Transport of water

Question 1

What are the two main functions of the xylem?

Answer 1

The transport of water and mineral ions

Support for the plant

Question 2

What is the direction of flow in the xylem?

Answer 2

From roots up to shoots and leaves

Question 3

What is the xylem made up of?

Answer 3

Dead, hollow cells with no end cell walls. This forms one continuous tube when the xylem cells are stacked on top of each other and fused together

Question 4

What are the two other types of tissues associated with the xylem?

Answer 4

Xylem parenchyma

Lignin

Question 5

What is xylem parenchyma?

Answer 5

Xylem parenchyma packs around xylem vessels, and stores food and contains tannin deposits

Question 6

What is tannin?

Answer 6

A bitter tasting chemical that protects plant tissues from attack by herbivores

Question 7

What is the function of lignin in the xylem?

Answer 7

Lignin lines the walls of the xylem.
It is important in:
- Supporting and strengthening the xylem
- Impermeable so prevents incorrect water loss

Question 8

Where does water leave the xylem?

Answer 8

Un-lignified bordered pits

Question 9

How is lignin arranged in the xylem?

Answer 9

It can form rings, spirals or solid tubes

Question 10

How does the structure of xylem cells allow for the fastest flow of water?

Answer 10

They have no cytoplasm, cell organelles, or end walls, so there is nothing to slow the flow of water.

Question 11

What occurs at the unlignified bordered pits in the xylem?

Answer 11

Water and minerals can leave the xylem tube it’s currently in, either entering a new xylem tube, or entering cells

Question 12

4 reasons why water is important in plants

Answer 12

• Turgor pressure provides a hydrostatic skeleton
• Turgor pressure drives root expansion
• Mineral ions and products of photosynthesis are transported in water
• Water is a raw material for photosynthesis

Question 13

What cell is responsible for the uptake of water into the plant?

Answer 13

Root hair cells are the exchange surface in plants where water is taken into the plant from the soil

Question 14

What do root hair cells have attached to them?

Answer 14

Root hairs

Question 15

Why are root hairs well adapted exchange surfaces?

Answer 15

• Small size can easily penetrate soil
• Large SA:V ratio
• Thin layer
• High water potential gradient between the soil water and the cell

Question 16

Why is there a high water potential gradient between soil water and root cells?

Answer 16

Soil water has a very low concentration of dissolved minerals so a high water potential.
The cytoplasm and cell sap of the root hair cells has many different solutes (e.g sugars, minerals), and so has a low water potential.
Therefore water moves into the root hair cells via osmosis

Question 17

Once water has entered the root hair cell, how does it move across the root to the xylem?

Answer 17

The water can travel either through the symplast or apoplast pathway to reach the xylem

Question 18

How does water move in the symplast pathway?

Answer 18

It moves through the continuous cytoplasm of living plant cells by diffusion and osmosis

Question 19

What joins cytoplasms together in the symplast pathway?

Answer 19

Plasmodesmata is a fine strand of cytoplasm that act as a bridge linking adjacent cells

Question 20

How does water move in the apoplast pathway?

Answer 20

It moves through the cell walls and the intercellular spaces. It does this through water tension

Question 21

What is the endodermis?

Answer 21

The layer of cells that surround the vascular tissue

Question 22

What is more common for water to move through, symplast or apoplast pathway?

Answer 22

Most water goes from the root cell to the xylem through the apoplast pathway

Question 23

What is the Casparian strip?

Answer 23

A band of waxy material called suberin, which runs around the endodermal cell walls, forming a waterproof barrier

Question 24

What happens in the apoplast pathway when the water reaches the Casparian strip?

Answer 24

It cannot pass through the Casparian strip, so must join the symplast pathway

Question 25

What is the significance of apoplast water joining the symplast pathway?

Answer 25

The water must pass through the selectively permeable cell surface membranes; this excludes any potentially-toxic solutes in the soil water

Question 26

What happens once the water enters the endodermal cells, just outside the xylem?

Answer 26

Endodermal cells actively transport mineral ions into the xylem

Question 27

What is the impact of the active pumping of minerals into the xylem?

Answer 27

It lowers the water potential in the xylem, causing water to enter the xylem via osmosis

Question 28

What is root pressure?

Answer 28

The movement of water up the xylem due to the active pumping of mineral ions. Root pressure is independent of transpiration

Question 29

What is evidence to suggest that active transport has a role in root pressure?

Answer 29

- When ATP inhibiting poisons like cyanide are added to root cells, there is no energy so root pressure disappears - Root pressure increases with a rise in temperature and falls with a fall in temperature

Question 30

What is the definition of transpiration?

Answer 30

The loss of water vapour by evaporation through stomata

Question 31

What are the pros of transpiration?

Answer 31

- Cools plants down (prevents heat damage) | - Causes the transpiration pull, which allows minerals and water to be delivered to cells

Question 32

What is the transpiration pull?

Answer 32

Water is continuously drawn up the xylem to replace the water lost by evaporation/transpiration

Question 33

What are the cons of transpiration?

Answer 33

Loss of water = loss of turgor pressure = plant wilting

Question 34

What is the cohesion of water molecules in plants?

Answer 34

Water molecules form H bonds with each other, so are able to stick together and pull each other

Question 35

What is the adhesion of water molecules in plants?

Answer 35

Water molecules form H bonds with the carbohydrates in the walls of the narrow xylem vessels

Question 36

What is the cohesion-tension theory?

Answer 36

The model of water moving from the soil in a continuous stream up the xylem and across the leaf due to the cohesive property of water

Question 37

What is capillary action, and what properties of water allow it to occur?

Answer 37

Capillary action is the process by which water can move up a narrow tubes against the force of gravity. This is made possible by the combined effects of the adhesion and cohesion of water moving up the xylem

Question 38

Why is it important that the xylem tubes are narrow enough?

Answer 38

So the water can flow in a continuous stream

Question 39

How does water leave the plant?

Answer 39

Water molecules evaporate from mesophyll cells into the air spaces of the leaf and move out of the stomata into the outside air by DIFFUSION

Question 40

What happens once the water evaporates from the mesophyll cell out of the plant?

Answer 40

The water potential of the mesophyll cell is lowered, causing water to move from an adjacent cell to replace the lost water by osmosis. This has a knock on effect, and this process repeats throughout the plant continuously (this is the transpiration pull).

Question 41

Evidence of the cohesion-tension theory

Answer 41

If a xylem vessel is broken, water can no longer move water up as the continuous stream of water held together by cohesive forces is broken

Question 42

Why must transpiration occur?

Answer 42

Transpiration is a consequence of photosynthesis, as stomata must open to allow CO2 to enter and oxygen to leave, which allows transpiration to occur

Question 43

Why does increasing the temperature increase transpiration rate?

Answer 43

Water molecules have more kinetic energy, so evaporate more quickly

Question 44

Why does a lower humidity increase transpiration rate?

Answer 44

There is a higher water vapour concentration gradient, so water vapour will diffuse out more quickly

Question 45

Why does a higher light intensity increase transpiration rate?

Answer 45

More stomata will open to allow more CO2 so the plant can photosynthesise more

Question 46

Why does higher air movement increase transpiration rate?

Answer 46

Stronger winds will more quickly get rid the moist water vapour air from around the lead, increasing the water vapour concentration gradient

Question 47

Why does a lower soil-water availability decrease transpiration rate?

Answer 47

The plant will be under water stress, so stomata will close and rate of transpiration will decrease

Question 48

What instrument can be used to estimate the rate of transpiration?

Answer 48

Potometer

Question 49

Why is using a potometer an estimation not a measure of transpiration?

Answer 49

Potometers measure the rate of water uptake by a plant, and not all water taken up by the plant will transpire; some will be used in photosynthesis or in maintaining turgor pressure. Therefore not all water taken up will transpire

Question 50

How do potometers work?

Answer 50

They measure the distance travelled by the air bubble, and use this to find the volume of the water taken up

Question 51

How do you convert the distance travelled by the bubble into volume of water taken up?

Answer 51

Distance travelled by the bubble x (pi x radius^2) = volume of water

Question 52

To undergo an accurate potometer practical, what steps must you take?

Answer 52

- Cut leafy shoots underwater - Place the shoot into a tube with a bung for a tight seal; seal joints with waterproof jelly so any water loss is from the plant - Do not get water on the leaves, or if you do leave it to dry

Question 53

Issues with using a potometer

Answer 53

Rate of water uptake does not equal water transpired | It is a complex set up, in which leaks are common and hard to detect