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Nutrition and Digestion Functions of the Digestive System 1. To break down the foods the body ingests into a form that the body cells can use 2. Transfers nutrients in this form out of the digestive tract and into the blood circulation So food to usable nutrients Polymers to monomers Polysaccharides to monosaccharides Proteins to amino acids Fats (triglycerides) to monoglycerides and fatty acids Functions 3. To retain the waste that remains after nutrients from the food have been transferred 4. To eliminate this waste from the body The Digestive System The digestive tract is essentially a long muscular tube with two openings mouth and anus It receives input along its length from accessory organs and glands There is mechanical digestion, breaking large chunks of food into smaller pieces, providing greater exposure of the molecules to enzymes There is chemical digestion, hydrolysis reactions that break macromolecules using water These reactions are catalyzed by enzymes How can all of this chemical digestion occur without the body digesting its own macromolecules? Compartmentalization Compartments The simplest type of digestion occurs within a cellular organelle As a cell engulfs food by phagocytosis, a food vacuole forms, which then fuses with a lysosome containing enzymes to form a digestive compartment As the food is digested, small food molecules pass into the cytoplasm Compartments Sponges are the only animals that digest food solely within their cells Most animals use a digestive compartment to process food Compartments Simpler animals, including cnidarians (such as hydras and jellies) and flatworms, have a gastrovascular cavity, a digestive compartment with a single opening that functions as both the entrance for food (like a mouth) and the exit for undigested wastes Compartments The vast majority of animals have a digestive tube with two separate openings, a mouth and an anus Such a tube is called an alimentary canal, or digestive tract Food moves through specialized regions that digest and absorb nutrients in a stepwise fashion This adaptation allows for much more efficient food processing Human Digestive System Mouth Food is chewed (mechanical breakdown) Complex carbohydrate digestion begins Saliva secreted from the salivary glands contains salivary amylase The tongue mixes the chewed food with saliva forming a bolus Pharynx The pharynx connects the mouth to the esophagus and opens to the trachea, or windpipe, which leads to the lungs During swallowing, a reflex moves the opening of the trachea upward and tips a door-like flap called the epiglottis to close the entrance to the trachea Esophagus Peristalsis moves the bolus through the esophagus and into the stomach Stomach The bolus moves past the cardiac sphincter into the stomach The stomach is highly elastic and convoluted, capable of expanding to accommodate the amount of food consumed Storage of food material “Time-release ” transfer into the small intestine Secretions include mucus, HCl, enzymes (pepsin), and hormones (gastrin) This mixture is now called chyme Small intestine Accessory organs aid the digestion that occurs in the small intestine The pancreas produces enzymes for the digestion of carbohydrates, fats, proteins, and nucleic acids and buffer to neutralize the chyme The liver produces bile that is stored in the gallbladder - important for the emulsification of fats Emulsifiers (bile) break up large globs of fat into smaller droplets This increases the number of fat molecules available to be digested Small intestine The chyme is stored in the stomach where there is continued digestion Small amounts move past the pyloric sphincter into the small intestine The small intestine is the site of the completion of digestion The typical meal takes 3-6 hours to move out of the stomach ~4 tablespoons at a time Completed digestion leaves monomers from those original polymers Absorption Monosaccharides and amino acids cross the intestinal lining into the circulatory system for distribution Fatty acids move into the lymphatic system The intestinal lining is covered with villi and microvilli to increase the surface area facilitating absorption Large intestine (colon) Material that is not absorbed from the small intestine is moved into the large intestine It is the site of concentration and storage of feces Undigested and unabsorbed material, water , and bacteria Water is reclaimed leading to concentration of feces Also absorbs vitamins produced by our resident bacteria Rectum and anus Material that collects in the colon eventually moves into the rectum The nervous system controls the sphincter muscle at the anus to retain/expel the fecal material Bulk, roughage, or fiber is fibrous material that makes the feces more bulky leading to a regular need to expel Human Nutrition A nutrient must do at least one of the following: Provides energy Acts as a structural building block for the body Regulates a process in the body There are 6 categories of nutrients Water, minerals, vitamins, carbohydrates, proteins, lipids Calories All of the calories that we take in during the day come from proteins, carbohydrates, or lipids A calorie is the amount of energy required to raise the temperature of 1 gram (ml) of water by 1°C A nutritional calorie (Calorie) is the amount required to raise 1000 grams (1 liter) by 1°C Metabolic Rate The rate at which your body consumes energy is called your metabolic rate A person’s metabolic rate consists of the basal metabolic rate (BMR), the amount of energy it takes to maintain body functions, and any additional energy consumption above that base rate The more active you are, the greater your actual metabolic rate and the greater the number of calories your body uses per day Any excess Calories that we consume during the day gets stored Mainly as fat, very little as glycogen Each gram of fat stores and yields 9 Calories Each gram of carbohydrate and protein stores and yields 4 Calories Proteins Our digestive system breaks proteins from food down to individual amino acids Although we have the ability to make 12 of the amino acids from scratch, there are 8 that we cannot make - essential amino acids Most proteins from animal products are complete, containing all 8 essential amino acids Most proteins derived from plant products are not Carbohydrates Can be divided into simple sugars (mono- and disaccharides) and complex carbohydrates consisting of digestible (starch) and indigestible (fiber) Nutritionists recommend a diet that is Low in simple sugars added to processed foods High in complex carbohydrates from fresh fruits, vegetables, and wholegrain foods Simple sugars represent “empty” Calories without vitamins and minerals They can result in a surge of glucose in the bloodstream Complex carbohydrates require more digestion resulting in a slow and steady supply of glucose Water For most people under most circumstances, thirst is an accurate guide to ensure we take in enough water to stay sufficiently hydrated Vitamins Vitamins are needed by the body to facilitate chemical reactions People who eat a balanced diet should be able to obtain enough of all needed nutrients in their food Minerals Minerals are needed by the body to either help form bodily structures or to facilitate chemical reactions Lipids Excellent source of energy but also play many roles in the body The form for much of our dietary lipids is the triglyceride The amount of saturation of those fatty acids is key Trans fats and saturated fats increase the risk of cardiovascular disease Polyunsaturated and monounsaturated fats are neutral Unsaturated fats rich in omega-3 fatty acids are protective A Healthy Diet Diets can be judged as healthy to the extent that they contain a high proportion of healthy foods Food pyramids are a useful visual guide to the intake of different foods Nutritional Disorders Dietary problems can have severe health consequences Obesity Obesity is a too-high body mass index (BMI), is the nutritional disorder of greatest concern, increases the risk of heart attack, diabetes, cancer, and several other diseases