3.4.2 - Mass transport in plants

Question 1

What is the xylem

Answer 1

A dead tissue made up of dead, hollow cells, which form a continuous tube .

Question 2

Function of xylem

Answer 2

Enables the transport of water and mineral ions from roots to rest of plant

Question 3

Describe what causes water to move up the xylem, according to the cohesion-tension theory

Answer 3

MP1 - water vapour evaporates out of the stomata on leaves.
MP2 - this decreases the water potential in mesophyll cells in leaves.
MP3 - water moves out of the xylem by osmosis into mesophyll cells down the conc grad.
MP4 - as this column of water is pulled up the xylem, it creates tension, pulling the xylem in to become narrower.
MP5 - due to hydrogen bonds between water molecules, there are cohesive, creating a column of water within the xylem.
MP6 - water molecules also adhere to the walls of xylem. This helps to pull water column upwards and ensure the column doesn’t break

Question 4

Why is the process that transport water up the xylem called the cohesion - tension theory ?

Answer 4

1. Cohesion - water molecules are polar and so are able to form hydrogen bonds with each other.
2. Cohesion is the internal force keeping the water column in tact, as it moves upwards.
3. Tension - water evaporates from the stomata in the leaves during transpiration. This expiration creates a negative pressure in the leaves air space.
   This negative pressure pulls up H2O from the xylem

Question 5

How does light intensity affect the rate of transpiration and why

Answer 5

1. Increased light intensity increases rate of photosynthesis
2. So more stomata are open to take in more CO2
3. However, this also allows more water vapour to evaporate from leaves.
4. So more tension in the xylem
5. So bigger transpirational pull
6. Increases rate of transpiration

Question 6

How does temperature affect rate of transpiration?

Answer 6

1. Higher temperature increases the kinetic energy of water molecules. This causes faster evaporation from mesophyll cells, and increased diffusion of water vapour out of stomata.
2. More tension in the xylem
3. So bigger transpirational pull
4. Higher rate of transpiration

Question 7

How does humidity affect rate of transpiration

Answer 7

1. If there is a high humidity outside the leaf, this reduces the conc grad of water vapour and reduces the rate of transpiration
2. Because there is less tension in the xylem and so less of a transpirational pull

Question 8

How does air movement affect rate of transpiration?

Answer 8

1. Wind removed the humid layer of air near the leaf’s surface
   This establishes and maintains a steel water vapour gradient
2. So more water vapour diffuses out of the leaf
3. More tension in the xylem
4. So bigger transpirational pull.
5. Higher rate of transpiration

Question 9

What is the phloem

Answer 9

A living tissue responsible for the transport of organic substances, like sucrose throughout the plant.

It is a part of the vascular bundle, along with the xylem

Question 10

What is the function of the phloem

Answer 10

Translocation - transports the sucrose from source to sink

This transportation is bidirectional

Question 11

In translocation, what is source?

Answer 11

Where sucrose is made
For example - in the leaves during photosynthesis

Question 12

In translocation what is the sink

Answer 12

Where sucrose would be used or stored.

Question 13

Describe the structure of phloem

Answer 13

1. Sieve tube element - long, tube-like cells joint end to end
2. The sieve tube element has no nucleus, little cytoplasm and sieve plates at ends.
3. Companion cells have nucleus and lots of mitochondria - for the active transport of sucrose.

Question 14

Describe the process of translocation in plants.

Answer 14

1. The active moment of sucrose through the phloem from the source to sink
2. It is bidirectional and requires energy because loading sucrose at the source involves active transports.
3. At the source - sucrose is actively transported from mesophyll cells - companion cells - sieve tube elements.
   Water enters by osmosis from the adjacent xylem. This causes high hydrostatic pressure in the phloem at the source end.
4. At the sink - sucrose is removed from the phloem for use in respiration or to be stored as starch. This increases water potential in the phloem. Water moves out, back into the xylem by osmosis. This lowers the hydrostatic pressure at the sink end.

The difference in pressure between source and sink causes a mass flow of phloem sap along the sieve tube.

Question 15

Describe how a high pressure is created in the phloem near the source.

Answer 15

1. By active transport, sucrose is transport from mesophyll cells - companion cell - into the sieve tube element
2. This reduces the water potential in the sieve tube element
3. So by osmosis, water moves from higher WP in xylem to lower WP in the sieve tube element.
4. This creates high hydrostatic pressure in sieve tube element because volume of h2o in phloem has increased.

Question 16

Phloem tissue is found j. Close association with xylem tissue. Explain the importance of this

Answer 16

1. When the organic substances enter the phloem, the water potential decrease and water enters the phloem from the xylem by osmosis down water potential gradient.
2. This causes high hydrostatic pressure to increase
3. So sucrose can move by mass transport from source to sink

Question 17

What does it mean to ring a tree

Answer 17

Ringing a tree means removing a complete ring of bark, including the phloem layer, from around the circumference of the trunk or branch of a tree

The xylem is still left in tact, so water can move up from the roots still.

However the phloem is interrupt so sugars, and other organic substances can’t be transported downwards from the leaves

Question 18

What happens to the tree after it has been ringed

Answer 18

1. Swelling above the ring
2. Tissue below the ring dies

Question 19

Why does swelling occur the ring occur when the tree has been ringed?

Answer 19

Sugars accumulate because they cannot move down past the cut

Question 20

Why does the tissue below the ring die after the tree is ringed ?

Answer 20

No supply of sugar to this tissue
No aerobic respiration
No atp produced
No atp hydrolysed
No energy released for metabolic reactions

Question 21

Conclusion that can be made after a tree has been ringed

Answer 21

this shows that the phloem is responsible for translocation of sugars and once it is removed, translocation stops below the ring, supporting the mass - flow theory

Question 22

What is a tracer?

Answer 22

A tracer is a radioactively labeled substance used to track the movement of molecules in biological systems.

In translocation experiments, scientists use a radioactive isotope of carbon — carbon-14 (¹⁴C) to label carbon dioxide (¹⁴CO₂). When a plant photosynthesizes using ¹⁴CO₂, it incorporates the radioactive carbon into sugars like sucrose.

Question 23

How was a tracer used to study translocation ?

Answer 23

A plant is exposed to ¹⁴CO₂ in a sealed chamber.

The plant photosynthesises, and the radioactive carbon is incorporated into sugars in the leaves.

These sugars are then transported via the phloem to other parts of the plant.
After a set time, the plant is frozen and sectioned.

Autoradiography is used: the sections are placed on photographic film, and areas with radiation turn the film dark.

Question 24

What Did Tracer Experiments Show About Translocation?

Answer 24

The darkened areas on the film match the position of the phloem.
This shows that radioactively labeled sugars travel only in the phloem, not the xylem or other tissues.
It supports the conclusion that phloem is the tissue responsible for translocation of sugars.

Question 25

What is autoradiography?

Answer 25

Autoradiography is a technique used to visualize the location of radioactive substances in a biological specimen (like a plant or tissue section). It’s especially useful in experiments using radioactive tracers (e.g. carbon-14) to track the movement of molecules such as sugars during translocation.

Question 26

Mass transport definition

Answer 26

The bulk movement of substances over long distances, at the same speed and in the same direction, this movement is due to differences in pressure.

Question 27

Movement of water out of a leaf

Answer 27

1. Atmosphere is less humid than air in leafs air space 2. Water vapour in air spaces diffuses out of the open stomata into air. 3. Water evaporates from cell walls of mesophyll cells into air spaces. This is to replace the lost water 4. Water potential in the mesophyll cells decrease. 5. Water moves from the xylem to the mesophyll cells by osmosis.

Question 28

In the photometer experiment, describe how the student would have return he air bubble to the start of the capillary tube in this investigation

Answer 28

The student would open the reservoir tal to allow the water to push the air bubble back to the start of the capillary tube

Question 29

In the photometer experiment, give 2 precautions the students should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot

Answer 29

1. Ensure all the joints are air tight to prevent any air leaks, so that only water is lost through the plant 2. Cut the shoot under water, to prevent air bubbles from entering the xylem, this would block the uptake of water

Question 30

A potometer measures the rate of water uptake rather than the rate of transpiration. Give 2 reasons why the potometer does not truly measure the rate of transpiration

Answer 30

1. It measures water uptake and not the water loss by transpiration for photosynthesis, turgid pressure 2. Water is produced in respiration 3. It assumes no water loss is caused by other sources, such as through the cut stem or water that evaporates from the apparatus

Question 31

In a potometer investigation, the student wanted to calculate the rate of water uptake by the shoot in cm^3 per minute. What measurements would she need to take?

Answer 31

1. Distance the air bubble moves in the capillary tube 2. Time taken for the air bubble to move that distance 3. Diameter of capillary tube