The Digestive System

The digestive system is responsible for obtaining and digesting food for the purpose of supplying nutrients and energy to the remainder of the body. The process of digestion is both mechanical and chemical, and both forms of digestion begin in the mouth. Food that enters the mouth is broken down into smaller particles and mixed with saliva as it is chewed. Saliva serves as a lubricant and a source of starch-digesting enzymes. Enzymes are special proteins that speed up chemical reactions. Once swallowed, food enters the esophagus and then the stomach, a highly acidic environment where enzymes begin digesting other nutrients.

From there food moves into the small intestine, which is the major site of both digestion and absorption. The small intestine is lined with tiny finger-like projections called villi that greatly increase the amount of intestinal surface that comes in contact with nutrients, allowing more nutrients to be absorbed than a smooth surface would allow. Any undigested food and unabsorbed nutrients pass through the large intestine for a last chance at absorption before being expelled as waste. Most water is absorbed here, along with B vitamins and vitamin C.

Energy

Although all nutrients are important, a significant role of digestion is to provide chemical energy for the body. Carbohydrates are the primary source of chemical energy for the body, with fats serving as energy storage. Fats are used as energy after muscles and other tissues use the body’s carbohydrate supply. The body is also capable of breaking down proteins for energy when needed. Thus food is considered energy input. Notice that the amount of carbohydrates, fats, and proteins in the apple nutrient example above are 13.81 g, 0.17 g, and 0.26 g, respectively per 100 g of apple and that combined, these nutrients provide a total of 50 ckal (kilocalorie) of energy. This amount of energy per 100 g of nutrients for an apple is compared to other examples of food in the chart below:

In this graphic, the example foods are arranged in two different ways. On the top, the foods are arranged in the highest to lowest amount of energy contained in 100 grams of the product. The bottom chart contain exactly the same information but is arranged in the highest to lowest amount of protein per 00 grams of product. Thus, while chocolate may provide a lot of energy, it contain relatively less protein. Therefore, while the energy may be immediately used in exertion and exercise, it is of much less use in building muscle. It is the differences in food nutrient content that must be taken into account when designing a balanced and healthy diet.

Uses of Energy by the Body

The energy released by the breakdown (digestion) of these three types of nutrients (carbohydrates, fats, and protein) is used for survival and for activity. The energy devoted to the involuntary actions involved in the body’s day-to-day survival is called the energy of maintenance or basal metabolism. The rate at which these maintenance functions use energy is called basal metabolic rate.

Maintenance functions include brain and nerve activity, contraction of smooth and cardiac muscle, the energy needed for building and repairing cells and tissues, and other activities of the body which are not under a person’s conscious control. Energy used for voluntary activity is that energy expended beyond what is needed for maintenance. This energy is mostly used by skeletal muscle during physical activity, but also is used for active thinking!

The total energy needed for maintenance and activity together is called energy output. Ideally, the body should be in a state of energy balance—energy input should be equal to energy output. This can be stated as “energy input minus energy output equals energy balance”. Therefore, when the units of energy input equal the units of energy output, energy balance mathematically equals “0”.

Energy balance is achieved through a combination of healthy food choices and adequate amounts of physical activity. When energy input exceeds energy output, the body is in positive energy balance and excess energy is stored as fat. When energy input is less than energy output, fat stores are used and the body is in negative energy balance. A physically fit person maintains energy balance in their body by making healthy food choices and engaging in physical activities that actively use energy.

In Investigation Four, students will review the role of the digestive system in nutrient absorption and perform experiments to examine how food choices and level of physical activity determine a person’s energy balance.