Which of the following enzymes is very important in determining the partitioning
ID: 201408 • Letter: W
Question
Which of the following enzymes is very important in determining the partitioning of glyceraldehyle-3-phosphate/dihydroxy acetone phosphate between starch synthesis and sucrose synthesis
Aquaporins
G3P/DHAP efflux channel
RubisCO
Triose-phosphate translocator
ATPases
The accumulation of phosphorylated sugars in the cytoplasm will
Increase the activity of certain key Calvin Cycle enzymes
Promote export of phosphorylated sugars in the phloem
Promote export of triose-P from the chloroplast
Promote starch synthesis in the chloroplast
Promote starch degradation in the chloroplast
A tomato fruit growing on a vine will receive most of its nutrients and water from
The xylem
Storage reserves such as seeds and tubers
All regions of the plant more or less equally
Its own photosynthetic capabilities
The mature leaves closest to it
What is true of phloem sieve tubes
They are dead at maturity
They are cytoplasmically dense
They are membrane lined
None of the above
B and C only
Explanation / Answer
Which of the following enzymes is very important in determining the partitioning of glyceraldehyde-3-phosphate/dihydroxyacetone phosphate between starch synthesis and sucrose synthesis
Answer: Triose-phosphate translocator
Explanation: Starch synthesis takes place inside the chloroplast, while sucrose synthesis takes place in the cytosol of green cells, both of them need for dihydroxyacetone phosphate (a triose phosphate) to be transported to the chloroplast or the cytosol, depending on what will be synthesized. The triose phosphate translocator located in the chloroplasts' membrane exports dihydroxyacetone phosphate and therefore is very important in determining the partitioning of glyceraldehyde-3-phosphate/dihydroxyacetone phosphate between starch synthesis and sucrose synthesis.
The accumulation of phosphorylated sugars in the cytoplasm will
Answer: Promote starch synthesis in the chloroplast
Explanation: Accumulation of phosphorylated sugars (glucose and fructose) is caused by high concentrations of sucrose in phloem, sucrose that is later transported inside the cytoplasm, broken down into fructose and glucose, and then both are phosphorylated. Therefore accumulation of phosphorylated sugars in cytoplasm wouldn't increase the activity of certain key Calvin cycle enzymes (that option is not the answer) as the main purpose of the Calvin cycle is to produce glucose in the cytoplasm and there is already a high concentration. Accumulation of phosphorylated sugars in cytoplasm wouldn't promote export of phosphorylated sugars in the phloem as is only sucrose the one transported in the phloem and there is already too much which caused accumulation of phosphorylated sugars. It wouldn't promote export of triose-P from the chloroplast, as this is done to synthesize sucrose in the cytoplasm but we already have too much. Finally, it wouldn't promote starch degradation in the chloroplast, as that is for when there are low sucrose levels in the cytoplasm. So, basically an accumulation of phosphorylated sugars in the cytoplasm is caused by the accumulation of sucrose in the cytoplasm, and when having high levels of sucrose, starch synthesis is stimulated to store the accumulated sugars.
A tomato fruit growing on a vine will receive most of its nutrients and water from
Answer: The mature leaves closest to it
Explanation: The xylem only doesn't transport nutrients only water so that is not the answer. In a plant, some regions contribute more than others in providing nutrients and/or water to other specific regions, and in this case, the tomato fruit receives it's nutrients from a specific region and not from all regions. Although yes, seeds are inside the fruit and rich in nutrients they don't provide nutrients to it, but to the new growing plant, and tubers which also have lots of nutrients, they are not part of the fruit (but stems or roots) and provide nutrients to the plant in general not just the fruits. Mature leaves close to the fruit provide are the main providers of water and nutrients like nitrogen, phosphorous, and potassium to the growing fruit, therefore, a 3:1 leaves to fruit ratio is recommended, in order to obtain healthy fruits.
What is true of phloem sieve tubes
Answer: They are membrane lined
Explanation: Sieve tubes are living cells when mature (unlike xylem vessels) but just don't have some organelles, therefore they are dead at maturity is not the answer. Sieve tubes have a low cytoplasmic density as they don't have organelles like a nucleus, ribosomes, chloroplasts, vacuoles, and Golgi apparatus, and structures like microtubules that could obstruct the flow of nutrients, therefore, they are cytoplasmically dense is not the answer. Sieve tubes are membrane lined with companion cells (adjacent to them) through channels. This membrane-lined channels connecting sieve tubes with companion cells to transfer large molecules are named plasmodesmata.
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