Energy production
(Nunn p.276)
Use of glucose to produce energy involves 3 stages:
- Glycolysis
- Citric cycle (Krebs' cycle)
- Oxidative phosphorylation
1. Glycolysis
Under anaerobic conditions
- Occurs entirely within the cytoplasm.
- Does not involve O2
- Converse glucose into lactic acid
- The pathway only completes all the way to lactic acid when
- PO2 falls below critical level
- there is an absence of respiratory enzymes (e.g. in RBC)
- 6-phosphofructokinase = rate-limiting enzyme
- 4 ATP is produced, but 2 were used near the beginning
- 2 NADH is produced (from NAD+), which cannot diffuse into mitochondria, and is used in conversion of pyruvic acid into lactic acid.
Lactic acid
Lactic acid can be:
- converted into liver glycogen
- re-converted by to pyruvic acid in the presence of O2, then oxidized in the citiric cycle
Overall effect
For each molecule of glucose
- 2 ATP produced
- 2 H2O and 2 lactic acid are also produced
Under aerobic conditions
Same as per anaerobic condition, except:
- Conversion of glyceraldehyde-3-phosphate to 3-phosphoglyceric acid (which produces the 2 NADH) occurs in the mitochondria.
=> the 2 NADH are produced within mitochondria
=> may enter oxidative phosphorylation, rather than producing lactic acid.
- Pyruvate does not get converted into lactic acid, but diffuses into mitochondria and enters the citric cycle.
2. Citric cycle (Krebs' cycle)
Occurs within the mitochondria.
Produces:
- 6 CO2
- further 8 NADH
- 1 FADH2
So far, each molecule of glucose produces:
- 2 ATP
- 6 CO2
- ????? 2 H2O
- 12 hydrogen in 10 NADH and 1 FADH2
3. Oxidative phosphorylation
- Occurs within the mitochondria.
- Hydrogens on NADH and FADH2 are passed along a chain of hydrogen carriers to combine with O2, forming H2O.
- 3 ATP is formed during transfer of each hydrogen ion
=> 36 ATP is formed in this step for each molecule of glucose
- Oxidative phosphorylation can only take place when PO2 within the mitochondrion is above a critical level, i.e. Pasteur point (0.13kPa or 1mmHg).
Summary
Anaerobic pathway
Each molecule of glucose
=> 2 pyruvic acid
=> 2 lactic acid + 2 ATP + 2 H2O
(67kJ per mole)
Aerobic pathway
Each molecule of glucose
=> 2 pyruvic acid
=> 38 ATP + 6CO2 + 6H2O
(1270kJ per mole)
Combustion yields 2820kJ per mole
=> aerobic efficiency = 45%