[7.7] transport of water in the xylem

Question 1

what does the xylem transport?

Answer 1

• water
• mineral ions

Question 2

what does a mature xylem vessel look like?

Answer 2

• a stack of cell walls on top of each other
• there are no internal structure so there is an unimpeded stream of water flowing through it

Question 3

how is the xylem vessel adapted for it function?

Answer 3

• there is lignin in the walls of xylem vessels
• this is extremely difficult to break down and waterproofs it so water doesn’t leak out
• spiralised thickening gives plant extra strength so pressure inside doesn’t cause the xylem vessel to collapse inwards due to negative pressure

Question 4

what is the role of the xylem?

Answer 4

to transport water through the stem in a process called transpiration, until it diffuses through the stomata

Question 5

describe the movement of water through the stomata

Answer 5

• higher water potential in atmosphere than in air spaces next to stomata
• water potential gradient from the air spaces through the stomata to the air
• provided the stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air
• water lost by diffusion from the air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells
• by changing the size of the stomata, plants can control their rate of transpiration

Question 6

how can water travel through the plant?

Answer 6

• apoplast pathway (cell wall pathway)
• symplast pathway (cytoplasm pathway)

Question 7

how does water enter the root hair cell from the soil?

Answer 7

• through osmosis, from low to high water potential
• sometimes, active transport of mineral ions is needed to manipulate the water potential for their own benefit, but mineral ions are also beneficial

Question 8

what are mineral ions needed for?

Answer 8

• potassium: for sodium-potassium pump in co-transport mechanism
• magnesium: to make chlorophyll, which absorb sunlight for photosynthesis
• nitrate: for amino acids, which form proteins
• phosphate: for nucleic acids (structure) and the phospholipid bilayer

Question 9

due to what force is water effectively drawn up the transpiration stream?

Answer 9

• cohesive forces between water molecules
• this is due to hydrogen bonds between water molecules

Question 10

why is water needed?

Answer 10

• metabolite (eg. photosynthesis)
• keeps mesophyll turgid so it is upright so maximum sunlight can be absorbed for photosynthesis
• cooling. reduces heat generated from plant’s metabolic reactions

Question 11

describe the movement of water across the cells of a leaf (symplast / cytoplasmic pathway)

Answer 11

• mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun
• these cells now have a lower water potnetial so water enters by osmosis from neighbouring cells
• loss of water from these neighbouring cells lowers their water potential
• they, in turn, take in water from their neighbours by osmosis
• in this way, a water potential gradient is established that pulls water from the xylem, across the leaf mesophyll, and finally out into the atmosphere

Question 12

describe the apoplast / cell wall pathway

Answer 12

• water moving in apoplast pathway is done almost exclusively through cell walls
• after the root cortex cells, there is an epidermic later
• this barrier prevents apoplast pathway through epidermis so water must go via the cells
• therefore, must cross the selectively permeable cell membrane
• this means that water, and anything dissolved in it, must cross at least one cell membrane before it gets into the xylem vessels, at which point water will be distributed everywhere else in the plant

Question 13

how can water moving in apoplast pathway be done almost exclusively through cell walls?

Answer 13

because cytoplasm is freely permeable

Question 14

what happens when unwanted things build up in the epidermic layer?

Answer 14

it has enzymes which can break them down

Question 15

what does the cohesion-tension mechanism help with?

Answer 15

moving water up plants, from roots to leaves, against the force of gravity

Question 16

describe the movement of water up the stem in the xylem based on the cohesion-tension mechanism

Answer 16

• water evaporates from the mesophyll cells in the leaves
• humidity of the atmosphere is usually less than tthat of air spaces, so water vapour then diffuses our through stomata
• this lowers the water potnetial of these cells, creating tension (ie. a pulling force) that pulls more water in from neughouring cells by osmosis
• water molecules are cohesive (stick together easily due to H bonds between water molecules) so when one water molecule leaves the leaf, another one is pulled along with it, drawing more water molecules up into the leaf from the stem and roots
• this means a continuous column of water in the xylem is pulled upwards to the leaves from the roots: the transpiration pull / stream

Question 17

how is the transpiration stream passive?

Answer 17

relies on energy from the sun

Question 18

what is negative pressure (in the context of water movement through a plant)

Answer 18

• water is pulled upwards from the roots due to a reduction in pressure in the leaves
• water moves towards an area of lower pressure

Question 19

what evidence is there to support the cohesion-tension theory?

Answer 19

• change in the diameter of tree trunks according to the rate of transpiration
• if a xylem vessel is broken and air enters it, the tree can no longer draw up water. this is because the continuous column of water is broken so water molecules can no longer stick together
• when a xylem vessel is broken, water does not leak out, as would be the case if it were under pressure. instead, air is drawn in, which is consistent with it being under tension

Question 20

describe the changes in a tree’s diameter, which supports the cohesion-tension theory

Answer 20

• during the day, when transpiration is at its greatest, there is more tension (more negative pressure) in the xylem
• this pulls the walls of the xylem vessels inwards and causes the trunk to shrink in diameter
• at night, when transpiration is at its lowest, there is less tension in the xylem so the diameter of the trunk increases

Question 21

what is a potometer?

Answer 21

• a piece of apparatus used to estimate the effect of various factors on the rate of transpiration
• it measures a plant’s water uptake and applies the idea that this will be directly proportional to, but not exactly the same as, water leaving the leaf by transpiration

Question 22

describe a method to measure the uptake of water using a potometer

Answer 22

1. cut shoot underwater to prevent air entering the xylem
2. fill potometer with water, making sure there ar eno air bubbles
3. place shoot into capillary tube in potometer underwater
4. remove potometer from under the water
5. seal joints with eg. vaseline to prevent water loss
6. air bubble is introduced into capillary tube
7. record start position of air bubble in the graduated capillary tube
8. using a stopwatch, record the bubble movement in a set time
9. rate of bubble movement is an estimate of rate of transpiration (only change one variable at a time)

Question 23

how do you calculate water uptake?

Answer 23

record:

• distance moved by bubble
• time taken to move
• volume of capillary tube
• units = something that incoporates volume and time eg. cm³ s⁻¹

calculation:

• volume of capillary tube = πr² × distance bubble moved (h)
• rate = (volume of capillary tube / scale) / time taken

Question 24

what factors affect the rate of transpiration?

Answer 24

• temperature
• humidity in the atomsphere
• wind
• light intensity
• concentration of carbon dioxide

Question 25

how does increased temperature affect the rate of transpiration?

Answer 25

• increased rate of transpiration
• rate of evaporation and diffusion of water out of stomata increases
• higher temp = higher KE

Question 26

how does increased humidity in the atompshere affect the rate of transpiration?

Answer 26

• decreased rate of transpiration
• lower water potential gradient between leaf and atmosphere

Question 27

how does increased wind / air movement affect the rate of transpiration?

Answer 27

• increased rate of transpiration
• moist air cannot cumulate outside stomata
• this creates a steep water potential gradient

Question 28

how does increased light intensity affect the rate of transpiration?

Answer 28

• increased rate of photosythesis so increased rate of transpiration
• increase in heat energy from sun hitting the leaf

Question 29

how does increased concentration of carbon dioxide affect the rate of transpiration?

Answer 29

• increased rate of photosynthesis so increased rate of transpiration
• stomata have to be open to allow carbon dioxide in so water can also leave

Question 30

why is uptake of water not necessarily equal to transpiration?

Answer 30

• water is used in photosynthesis
• water is used in hydrolysis
• water is used in vacuoles ro maintain cell turgidity
• in a potometer system, there could be leaks