Transpiration and Translocation

Question 1

What is the humidity of the atmosphere usually like?

Answer 1

Less than that of the air spaces next to the stomata

Question 2

What is created given that the humidity of the atmosphere is less than that by the stomata?

Answer 2

A water potential gradient from air spaces though stomata to the air

Question 3

What happens due to the water potential gradient between stomata and air?

Answer 3

Provided stomata are open, water vapour molecules diffuse out of air spaces into air

Question 4

How is water lost by diffusion from air spaces replaced?

Answer 4

Evaporating from the cell walls of the surrounding mesophyll cells

Question 5

How do plants control their rate of transpiration?

Answer 5

By changing the size of the stomatal pores

Question 6

How is water lost from mesophyll cells?

Answer 6

Evaporation from their cells walls to the air spaces in the leaf

Question 7

How is water lost from mesophyll cells replaced?

Answer 7

Water reaching the mesophyll cells from the xylem via cells walls or the cytoplasm

Question 8

How does the movement occur via the cytoplasmic route?

Answer 8

• Mesophyll cells lose water to air spaces by evaporation
• These cells now have a lower water potential
• Water enters by osmosis from neighbouring cells
• Water potential lowers in these cells
• So take water from their neighbours via osmosis

Question 9

What is established due to the movement of water via the cytoplasmic route to replace the water in the mesophyll cells?

Answer 9

A water potential gradient that pulls water from the xylem, across the leaf mesophyll, and into the atmosphere

Question 10

What is the main factor that is responsible for the movement of water up the xylem?

Answer 10

Cohesion - tension

Question 11

How do water molecules have cohesion?

Answer 11

They from hydrogen bonds so tend to stick together

Question 12

What does water form across the mesophyll cells and down the xylem?

Answer 12

A continuous, unbroken column

Question 13

What happens as water evaporates from the mesophyll cells in terms of the column of water?

Answer 13

More molecules of water are drawn up behind as a result of cohesion

Question 14

What is pulled up the xylem as a result of transpiration?

Answer 14

A column of water - called the transpiration pull

Question 15

What does the transpiration pull put the xylem under?

Answer 15

Tension - there is a negative pressure within the xylem
(hence the name cohesion-tension)

Question 16

How does the diameter of a tree trunk change between day and night?

Answer 16

• In the day, transpiration at its greatest, so more tension in xylem, so pulls walls inwards and causes trunk to shrink in diameter
• At night, transpiration at lowest, so less tension and diameter of trunk increases

Question 17

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

Answer 17

Tree can no longer draw up water as continuous column is broken and water molecules can’t stick together

Question 18

What proves that the xylem vessel is under tension?

Answer 18

When it is broken, air does not leak out but is drawn in

Question 19

Does the transpiration pull require energy?

Answer 19

No - it is passive
Energy is still needed to drive the process - comes in the form of heat that evaporates water from leaves which comes from sun

Question 20

Are the xylem vessels alive or dead?

Answer 20

Dead

Question 21

How do xylem vessels form a series of continuous, unbroken tubes from roots to leaves?

Answer 21

Vessels have no end walls - essential to cohesion-tension theory

Question 22

What is used to strengthen the xylem vessels?

Answer 22

Lignin - often forms rings or spirals around the vessel

Question 23

What do we use for the measurement of water uptake?

Answer 23

A potometer

Question 24

Where do we measure water uptake from?

Answer 24

The shoot

Question 25

What different conditions can we measure water uptake under?

Answer 25

Various humidities, wind speeds or temperatures

Question 26

How is the leafy shoot cut in the potometer experiment?

Answer 26

Cut under water - care is taken to not get water on leaves

Question 27

What is the potometer filled with at the beginning of the experiment and what are we careful about?

Answer 27

Filled with water, ensuring no air bubbles

Question 28

How is the leafy shoot fit into the potometer?

Answer 28

Using a rubber tube, and fitted under water

Question 29

What happens after the leafy shoot is fitted into the potometer?

Answer 29

Potometer is removed from under the water and all joints are sealed with waterproof jelly

Question 30

How do we start the potometer experiment after the apparatus has been set up?

Answer 30

An air bubbles is introduced into the capillary tube

Question 31

What do we measure in the potometer experiment?

Answer 31

The distance moved by the air bubble in a given time
Measured a number of times to calculate a mean

Question 32

What does the mean value give us in the potometer experiment?

Answer 32

Volume of water lost, which is plotted against time on a graph

Question 33

What happens as the air bubble nears the junction between the reservoir tube and the capillary tube in the potometer experiment?

Answer 33

Tap on reservoir opened and syringe pushed down until bubble is pushed back to start of scale on capillary tubes