Mass Transport In Plants

Question 1

In plants, where is water absorbed from?

Answer 1

It is absorbed from extensions on the roots called root hair cells.

Question 2

What are xylem vessels?

Answer 2

These are thick hollow tubes that transport water through the plant.

Question 3

What is the force that pulls water through the plant?

Answer 3

This is transpiration.

Question 4

What causes transpiration?

Answer 4

Transpiration is caused by the evaporation of water from the leaves.

Question 5

Is transpiration passive or active?

Answer 5

It is passive as the energy comes from the sun.

Question 6

What has a higher humidity, the atmosphere or air spaces next to the stomata?

Answer 6

The air spaces next to the stomata have a higher humidity.

Question 7

What causes the water potential gradient from the air spaces through the stomata to the air?

Answer 7

The water potential gradient is because of the differences in humidity, water therefore diffuses from the air spaces into the atmosphere.

Question 8

In order for the water vapour molecules to diffuse into the air, what do the stomata need to be?

Answer 8

The stomata need to be open.

Question 9

What replaces water that is lost by diffusion?

Answer 9

Water that is evaporated from the surrounding mesophyll cells replaces the water lost in the air spaces as a result of diffusion into the atmosphere.

Question 10

How do plants control their rate of transpiration?

Answer 10

They control the rate of transpiration by opening and closing their stomata.

Question 11

What replaces the water lost from he mesophyll cells?

Answer 11

The water is replaced by the xylem either by the cell wall or cytoplasmic route.

Question 12

How does the cytoplasmic route move the water?

Answer 12

Mesophyll cells have a lower water potential because of evaporation, therefore the neighbouring cells transfer water by osmosis to the mesophyll cells, this results in the neighbouring cells having a lower water potential so osmosis from other cells occurs. This creates a water potential gradient that pulls water from the xylem.

Question 13

What path does the cytoplasmic route take?

Answer 13

The water goes from the xylem, across the leaf mesophyll and into the atmosphere.

Question 14

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

Answer 14

This is caused by cohesion-tension.

Question 15

What is cohesion?

Answer 15

This is where the hydrogen bonds between the molecules cause the water molecules to stick together.

Question 16

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

Answer 16

A continuous unbroken column.

Question 17

What happens to the air spaces and what is this due to?

Answer 17

More water molecules are drawn up due to cohesion.

Question 18

What is the transpiration pull?

Answer 18

Where a column of water is pulled up the xylem as a result of transpiration.

Question 19

What is the cohesion-tension theory?

Answer 19

This is the idea that transpiration puts the xylem under tension so it has negative pressure allowing the water to move up through the xylem.

Question 20

What evidence supports the cohesion-tension theory?

Answer 20

Trees have different widths depending on the time of day because there is more negative pressure during the day so the trunks are smaller because more transpiration occurs.
Water does not leak out if the xylem breaks, instead air is drawn in.
If air enters the xylem then no water will be dawn up to the mesophyll cells.

Question 21

Are the xylem vessels dead or alive?

Answer 21

They are dead.

Question 22

Do xylem have end walls?

Answer 22

No, there are no end walls so the xylem are a series of connected tubes from the roots to the leaves.

Question 23

Why is it difficult to measure the water loss from a plant?

Answer 23

It is almost impossible to condense all of the water vapour and collect all of the water produced.

Question 24

Why is the leafy shoot cut under water rather than in the air?

Answer 24

As xylem is under tension, cutting the shoot in air would lead to air being drawn into the stem, this would stop the transport of water up the shoot. Cutting underwater means that the continuous column can be maintained.

Question 25

Why are all of the potometer joints sealed with waterproof jelly?

Answer 25

This is done because it stops air being drawn into the xylem which would result in the continuous water column being disrupted.

Question 26

Why might results obtained from a laboratory experiment be different to those from a plant in the wild?

Answer 26

An isolated shoot is much smaller than a whole plant and so it may not be representative of the plant.
Conditions in the lab may be different e.g light intensity, air movement and humidity.

Question 27

What are the features of root hair cells that make them good for absorbing water and mineral ions?

Answer 27

Thin cell wall
Large surface area
Long, hair like extension

Question 28

What are the xylem vessels made up of?

Answer 28

As cells mature, their walls incorporate lignin and the cells die. The lignin forms rings or spirals around the vessel.

Question 29

What are two features of the xylem vessel that are needed for their function?

Answer 29

They are hollow and elongated.

Question 30

Why is it an advantage that the xylem vessels are dead?

Answer 30

The components of living cells would slow water movement.

Question 31

Give two features of lignin that make it suitable to its function.

Answer 31

Waterproof

Strong

Question 32

Why is it better for the xylem to have a spiral thickening of the cell wall?

Answer 32

Allows the vessel to elongate as the plant grows,
Less wasteful as less material is used,
Plant will have a lower mass,
Allows stems to be flexible.

Question 33

What is translocation?

Answer 33

This is where organic molecules and some mineral ions are transported from one part of a plant to another.

Question 34

What transports biological molecules in flowering plants?

Answer 34

Phloem

Question 35

What is phloem made up of?

Answer 35

It is made of sieve tube elements which are long thin structures that are arranged end to end. Their end walls are perforated to give sieve plates.

Question 36

What type of cells are associated with sieve tube elements?

Answer 36

Companion cells.

Question 37

What are sources?

Answer 37

These are the site of molecule production.

Question 38

What are sinks?

Answer 38

These are places where molecules are either stored or used directly.

Question 39

Where does phloem transport sugars?

Answer 39

It transports sugars from sources to sinks.

Question 40

In which direction does phloem transport biomolecules?

Answer 40

In both directions as the sinks can be both below and above the source in the plant.

Question 41

What does phloem transport?

Answer 41

Organic molecules: sucrose, amino acids;

Inorganic ions: potassium, chloride, phosphate and magnesium ions.

Question 42

Why is it that the molecules are not transported by diffusion?

Answer 42

The molecules move too fast.

Question 43

What are the three phases of the mass flow theory?

Answer 43

1. Transfer of sucrose into sieve elements from photosynthesising tissue.
2. Mass flow of sucrose through sieve tube elements.
3. Transfer of sucrose from the sieve tube elements into storage or other sink cells.

Question 44

Explain phase 1 of the mass flow theory.

Answer 44

Sucrose is manufactured as result of photosynthesis and diffuses down the concentration gradient by facilitated diffusion from the photosynthesising cells to the companion cells.
Hydrogen ions are then actively transported from the companion cells to the spaces within cell walls.
The hydrogen ions then diffuse down a concentration gradient through carrier proteins into the sieve tube elements.
Sucrose molecules are then transported along with the hydrogen ions in a process known as co-transport by co-transport proteins.

Question 45

Explain phase 2 of the mass flow theory.

Answer 45

The sucrose in the sieve tubes creates a more negative water potential, as the xylem have a higher water potential, water moves from the xylem into the sieve tubes by osmosis, this creates a high hydrostatic pressure within them.
At sinks, sucrose is either used up during respiration or converted into starch for storage.
These cells therefore have a low sucrose content so sucrose has to be actively transported into them from the sieve tubes, giving the sieve tubes a low water potential.
As a result of the lowered water potential in the respiring cells, water moves into them by osmosis from the sieve tubes. The hydrostatic pressure of the sieve tubes is therefore lowered.
There is a high hydrostatic pressure at the source and a low hydrostatic pressure at the sink.
This creates a mass flow of sucrose solution down this hydrostatic gradient in the sieve tubes.

Question 46

What is phase 3?

Answer 46

This is where the sucrose is actively transported by companion cells, out of the sieve tubes and into the sink cells.

Question 47

What evidence supports the mass flow hypothesis?

Answer 47

When sieve tubes are cut, sap is released which shows that there is a pressure.
Sucrose concentration is higher in sources than in sinks.
Downward flow in phloem only occurs during the day.
Increased sucrose levels in the leaf are followed by increased sucrose levels in phloem after.
Metabolic poisons and a lack of oxygen can inhibit translocation of sucrose.
Companion cells produce ATP.

Question 48

What evidence is there that questions the mass flow hypothesis?

Answer 48

The function of sieve plates is unclear because in theory they hinder mass flow.
Not all solutes move at the same speed but they should in the case of mass flow.
Sucrose is delivered at the same rate to all regions, regardless of the sucrose concentration in these areas.

Question 49

What is on the inside of the protective bark layer?

Answer 49

A layer of phloem which extends all round the stem, inside the phloem layer is the xylem.

Question 50

What is done at the start of a ringing experiment?

Answer 50

The outer layers (protective layers and phloem) are removed around the complete circumference of the woody stem while it is still attached to the rest of the plant.

Question 51

In a ringing experiment, what happens after a period of time?

Answer 51

The stem region immediately above the missing ring of tissue starts to swell.

Question 52

What are the samples of the ringing experiment found to contain?

Answer 52

They are rich in sugars and other dissolved organic substances.

Question 53

What happens to some of the non-photosynthetic tissues in the region below the ring?

Answer 53

They wither and die whilst those above the ring continue to grow.

Question 54

What does removing the phloem of the stem lead to?

Answer 54

The phloem of the stem has lead to an accumulation of sugars above the ring and an interruption of the flow of sugars in the region below the ring.

Question 55

What does an interruption in the flow of sugars below the ring lead to?

Answer 55

This leads to the death of tissues in this region.

Question 56

What does the accumulation of sugars above the ring lead to?

Answer 56

This causes a swell in the tissues above the ring.

Question 57

What conclusion can be drawn from ringing experiments?

Answer 57

Phloem is responsible for the translocation of sugars in plants, not xylem.

Question 58

Why can this conclusion be drawn?

Answer 58

This is because the xylem had not been removed so the continuity hadn’t been broken. If xylem was responsible for the translocation of sugars, there wouldn’t have been an accumulation above the ring.

Question 59

Is mass flow a passive process?

Answer 59

Mass flow is a passive process however, some aspects of it are active, this means that the overall process is active and why it can be affected by factors such as temperature.

Question 60

What are used in tracer experiments?

Answer 60

Radioactive isomers are used.

Question 61

Why are radioactive isotopes used?

Answer 61

This is because they can trace the movement of substances in plants by producing radioactive substances such as 14CO2 which will become incorporated into the sugars produced by photosynthesis.

Question 62

What happens to the radioactive sugars?

Answer 62

The radioactive sugars can be traced as they move through the plant using autoradiography.

Question 63

How does autoradiography work?

Answer 63

A cross section of the plant stem is taken and then placed into an X Ray film. The film becomes blackened where it has been exposed to radioactive substances.

Question 64

What do the blackened regions correspond to?

Answer 64

They correspond to where the phloem tissue is in the stem. As the other tissues do not blacken the film it means that the radioactive sugars are not carried by any other tissues and that it is solely phloem that is responsible for translocation.

Question 65

In order to incorporate radioactive isotopes, what conditions should the plant be grown in?

Answer 65

They should be grown in an atmosphere containing 14CO2.

Question 66

What are the four pieces of evidence that the translocation of organic molecules occurs in the phloem?

Answer 66

When phloem cut, a solution of organic molecules flow out.
Plants provided with radioactive carbon dioxide are shown to have radioactively labelled carbon in the phloem after a short amount of time.
Aphids have needle like mouthparts that allow them to reach the phloem and extract the contents. These contents show daily variations in the leaves and a little later on, identical changes in the sucrose content in the phloem.
The ringing experiment.