Enzymes and Vitamins
Without some help, many of the reactions that take place in your cells would not happen fast enough to keep the cells alive. Your internal temperature should not vary more than a few degrees from 37°C. So you cannot increase the temperature to speed up a reaction in your cells. 
Enzymes and vitamins are compounds that help cells function efficiently at normal body temperature.
Figure 20 Lemons, limes, and oranges are sources of vitamin C. Drawing Conclusions Is vitamin C soluble in fat or in water?
Enzymes
Without enzymes, cells could not digest food or extract energy from food. Enzymes are proteins that act as catalysts for reactions in cells. Enzymes allow reactions to proceed faster at much lower temperatures than would normally happen. Your body uses thousands of enzymes to control reactions within your cells. Pepsin, for example, is an enzyme that breaks apart proteins. It works only in the acidic environment of the stomach. Some enzymes require a co-enzyme to function, often a metal ion or a water-soluble vitamin.
Vitamins
In the 1800s, British sailors ate limes on long sea voyages. The limes were protection against scurvy, which causes severe pain and weakness. Limes contain vitamin C. Vitamins are organic compounds that organisms need in small amounts, but cannot produce. A vitamin that dissolves in water, such as vitamin C, gets eliminated from the body and must be replaced daily. A vitamin that dissolves in fats, such as vitamin A, can build up over time in body tissues. Taking excess amounts of fat-soluble vitamins may be harmful.
Section 9.4 Assessment
Reviewing Concepts
- What happens to sunlight during photosynthesis?
- Describe the relationship between photosynthesis and cellular respiration.
- What molecules improve the efficiency of cell functions?
What happens to starch during digestion?
Why can't an increase in temperature be used to speed up reactions in cells?
Critical Thinking
Formulating Hypotheses Why must combustion in cells take place in multiple steps?
Drawing Conclusions Some bacteria do not require oxygen. How would the disappearance of plants affect these bacteria?
Inferring What affect could a lack of essential amino acids have on reactions in a person's body?
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