(b) Food is masticated by teeth and moistened by saliva secreted from the salivary glands. Figure 11.5 (a) Digestion of food begins in the mouth. The epiglottis is a flap of tissue that covers the tracheal opening during swallowing to prevent food from entering the lungs. The esophagus leads to the stomach and the trachea leads to the lungs. The pharynx opens to two passageways: the esophagus and the trachea. The tongue helps in swallowing-moving the bolus from the mouth into the pharynx. The chewing and wetting action provided by the teeth and saliva prepare the food into a mass called the bolus for swallowing. Another enzyme called lipase is produced by cells in the tongue to break down fats. It also contains an enzyme called salivary amylase that begins the process of converting starches in the food into a disaccharide called maltose. Saliva also contains lysozyme, which has antibacterial action. Saliva contains mucus that moistens food and buffers the pH of the food. The chemical process of digestion begins during chewing as food mixes with saliva, produced by the salivary glands ( Figure 11.5). All mammals have teeth and can chew their food to begin the process of physically breaking it down into smaller particles. The food is broken into smaller particles by mastication, the chewing action of the teeth. Figure 11.4 The components of the human digestive system are shown.īoth physical and chemical digestion begin in the mouth or oral cavity, which is the point of entry of food into the digestive system. The waste material travels on to the large intestine where water is absorbed and the drier waste material is compacted into feces it is stored until it is excreted through the anus. The smaller molecules are absorbed into the blood stream through the epithelial cells lining the walls of the small intestine. Further breakdown of food takes place in the small intestine where bile produced by the liver, and enzymes produced by the small intestine and the pancreas, continue the process of digestion. This acidity kills microorganisms, breaks down food tissues, and activates digestive enzymes. The stomach contents are extremely acidic, with a pH between 1.5 and 2.5. Using peristalsis, or wave-like smooth-muscle contractions, the muscles of the esophagus push the food toward the stomach. The food is then swallowed and enters the esophagus-a long tube that connects the mouth to the stomach. The enzymes present in saliva also begin to chemically break down food. The teeth play an important role in masticating (chewing) or physically breaking food into smaller particles. The process of digestion begins in the mouth with the intake of food. The rise in obesity and the resulting diseases like type 2 diabetes makes understanding the role of diet and nutrition in maintaining good health all the more important. Taking in more food energy than is used in activity leads to storage of the excess in the form of fat deposits. One of the challenges in human nutrition is maintaining a balance between food intake, storage, and energy expenditure. This happens by both physical means, such as chewing, and by chemical means. During digestion, food particles are broken down to smaller components, which are later absorbed by the body. The conversion of the food consumed to the nutrients required is a multistep process involving digestion and absorption. Animals must convert these macromolecules into the simple molecules required for maintaining cellular function. However, the food consumed consists of protein, fat, and complex carbohydrates. At the cellular level, the biological molecules necessary for animal function are amino acids, lipid molecules, nucleotides, and simple sugars. While plants can obtain nutrients from their roots and the energy molecules required for cellular function through the process of photosynthesis, animals obtain their nutrients by the consumption of other organisms.
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