Anaerobic Respiration

This lesson covers: 

  1. Differences between anaerobic and aerobic respiration
  2. Alcohol fermentation and lactic acid fermentation
  3. Adaptations for anaerobic respiration in rice plants

Differences between anaerobic and aerobic respiration

Anaerobic respiration acts as an emergency measure for ATP production when oxygen levels are low, as it does not require oxygen.


Differences between aerobic and anaerobic respiration:

  • Some steps of aerobic respiration occur within mitochondria, while anaerobic respiration only occurs in the cytoplasm.
  • Only aerobic respiration includes the link reaction, the Krebs cycle, and oxidative phosphorylation.
  • Aerobic respiration yields approximately 34 ATP per glucose molecule, while anaerobic respiration yields only 2 ATP per glucose molecule.

Alcohol fermentation and lactic acid fermentation

In anaerobic conditions, which are those without oxygen, cells can produce a small yield of ATP through glycolysis. This can only continue if the reduced NAD that is produced can be oxidised again.


The link reaction and Krebs cycle cannot continue if all the FAD and NAD are reduced, because they cannot accept any more protons and electrons. Oxidative phosphorylation cannot occur without oxygen as the final electron acceptor.

There are two major pathways through which cells can undergo anaerobic respiration: alcohol fermentation and lactic acid fermentation.


Alcohol fermentation:

  • Occurs in yeasts and some plants.
  • Pyruvate is decarboxylated, losing a molecule of CO2 and is converted into ethanal.
  • Ethanal is reduced, and accepts a hydrogen from reduced NAD, catalysed by alcohol dehydrogenase.
  • This produces ethanol, and regenerates NAD.
  • Ethanol cannot be further metabolised so must be removed as a waste product.


Lactic acid fermentation:

  • Occurs in some animals and bacteria.
  • Pyruvate accepts a hydrogen from reduced NAD, catalysed by lactate dehydrogenase.
  • This forms lactic acid, and regenerates NAD.
  • Lactic acid can be oxidised back to pyruvate to generate more ATP, or it can be stored as glycogen.

Anaerobic adaptations in flooded rice

Rice plants exhibit remarkable adaptations to thrive in anaerobic conditions of paddies (flooded fields).


Adaptations of rice plants to grow in flooded fields:

  • Stems elongate as water levels rise to keep leaves and reproductive structures above water.
  • Aerenchyma tissue forms to facilitate the transport of oxygen throughout the submerged plant parts.
  • Alcoholic fermentation allows rice to generate ATP even with limited oxygen availability.
  • The plants can tolerate the accumulation of ethanol.
  • The plants can produce more ethanol dehydrogenase to break down ethanol.


These adaptations are essential for rice survival in environments where flooding is common.