1. Cellular Respiration: This is the primary way cells obtain energy from food. It's a complex process that breaks down glucose (a sugar) in the presence of oxygen to produce ATP, the cell's energy currency. Cellular respiration can be divided into three main stages:

* Glycolysis: Glucose is broken down into pyruvate, generating a small amount of ATP.

* Krebs Cycle (Citric Acid Cycle): Pyruvate is further broken down, generating more ATP and electron carriers.

* Electron Transport Chain: Electron carriers deliver electrons, driving the production of a large amount of ATP.

2. Fermentation: This process occurs when oxygen is limited. It's less efficient than cellular respiration, producing far less ATP. There are two main types of fermentation:

* Lactic Acid Fermentation: This is used by muscle cells when oxygen is depleted. Pyruvate is converted into lactic acid, which can build up in the muscles causing fatigue.

* Alcoholic Fermentation: This is used by yeast and some bacteria. Pyruvate is converted into ethanol (alcohol) and carbon dioxide.

Let me know if you'd like more detail on any of these processes!