Cells take on food in the form of carbohydrates, lipids and proteins and converts the energy it contains into energy carried in the molecule ATP.

Food molecules are modified by a large number of reactions which occur in small steps, each catalysed by an enzyme.
These are the basis of metabolism. Notice that all three types of food molecules (carbohydrates, lipids and proteins) pass through a common pathway to make ATP called the citric acid cycle (tricarboxylic acid, Krebs cycle) and electron transport chain.
These 2 processes occur in the mitochondria.

Process of making ATP in the mitochondria is called oxidative phosphorylation.
Making ATP outside the mitochondria are relatively small without the presence of air and occurs via glycolysis and fermentation which is also called substrate-level phosphorylation.
(Oxygen is needed to make ATP in the respiratory chain and carbon from the food is released in the form of carbon dioxide. This is why we breath oxygen and breathe out carbon dioxide)

Metabolism also allows the inter-conversion of different types of food molecules. For example if proteins are low in a diet they can be made from carbohydrates or lipids and vice versa. Interconversion between carbohydrates, lipids and proteins occur via the key molecules pyruvic acid and acetyl CoA.

Citric Acid Cycle

The citric acid cycle is a cyclic series of enzyme catalysed reactions that are of fundamental important to metabolism of aerobic organisms. Enzymes for the citric acid cycle are found in the mitochondrion.

2 carbon acetyl-CoA, react with four carbon oxaloacetate to form 6 carbon citrate. In a series of seven reactions, this is reconverted to oxaloacetate and produces 2 molecules of carbon dioxide. But more importantly, the cycle produces 3 molecules of NADH and 1 mole of FADH2, which are subsequently used in electron transport chain to generate ATP.