transport of water in the xylem

Question 1

what is transpiration?

Answer 1

evaporation and loss of water from a leaf via diffusion through stomata

Question 2

what are the factors affecting transpiration rate?

Answer 2

-humidity
-temperature
-light intensity
-wind speed

Question 3

how does high humidity affect rate of transpiration?

Answer 3

there is a less steep water potential gradient between leaf and air, so there would be a slower rate of diffusion and rate of transpiration decreased

Question 4

how does high temperature affect rate of transpiration?

Answer 4

water particles have a greater KE

so would evaporate and diffuse from the stomata in the leaves quicker

so rate of transpiration increases

Question 5

how does high light intensity affect rate of transpiration?

Answer 5

higher light intensity means stomata are open for longer to let in more CO2 for photosynthesis

and this also means more water can leave the leaf so faster rate of transpiration

Question 6

how does high wind speed affect rate of transpiration?

Answer 6

maintains water potential gradient so faster rate of diff of water out of leaf so faster rate of transpiration

Question 7

what is the xylem?

Answer 7

a tissue that transports water in the stem and leaves of plants

Question 8

what is in the cell wall of xylem that makes it waterproof and supports it to prevent it from collapsing under pressure?

Answer 8

lignin

Question 9

what does the xylem form?

Answer 9

a continuous hollow tube for efficient transport of water by lacking end walls

Question 10

what do pits in the xylem do?

Answer 10

allow for lateral movement of water in and out of vessels

Question 11

what is the vascular bundle of a plant made of?

Answer 11

xylem (that transports water from roots to leaves via transpiration)

phloem (transport of organic substances up and down the plant via translocation)

Question 12

what theory describes how water moves up the xylem?

Answer 12

cohesion-tension theory

Question 13

during the day when light is available for photosynthesis, what does a plant do?

Answer 13

the stomata are open to allow CO2 to enter for photosynthesis

the water in the intercellular spaces is at a higher water potential so it diffuses into air surrounding leaf which has a lower water potential

water evaporates from the spongy mesophyll cells to replace the water that has diffused out of the stomata

Question 14

how is the water potential of the spongy mesophyll cells affected by water leaving the plant?

Answer 14

the water potential of the leaves becomes more negative

so water moves into the leaf from the xylem via osmosis

Question 15

what does water leaving the xylem and entering leaf via xylem create?

Answer 15

creates tension in the column of water in the xylem with the transpiration pulling water upwards and gravity pulling it down

a negative pressure is present is present at the top of the xylem and so water moves upwards

Question 16

what property of water links to the transpiration stream and why?

Answer 16

COHESION

the water in the xylem is a continuous column of water because of hydrogen bonding sticking the water molecules to each other (cohesion)

this means that as one water molecule is drawn to the top of the xylem, all the rest of the molecules are drawn up with it

Question 17

how is water drawn into the xylem and what effect does this have?

Answer 17

from root cells

the water potential of the root cells becomes more negative as water is drawn into the xylem,

therefore water moves into the root cells from the soil by osmosis

Question 18

what kind of process is the transpiration pull?

Answer 18

passive process that doesn’t require metabolic energy to take place

the xylem vessels through which water passes are dead and so cannot actively move the water

Question 19

what is needed for the process of transpiration?

Answer 19

energy in the form of heat and comes from the sun

Question 20

how does the diameter of a tree trunk change depending on rate of transpiration?

Answer 20

during the day, when transpiration is at its greatest, there is more tension (more negative pressure) in the xylem

this pulls walls of xylem vessels inwards and causes trunk to shrink in diameter

at night, when transpiration is lowest, there is less tension in the xylem and the diameter of the trunk increases

Question 21

what happens if a xylem vessel is broken and air enters?

Answer 21

tree can no longer draw up water because the continuous column of water is broken and water molecules can no longer stick together

Question 22

what are xerophytes?

Answer 22

plants that are adapted to arid (hot and dry) conditions

Question 23

why do xerophytes need adaptations?

Answer 23

to absorb as much water as possible when water is available

and to reduce water loss via transpiration

Question 24

how does a xerophytic plant having rolled leaves, stomata in sunken pits or trichomes (hairs) reduce water loss?

Answer 24

air is trapped

water transpires into trapped air

an area of local humidity is created

the water potential gradient is less steep

the rate of transpiration is lower due to a lower rate of diffusion

Question 25

why do some xerophytic plants only have stomata on the underside of a leaf?

Answer 25

this side is in the shade so heat energy levels are lower this lowers rate of evaporation as there is less energy to change state of water from liquid to gas it also lowers rate of diffusion as particles have less KE

Question 26

why do some xerophytic plants have stomata that close at midday?

Answer 26

stomata are only open when temperatures are cooler e.g. at night so rates of evaporation and diffusion are lower- so less water loss plant is able to store CO2 obtained during the night when stomata are open, this can then be used during the day for photosynthesis

Question 27

why do some xerophytic plants have deep roots?

Answer 27

water can be obtained from the water table (deep underground)

Question 28

what is the purpose of thicker waxy cuticle on xerophytic plants?

Answer 28

greater waterproofing and longer diffusion pathway reducing water loss

Question 28

why might some xerophytic plants have shallow roots with lots of branching close to the surface?

Answer 28

when it rains water can be absorbed quickly before it drains away and the amount that is absorbed is maximised by large surface area

Question 29

what can be used to measure rate of transpiration? how?

Answer 29

a potometer as more water is taken up by the plant (as a result of water being lost from the leaf via transpiration), the air bubble will move along the ruler or scale

Question 30

what measurements will be needed from a potometer to calculated volume of water lost from a leaf during transpiration?

Answer 30

measure distance the bubble has moved over a set period of time need to know diameter of the capillary tube and do Pi x r^2 to work out area of the end of the cylinder multiply area of cylinder by distance the bubble travelled

Question 31

how would you find rate of transpiration from volume of water lost in a potometer?

Answer 31

divide the volume by the period of time

Question 32

what is the role of the reservoir in a potometer?

Answer 32

to add water back into the potometer to push the bubble back to the start to do repeats with the same plant

Question 33

why do we want to do potometer repeats using the same plant?

Answer 33

there is the same SA for the leaves and same number of stomata, so rate of transpiration should be similar

Question 34

if we have to use different plants for repeats in a potometer, what additional measurement would be needed?

Answer 34

total SA of the leaves of the plant so rate can be compared per mm^2