Cellular Respiration Overview

Overview

This lecture covers anaerobic pathways after glycolysis, how other nutrients enter cellular respiration, and key mechanisms regulating cellular respiration.

Fermentation and Anaerobic Respiration

• Glycolysis does not require oxygen and uses NAD+ as a reactant.
• In the presence of oxygen, NAD+ is regenerated by the electron transport chain.
• Without oxygen, cells regenerate NAD+ via fermentation to allow glycolysis to continue.
• Fermentation occurs in the cytoplasm and enables substrate-level phosphorylation, producing ATP.
• Two types of fermentation: lactic acid fermentation (in humans and some bacteria) and alcohol fermentation (in yeast).
• Lactic acid fermentation converts pyruvate to lactate, oxidizing NADH back to NAD+.
• Alcohol fermentation (yeast) converts pyruvate to ethanol and CO₂, also regenerating NAD+.

Metabolism of Proteins and Lipids

• Glucose is not the only substrate for cellular respiration; other carbohydrates, proteins, and lipids can enter the pathway.
• Disaccharides (like sucrose and lactose) and polysaccharides (like starch and glycogen) can be broken to enter glycolysis.
• Amino acids (from proteins) and glycerol (from lipids) can enter glycolysis or the citric acid cycle after modifications.
• Fatty acids and some amino acids are metabolized and enter the citric acid cycle.
• Nucleic acids can also be broken down to enter the respiratory pathway if needed.
• Deamination removes amino groups from amino acids, releasing ammonia (converted to urea and excreted).

Regulation of Cellular Respiration

• Cellular respiration is regulated by the entry of glucose into the cell, often controlled by hormones (e.g., insulin).
• Insulin increases glucose uptake by stimulating GLUT4 transporters on the cell membrane.
• Enzyme reversibility determines if a reaction is reversible or requires different enzymes for forward and reverse directions.
• Enzyme activity can be affected by changes in pH (e.g., due to lactic acid accumulation).
• Glucose is phosphorylated in the first glycolysis step to trap it inside the cell.
• Increased reactant (e.g., ADP) levels speed up pathways, while increased product (e.g., ATP) levels slow them down.

Key Terms & Definitions

• Fermentation — anaerobic process regenerating NAD+ for glycolysis.
• Lactic Acid Fermentation — conversion of pyruvate to lactate, common in muscles and some bacteria.
• Alcohol Fermentation — conversion of pyruvate to ethanol (and CO₂), seen in yeast.
• Substrate-Level Phosphorylation — ATP production directly from substrate reactions, not via electron transport.
• Deamination — removal of an amino group from amino acids, producing ammonia.
• Insulin — hormone that promotes cellular glucose uptake via GLUT4 transporters.

Action Items / Next Steps

• Review metabolic pathways for proteins and lipids entering cellular respiration.
• Prepare for next chapter: photosynthesis.