3.4.2 Mass transport in plants

Question 1

How is water transported in plants?

Answer 1

Through xylem vessels

Question 2

Describe the structure of xylem vessels

Answer 2

Long, continuous columns that also provide structural support to the stem

Question 3

Explain the cohesion-tension theory

Answer 3

Water molecules form hydrogen bonds with each other, causing them to ‘stick’ together. The surface tension of the water also creates this sticking effect. Therefore as water is lost through transpiration, more can be drawn up the stem

Question 4

What are the three components of phloem vessels?

Answer 4

1. Sieve tube elements
2. Companion cells
3. Plasmodesmata

Question 5

Explain sieve tube elements in the phloem

Answer 5

They form a tube to transport sucrose in the dissolved form of sap

Question 6

Explain companion cells in the phloem

Answer 6

Involved in ATP production for active loading of sucrose into sieve tubes

Question 7

Explain plasmodesmata in the phloem

Answer 7

Gaps between cell walls where the cytoplasm links, allowing substances to flow

Question 8

Name the process whereby organic materials are transported around the plant

Answer 8

Translocation

Question 9

How does sucrose in the leaf move into the phloem?

Answer 9

Sucrose enters companion cells of the phloem vessels by active loading, which uses ATP and a diffusion gradient of hydrogen ions. Sucrose then diffuses from companion cells into the sieve tube elements through the plasmodesmata

Question 10

How does phloem vessels transport sucrose around the plant?

Answer 10

As sucrose moves into the tube elements, water potential inside the phloem is reduced. This causes water to enter via osmosis from the xylem and increases hydrostatic pressure. Water moves along the sieve tube towards areas of lower hydrostatic pressure. Sucrose diffuses into surrounding cells where it is needed

Question 11

Give evidence for the mass flow hypothesis of translocatoin

Answer 11

• Sap is released when a stem is cut, therefore there must be pressure in the phloem
• There is a higher sucrose concentration in the leaves that the roots
• Increasing sucrose levels in the leaves results in increased sucroses in the phloem

Question 12

Give evidence against the mass flow theory of translocation

Answer 12

• The structure of sieve tubes seems to hinder mass flow
• Not all solutes move at the same speed, as they would in mass flow
• Sucrose is delivered at the same rate throughout the plant, rather than to areas with the lowest sucrose concentration first

Question 13

How can Ringing experiments be used to investigate transport in plants?

Answer 13

The bark and phloem of a tree are removed in a ring, leaving behind the xylem. Eventually the tissues above the missing ring swells due to accumulation of sucrose as the tissue below begins to die. Therefore sucrose must be transported in the phloem

Question 14

How can Tracing experiments be used to investigate transport in plants?

Answer 14

Plants are grown in the presence of radioactive carbon dioxide, which will be incorporated into the plant’s sugars. Using autoradiography, we can see that the areas exposed to radiation correspond to where the phloem is