Module 3 - Mass transport in plants

Question 1

What is the role of the xylem tissue?

Answer 1

Transports water and mineral ions in solution

Question 2

What is a xylem vessel?

Answer 2

They are the long tubes made of lignin that transport the water and minerals. They are made from dead cells called vessel elements.
There are no end walls.

Question 3

Explain the cohesion-tension theory of water?

Answer 3

1. Water evaporates from leaves via transpiration.
2. This creates tension in the xylem vessels which pulls water up the leaf
3. Water molecules form H bonds through cohesion which carries more water up the xylem vessel.
4. The water potential lower down the stem causes water to move up from the roots.

Question 4

Describe the cohesion-tension theory of water transport in the xylem (5)

Answer 4

1. Water lost from leaf because of transpiration
2. Lowers water potential leaf cells
3. Water pulled up xylem (creating tension)
4. Water molecules cohere / ‘stick’ together by hydrogen bonds;
5. (forming continuous) water column;
6. Adhesion of water (molecules) to walls of xylem;

Question 5

What is transpiration?

Answer 5

The evaporation of water from a plants surface.
Water evaporates from the moist cell walls and accumulates in spaces around cells. Stomata opening causes water to leave the leaf, down the concentration gradient

Question 6

What are the 4 main factors that affect transpiration rate?

Answer 6

Light, temperature, humidity and wind

Question 7

Explain how light affects transpiration rate

Answer 7

When there is light, stomata open to let CO2 in for photosynthesis so, water also leaves the leaf. When it is dark, the stomata close

Question 8

Explain how temperature affects transpiration rate

Answer 8

The higher the temperature, water molecules have more energy so they evaporate from the cells faster.

Question 9

Explain how humidity affects transpiration rate

Answer 9

If the air is dry, the concentration gradient between the leaf and air is increased, increasing transpiration rate so, a lower humidity is required.

Question 10

Explain how wind affects transpiration rate.

Answer 10

Lots of air movement blows away water molecules from around the stomata which increases concentration gradient.

Question 11

What is a potometer and what does it measure?

Answer 11

An apparatus to measure water uptake by a plant which is directedly proportional to transpiration rates.

Question 12

Describe how a potometer can be used to estimate transpiration rates.

Answer 12

1. Cut a shoot underwater to prevent any air from entering the xylem - cut at a slant to increase SA.
2. Remove the end of the capillary tube from the beaker of water and wait for an air bubble to form
3. Then, place the capillary tube back into the water
4. Mark the starting position of the air bubble and start stop watch
5. Record distance of air bubble over a given period
6. Calculate the rate of bubble movement

Question 13

What is the role of the phloem in plants?

Answer 13

They transports solutes around the plant

Question 14

What is the phloem made up of?

Answer 14

• Sieve tube elements are living cells that form the tube for transporting elements
• Companion cells are contained for each sieve tube element. They carry out functions for the sieve tubes such as providing energy for active transport

Question 15

What is translocation?

Answer 15

The movement of solutes to exactly where they are needed in the plants - also known as assimilation

Question 16

What is meant by the source and sink.

Answer 16

The source is where the solutes are made (mainly the leaves) and the sink is where the solutes are used up (other parts of the plant).

Question 17

How is a concentration gradient maintained from the source to the sink.

Answer 17

At the sink end, enzymes may break down the solutes or make them into something else. This lowers the concentration gradient at the sink.

Question 18

What is stage one of the mass flow hypothesis?

Answer 18

Active transport loads solutes from companion cells into sieve tubes. This lowers water potential in sieve tubes so water moves in from xylem and companion cells via osmosis. This then creates high pressure in the sieve tubes at the source end

Question 19

What is stage two of the mass flow hypothesis?

Answer 19

At the sink, solutes are removed which increases water potential in the sieve tubes so water leaves via osmosis. This lowers the water potential.

Question 20

What is the third stage of mass flow hypothesis?

Answer 20

A pressure gradient is created between the source and sink. Pressure pushes solutes away from the source to the sink.

Question 21

Evidence that supports the mass flow hypothesis

Answer 21

• If a ring of bark was removed from a woody stem, a bulge of fluid is formed above the ring containing a high concentration of solutes. There is a downward flow of sugars.
• Aphids pierce the phloem. Their body is removed but the mouthpart is left behind which means sap flows quicker near the leaves than down the stem….. shows a pressure gradient
• A metabolic inhibitor stops ATP production. Placed into a phloem then translocation stops which shows ATP is used

Question 22

Objections of the mass flow hypothesis

Answer 22

Solutes move to many different sinks, not just one with the highest water potential.
Sieve tubes create a barrier to mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate.

Question 23

What is he experiment that shows translocation of solutes?

Answer 23

1. Supply part of a plant with a substance that has a radioactive label. Can be supply to a single leaf that has been pumped into a container that surrounds the leaf
2. Radioactive carbon is incorporated into organic substances produced by the leaf. Moves around in translocation
3. Movement can be tracked using autoradiography. Plant is killed and placed onto a photographic film. If film turns black, radioactivity present