Key concepts:

• Polysaccharides (chains of sugars), proteins (chains of amino acids), polynucleotides (chains of nucleotides) and lipids (chains of CH2 units) are the major types of food molecules.
• These major food molecules are all polymers with lots of stored potential energy.
• For example, one gram molecule of a common type of lipid contains over 2 million calories of energy. This makes lipid molecules a good source of energy and a good molecule for storing energy.
• In a chemical reaction of oxygen with lipid, all this energy would be released into the environment.
• A sudden release of all this energy in an instant of time would be very explosive and dangerous. It would be like a bomb going off.
• Cells must therefore release the stored energy in food slowly, so they don't blow up or burst into flames.