Mass transport in plants

Question 1

What are the two types of tissues involved in transport in plants?

Answer 1

Xylem, Phloem

Question 2

What is the xylem?

Answer 2

The tissue that transports water and dissolved mineral ions from the roots, up the stem, into the leaves.

Question 3

What is the phloem?

Answer 3

The tissue that carries organic substances from the source to the sink in plants.

Question 4

What is the structure of the xylem?

Answer 4

They make long hollow tubes formed from dead cells joined together. They are one way only and there are no end walls between cells. The walls contain lignin for strength and structural support to prevent the tubes from collapsing.

Question 5

What is cohesion?

Answer 5

When water molecules are attracted to other water molecules by hydrogen bonding.

Question 6

What is the transpiration stream?

Answer 6

The movement of water and dissolved mineral ions through xylem vessels, from the roots to the leaves.

Question 7

What is transpiration?

Answer 7

The evaporation of water from a plants surface, usually the leaves

Question 8

What is the cohesion-tension theory about?

Answer 8

Explains how water and dissolved mineral ions move from the roots, up the stem and to the leaves in plants.

Question 9

Explain the cohesion-tension theory.

Answer 9

1. Water transpires from the leaves which means the water potential in the leaf cells decreases which causes water to be drawn out of the xylem by osmosis.
2. This creates tension and negative pressure which means more water is drawn up through the xylem. This happens because water molecules are cohesive due to hydrogen bonding which means water in the xylem moves as a continuous column.
3. This allows more water to enter the roots by osmosis because the water potential in the root cell is low.

Question 10

What are the factors affecting rate of transpiration?

Answer 10

Light intensity
Humidity
Temperature
Wind

Question 11

How does light intensity affect the rate of transpiration?

Answer 11

More light intensity = faster rate of transpiration
- More light intensity means that stomata will open to allow CO2 to diffuse in for photosynthesis. This means there is a greater rate of diffusion of water vapour out of the leaf which means greater rate of transpiration.

Question 12

How does humidity affect the rate of transpiration?

Answer 12

Increased humidity = slower rate of transpiration
- More humidity means there is a lower concentration gradient of water vapour inside and outside the leaf which means slower rate of diffusion of water vapour out of the leaf.

Question 13

How does temperature affect the rate of transpiration?

Answer 13

Higher temperature = faster rate of transpiration
- Water molecules have more kinetic energy so more of them are able to evaporate. Higher concentration gradient of water means faster diffusion.

Question 14

How does wind affect the rate of transpiration?

Answer 14

More wind = faster rate of transpiration
- Water molecules are removed around the stomata with increased wind so there is a greater concentration gradient of water vapour which increases the rate of diffusion and transpiration.

Question 15

What can be used to investigate the rate of transpiration?

Answer 15

A potometer

Question 16

What is an assumption we make about the water and the capillary tube in a potometer experiment?

Answer 16

The amount of water taken up by the capillary tube is equal to the amount of water lost in transpiration.

Question 17

Why is the amount of water taken up from the capillary tube in a potometer not exactly the amount of water lost in transpiration

Answer 17

Some water is used in photosynthesis or hydrolysis reaction.
Some water could enter the cells by osmosis causing them to become turgid if they started off with a lower water potential than their surroundings.

Question 18

What is the source?

Answer 18

Anywhere in a plant where sucrose is released into the phloem and e.g. leaf

Question 19

What is the sink?

Answer 19

Anywhere in a plant where sucrose is removed from the phloem e.g. roots

Question 20

What are the adaptations of the phloem?

Answer 20

1. They are made up of sieve tube elements which are living cells that form long tubes through which organic substances can pass
2. Consist of companion cells which have many mitochondria to provide ATP because translocation is a passive process.
3. Thick cell walls offer structural support

Question 21

Explain the mass flow hypothesis

Answer 21

1. Companion cells contain mitochondria which produce ATP to actively transport hydrogen ions into the surrounding tissue.
2. This creates a diffusion gradient with a high concentration of hydrogen ions outside the companion cells
3. Hydrogen ions move back into the companion cells via co transport and sucrose also goes along.
4. This causes the concentration of sucrose in the companion cells to increase
5. Sucrose moves from the companion cells to the sieve tube elements using the plasmodesmata or a carrier protein

Question 22

Explain the evidence supporting mass flow hypothesis.

Answer 22

1. Ringing experiments
   - A ring of bark which only contains the phloem is removed. A bulge forms above the ring due to pressure being applied from above which means that sucrose accumulates in the phloem. The concentration of sucrose above the bulge is greater than below it which suggests that sucrose moves down the phloem.
2. Radioactive traces
   - A plant is supplied with radioactive carbon dioxide that contains carbon-14. The radioactive carbon incorporates into glucose when a plant undergoes photosynthesis. This is converted into sucrose. The radioactive sucrose moves around the plant by translocation which can be detected using radioactive tracers.

Question 23

Explain evidence against mass flow hypothesis.

Answer 23

1. Organic substances have to reach the sinks ranging from the roots to the flowers so how can sap move up and down at the same time?
2. Sucrose moves at the same standard speed instead of going more quickly to the places where sucrose is needed which is what the mass flow hypothesis should suggest.

Question 24

Explain how pressure is generated inside the phloem.

Answer 24

• Sucrose is actively transported into the phloem.
• This reduces the water potential
• Water will move into the phloem via osmosis from the xylem.

Question 25

Describe the mass flow hypothesis for the mechanism of translocation in plants.

Answer 25

The companion cells actively transport leaf sugars into the phloem. This lowers the water potential in the sieve tube elements causing water to move in by osmosis. This increases pressure in the phloem causing mass movement towards the roots. The sugars in the roots are used for respiration for storage.