Five external factors that must be present or provided to sustain life include: 1. Nutrients (food) contain the chemical substances used for energy and building cells. 2. Oxygen is required because the reactions within cells that release energy from food are oxidative reactions, meaning they need oxygen to proceed. 3. Water provides the environment necessary for chemical reactions and the fluid base for body excretions and secretions. 4. Normal body temperature is necessary because chemical reactions occur best at certain temperatures. Too low and the reactions will slow down and eventually stop. Too high and the reactions will occur too fast and then fail as proteins lose their shape. 5. Appropriate atmospheric pressure is necessary in order for sufficient gas exchange to occur through breathing.
Work Step by Step
Each factor is discussed on page 8. Life requires five external factors or survival needs. However, it is not enough that each external factor merely be present. They must also be present in the right amount.