Nutrition module 2 study guide

2.1 Digestive: From Food to Fuel What is digestion? Digestion is the process of transforming food into basic nutrients that can be absorbed and used by the body. Just as a car needs gas to move, our bodies require fuel to function and perform daily activities. Once absorbed, these nutrients can be used to fuel cellular growth, fight infections, and provide energy to our muscles and brain. Without the process of digestion, the nutrients we initially consume in food would be useless. Before food even enters the mouth, a cascade of events involving the nervous system and a variety of hormones are set of based on a variety of stimulus: • cognition: thoughts about food can start the flow of saliva • sound: hearing a description of the meal • odor: smells stimulate a hunger response and influence its taste • appearance: seeing the food • taste: begins as the food enters your mouth and also how it feels Once food enters the mouth, it begins to break down into smaller units in both mechanical and chemical processes. The mechanical process begins in the mouth with chewing. From there, involuntary muscle contractions are used to move food mixtures along the tract, a process referred to as peristalsis. However, it should also be noted that chemical aids may be present along the various stages of digestion. Enzymes, acid, bile, and mucus are all chemical means the body has at its disposal to further aid in breaking down food. Notably, enzymes are proteins that catalyze (speed up) chemical reactions. Relative to digestion, enzymes catalyze the chemical reactions required to break down food particles into smaller parts, which prepares the nutrients for absorption. Absorption Nutrient absorption occurs across the wall of the gastrointestinal (GI) tract—a long hollow tube consisting of several layers of tissue that begins with the mouth and ends at the anus. The mucosa (intestinal wall) is the inner-most layer, and it is made of absorptive cells and glands. Circular and longitudinal muscles comprise the outer layers, both of which function to mix and move food along the GI tract. Notably, circular bands of muscle are most often found where one part of the tract connects to another, serving as valves to control the flow of the food particles. As shown in Figure 2.1, there are three processes the body can use to move nutrients from the GI tract into the blood/lymph system and eventually into the cells: 1. Passive diffusion: Substances move easily in and out of cells without the use of energy. Nutrients move from high to low concentrations. 2. Facilitated diffusion: No energy is required, but a special protein carrier is required to help substances cross in or out of the cell. Nutrients move from high to low concentrations. 3. Active transport: Energy is required to move substances in or out of the cell. Nutrients move from low to high concentrations. As summarized in Figure 2.2, neither passive nor facilitated difusion require an initial input of energy to transport nutrients across a membrane. However, a specialized carrier (or gatekeeper) protein is required for facilitated difusion. In both cases, nutrients naturally move from an area of high concentration to an area of low concentration. Unlike passive transport, active transport not only requires an input of energy, it also moves nutrients against the natural gradient, moving from areas of low concentration to areas of high concentration.