translocation

Question 1

What is translocation?

Answer 1

Movement of sugars from source to sink.

Question 2

Process of translocation.

Answer 2

1 - photosynthesis occurs. Sucrose made via condensation reaction. Does not get used whilst being transported.
2 - Sucrose moves down its concentration gradient to the companion cell by diffusion through cytoplasm (symplastic). Higher concentration of sucrose in leaf cells than phloem.
3- Sucrose can move through the cell walls via the apoplast pathway and is then actively loaded into the companion cell.
4- Sucrose enters the phloem via symplastic pathway and this lowers the water potential of the phloem.
5- water moves by osmosis from xylem to phloem down water potential gradient.
6- Hydrostatic pressure increases in the phloem.
7- Fluid moves down pressure gradient.
8 - Sucrose diffuses out of the phloem into the sink for use in respiration.
9 - Sucrose conc decreases so water moves by osmosis back into xylem.

Question 3

Process of active loading of sucrose into phloem.

Answer 3

1- Hydrogen ions are actively pumped using proton pump out of the companion cells into the surrounding tissue using energy from ATP.
2 - Accumulation of hydrogen ions outside cell, so high H+ ions concentration. Due to this, H+ ions can diffuse back into the cell via co-transporter. This requires a co-transporter molecule, which is sucrose.
3- Sucrose moves against conc gradient. H+ ions diffuses down conc gradient back into the cell, at the same time, sucrose is co-transported into the cell.
4- High concentration of sucrose in the cytoplasm. Sucrose is transported via plasmodesmata from companion cells into the phloem.

Question 4

Evidence showing translocation.

Answer 4

• Advances in microscopy allow us to see the adaptations of the companion cell for active transport.
• It the mitochondria of the companion cells are poisoned, translocation stops.
• The flow of sugars in the phloem is about 10000 times faster than it would be by diffusion alone, suggesting an active process is driving the mass flow.

Question 5

How can aphids be used to demonstrate the translocation of organic solutes in the phloem?

Answer 5

• It has been shown that there is a positive pressure in the phloem that forces the sap out through the stylet.
• The pressure and therefore the flow rate in the phloem is lower closer to sink than it is near the source.
• The concentration of sucrose in the phloem sap is also higher near to the source than near the sink.

Question 6

What are Xerophytes?

Answer 6

Xerophytes are plants that are adapted to dry and arid conditions