Chapter 6: Metabolism

Heterotrophs consume other organisms to obtain sugars, proteins, fats, and DNA. Their bodies can break down these molecules to recycle and reuse. The process of breaking down molecules to get energy is a part of metabolism. Metabolism encompasses all sife-sustaining processes that occur within a cell, but can be broken down into 2 halves. The reactions that use energy to build the components of a functioning cell, like DNA and proteins fall under the umbrella of anabolism. The reactions that break down organic matter to harvest the energy from them are a part of catabolism.

All living organisms need energy to survive and therefore all living organisms must you some catabolic pathway to get to it. The one humans use is aerobic respiration. From a human perspective, we see respiration as breathing, but this act is only present because, at the cellular level, oxygen and carbon dioxide are necessary components of the aerobic respiration reaction. This is the reason gills, lungs, and breathing in general evolved in so many species. However, these structures aren’t necessary for energy harvesting. Single-celled organisms like yeast perform energy harvesting fine without oxygen. The catabolic processes can be broken down into 3 basic categories: fermentation, anaerobic respiration, and aerobic respiration.

Aerobic respiration involves oxidation of organic molecules (sugars), which causes a natural release of energy. Cells can’t use the energy directly from the sugar molecule’s bonds, so they transfer it to ATP.

In most cells, respiration begins with the oxidation of glucose to pyruvate via a set of chemical reactions called glycolysis. During glycolysis, some of the energy released from each glucose molecule is stored in ATP. Glycolysis occurs with or without oxygen. If oxygen is present, most organisms continue respiration by oxidizing pyruvate to CO2 via chemical reactions of the citric acid cycle (also known as the tricarboxylic acid cycle and Krebs cycle). Organisms that use oxygen for respiration beyond glycolysis are called aerobes. 

As aerobes oxidize pyruvate in the citric acid cycle, they store energy in the electron carriers such as NAD (nicotinamide adenine dinucleotide). Aerobes store energy by reducing NAD and related compounds. These compounds later transfer their high-energy electrons to a series of compounds collectively called the electron transport chain, which generates proton gradients from energy stored in reduced NAD and related compounds to form ~18x more ATP than that formed in glycolysis.

Without oxygen to accept electrons passed through the electron transport chain, the chain is not functional and an aerobic organism will quickly die.

Other organisms, called anaerobes, live without oxygen and may even die if they’re exposed to it. Some of them are primitive bacteria that gather their energy with a pathway of anaerobic respiration that uses inorganic electron acceptors other than oxygen. For example, many bacteria use nitrate, sulfate, or other inorganic compounds as the electron acceptor instead of oxygen. Other anaerobes use glycolysis, but the pyruvate from glycolysis is reduced via anaerobic fermentation to either CO2 and ethanol (in plants and some microbes, like yeast) or lactic acid (in other microbes and oxygen-stressed muscles of animals). Anaerobic fermentation does not involve or benefit from the additional ATP produced by the citric acid cycle or electron transport chain. Thus, anaerobic fermentation produced 18x fewer ATP per glucose molecule than aerobic respiration does.