Metabolism in two directions
Metabolism is the sum of a cell's chemical reactions. Catabolism breaks molecules down and releases energy; anabolism builds molecules and uses energy. The cell's energy currency is ATP. Nearly every reaction is sped up by an enzyme, a protein catalyst that lowers the activation energy by binding the substrate at its active site. A competitive inhibitor resembles the substrate and blocks the active site. Because enzymes are proteins, heat or extreme pH can denature them.
Getting ATP from glucose
Glycolysis occurs in the cytoplasm and splits one glucose into two pyruvate. It uses 2 ATP and makes 4, so the net gain is 2 ATP, plus 2 NADH. What happens next depends on the final electron acceptor:
- Aerobic respiration. Pyruvate is fully oxidised in the citric acid cycle. NADH and similar carriers hand electrons to an electron transport chain. In bacteria the chain sits in the plasma membrane. Oxygen is the final electron acceptor, and the proton gradient drives ATP synthase. This gives by far the most ATP.
- Anaerobic respiration. Same chain idea, but a different inorganic acceptor, such as nitrate or sulfate, is used in place of oxygen.
- Fermentation. No chain is used. Pyruvate or a derivative accepts the electrons from NADH, regenerating NAD+ so that glycolysis can continue. Fermentation yields only the 2 ATP from glycolysis. Products include lactic acid (in Lactobacillus) and ethanol plus carbon dioxide (in yeast).
Nutritional types
Microbes are named by energy source and carbon source. Phototrophs use light; chemotrophs use chemical energy. Autotrophs build their carbon compounds from carbon dioxide; heterotrophs need organic carbon. Cyanobacteria are photoautotrophs. Most medically familiar bacteria are chemoheterotrophs.
Worked example
A culture of Lactobacillus in milk runs out of oxygen, yet keeps making ATP. Walk through it.
- Glycolysis converts glucose to pyruvate and nets 2 ATP, but each round turns NAD+ into NADH.
- Cells hold only a small pool of NAD+. Without a way to refill it, glycolysis would stop.
- Pyruvate accepts electrons from NADH and becomes lactate, restoring NAD+.
- Glycolysis continues, and 2 ATP per glucose keeps flowing, at the cost of a lot of glucose being used.
The lesson is that fermentation does not make extra ATP. It keeps glycolysis alive. The lactic acid it releases lowers the pH of the milk.