Transpiration

Question 1

What is transpiration?

Answer 1

The loss of water from the leaves of a plant by evaporation

Question 2

What is a transpiration stream?

Answer 2

The constant movement of water from the roots to the leaves driven by evaporation from the stomata

Question 3

What leaf feature limits the water loss from the upper leaf surface?

Answer 3

Waxy cuticle

Question 4

Why is it that majority of water vapour is lost during the day?

Answer 4

Because photosynthesis only takes place in light

Question 5

Are transpiration streams active or passive and why?

Answer 5

Passive as the xylem are non-living

Question 6

Describe how transpiration takes place in leaves

Answer 6

Water molecules move across leaves by the apoplectic pathway by mass flow. The water molecules evaporate from the surface of the mesophyll cells in the air into the spaces in the leaf and move out the stomata into the surrounding air down a concentration gradient

Question 7

Describe how other adjacent mesophyll cells respond to another mesophyll cell loosing water to transpiration

Answer 7

The loss of water by evaporation from a mesophyll cell lowers the water potential of the cell so water moves into the cell from an adjacent cell by osmosis. This is repeated across the leaf to the xylem. Water moves out of the xylem by osmosis into the cells of the leaf.

Question 8

What is capillary action?

Answer 8

The combined effect of cohesion and adhesion allowing water to rise up a narrow tube against gravity

Question 9

Describe cohesion in the xylem

Answer 9

Water molecules for hydrogen bonds with each other and tend to stick together to form a stable structure

Question 10

Describe adhesion in the xylem

Answer 10

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

Question 11

What is transpiration pull?

Answer 11

Water is drawn up the xylem by mass flow in a continuous stream to replace the water lost by evaporation

Question 12

What feature of the xylem withstands the tension created by transpiration pull

Answer 12

Lignin

Question 13

Give two pieces of evidence for cohesion tension theory

Answer 13

Changes in tree diameter, cut flowers

Question 14

How does changing tree diameter prove the cohesion tension theory

Answer 14

When transpiration is at its height during the day, tension in the xylem is at its highest and as a result, the tree shrinks in diameter. In the night during a low transpiration rate, the diameter will increase

Question 15

How can cut flowers prove the cohesion tension theory

Answer 15

Cut flowers tend to draw in air rather than leaking water out because water continues up the stem. These cut xylems stop drawing up water as the air drawn in breaks the transpiration stream - air interferes between the cohesion between water molecules

Question 16

Give four examples of how transpiration is useful to plant function

Answer 16

It transports useful mineral ions up the plant, maintains cell turgidity, supplies water for growth, cell elongation and photosynthesis, supplies water that evaporates and keeps the plant cool

Question 17

How does the stomata open in favourable environments?

Answer 17

The guard cells pump solutes by active transport which reduces the water potential of guard cells. This causes more water to move in by osmosis and increases turgor

Question 18

What is the role of cellulose hoops on guard cells?

Answer 18

They prevent the guard cells from swelling in width so that they extend lengthways

Question 19

Why is it that guard cells become bean shapes when they open?

Answer 19

Because the inner wall of the guard cell is less flexible than the outer wall

Question 20

List 4 ways in which plants control water loss:

Answer 20

waxy cuticles, wilting, stomata, deciduous plants losing leaves in the winter

Question 21

How does wilting control water loss in plants?

Answer 21

wilting is a protection mechanism against further water loss- when leaves hang down and collapse, there is a smaller surface area for evaporation

Question 22

How does losing leaves in the winter control water loss in deciduous plants?

Answer 22

the ground may be frozen and temperatures are too low for photosynthesis so water should be preserved

Question 23

Give the five main factors that affect the rate transpiration?

Answer 23

light intensity, temperature, relative humidity, air movement, soil water availability

Question 24

How does light intensity affect the rate of transpiration?

Answer 24

an increased light intensity increases the rate of transpiration

Question 25

Why does an increased light intensity increase the rate of transpiration?

Answer 25

light is a requirement of photosynthesis and so in light conditions the stomata open for the gas exchanges needed for photosynthesis. In the dark, most stomata close.

Question 26

How does temperature affect the rate of transpiration?

Answer 26

an increased temperature increases the rate of transpiration

Question 27

Why does an increased temperature increase the rate of transpiration?

Answer 27

an increased temperature increases the kinetic energy of the water molecules and therefore increases the rate of evaporation out of stomata and also increases the concentration of water vapour that the external air can hold before it becomes saturated

Question 28

Why does an increased movement of air increase the rate of transpiration?

Answer 28

air moving outside the leaf will carry water vapour that has just diffused out of the leaf which maintains a higher water potential gradient

Question 29

Why does insufficient soil water reduce transpiration?

Answer 29

the plant is put under water stress and cannot replace the water it has lost so the stomata close and the leaves wilt