Cell Respiration Limitations

Respiration Limitations

• Cells may struggle with respiration due to a lack of the required inorganic final electron acceptors.
• Genetic limitations can prevent cells from synthesizing essential complexes and electron carriers needed for the electron transport system.
• Cells may also lack genes for enzymes necessary for the Krebs cycle, affecting their respiratory capabilities.
• The inability to utilize an electron acceptor can depend on environmental conditions; genetic limitations are permanent.

Prokaryotic Respiration

• Some prokaryotes, such as Streptococcus, are incapable of respiration even in the presence of oxygen.
• Facultative prokaryotes can switch to respiration when conditions permit, allowing for increased ATP production from glucose.

Glycolysis and NADH Recycling

• In the absence of respiration, NADH must be converted back to NAD+ to sustain glycolysis, which is the sole method for ATP production under these circumstances.

Fermentation as an Alternative

• Fermentation uses organic molecules like pyruvate as final electron acceptors, bypassing the electron transport system.
• This process yields only up to 2 ATP per glucose through glycolysis, making it less efficient than respiration.

Microbial Fermentation Applications

• Fermentation is significant in producing food, pharmaceuticals, and for microbial identification.

Lactic Acid Fermentation

• Occurs in low-oxygen environments and involves organisms like yogurt bacteria and human muscle cells.
• Chemical Reaction: Pyruvate + NADH ↔ Lactic Acid + NAD+.

Lactic Acid Bacteria (LAB)

• Includes species like Lactobacillus, Leuconostoc, and Streptococcus, crucial for yogurt and cheese production by coagulating milk proteins through acidity.
• Two types of fermentation:
  • Homolactic Fermentation: Produces only lactic acid (e.g., Lactobacillus delbrueckii).
  • Heterolactic Fermentation: Generates lactic acid, ethanol, acetic acid, and CO2 (e.g., Leuconostoc mesenteroides used in pickles and sauerkraut).

Medical Importance of LAB

• LAB help inhibit pathogen growth by creating a low pH environment in the body.
• Crucial for maintaining vaginal health, as imbalances can lead to yeast infections.
• LAB are essential components of probiotics, supporting gastrointestinal health.

Alcohol Fermentation

• This process converts pyruvate into ethanol.
• Chemical Reactions:
  • Pyruvate → Acetaldehyde + CO2 (via pyruvate decarboxylase).
  • Acetaldehyde + NADH → Ethanol + NAD+ (via alcohol dehydrogenase).
• Alcohol fermentation is utilized in the production of alcoholic beverages and baking, with CO2 contributing to the rise of bread.
• It is also important for biofuel generation from plant materials.