The Digestive System and Body Metabolism Digestion Breakdown of ingested food Absorption of nutrients into the blood Metabolism Production of cellular energy (ATP) Constructive and degradative cellular activities Digestion is primarily extracellular, with the end of digestion at the pt when nutrients are absorbed into cells. Metabolism occurs within cells, involving release of chemical energy from food AND building of biological molecules.

Processes of the Digestive System Mechanical propulsion Secretion Digestion - mechanical and chemical breakdown Absorption Elimination One way disassembly line. Several steps done by dift organs. Some overlap in function. Simpler systems are sacs, with batch processing only. One-way systems allow specialization, continuous processing, th4, constant or irregular feeding. Digestive systems have evolved to match feeding habits/ opportunities. So, bird with expandable crop to collect seeds, grind them up in safer location. Carnivores with shorter intestines, as meat is quicker to digest than plant material with cellulose. Ruminants that rechew food to break down plant fibers, get max nutrition w/ aid of microbes.

Mechanical Processing Teeth break down food molars (12) Teeth - Permanent teeth replace deciduous teeth beginning between the ages of 6 to 12 A full set is 32 teeth, but some people do not have wisdom teeth Shape of tooth reflects type of cutting/grinding: Types: incisors, canines, premolars, molars Structure: crown, root Tongue: skeletal muscle, moves food in mouth, taste Saliva: Mixture of mucus and serous fluids Helps to form a food bolus Contains salivary amylase to begin starch digestion Dissolves chemicals so they can be tasted Source: parotid, submandibular, sublingual salivary glands Composition: mucin, salivary amylase, bicarbonate, lysozyme Permanent teeth Replace deciduous teeth beginning between the ages of 6 to 12 Activities of the Pharynx and Esophagus These organs have no digestive function Serve as passageways to the stomach Voluntary phase: tongue pushes bolus of food into pharynx Involuntary phase/swallowing reflex: receptors in pharynx stimulated by presence of food Soft palate rises, Larynx rises slightly Tongue pushes food further, Food enters esophagus Tongue blocks off the mouth Soft palate (uvula) blocks the nasopharynx Epiglottis blocks the larynx Peristalsis moves the bolus toward the stomach The cardioesophageal sphincter is opened when food presses against it premolars (8) canines (4) incisors (8) lower jaw upper jaw

Stomach Functions Acts as a storage tank for food Mechanical, chemical breakdown of protein begins Delivers chyme to the small intestine Chyme = processed food Convolutions of interior surface allow stomach to expand. Gastric juice: specific cells secrete Hydrochloric acid: produces a pH of about 2, breaks down large bits of food Intrinsic factor; made by same cells making acid, needed to absorb Vitamin B12 Mucus: protects stomach lining from acid Pepsinogen: activated by acid to pepsin, begins protein breakdown Rennin – works on digesting milk protein The only absorption that occurs in the stomach is of alcohol and aspirin Necessity of an Extremely Acid Environment in the Stomach Activates pepsinogen to pepsin for protein digestion Provides a hostile environment for microorganisms

Propulsion in the Stomach Food must first be well mixed Rippling peristalsis occurs in the lower stomach The pylorus meters out chyme into the small intestine (30 ml at a time) The stomach empties in four to six hours Stomach contractions: blend food and propel forward Structural adaptation: third muscularis layer Direction: from lower esophageal sphincter to pyloric sphincter Chyme: result of mixing, affects hormone secretions regulating peristalsis and emptying of stomach Gastric fluid is regulated by neural and hormonal factors Presence of food or falling pH causes the release of gastrin Gastrin causes stomach glands to produce protein-digesting enzymes Figure 14.15

Chemical Digestion in the Small Intestine Pancreatic enzymes digest starch, proteins, nucleic acids, fats; reset pH. Gall bladder - Pancreatic enzymes play the major digestive function Help complete digestion of starch (pancreatic amylase) Carry out about half of all protein digestion (trypsin, etc.) Responsible for fat digestion (lipase) Digest nucleic acids (nucleases) Alkaline content neutralizes acidic chyme Endocrine products of pancreas Insulin Glucagons Enzymes from the brush border Break double sugars into simple sugars Complete some protein digestion Figure 14.6

Propulsion in the Small Intestine Peristalsis is the major means of moving food Segmental movements Mix chyme with digestive juices Aid in propelling food Esophagus moves food bolus by peristalsis Small intestine uses segmental movements to mix food, push it up agst lining for better absorption.

Absorption in the Small Intestine Site of nutrient absorption into the blood Villi = Fingerlike structures on the mucosa surface Microvilli on cells All increase surface area Each villus is made of many cells. Each absorptive cell has tiny microvilli (1700) on its free surface. = brush border. Result is increase about 600-fold of surface area. Note vascular connections and lymph collecting vessels (green) Most absorbed by active transport through cell membranes Substances are transported to the liver by the hepatic portal vein or lymph Figure 14.7a

Absorption of Proteins and Carbohydrates Amino acids, sugars move by active transport into blood vessels Next stop: liver, where glucose glycogen amino acids protein Excess molecules are converted to fats Figure 14.13

Absorption of Fats Lipids are absorbed into lymph system, which drains into heart Emulsified with bile salts Digested into fatty acids, glycerol With bile salts, diffuse into cells Reassembled into fats, exocytosis Figure 14.14

Food Breakdown and Absorption in the Large Intestine No digestive enzymes are produced Resident bacteria digest remaining nutrients Produce some vitamin K and B Release gases Water and vitamins K and B are absorbed Undigested fiber keeps materials moving and is eliminated via feces

Control of Digestive Activity Regulation dependent on volume and content of food Nervous system: sight, smell of food, stretch receptors in stomach Hormones: Gastrin: stimulates release of gastric juice Secretin: stimulates pancreas to secrete water and bicarbonate Cholecystokinin (CCK): signals pancreas to secrete digestive enzymes Chemical and mechanical receptors in organ walls trigger reflexes Table 14.1, p475. Stomach is stimulated by food entering it. Produces 3 hormones: Gastrin - stimulates release of gastric juice, stimulates small intestine Histamine - stimulates parietal cells to release HCl. pH drops Somatostatin - inhibits secretion of gastric juice and pancreatic juice. Inhibits emptying of stomach. 2-3 L of gastric juice per day, normally. Stimuli include: Stretch of the organ pH of the contents Presence of breakdown products Reflexes include: Activation or inhibition of glandular secretions Smooth muscle activity NOTE: this is not appetite control mechanism. Separate system. Appetite stimulated by: CCK (from pancreas) , low glucose levels in blood. Suppressed by leptin (from adipose cells). Phen-fen drug.

Nutrition Carbohydrates: major energy source, simple or complex Lipids: cell components and energy sources, saturated or unsaturated Proteins: 20 amino acids Vitamins: fat soluble and water soluble Minerals: recommended daily allowance Fiber Major biological polymers, made of repeated monomers. Same categories as listed on food nutrition labels. These molecules make up the vast bulk of our food, organic or otherwise. Carbs - energy, fiber Lipids, energy and membs Proteins - made up of amino acids.20 essential our bodies must ingest 9 of these, can make the rest. Vitamins - coenzymes. Essential to function of other enzymes, but not consumed in reaction. Minerals - specific functions. Fe in hemoglobin. Water - vital. Part of everything

Food Guide Pyramid Carbohydrates are The body’s preferred source to produce cellular energy (ATP) Glucose (blood sugar) is the major digestive product and serves as fuel to make ATP Brain uses 65% of blood glucose. Neurons take lots of energy, prefer to use glucose. Other cells will use fats, proteins if glucose is unavailable. Caffeine stimulates enzymes which break down fats AND those which make storage fats. No net gain. Figure 14.16

Body Energy Balance Energy intake = total energy output (heat + work + energy storage) Energy intake from food oxidation Energy output Heat is usually about 60% Storage energy is in the form of fat or glycogen Energy intake - combat obesity by blocking conversion of food. Olestra - fat which cannot be broken down (5 Carbon sugar with 5 fatty acids) gastric bypass surgery artificial sweeteners - same sweetness, fewer molecules = fewer Calories Energy output - maintain body temp. Heat is usly lost to environment. So if two people weigh the same, which uses more energy for body temp stability: short, wide vs tall, thin? Why? Storage energy - carbo loading before a competition is putting glycogen into muscle cells and liver. Longer term storage uses fats in adipose tissue, usly underneath skin.

Regulation of Food Intake Mechanisms that may regulate food intake Levels of nutrients in the blood Hormones Body temperature Psychological factors Anorexia - may involve some a transport protein for norepinephrine.neurotransmitter, Bullemia Hormones : ghrelin, from stomach and brain cells, makes you feel hungry. Goes down after a meal. Increases with dieting. Leptin - secreted by fat cells. Dampens appetite center in brain. Not a candidate for weight control. Overweight people may beless sensitive to leptin. CCK - cholecystokinin - from small intestine to stimulate pancreas to release enzymes. Also signals brain to dampen appetite. Perhaps some potential as anti-obesity drug. PYY3-36 - another candidate for appetite control. Dampens.

Basal Metabolic Rate BMR– amount of heat produced by the body per unit of time at rest Factors that influence BMR Surface area Gender Age – children and adolescents have a higher BMR thyroxine from thyroid gland Give reasons why each one of these factors should influence BMR. Factors that influence BMR Surface area - loss of body heat. Surface area/volume Gender - males highe b/c of lower % BODY FAT. Adipose tissue w/ low metabolic rate. Age – children and adolescents have a higher BMR b/c of growth, not just repair of cells. thyroxine from thyroid gland.

Total Metabolic Rate (TMR) Total amount of kilocalories the body must consume to fuel ongoing activities TMR increases with an increase in body activity