13.7 Exchange And Transport

Question 0

What are the root hairs of a plant responsible for?

Answer 0

The absorption of water and mineral ions.

Question 1

What are the exchange surfaces of a plant?

Answer 1

Root hairs

Question 2

By which process do plants constantly lose water?

Answer 2

Transpiration.

Question 3

How much water is lost by plants per day by transpiration?

Answer 3

As much as 700 dm^3 per day in a large tree.

Question 4

Why are root hair cells efficient surfaces for the exchange of water and mineral ions?

Answer 4

• they provide a large surface area as they are very long extensions and occur in thousands on each of the branches of a root.
• they have thin surface layer across which materials can move easily.

Question 5

What is the water potential of the soil solution surrounding root hair cells?

Answer 5

Slightly less than zero (mostly water) it contains small quantities of mineral ions.

Question 6

How does water move from the soil into the root hair cells?

Answer 6

By osmosis.

Question 7

What do root hair cells contain which make them have a low water potential?

Answer 7

Sugars, amino acids and mineral ions are dissolved in them. Water moves by osmosis down the water potential gradient.

Question 8

How does water continue it’s journey across the root after moving into it from the soil by osmosis?

Answer 8

• the apoplastic pathway

- the symplastic pathway

Question 9

Describe the apoplastic pathway.

Answer 9

As water is drawn into endodermal cells it pulls more water along behind it due to cohesive properties of water molecules. Therefore water is drawn along the cell walls of the cells of the root cortex.

(Little or no resistance to the pull of water because of cellulose cell walls with water filled spaces.)

Question 10

Describe the symplastic pathway.

Answer 10

This takes place across the cytoplasm of the cells of the cortex due to osmosis.

The water passes through the cell walls along tiny openings called plasmodesmata which are each filled with a thin strand of cytoplasm. So, there is a continuous column of water from the root hair cell to the xylem at the centre of the root.

Question 11

How does water move through the column in symplastic pathway?

Answer 11

Water entering by osmosis increases the water potential of the root hair cell.

The root hair cell now has higher water potential than the first cell in the cortex.

Water therefore moves from the root hair cell to the first cell in the cortex by osmosis down, the water potential gradient.

The first cell now has a higher water potential than its neighbour to the inside of the stem.

This chain continues, a water potential gradient is set up across all the cells of the cortex, which carries water along the cytoplasm from the root hair cell to the endodermis.

Question 12

What blocks water travelling by the apoplastic pathway after it reaches the endodermis?

Answer 12

The waterproof band that makes up the casparian strip in the endodermal cells.

Question 13

Where does the blocked water on the apoplastic pathway go?

Answer 13

It is forced into the living protoplast of the cell where it joins water that has arrived there by the symplastic pathway.

Question 14

What is the most likely way that water gets into the xylem after it has reached the living protoplast?

Answer 14

The active transport of salts - carrier proteins actively transport salts into the xylem from the endodermal cells. This creates a lower water potential in the xylem. Water moves by osmosis down a concentration gradient into the xylem. (Helps create root pressure).

Question 15

Give evidence for root pressure.

Answer 15

• the pressure increase with a rise in temperature and decrease at lower ones.
• metabolic inhibitors such a cyanide prevent most energy release by respiration and also cause root pressure to cease.
• a decrease in the availability of oxygen or respiratory substrates causes a reduction in root pressure.

Question 16

Learn the root picture on page 190

Answer 16

Root hair
Cortex
Endodermis
Phloem
Xylem

Question 17

What is the main force that pulls water up a stem?

Answer 17

The evaporation of water from the leaves. Transpiration.

Question 18

The humidity of the atmosphere is usually less than that of the…

Answer 18

Air spaces next to the stomata. Water vapour molecules diffuse out of the air spaces into the surrounding air.

Question 19

By which method is water lost from the mesophyll cells of the leaf?

Answer 19

By evaporation from their surfaces to the air spaces of the leaf. This is replaced by water reaching the mesophyll cells from the xylem by apoplastic or symplastic pathways.

(Remember symplastic method or water potential gradients etc).

Question 20

Name the two main factors responsible for the movement of water up the xylem.

Answer 20

Root pressure

Cohesion-tension theory

Question 21

Describe cohesion-tension theory.

Answer 21

Water evaporates from the leaves as a result of transpiration.
Water molecules form hydrogen bonds between one another and stick together (cohesion).
Water forms a continuos pathway across the mesophyll cells and down the xylem.
As water evaporates, more molecules of water are drawn up behind it as a result of cohesion.
Water is hence pulled up the xylem as a result of transpiration - transpiration pull - this puts the xylem under tension.

Question 22

Give evidence for the cohesion-tension theory.

Answer 22

• change in diameter of tree trunks according to the rate of transpiration. During the day when transpiration is at it’s greatest there is more tension in the xylem. The tree trunk shrinks in diameter.
• if a xylem vessel is broken and air enters it, the tree can no longer draw up water. The column is broken- water can’t stick together.
• when a xylem vessel is broken, water does not leak out, instead air is drawn in which is consistent with it being under tension. (If it was under pressure, water would leak out).

Question 23

What type of process if transpiration?

Answer 23

Passive.

(Xylem vessels are dead anyway). (Heat does come from the sun to cause evaporation though).

Question 24

What are the benefits of transpiration?

Answer 24

Mineral ions, sugars and hormones are moved around the plant dissolved in water. Without transpiration, water would not be so plentiful and the transport of materials would not be as rapid.

Question 25

What is a down side to transpiration?

Answer 25

Huge water loss due to open stomata.

Question 26

List four factors which affect the rate of transpiration.

Answer 26

Light
Temperature
Humidity
Air movement

Question 27

How does light affect transpiration?

Answer 27

Stomata open in the light and close in the dark.
Therefore, higher light intensity = increase in transpiration
Lower light intensity = decrease in transpiration

Question 28

How does temperature affect transpiration?

Answer 28

Alters the kinetic energy of the water molecules and the relative humidity of the air.
Therefore, higher temperatures= increase in transpiration
Lower temperatures = decrease in transpiration

Question 29

How does humidity affect transpiration?

Answer 29

Affects the water potential gradient between the air-spaces in the leaf and the atmosphere.
Therefore, lower humidity = increase in transpiration
Higher humidity = decrease in transpiration

Question 30

How does air movement affect transpiration?

Answer 30

Changes the water potential gradient by altering the rate at which moist air is removed from around the leaf.
Therefore, more air movement = increase in transpiration
Less air movement = decrease in transpiration

Question 31

What adaptions do plants with a plentiful water supply have to reduce water loss?

Answer 31

Waterproof covering over parts of the leaf.

The ability to close stomata when necessary.

Question 32

What are plants called that do not have a plentiful water supply and have many adaptions to prevent water loss?

Answer 32

Xerophytes.

Question 33

List the adaptions of xerophytes.

Answer 33

A thick cutile - thick waterproof barrier.

Rolling up of leaves- protects lower epidermis where stomata are. Traps a region of still air. This region becomes saturated with water vapour and so there is no water potential gradient and reduced rates of transpiration.

Hairy leaves - traps moist air next to the leaf surface. Water potential gradient reduced = transpiration rates reduced.

Stomata in pits or grooves - traps moist air - water potential gradient - decreased transpiration rates.

A reduced surface area to volume ratio if the leaves - less transpiration but this must be balanced against the need for a sufficient area for photosynthesis.