1.2.3 - Transport in Plants (Translocation) Part 19

Question 1

Define the term ‘sink’.

Answer 1

Sink - What removes sucrose from the phloem (eg. roots, meristem).

Question 1

List the evidence that proves ATP is used in the process of translocation.

Answer 1

• Companion cells have numerous mitochondria.
• Metabollic poison stops translocation.
• Rate of the flow of sugars is too high. This means energy is needed to drive the flow.

Question 2

Give evidence that goes against the theory of translocation.

Answer 2

• Not all solutes move at the same rate.
• Sucrose moves to parts of the plant at the same rate. They do not move quicker to area with a lower concentration of sucrose.
• The role of sieve plates are also unclear.

Question 3

Describe the movement of sucrose from the source to the sink.

Answer 3

• Sucrose is actively loaded into the sieve tube elements, reducing the water potential.
• Water enters by osmosis. This increases the hydrostatic pressure in the sieve tube.
• Water moves down the sieve tube from a higher hydrostatic pressure to a lower hydrostatic pressure at the sink.
• Sucrose is removed by surrounding cells. This increases the water potential.
• Water leaves the phloem by osmosis. The hydrostatic pressure reduces.

Question 4

List the evidence that proves that translocation uses this mechanism.

Answer 4

• The pH of the companion cells is higher thatn the surrounding cells.
• The conentration of sucrose is higher in the source than in the sink.

Question 5

Define the term ‘translocation’.

Answer 5

Translocation - An energy-requiring process transporting assimilates like sucrose in the phloem.

Question 6

Define the term ‘mass flow’.

Answer 6

Mass flow - The flow of water that caries sucrose and other assimilates along the phloem. It can move up or down the plant, depending on where the sugars are needed.

Question 7

Define the term ‘source’.

Answer 7

Source - What releases sucrose into the phloem (such as the leaves).

Question 9

Describe how sucrose enters the phloem.

Answer 9

• Companion cells release ATP to actively transport H⁺ ions out.
• This sets a concentration gradient, forcing the H⁺ ions to diffuse back into the companion cells.
• The ions have co-transporter proteins that enable them to bring sucrose along with them into the companion cell.
• This builds up a high concentration of sucrose, making the sucrose diffuse into the sieve tube elements via the plasmodesmata that connects the two.

Question 10

List the evidence that proves the phloem is involved in the process of translocation.

Answer 10

• If a plant is supplied with radioactive carbon, this will appear in the phloem.
• Ringing a tree removes the phloem. This results in the sugars collecting above the ring.
• Aphids’ mouthparts take food from the phloem. This means the transportation of sugars must take place in the phloem.