6.2- Transpiration

Question 1

Define transpiration

Answer 1

loss of water from leaves and stems of plants

consequence of gas exchange in the leaf

Question 2

Why do leaves have mechanisms to open and close the openings (guard cells)

Answer 2

because they cannot filter what gases pass through the stomata

Question 3

PROCESS of transpiration

Answer 3

Transpiration begins with light energy converting water into water vapour in the humid spongy mesophyll of the leaf. This water vapour then dissolves and evaporates out of the leaves. This creates a negative pressure gradient causing water to be pulled up through the xylem vessels and through the cell walls via capillary actions. This generates tension which then drags water up the xylem in opposition to gravity. This is known as the cohesion-tension theory. Adhesive property of water and evaporation generate tension forces in leaf cell walls. Cohesive property of water and structure of xylem vessels allow transport under this tension.

Question 4

What is abscisic acid

Answer 4

produced within the plant to force the closure of the stomata to prevent dehydration

Question 5

Why do xylem vessels have no cell content and absent/incomplete end walls

Answer 5

to allow water to flow through unimpeded

Question 6

Why do xylems have lignified walls

Answer 6

to withstand internal negative pressures (tensions) without collasping

Question 7

Why do xylem vessels have pits

Answer 7

to allow water to move sideways in and out of cells

Question 8

Describe xylem vessel structure

Answer 8

non-living continuous and hollow tubes that lack cell contents and end walls.
As they develop, cell walls and contents degrade allowing for unimpeded flow

cell walls are thickened with cellulose and strengthened with lignin

Question 9

What factors can transpiration rate be affected by?

Answer 9

Light
Temperature
Wind Speed
Humidity

Question 10

How does light affect transpiration rates

Answer 10

increases as guard cells open the stomata due to increased rates of photosynthesis

Question 11

How does temperature affect ROT

Answer 11

increases the molecular movement which includes evaporation of water

Question 12

How does wind speed effect ROT

Answer 12

increases, moves water vapour away from opening of stomata which increases the pressure gradient between in/out of leaf

Question 13

How does humidity affect ROT

Answer 13

decreases, air outside stomata opening is saturated with air which decreases the pressure gradient

Question 14

What can be ROT be measured

Answer 14

bubble and mass potometers

Question 15

How does a mass potometer work

Answer 15

measures the change in mass of a plant as the measure of water evaporated from the plant

Question 16

How does the bubble potometer work

Answer 16

measures the uptake of water by a plant as the measure of water loss to evaporation.

Question 17

Units of transpiration

Answer 17

mL/hr