Question

If you analyzed the phloem sap of a plant and found high concentrations of ________________, the plant most likely phloem loads through the symplasm.

a. sucrose

b.starch

c. ruffinose family oligosaccharides

d. sugar alcohols

e. glucose

(I choose c as my answer)

Phloem transport is expected to remain fairly constant during diurnal cycles (day/night cycles) because.....

a. the statement is false

b. sucrose is always abundant in leaf cells

c. sucrose and starch are produced during the day, and starch is converted to sucrose at night.

d. starch, the main product of photosynthesis, is degraded more slowly than it is synthesized to provide a constant supply of sucrose.

e. the oppp oxidizes glucose to provide substrates to make glucose

(I choose c as my answer)

In plastids, the anabolic reactions of the Calvin Cycle (sometimes, albeit rarely, referred to as the reductive pentose phosphate pathway) need to be kept separate from the catabolic reactions of the oxidative pentose phosphate pathway (OPPP). How is this achieved?

a. disulfide bridges on enzymes in the calvin cycle need to be oxidized

b. disulfide bridges on enzymes in the oppp need to be oxidized

c. calvin cycles are in the stroma; oppp enzymes are in the thylakiod lumen

d. both reactions are ccontrolled by substrate and product concentration favoring either the foward (anabolic) or reverse (catabolic) reactions.

please explain. thank you

Answer 1

1) phloem sap when it is transported by symplastic pathway it have raffinose family oligosaccharides in high concentration. special type pf companion cells called as intermediatory cells are involved in symplastic phloem loading.

2)Answer is A . statement is false because in diurnal cycle phloem transport is low in morning , reaches to maximam by late afternoon in response to photosynthesis and lowers in the evening and night.

3)Answer is A. when the disulfide bridges of enzymes of calvin cyle are in oxidized form( s-s) they are inactive. at that time enzyme of oppp are in active form and vice versa. when disulfide bridges are in reduced form(SH- SH) the enzymes of calvin cycle are in active state. at that time key enzyme of OPPP glucose-6-phosphate dehydrogenase are in inactive form. this is how enzymes are regulated. it is FD-TD system of enzyme regulation and it is light regulation of chloroplast enzymes.