The overall function of the human digestive system (Figure 6-1) is to provide materials to be used by the individual cells of the body. These materials are used by the cells:
a. As energy for life processes.
b. For growth and repair of body tissues. 6-2. THE ENERGY CYCLE
The body requires that all of its energy be brought into it from external sources.
a. Solar Radiation. The ultimate source of all energy for living things on Earth is the Sun. This energy reaches the Earth in the form of solar radiation.
b. Photosynthesis. This radiant energy is stored by plants as the chemical bonds of glucose molecules. The process for doing this is called photosynthesis.
PHOTO = light SYNTHESIS = put together This takes place in the presence of the green substance called chlorophyll.
Carbon Dioxide + Water + Energy YIELDS Glucose + Oxygen c. Food Consumption. The green plants are then utilized as food by various animals. Ultimately, either the green plants or the animals that ate the green plants are consumed by humans.
d. Digestion and Metabolic Oxidation. Through the processes of digestion, the glucose is released. It is then delivered to the cells of the body by the circulatory system. Within the cells of the body, the energy is released from the glucose by the chemical process known as metabolic oxidation:
Glucose + Oxygen YIELDS Carbon Dioxide + Water + Energy e. Production and Use of ATP. The released energy is then used to produce the compound known as ATP (adenosine triphosphate). This metabolic oxidation and the production of ATP occur in the mitochondria. For this reason, the mitochondria are known as the "powerhouses" of the cell. When energy is required for carrying on any of the life processes, it is obtained from the ATP.
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