Cells must constantly generate energy to sustain life. The processes that produce and consume energy all make up cellular metabolism. Metabolism includes all chemical reactions that occur according to the laws of thermodynamics. Animals obtain energy from consuming organic molecules and breaking them down to release the energy stored within their chemical bonds. Energy is generated via cellular respiration through three major pathways: glycolysis, the citric acid cycle, and oxidative phosphorylation. Through a series of exothermic reactions, these pathways release energy in the form of ATP, which can be used to power biosynthesis.
At A Glance
- In a series of steps, energy is released from food and used to drive the reactions that build the molecules cells need.
- Free energy change can be used to predict whether a chemical reaction will occur spontaneously.
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Enzymes degrade the carbohydrates, proteins, and lipids in food into simple subunits that will be further broken down either in glycolysis or in the citric acid cycle.
- During glycolysis, glucose is oxidized to pyruvate.
- The first phase of glycolysis is called "energy investing" because energy is required to form fructose from glucose.
- The second phase of glycolysis is called "energy harvesting" because energy in the form of ATP and NADH is produced.
- Pyruvate is oxidized to form acetyl-CoA, which enters the citric acid cycle to produce ATP, electron carriers, and CO2 as a waste product.
- During oxidative phosphorylation, the oxidation of electron carriers is coupled to the phosphorylation of ADP to produce most of the ATP made by the cell.
