Question 27

What are dissolved substances sometimes called?

Answer 27

Assimilates

Question 28

Where does translocation happen?

Answer 28

In the phloem

Question 29

What is the source and sink of a substance?

Answer 29

Source - Where it’s made (so it’s at a high concentration there)
Sink - Where it’s used up (so it’s at a low concentration there)

Question 30

What role do enzymes play in translocation?

Answer 30

They maintain a concentration gradient. They change the dissolved substances at the sink, this makes sure there’s always a lower concentration at the sink than at the source.

Question 31

What is the mass flow hypothesis?

Answer 31

1) Active transport is used to load the dissolved solutes into the sieve tubes of the phloem at the source
2) This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis
3) This creates a high pressure inside the sieve tubes at the source

4) At the sink, solutes are removed from the phloem to be used up
5) This increases water potential inside the sieve tubes so water leaves by osmosis
6) This lowers the pressure inside the sieve tubes

7) This results in a pressure gradient from source to sink end
8) This gradient pushes solutes along the sieve tubes to where they are needed

Question 32

What is the evidence for the mass flow hypothesis?

Answer 32

1) If you remove a ring of bark including the phloem but not the xylem, a bulge forms above the rind, if you analyse the fluid from this bulge it has a higher concentration of sugars than the fluid below the ring, this supports the mass flow hypothesis.
2) You can investigate the pressure in the phloem using aphids, they pierce the phloem, then their bodies are removed leaving the mouthparts behind, which allows the sap to flow out. The sap flows out nearer the leaves (source) than further down the plant, this shows that there is a pressure gradient
3) If you put a metabolic inhibitor (which stops ATP being produced) into the phloem then translocation stops, this shows that active transport is involved

Question 33

What is the evidence against the mass flow hypothesis?

Answer 33

1) Sugars travel to many different sinks, not just the one with the highest water potential
2) The sieve plates would create a barrier to mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate.