Mass transport in plants

Question 1

What is the structure of the xylem

Answer 1

Xylem vessels have long hollow tubes with no cell contents.
They are dead
The cell walls are strengthened with lignin
There are no walls where the cells join together

Question 2

How is the xylem adapted for its function

Answer 2

Thick lignin walls make it waterproof and very strong so withstands tension
As it is dead material there is no water potential gradient caused by a living cell
It has no cross walls or cell contents so no resistance to flow for a continuous column of water
Pits are holes that allow water to flow sideways into other vessels for water transfer around the whole plant

Question 3

What makes water go up a plant

Answer 3

Water travels up the stem in a plant in the xylem vessels
It travels via a method called Cohesion Tension

Question 4

What is transpiration

Answer 4

The evaporation of water from the leaf through the stomata
It drives water transport up the xylem

Question 5

What is a differentiated cell

Answer 5

They have specific adaptations to help carry out their function

Question 6

What happens in the cohesion tension theory

Answer 6

1)Water evaporates from the mesophyll cells and diffuses out of the open stomata down a water potential gradient which is known as tranpiration
2) This lowers the water potential of the mesophyll cells

Question 7

What happens in the cohesion tension theory
(2)

Answer 7

3)So water is drawn out from the xylem which has a higher water potential into the cells via osmosis
4) Water is therefore pulled up the xylem creating a negative pressure called tension

Question 8

What happens in the cohesion tension theory
(3)

Answer 8

5)Due to the hydrogen bonds between water molecules called cohesion
6) Forming a continuous column of water
7) Water molecules are also attracted to the walls of the xylem ADHESION

Question 9

What is a factor that proves cohesion tension

Answer 9

On a hot day during rapid transpiration the diameter of a tree trunk will reduce slightly due to the adhesion with a walls of the xylem and negative pressure making the xylem vessels slightly narrower

Question 10

What happens in trasnpiration

Answer 10

Water evaporates from the moist cell walls and accumulates in the spaces between cells in the leaf
When the stomata opens it moves out of the leaf down the concentration gradient

Question 11

If a xylem vessel is broken and air enters it transpiration can no longer happen. Why?

Answer 11

Because the continuous column of water has been broken

Question 12

When a xylem vessel is broken water does not leak out which would happen if the xylem was under pressure. Instead air is drawn in. This is due to which process?

Answer 12

Due to tension

Question 13

Explain why transpiration is important (3)

Answer 13

Provides the leaves with water for photosynthesis
Provides water to make cells turgid
Provides water for hydrolysis reactions for cells
Mass transport of water through the plant transports ions and growth factors for cells

Question 14

Explain why the rate of water flow will peak at mid day

Answer 14

In the middle of the day it is warmer/ lighter
The stomata opens
Transpiration is highest that day
So cohesion creates a continuous column of water

Question 15

Why does pressure decrease as the rate of water flow increases

Answer 15

Increased tension due to evaporation of water at the leaves increasing the flow of water up the xylem

Question 16

Describe the ways in which water could be used in plants

Answer 16

Used in photosynthesis
Produced in respiration
Used in hydrolysis
Used for support or turgidity
As a solvent for transport
As a solvent for faster chemical reactions

Question 17

Describe how ions are actively transported from the roots to the xylem

Answer 17

Active transport uses carrier proteins and energy from the hydrolysis of ATP
Made in aerobic respiration from the mitochondria
To move salts from the roots to the xylem
So water moves by osmosis into the xylem
Down a water potential gradient

Question 18

What are the 4 main factors that affect transpiration

Answer 18

Light intensity
Temperature
Humidity
Air movement

Question 19

How dos water evaporating out of the stomata affect the rate of transpiration

Answer 19

As water evaporates out of the stomata it accumulates as vapor around the stomata outside the leaf
So the water potential around the stomata is increased
This reduces the water potential gradient betweeen the inside and outside of the leaf
So the transpiration rate is reduced

Question 20

Explain why the faster the air movement around the stomata the higher the rate of transpiration

Answer 20

The movement of air around the leaf decreases the water potential
So the water potential gradient increases
This increases the rate of transpiration
The faster the air movement the greater the rate of transpiration

Question 21

Explain how a rise in temperature increases the rate of transpiration

Answer 21

A rise in temperature increases the kinetic energy the molecules have
which increases the rate of evaporation of water molecules
also decreases the humidity increasing water potential gradient
Therefore it increases the rate of evaporation

Question 22

What is humidity a measure of

Answer 22

The number of water molcules in the air

Question 23

How does humidity affect the rate of transpiration

Answer 23

An increase in humidity decreases the rate of transpiration
A greater humidity reduces the water potential gradient between the inside and outside of the leaf at the stomata due to the larger number of water vapour molecules in the air
So the rate of diffusion of water vapour out of the leaf is reduced

Question 24

When does the stomata usually open and close

Answer 24

Open in the light and close in the dark

Question 25

How does light intensity affect the rate of transpiration

Answer 25

An increase in light intensity increases the rate of transpiration
More stomata open in the light to allow more carbon dioxide to enter for photosynthesis
So more water evaporates out of the stomata causing a higher rate of transpiration

Question 26

How does a potometre work

Answer 26

1) A leafy shoot is cut underwater instead of in air to prevent air from entering the xylem which would break the continuous water column. The shoot is place in a rubber tube
2) The potometer is filled completely with water making sure there are no air bubbles
3) The potometer is removed from under the water and all joints are sealed with waterproof jelly to prevent water leaking out which would produce an inaccurate results

Question 27

How does a potometre work (2)

Answer 27

4) An air bubble is introduced into the capillary tube
5) As transpiration occurs water moves through the capillary tube and into the plant and the bubble of air moves with it
6) The distance moved over a period of time is recorded and the mean is calculated of a number of repeats
7)The volume of water lost over a period of time can be calculated by knowing the radius of the tube and thew distance the bubble has moved in mm

Question 28

What is the volume of a cylinder

Answer 28

pi r2 l

Question 29

What assumption must be made if you are using a potometer to measure the rate of transpiration

Answer 29

It is assumed the volume of water taken up by the plant equals the volume of water lost by transpiration

Question 30

Why can this not be assumed (card 29)

Answer 30

Some water entering the plant is used in photosynthesis or used for turgor pressure and some is produced in respiration

Question 31

Why might the results of a laboratory potometer not be representative of the same plant in the wild

Answer 31

The isolated shoot is much smaller/ may be damaged when cut
Conditions in the lab may be different than in the wild like with temperature light intensity of air flow

Question 32

How do plants reduce water loss in dry environments

Answer 32

Reduced number of stomata
Stomata in pits to trap water vapour and increase humidity
Hairs to trap water vapour and increase humidity
Leaves reduced to spines to reduce SA:V ratio
Thick waxy cuticle to reduce evaporation

Question 33

How can you make the potometer experiment more reliable

Answer 33

Seal joints
Dry off leaves
Cut shoot under water
Shut tap
Ensure no air bubbles are present

Question 34

What measurements do you need to make to measure the rate of water uptake of the shoot

Answer 34

The radius/diameter of capillary tube
Distance and time

Question 35

What are solutes

Answer 35

Dissolved substances

Question 36

What are solutes sometimes called

Answer 36

Assimilates

Question 37

What 2 types of cells are phloems formed from

Answer 37

Sieve tube elements and companion cells

Question 38

Explain how phloem tissue is adapted for its function (3 marks)

Answer 38

Sieve tubes have no nucleus little cytoplasm and few organelles so it allows unobstructed flow of solutes
End walls of sieve tubes perforated with holes so it allows the continuous flow of substances through sieve tubes
Companion cells contain many organelles like mitochondria so it carries out functions for the sieve tubes like mitochondria making ATP for active transport

Question 39

What is translocation

Answer 39

The movement of solutes to where they are needed in a plant

Question 40

What is the source

Answer 40

Where sugars are made and stored and are therefore in high concentration

Question 41

What is the sink

Answer 41

Where the sugars are used in respiration or converted to storage so are therefore in low concentration
There is therefore a concentration gradient from source to sink

Question 42

In the mass flow hypothesis what happens at the source

Answer 42

Active transport is used to load solutes from companion cells in to the sieve tubes of the phloem at the source
This lowers the water potential in sieve tubes
Water enters the sieve tubes by osmosis from the xylem
This creates a high hydrostatic pressure inside the sieve tubes at the source

Question 43

In the mass flow hypothesis what happens at the sink

Answer 43

At the sink end sucrose is removed from the phloem to be used in respiration in respiring cells or stored as starch in storage organs
This increases the water potential inside the sieve tubes so water leaves the tubes by osmosis
This lowers the hydrostatic pressure inside the sieve tubes

Question 44

What does mass flow result in

Answer 44

The result is a pressure gradient from the source end to sink end
The gradient from the site of photosynthesis pushes solutes along the sieve tubes towards the respiring cells or storage organs by mass flow
The higher concentration of sucrose at the source the higher the rate of translocation

Question 45

What is the supporting evidence for the mass flow hypothesis

Answer 45

If a ring of bark which includes the phloem but not the xylem is removed from a woody stem a bulge forms above the ring
If a metabolic inhibitor is put into the phloem then translocation stops which is evidence that active transport is involved
A radioactive tracer such as radioactive CO2 can be used to track the movement of organic substances in a plant
The radioactive carbon is incorporated into sucrose at the source when the cells photosynthesis and it can be traced proving translocation can occur both up and down the phloem

Question 46

How can aphids prove mass flow hypothesis

Answer 46

They pierce the phloem then their bodies are removed leaving mouth parts behind which allows sap to flow out. The sap flows out quicker nearer the leaves than further down the stem which is evidence that there is a pressure gradient

Question 47

What is evidence against the mass flow hypothesis

Answer 47

Sugars move to all areas of the plant not just from source to sink
The sieve plates would create a barrier to mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate