Free Version
Moderate

# Photosynthesis: Link Between Starch and Sucrose Synthesis

BIOCHM-ZJOJ1X

During photosynthesis, most plants synthesize and store starch in the chloroplast. Some of this starch is broken down in the light and exported to the cytosol as triose-phosphate (triose-P) in exchange for ${ P }_{ i }$ on the phosphate translocator. The exported triose-P is then converted to sucrose in the cytosol.

See the simplified schematic below illustrating starch and sucrose synthesis from photosynthetically assimilated carbon.

Sucrose is the major form of sugar transported in the phloem of most plants for anabolic metabolism in other parts of the plant. When the demand for sucrose decreases, less sucrose is transported out of the cell and this leads to an excess accumulation of photosynthetically produced starch in the chloroplast.

Which of the following is the MOST likely biochemical explanation for these observations?

A

The accumulation of sucrose results in feedback inhibition of the cytosolic sucrose synthesis pathway. The high levels of sucrose competitively inhibit the phosphate translocator.

B

The accumulation of sucrose results in feedback inhibition of the sucrose synthesis pathway thereby decreasing the availability of cytosolic (${ P }_{ i }$) needed by the phosphate translocator for antiport activity with triose-P.

C

The accumulation of starch and sucrose results in increased levels of ${ P }_{ i }$ in the chloroplast stroma. This inhibits phosphate translocator activity.

D

The export of triose-P requires chloroplast ATP for phosphate translocator antiport activity. ATP is less available due to decreased availability of ${ P }_{ i }$.

E

The accumulation of starch and sucrose results in decreased levels of ${ P }_{ i }$ in the chloroplast stroma. This inhibits phosphate translocator activity.