Transports in Plants - Xylem

Question 1

what is transpiration

Answer 1

the loss of water vapour from the stomata by evaporation

Question 2

what are the 4 factors that affect the rate of transpiration

Answer 2

light intensity
temperature
humidity
wind

Question 3

what effect does light intensity have on the rate of transpiration and why

Answer 3

an increase in light intensity will increase the rate of transpiration
- this is because the guard cells will swell which means MORE stomata are open so there’s a larger surface area for evaporation

Question 4

what effect does temperate have on the rate of transpiration and why

Answer 4

an increase in temperature will increase the rate of transpiration
- this is because more heat means more kinetic energy so the molecules are moving faster and therefore evaporating faster and therefore there is more evaporation

Question 5

what effect does humidity have of the rate of transpiration and why

Answer 5

an increase in humidity decreases the rate of transpiration
- this is because there’s more water vapour surrounding the outside of the plant meaning that there’s a reduced water potential gradient between the inside and outside of the cell. Water will not be able to evaporate out

Question 6

what effect does wind have on the rate of transpiration and why

Answer 6

an increase in wind will increase the rate of transpiration
- this is because the water molecules around the leaf are blown away, so the concentration gradient is maintained, allowing more evaporation of water from the inside to outside

Question 7

what are the key points of the cohesion tension theory

Answer 7

cohesion
capillarity (adhesion)
root pressure

Question 8

explain the cohesion in water molecules

Answer 8

water is dipolar
hydrogen bonds are able to form between the oxygen and hydrogen of different water molecules
this allows cohesion of the water molecules allowing a continuous stream through the xylem

Question 9

what are the charges on the hydrogen and oxygen

Answer 9

hydrogen = slightly positive
oxygen = slightly negative

Question 10

what is capillarity (adhesion)

Answer 10

the water is able to adhere to the xylem walls

Question 11

if the xylem is narrower, the impact of capillarity is …..

Answer 11

bigger

Question 12

what is root pressure

Answer 12

as water moves into the root by osmosis, the volume of liquid inside the root is increased and therefore the pressure inside of the root increases

Question 13

how does root pressure affect transpiration

Answer 13

the increases in pressure in roots forces the water above it, upwards

Question 14

describe the movement of water up the xylem

Answer 14

• water evaporates out of the stomata > loss of water volume creates a lower pressure
• due to negative pressure, water is pulled up the xylem to replace the water that was evaporated
• the water is pulled due to the cohesion property (hydrogen bonds between the oxygen and hydrogen) and the adhesion property (water sticks to the xylem wall which helps them to not fall)
• the pulling of the water column creates tension which pulls the xylem walls in to become narrower