Starch and sucrose are synthesized from excess triose phosphates produced during photosynthesis. Starch synthesis occurs in plastids and uses ADP-glucose as a substrate to form amylose and amylopectin polymers. Sucrose synthesis occurs in the cytoplasm and joins fructose and glucose monomers. The key enzymes involved in these pathways are regulated by light and dark conditions to preferentially form either starch or sucrose depending on the levels of phosphate and triose phosphates. Specifically, high phosphate in the dark favors sucrose synthesis while low phosphate in the light favors starch formation.

1. BIOSYNTHESIS OF STARCH
AND SUCROSE
KUNAL
MSc. IInd Year
Marine Biology
17373016

2. Starch and Sucrose Biosynthesis
• Excessive amounts of triose and monosaccharide phosphates are converted to
alternative forms in the light
• Liberates phosphate for ATP synthesis
Starch Biosynthesis Sucrose Biosynthesis
 Carbohydrate storage  Carbohydrate transport
 Occurs in plastids  Occurs in cytoplasm
 ADP-glucose substrate  Fructose 6-phosphate & UDP-glucose
 Adds to reducing end (unlike glycogen synthesis)  Joins reducing (anomeric) hydroxyls
 α(1→4) glucose (amylose) with α(1→6)
branches (amylopectin)
 Glucose(α1↔β2)Fructose

3. ENZYMES
1.ALDOLASE
2.FRUCTOSE-1,6-BISPHOSPHATASE
3.HEXOSE PHOSPHATE ISOMERSE
4.PHOSPHOGLUCOMUTASE
5. a. ADP GLUCOSE PYROPHOSPHORYLASE
b. UDP GLUCOSE PYROPHOSPHORYLASE
6. a. STARCH SYNTHASE
b. SUCROSE PHOSPHATE SYNTHASE
7. b. SUCROSE PHOSPHATE PHOSPHATASE

5. • Need phosphate for ATP synthesis and triose phosphate for
carbon fixation
• Fructose 2,6-bisphosphate (F2,6BP) activates pyrophosphate-
dependent phosphofructokinase-1 (PP-PFK-1) and inhibits
fructose bisphosphatase-1 (FBPase-1)
• Its synthesis by phosphofructokinase-2 is inhibited by triose
phosphates (light) and activated by phosphate (dark)
• In dark: ↑ Pi, ↑ F2,6BP, ↑ F1,6BP → glycolysis
• In light: ↑ triose phosphates, ↓ F2,6BP, ↑ F6P → sucrose
biosynthesis

6. • Sucrose 6-phosphate synthase (SPS) is
partially inactivated by phosphorylation by
SPS kinase
• In light: glucose 6-phosphate (high
gluconeogenesis) directly stimulates SPS and
inhibits SPS kinase activating SPS (sucrose
biosynthesis)
• In dark: phosphate directly inhibits SPS and
inhibits SPS phosphatase inactivating SPS (no
sucrose biosynthesis)

7. B. AMYLOPECTIN
A. AMYLOSE
A. AMYLOSE: D-Glucose residues in (α1 4) linkage.
B. AMYLOPECTIN: (α1 6) Branch Point of Amylopectin.

8. ADP-glucose pyrophosphorylase synthesizes starch precursor
• inhibited by high [Pi] accumulating in the dark (ATP hydrolysis)
• activated by high [3-phosphoglycerate] accumulating in the light (carbon
assimilation; diminished sucrose biosynthesis)

9. Starch biosynthesis is growing from reducing end

10. Starch vs. sucrose synthesis is regulated by level of
cytosolic Pi as it affects triose-P export from chloroplast.
 When [Pi] is high, triose-P is exported
in exchange for Pi & used to
synthesize sucrose.
 If [Pi] is low, then triose-P is retained
in chloroplast and used to synthesize
starch.

11. REFERENCES
• Nelson,D.L. and Cox,M.M.,2013.Principles of Biochemistry. W.H Freeman
And Compay,New York.
• Berg,J.M.,Tymoczko, J.L. and Styrer, 2002. Biochemistry. W.H, Freeman &
Co.