Biology Topic 12.5: Anaerobic Respiration

Oxygen is crucial for converting hydrogen to water, facilitating ATP production in glycolysis and Krebs cycle.
In the absence of oxygen, only glycolysis can provide ATP, but its continuation requires regeneration of NAD+ from reduced NAD.
Pyruvate accepts hydrogen from reduced NAD through fermentation, regenerating NAD+.

Alcoholic Fermentation:
- Occurs in specific bacteria, fungi (e.g., yeast), and some plant cells under anaerobic conditions.
- Involves decarboxylation of pyruvate, producing ethanol and carbon dioxide.
- Used in brewing and baking industries; yeast ferments carbohydrates into ethanol, which becomes toxic above 15% concentration.
Lactate Fermentation:
- Happens in animals during oxygen shortages, allows survival during strenuous activities (e.g., fleeing predators).
- Pyruvate forms lactate by accepting hydrogen from glycolysis.
- Lactate accumulation can cause muscle fatigue; it is transported to the liver where it is converted back to glucose in the Cori cycle.

Aerobic respiration produces 31 ATP per glucose, totaling 945.5 kJ/mol, indicating a 33% efficiency.
Anaerobic respiration yields only 2 ATP, amounting to 61 kJ/mol, showing about 2% efficiency.
Complete glucose breakdown theoretically yields 2870 kJ/mol.

Various substrates can be metabolized without converting to glucose:
- Lipids yield over twice the energy of carbohydrates (39.4 kJ/g vs. 15.8 kJ/g).
- Proteins are a last resort during starvation, yielding around 17.0 kJ/g.

Rice plants adapt to flooded conditions by developing aerenchyma tissue with air spaces, promoting oxygen diffusion.
Embryonic cells tolerate high ethanol concentrations from anaerobic respiration.
Alcohol dehydrogenase enzyme production helps break down ethanol.

RQ is the ratio of carbon dioxide produced to oxygen consumed, indicating substrate type.
Example: Glucose oxidation gives an RQ of 1.0; lipids yield lower RQs (<1.0).
RQ values vary; most animals have an RQ between 0.8-0.9 during rest, suggesting a mix of carbohydrates and lipids being respired.
Anaerobic respiration results in an infinite RQ due to the absence of oxygen intake.

RQ can misrepresent substrate use due to:
- Partial oxidation of substrates or mixed substrate respiration.
- Organisms rarely respiring a single food source leading to varied RQ due to mixed substrates.