Energy production

(Nunn p.276)

Use of glucose to produce energy involves 3 stages:

1. Glycolysis
2. Citric cycle (Krebs' cycle)
3. 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
  1. PO2 falls below critical level
  2. 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:

1. converted into liver glycogen
2. 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:

1. 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.
2. 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%