Xylem And Transport Of Water

Question 1

What is the function of xylem tissue?

Answer 1

Transports water and mineral ions up the plant from root to leaves = transpiration

Question 2

Describe and explain the structure and function of xylem vessels

Answer 2

bold = structure

• dead cells aligned end to end
• no end walls - forms continuous tubes from root to leaf, ease the flow of water
• no nucleus and no cytoplasm - hollow tubes to ease the flow of water
• walls thickened (lignified)- prevents tubes collapsing and waterproof the walls of the cells
• the lignin thickening in the cell walls forms patterns which can be spiral, rings (annular) or a network of broken rings (reticulate), this prevents the vessel from being too rigid and allows flexibility in the stem and branch
• bordered pits - in some places the lignification is incomplete leaving pores in the wall of the vessel, allowing water to pass into an adjacent vessel or a living part of a plant

Question 3

Explain the cohesion- tension theory

Answer 3

1. Water evaporates from the leaf surface in a process called transpiration ( forms water vapour)
2. This creates tension in the xylem
3. Water molecules are cohesive (they stick together) due to hydrogen bonds between molecules do when some are pulled into the leaf, others follow. Thsi means the whole column of water in the xylem is pulled upwards
4. Water then enters the stem through the roots

Question 4

What is transpiration?

Answer 4

The loss of water vapour from the aerial parts of the plant, such as through stomata in the leaf

Transpiration mainly occurs in the leaves

Water will move through the xylem from the roots to replace that which has been lost from the transpiration stream

Question 5

What are the 3 ways water moves through leaves?

Answer 5

1. Osmosis from the xylem to the mesophyll cells
2. Evaporation from the surface of the mesophyll cells into the intercellular spaces
3. Diffusion of water Vapor from the intercellular spaces out though the stomata

Question 6

Why is transpiration important?

Answer 6

• water is required in the leaves for PHOTOSYNTHESIS
• water is required to enable cells to GROW and ELONGATE
• water keeps cell TURGID
• th e flow of water carries USEFUL MINERAL IONS up the plant
• evaporation of water keeps the plant COOL

Question 7

What will happen to the diameter of xylem as rate of transpiration increases and why?

Answer 7

As the rate of t re aspiration increases the diameter of the xylem will decreases due to the increases tension in the xylem pulling the xylem walls inwards

Question 8

Name 8 factors that effect transpiration

Answer 8

1. Number of leaves/ size of leaf
2. Number/size/position of stomata
3. Waxy cuticle
4. Light intensity
5. Temperature
6. Relative humidity
7. Air or wind movement
8. Water availability

Question 9

How do the number of leaves/ size of leafs affect water loss?

Answer 9

More leaves/larger = increased surface area so more water is lost by transpiration

Question 10

How do number/size/position of stomata affect the water loss?

Answer 10

If leave shave many stomata then more water is lost quickly. If stomata are on the lower surface water vapour is lost slower

Question 11

How does the waxy cuticle affect water loss?

Answer 11

Thicker cuticle reduces evaporation

Question 12

How does light intensity affect water loss?

Answer 12

In the light, the stomate open to allow more gas exchange for photosynthesis therefore more water lost via evaporation

Question 13

How does temperature affect water loss?

Answer 13

Increased temp increases the kinetic energy of the water molecules. More water vapour is lost via the stomata

Question 14

How does relative humidity affect water loss?

Answer 14

Increased humidity, decreases evaporation form the leaf as there is a smaller water potential gradient

Question 15

How does air or wind movement affect water loss?

Answer 15

More wind removes more water vapour from the leaf’s surface. Therefore increases the water potential gradient and so increases the rate of transpiration

Question 16

How does water availability affect water loss?

Answer 16

Decreased water availability means stomata close so prevents gas exchange so less photosynthesis

Question 17

What are xerophytes?

Answer 17

Plants which can survive in dry places such as deserts and sand dunes. Water loss is unavoidable as the stomata need to be open to allow gas exchange to provide carbon dioxide for photosynthesis therefore they need adaptations to help reduce water loss

Question 18

Name 8 adaptations of xerophytes

Answer 18

1. Smaller leaves
2. Thick waxy cuticle
3. Stomata closing when very hot
4. Hairs on leaf surface
5. Stomata sunk in puts
6. Rolling of leaves
7. Loss of leaves
8. High salt concentration in cells

Question 19

Why do xerophytes adapt to have smaller leaves

Answer 19

Reduces surface area do decreases water vapour loss by transpiration
eg pine trees

Question 20

Why do xerophytes adapt to have a thick waxy cuticle

Answer 20

Reduces evaporation from the leaf surface, acts as a barrier

Eg holly leaves

Question 21

Why do xerophytes adapt to have stomata closing when very hot

Answer 21

Reduces water loss when water availability is low
Eg tree

Question 22

Why do xerophytes adapt to have hairs in leaf surface?

Answer 22

Traps a layer of water vapour close to the surface of the leaf decreasing the water potential gradient

Eg marram grass

Question 23

Why do xerophytes adapt to have stomata sunk in pits?

Answer 23

Traps water vapour in the pit close to the surface of the leaf decreasing the water potential gradient

Eg marram grass

Question 24

Why do xerophytes adapt to have rolled leaves?

Answer 24

Traps water vapour in the rolled leaf decreasing the water potential gradient. Reduces surface area

Eg marram grass

Question 25

Why do xerophytes adapt to have loss of leaves?

Answer 25

Cannot lose water through stomata in the leaf by transpiration

Eg cactus

Question 26

Why do xerophytes adapt to have a high salt concentration in cells

Answer 26

Decreases the water potential gradient in cells, less water is lost between cells

Question 27

How should photometer apparatus be set up to ensure that valid measurements can be made?

Answer 27

1. Cut healthy shoot under water( to stop air entering xylem vessels)
2. Cut shoot at a slant (to increase surface area)
3. Check apparatus is full of water
4. Insert shoot into apparatus under water ( to prevent air locks in xylem)
5. Remove potometer from water and ensure airtight joints around shoot (can be achieved using Vaseline)
6. Dry leaves
7. Keep conditions constant
8. Allow time for shoot to acclimatise
9. Shut screw clip
10. Keep ruler fixed and record starting position of air bubble on a scale
11. Start timing and measure/calculate distance moved per unit time

Question 28

Why does a potometer measure?

Answer 28

Rate of water uptake, as not all the water taken up by the plant is lost in transpiration, it is therefore only an estimate of the rate of transpiration

Question 29

Why is it necessary to form an airtight seal when setting up potometer?

Answer 29

• To ensure the water surface doesn’t come into contact with air as evaporation can affect results
• prevents leaks

Question 30

What are limitations of the potometer?

Answer 30

• measures water uptake not transpiration
• difficulty making an airtight seal which slows air bubble movement and may stop transpiration
• plant has no roots so no resistance to water being pulled up