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- 1. Carboxylation of and splitting: 6C → 2[3C]
- - ribulose 1,5-bisphosphate (5C) condenses with
- CO2 and H2O to form a 6C molecule and splits
- into 2x3C (catalyzed by Rubisco)
- 2. Reduction of 3C
- molecule by NADPH
- (ATP is used!)
- - 6 cycles provide 2x3C
- for making Glc
- 3. Regeneration of
- ribulose 1,5-bisphosphate
- The carboxysome takes up bicarbonate (HCO3
- –), which
- is then immediately converted to CO2 by carbonic
- The CO2 is then fixed by Rubisco.
The tricarboxylic acid cycle (TCA cycle) is a series of enzyme-catalyzed chemical reactions that form a key part of aerobic respiration in cells. This cycle is also called the Krebs cycle and the citric acid cycle.
reverse, TCA cycle
- - Uses ATPs and NADPH (NADH) to fix CO2
- - This allows building of acetyl-CoA and sugars
The reductive acetyl-CoA pathway:
- - Used by anaerobic soil bacteria, autotrophic
- sulfate reducers, and methanogens
- - Two CO2 molecules are condensed through
- converging pathways to form the acetyl group of
- - Reducing agent is H2! instead of NADPH.
The 3-hydroxypropionate cycle:
- - Used by thermophiles
- - Acetyl-CoA condenses with hydrated CO2
- (bicarbonate ion, HCO3
- –) and is reduced by
- 2 NADPH to 3-hydroxypropionate.
- - In all, three molecules of CO2 are fixed into one
- molecule of pyruvate, which serves as a
- substrate for biosynthesis.
Nitrogen gas (N2) is fixed into ammonium ion (NH4
only by some species of bacteria and archaea.
- Aquatic cyanobacteria develop special cells called
- heterocysts to fix N2. Photosynthesis is turned off to maintain
- anaerobic conditions.