3.3.4.2 Mass Transport in plants (C7)

Question 1

Explain how the active transport of mineral ions into the xylem vessels in the roots results in water entering these vessels and then being moved ho the xylem tissue (hint root pressure)

Answer 1

1. The water potential in the xylem is reduced by the entry of ions
2. So a water potential gradient is established between the xylem and surrounding cells
3. The plasma membranes of the surrounding cells are partially permeable
4. So water enters the xylem via osmosis
5. So the volume of water in the xylem increases
6. And so the pressure in the xylem increases as water cannot move back down due to the concentration gradient

Question 2

The presence of an air bubble in a xylem vessel in the stem blocked the movement of water through that vessel. Use the cohesion tension theory to explain why

Answer 2

1. Water evaporates from the leaves
2. So the water in the xylem is under tension causing the water molecules to stick together
3. So the water is stuck together in a single column, the air bubble breaks the column which prevents cohesion

Question 3

Explain how water enters a plant root from the soil and travels through to the endodermis

Answer 3

1. Water enters root hair cells by osmosis (this is because the active transport of mineral ions has a created a water potential gradient causing water to move into the root hair cell)
2. Then water moves through the cortex by osmosis down a water potential gradient
3. The water diffuses through the symplastic pathway (through the cells cytoplasm) and apoplastic pathway (through the cell walls)

Question 4

Describe one piece of evidence that supports the root pressure theory and explain how it supports the theory

Answer 4

Sap is discharged from a cut stem. Only an upward force could make this happen

Question 5

The diameter of a tree is less during the day than it is at night. Explain how this supports cohesion tension theory

Answer 5

1. Evaporation of water through the leaves mainly during the daytime
2. So the tension in the xylem creates an inward pull
3. So the xylem vessels become narrower due to the adhesion of water molecules to the walls of the xylem vessels

Question 6

The diameter of a tree is less during the day than it is at night. Explain how this does not support root pressure theory

Answer 6

1. Root pressure give out an OUTWARD force pushing on the walls of the xylem
2. So trees would become wider not narrower as the xylem should become wider

Question 7

Using the cohesion tension theory explain the changes in the circumference of a trees throughout the day

Answer 7

1. Increase of light, at the start of the day, causes the stomata to open.
2. So water evaporates out of the stomata causing water to move across the leaf by osmosis.
3. Evaporation exerts force causing tension in water columns
4. Cohesion holds water column together
5. Adhesion between walls of the xylem and the water molecules results in a pulling force which causes the water columns to decrease in volume
6. Hence the xylem vessels and the tree trunk both become narrower

Question 8

Explain how the rate of water movement is affected by an increase humidity of air around the leaves

Answer 8

1. High humidity decreases the water potential gradient so no net movement of water in or out the leaf.
2. This mean that the rate of water movement decreases as less water is lost by transpiration

Question 9

What is translocation?

Answer 9

Translocation is the transport of organic substances in the phloem from source to sink

Question 10

Give 2 features of xerophytic plants

Answer 10

• hairs
• stomata in pits
• thick way cuticle
• rolled leaves
• spines/fewer leaves

Question 11

Why do some leaves have hairs on them?

Answer 11

trap a layer of water vapour so WP gradient reduced

Question 12

Why do some leaves have stomata in pits?

Answer 12

trap a layer of water vapour so WP gradient reduced

Question 13

Why do some leaves have thick waxy cuticle?

Answer 13

increases diffusion distance

reduces evaporation/transpiration

Question 14

Why do some leaves have rolled leaves?

Answer 14

trap a layer of water vapour so WP gradient reduced

Question 15

Why do some plants have spines or fewer leaves?

Answer 15

reduce water loss by evaporation

Question 16

Why might the rate of water uptake by a plant NOT be the same as the rate of transpiration?

Answer 16

1. Water used for support/turgidity;
2. Water used in photosynthesis;
3. Water used in hydrolysis;
4. Water produced during respiration;

Question 17

Mass movement of sugars around the plant is known as..

Answer 17

translocation

Question 18

How are sugars transported in plant stems - describe the process.

Answer 18

1. (At source) sucrose is actively (transported) into the phloem/sieve element/tube;
2. By companion/transfer cells;
3. Lowers water potential in phloem/sieve element/tube and water enters by osmosis;
4. (Produces) high (hydrostatic) pressure;
5. Mass flow/transport towards sink/roots/storage tissue;
6. At sink/roots sugars are removed/unloaded;

Question 19

Describe cohesion-tension theory in the xylem

Answer 19

1. water lost due to transpiration from leaves
2. lowers WP in mesophyll cells
3. causes water to be drawn up the xylem (tension)
4. water molecules stick together due the the H bonds
5. creating a continuous column of water
6. water molecules also adhere to xylem wall

Question 20

Describe the mass flow hypothesis

Answer 20

1. Active transport is used to load solutes (e.g. sucrose) from companion cells into sieve tubes of the phloem at the source (e.g. leaves)
2. This lowers the water potential inside the sieve tubes so water enters by osmosis from xylem and companion cells
3. This creates high pressure inside the sieve tubes at the source end of the phloem
4. At the sink end, solutes are removed
5. This increases water potential so water leaves the tubes by osmosis
6. This lowers the pressure inside the sieve tubes
7. This results in a pressure gradient from the source end to the sink end
8. This gradient pushes solutes along the sieve tubes towards the sink
9. When they reach the sink, solutes will be used or stored

• The higher the concentration of sucrose at the source, the higher the rate of translocation

Question 21

Name the 5 pieces of evidance for the mass flow hypothesis

Answer 21

1. If a ring of bark (this includes phloem but not xylem) is removed from the woody stem, a bulge forms above the ring. The fluid from the bulge has a higher concentration of sugars than the fluid from the ring – evidence of downward flow of sugars
2. Radioactive tracer experiments (14C) that are used to track movement of organic substances
3. Using aphids – they pierce the phloem and if they are removed only with their mouthparts behind, the sap flows out quicker nearer the leaves further down the stem – evidence of a pressure gradient
4. Companion cells contain mitochondria to produce ATP to support translocation – if an inhibitor which stops ATP production is put into the phloem then translocation stops
5. Flow of sugars in the phloem is many times faster than simple diffusion – suggesting active transport

Question 22

Name the 3 pieces of evidance against the mass flow hypothesis

Answer 22

1. Solutes (e.g. sucrose) travel to many different sinks at the same time, not just the one with the lowest sucrose concentration as the model would suggest
2. The sieve plates would act as a barrier to mass flow – a lot of pressure would be needed for the solutes to pass through at a reasonable rate
3. Solutes don’t move through the plant at the same speed which they would do if they moved via mass flow

Question 23

Describe the ringing experiment

Experimental example of translocation

Answer 23

1. Removal of tissue: remove the outer tissue of the woody stem, consisting of the protecting layer and the phloem tissue around the complete circumference
2. Observe: watch the upper half of the stem, above when the layers are removed, to see swelling (over a week timeframe)
3. Fluid levels: by testing the fluid it can be found that they are high in sugars and other dissolved substances
4. Results: by viewing the whole plant, it can be seen that the area towards the roots which can’t perform photosynthesis, begins to wither and die, whilst the rest of the plant can continue to grow

These observations can support the conclusion that sugars in the phloem above the ring are responsible for the swelling and the interruption in the flow of these sugars below the ring causes the death of these tissues

Question 24

Describe the radioactive tracer experiment

Experimental example of translocation

Answer 24

1. Supply of plants with radioactive materials: e.g. the leaf can be supplied with organic substance that has a radioactive label. Like carbon dioxide containing 14C which is a radioactive isotope. This can be done to a single leaf by pumping into a container which completely surrounds the leaf.
2. Incorporation of radioactive CO2: The radioactive carbon will be incorporated into organic substances (e.g. sugars produced during photosynthesis) which will be moved around the plant by translocation.
3. Tracking of substances using autoradiography: A technique to reveal where the radioactive tracer has spread to in a plant. The plant is killing (e.g. rapid freezing) and the whole plant (or sections of it) is placed on photographic film – the radioactive substance is present wherever the film turns black.
4. Observation of results: The results show the translocation of substances from source to sink over time – e.g. autoradiographs of plants killed at different times show an overall movement of solutes from the leaves towards the roots.