Cells use energy in order to grow, regulate metabolism and reproduce. This energy is obtained from a source such as food molecules or light from the sun, and through processes like glycolysis, the citric acid cycle and oxidative phosphorylation, an energy-rich molecule is then created. The cell can then use the energy in the protein molecule to help it function.
The powerhouse of an animal cell is called the mitochondria. In plants, chloroplasts carry out a similar function. Eukaryotic cells, or cells that contain a nucleus (which includes both plant and animal cells), can use three different modes to create energy molecules from an energy source.
Glycolysis, also known as fermentation, involves dividing a glucose (sugar) molecule into two molecules of a new substance called pyruvate. No oxygen is needed for this reaction, but if some is present, then the pyruvate molecules can enter the mitochondria to be transformed into two acetyl-CoA molecules and one carbon dioxide molecule. This particular energy process is called the citric acid cycle.
The third and final process is oxidative phosphorylation, which involves a gradient of protons forming as electrons pass through protein complexes in the inner membrane of the mitochondria.
If there is an abundant energy source present, cells can also make larger molecules to store for later, called polysaccharides and lipids (sugars and fats).