2. Trapping and Using Energy All of the activities of an organism involve chemical reactions in some way. The total of all chemical reactions in an organism is called metabolism. Energy for Life 3 3 The chemical reactions of metabolism need enzymes.

3. Trapping and Using Energy Enzymes in cells cause a change, but the enzyme is not changed and can be used again. Enzymes also can cause molecules to join. Energy for Life 3 3 Without the right enzyme, a chemical reaction in a cell cannot take place. Each chemical reaction in a cell requires a specific enzyme. Click image to view movie.

4. Photosynthesis Living things are divided into two groups— producers and consumers—based on how they obtain their food. Organisms that make their own food, such as plants, are called producers. Energy for Life 3 3 Organisms that cannot make their own food are called consumers.

5. Photosynthesis Plants and many other producers can convert light energy into another kind of energy—chemical energy. The process they use is called photosynthesis. Energy for Life 3 3 During photosynthesis, producers use light energy to make sugars, which can be used as food.

6. Producing Carbohydrates Producers that use photosynthesis are usually green because they contain a green pigment called chlorophyll (KLOR uh fihl). Chlorophyll and other pigments are used in photosynthesis to capture to capture light energy. Energy for Life 3 3 In plant cells, these pigments are found in chloroplasts.

7. Producing Carbohydrates The captured light energy is used to drive chemical reactions during which the raw materials, carbon dioxide and water, are used to produce sugar and oxygen. For plants, the raw materials come from air and soil. Energy for Life 3 3

8. Producing Carbohydrates Some of the captured light energy is stored in the chemical bonds that hold sugar molecules together. Energy for Life 3 3

9. Storing Carbohydrates Plants make more sugar during photosynthesis than they need for survival. Energy for Life 3 3 Excess sugar is changed and stored as starches or used to make other carbohydrates. Plants use carbohydrates as food for growth, maintenance, and reproduction. Consumers take in food by eating producers or other consumers.

10. Respiration Some of the energy from the food you eat is used to make you move. Energy for Life 3 3 Some of it becomes thermal energy, which is why you feel warm or hot when you exercise. Most cells need oxygen to break down food.

11. Respiration During respiration, chemical reactions occur that break down food molecules into simpler substances and release their stored energy. Energy for Life 3 3 Just as in photosynthesis, enzymes are needed for the chemical reactions of respiration.

12. Breaking Down Carbohydrates The type of food molecules most easily broken down by cells are carbohydrates. Energy for Life 3 3 Respiration of carbohydrates begins in the cytoplasm of the cell. The carbohydrates are broken down into glucose molecules. Each glucose molecule is broken down further into two simpler molecules. As the glucose molecules are broken down, energy is released.

13. Breaking Down Carbohydrates The two simpler molecules are broken down again. This breakdown occurs in the mitochondria of the cells of plants, animals, fungi, and many other organisms. Energy for Life 3 3 This process uses oxygen, releases energy, and produces carbon dioxide and water as wastes.

14. Breaking Down Carbohydrates When you exhale you breathe out carbon dioxide and some of the water. Energy for Life 3 3 Respiration occurs in the cells of all living things.

15. Fermentation When cells do not have enough oxygen for respiration, they use a process called fermentation to release some of the energy stored in glucose molecules. Energy for Life 3 3 Like respiration, fermentation begins in the cytoplasm.

16. Fermentation Again, as the glucose molecules are broken down, energy is released. Energy for Life 3 3 But the simple molecules from the breakdown of glucose do not move into the mitochondria. Instead, more chemical reactions occur in the cytoplasm.

17. Fermentation These reactions release some energy and produce wastes. Energy for Life 3 3 Depending on the type of cell, the wastes may be lactic acid, alcohol, or carbon dioxide. The presence of lactic acid is why your muscle cells might feel stiff and sore after you exercise.

18. Related Processes During photosynthesis and respiration, what is produced in one is used in the other. Energy for Life 3 3 Photosynthesis produces sugars and oxygen, and respiration uses these products. The carbon dioxide and water produced during respiration are used during photosynthesis