Overview
This lecture covers anaerobic metabolism (fermentation), how proteins and lipids can enter cellular respiration pathways, and ways cellular respiration is regulated.
Anaerobic Respiration & Fermentation
- Glycolysis occurs without the need for oxygen, using NAD+ as a reactant.
- In the presence of oxygen, NAD+ is regenerated through the electron transport chain; without oxygen, fermentation regenerates NAD+.
- Fermentation allows glycolysis and ATP production to continue in anaerobic (no oxygen) conditions.
- Two types of fermentation: lactic acid fermentation (in humans, bacteria) and alcohol fermentation (in yeast).
- Lactic acid fermentation occurs in muscle cells during intense exercise, red blood cells, and some bacteria (e.g., yogurt).
- The enzyme lactate dehydrogenase catalyzes lactic acid fermentation in the cytoplasm, converting pyruvate to lactate while oxidizing NADH to NAD+.
- Alcohol fermentation (in yeast) converts pyruvate to acetaldehyde (via pyruvate decarboxylase) and then to ethanol (via alcohol dehydrogenase), also regenerating NAD+.
- Fermentation allows continued ATP production via substrate-level phosphorylation, yielding only 2 ATP per glucose.
Alternative Inputs for Cellular Respiration
- Not only glucose, but other carbohydrates (sucrose, lactose, starch), proteins, and lipids can enter cellular respiration pathways.
- Proteins are broken into amino acids, which can be modified to enter glycolysis or the citric acid cycle.
- Fats are split into glycerol (enters glycolysis) and fatty acids (can be metabolized and enter the citric acid cycle).
- Even nucleic acids can be degraded for energy if necessary.
Regulation of Cellular Respiration
- Cellular respiration can be regulated by controlling glucose entry into cells (e.g., via the hormone insulin).
- Insulin increases glucose uptake by triggering GLUT4 receptor insertion into the cell membrane.
- Enzyme reversibility: Reversible reactions (single enzyme) can reach equilibrium; irreversible reactions use different enzymes in each direction.
- Changes in pH (from lactic acid) can affect enzyme activity, slowing or altering metabolic rates.
- Amount of reactants (e.g., ADP increases pathway rate) and products (e.g., ATP decreases pathway rate) regulate pathway speed.
- Glucose is phosphorylated at the start of glycolysis (by hexokinase) to trap it inside the cell.
Key Terms & Definitions
- Fermentation — Anaerobic process that regenerates NAD+ to allow glycolysis to continue.
- Lactic Acid Fermentation — Converts pyruvate to lactate, regenerating NAD+ (in muscles, some bacteria).
- Alcohol Fermentation — Converts pyruvate to ethanol and CO₂, regenerating NAD+ (in yeast).
- Substrate-Level Phosphorylation — ATP formation directly in glycolysis/fermentation, not involving electron transport chain.
- Insulin — Hormone that promotes glucose uptake into cells.
- Hexokinase — Enzyme that phosphorylates glucose in the first step of glycolysis.
- GLUT4 Receptor — Cell membrane protein that facilitates glucose entry into cells.
Action Items / Next Steps
- Review diagrams of fermentation (lactic acid vs. alcohol) and cellular respiration entry points for proteins and fats.
- Prepare for Chapter 8 on photosynthesis.