Transpiration

Question 1

How are leaves adapted to reduce water loss?

Answer 1

covered with a waxy cuticle that makes them waterproof

reduces transpiration

guard cells control the diameter of the stomata, they close during times of low water

Question 2

How does water leave the leaf?

Answer 2

it evaporates from the freely permeable cellulose cell walls of the mesophyll cells into the air spaces

moved down a water VAPOUR potential gradient

this lowers the water potential of the mosophyll cells, so water moves into the cell from an adjacent cell

Question 3

What are cohesion and adhesion?

Answer 3

adhesion - water molecules for hydrogen bonds with the carbs in the wall of the xylem vessels

cohesion - water molecules form hydrogen bonds with each other and stick together

Question 4

What are the effects of cohesion and adhesion?

Answer 4

results in capillary action

water can rise up a narrow tube against the force of gravity

water is drawn up the xylem in a continous steam to replace the water lost by evaporation

this is known as transpiration pull

Question 5

What is the evidence for the cohesion-tension theory?

Answer 5

changes in the diameter of trees - when transpiration is at its highest, tension is highest, the tree shrinks in diameter

when a xylem vessel is broken, air is drawn in rather than water leaking out

when air is pulled in, water can no longer move up the stem as a continuous steam

Question 6

How is transpiration rates estimatated?

Answer 6

using a potometer

ensure there is a airtight seal around the stem

water level is zeroed on the scale on the meniscus

Question 7

Why can transpiration rates only being measured using this method?

Answer 7

around 99% of water lost through transpiration, we cannot be certain that it was all transpired

Question 8

The cohesion-tension theory is often used to explain the mechanism by which water moves up the xylem from the roots to leaves

use this to explain how water moves from roots to leaves

Answer 8

evaporation from top of plant

water molecules are cohesive

xylem is under tension

chain of molecules pulled up

Question 9

Use cohesion-tension theory to explain how water moves from roots to the leaves

Answer 9

evaporation at the top of the plant

creates tension in the xylem

water molc are cohesive

chain pulled up by tension