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Biochemistry - Lecture 21 + 22
Photosynthesis Light Dependent Reaction
Energy from sunlight is transformed into ATP and reducing power (NADPH).
Electrons are obtained from H2O oxidation and used to reduce NADP+ to NADPH.
The proton gradient is created using the free energy of electron transport. Gradient is later used to synthesize ATP.
2H2O + 2NADP+ + 8 photons → 2NADPH + 2H+ + O2 3Pi + 3ADP → 3ATP + 3H2O
The "dark phase" of photosynthesis
Old naming: "Dark" meant light-independent reactions. In fact it happens during the day!
Calvin-Benson cycle, pentose phosphate cycle, or simply Calvin cycle.
Powered by ATP and NADPH.
Converts CO2 (fully oxidized carbon atoms) into carbohydrates (more reduced state), that's why NADPH is needed.
Reactions take place in stroma of chloroplasts.
The Calvin cycle has three stages 1) Fixation of atmospheric CO2 by ribulose 1,5-bisphosphate (5C) to form two molecules of 3-phosphoglyerate (2×3C=6C).
2) Reduction of 3-phosphoglycerate to form glyceraldehyde 3-phosphate (G3P)
(3C) which can be converted to hexoses (6C).
3) Regeneration of ribulose 1,5-bisphosphate (5C) from 2G3P (2×3C) so that more CO2 can be fixed. Carbon Fixation
Ribulose 5-phosphate (5C) combines with CO2 to form a 6C
intermediate, which breaks into two molecules of 3-phosphoglycerate (3PG).
The enzyme responsible is Ribulose 1,5-Bisphosphate
RuBisCo is on the stromal surface of thylakoid membranes.
This reaction is the rate-limiting step in hexose synthesis.
Very inefficient enzyme, being slow and poorly selective, fixes ~3CO2 per second.
Occasionally it binds O2 instead of CO2 and carries out an oxygenase reaction instead of the correct carboxylase reaction, repair mechanisms → waste of energy.
Present at high levels in the chloroplast (>16% of total protein).
RuBisCo is the most abundant protein in plants and probably on the planet.
3-phosphoglycerate (3PG) is an intermediate in glycolysis and gluconeogenesis.
3PG is phosphorylated to 1,3-bisphosphoglycerate (1,3BPG) with consumption of ATP.
1,3BPG is reduced to glyceraldehyde 3-phosphate (G3P) with oxidation of NADPH.
In 3 turns of the cycle, 3 molecules of CO2 (3C) are combined with 3 molecules of R5P
(15C) to make 6 molecules of 3PG/G3P (18C).
Ribulose 5-phosphate (R5P) needs to be regenerated for the Calvin cycle to continue.
Five of the 6 G3P molecules are used to regenerate 3 molecules of R5P, one net molecule of G3P has been produced. Two net molecules of G3P can be converted to glucose.
Ultimately, production of one net molecule of glucose requires the cycle to occur six times. Glucose Storage in Plants
Making one molecule of glucose requires:
18ATP/12NADPH (light reactions).
Plants convert glucose/fructose into:
Sucrose (in the cytosol).
Starch (in the chloroplast).
Cellulose (in the cell wall).
Sucrose is a mobile form of carbohydrate which can circulate throughout the plant.
Synthesis of glucose from noncarbohydrate precursors, such as lactate and alanine, via pyruvate.
Liver is major site, small amount in kidney.
Gluconeogenesis maintain glucose levels in blood, provides energy to brain and muscle.
Pyruvate → Glucose, but not a complete reversal of glycolysis.
The 3 irreversible steps in glycolysis require separate enzymes for gluconeogenesis -
everything else is the same.
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