A. Organisms need energy for life processes

• Energy is the ability to move or change matter, and in living organisms involves chemical reactions
• Metabolism is the sum of all the chemical reactions occurring in an organim, some of which absorb energy, and others which release energy
• Activation energy is required to initiate any chemical reaction

• Enzymes are protein molecules that speed up chemical reactions by lowering activation energy
• Substrates are substances that enzymes act upon
• Enzymes and substrates interact if they have corresponding shape and charge
• While the substrate is altered by an enzyme's action, the enzyme itself is unchanged, and able to immediately act upon other substrate molecules
• Enzyme activity is sensitive to environmental factors such as temperature and pH, which alter the enzyme's shape so that it no longer fits the substrate