Chapter 6: Digestion, Absorption and Metabolism-The Link Between Food and Energy

Transcription

1 Chapter 6: Digestion, Absorption and Metabolism-The Link Between Food and Energy Food provides us with (1) energy (ATP) for body processes, (2) heat for body temperature regulation and (3) the chemical building blocks for growth and maintenance of body tissues. Food is intimately connected to our sense of taste, but also stimulates our senses of sight, smell, touch and hearing. The goals of digestion are to break large molecules down to smaller molecules and absorb the smaller molecules into the cells of the body. Molecules are the building blocks of cells. The foods we eat undergo three processes: (1) digestion, (2) absorption and (3) elimination These processes occur in the gastrointestinal tract. Gastrointestinal (GI) tract is a series of organs arranged as a long tube. Digestion is the process of breaking large food molecules down to smaller molecules. Digestion includes: (1) mechanical digestion, which is the physical breakdown of food and (2) chemical digestion, which includes the enzymatic reactions that break down food molecules. Sense of smell (olfaction) plays a role in the process of digestion. You use other senses as well, such as sight and touch) to determine if you should consume some sort of food. Taste receptors (taste buds) in the mouth also help with choosing foods. The GI tract includes: (1) Digestive Organs Mouth Esophagus 18

2 Stomach Cardiac Sphincter Pyloric Sphincter Small Intestine Protein digestion begins here - in the stomach. The stomach has a volume of 50 ml when empty and about 4 liters when filled. Large Intestine (Colon) Peristalsis is the muscular movement of materials through the GI Tract. Segmentation is the periodic squeezing motion of the intestines, which forces the materials backward to mix with digestive juices then forward again. (2) The Accessory Structures Tongue Liver Gall Bladder Pancreas Salivary Glands Digestive Glands (1) Located in the stomach. These glands produce and release: 19

3 Gastrin, a hormone that regulates acid. Hydrochloric acid (HCl). Enzymes for protein digestion. (2) Located in the intestines. These glands produce and release: Enzymes for all types of foods. CCK, a hormone from the walls of the intestine that stimulates the gall bladder to release bile. The duodenum also produces secretin, which is a hormone that regulates the release of pancreatic juice. Digestion begins in the mouth. Chewing is the mechanical digestion that breaks food into smaller pieces. Some chemical digestion takes place. Salivary amylase is an enzyme produced by the salivary glands that begins the chemical digestion of carbohydrates. Digestion in the stomach includes: (1) extensive mechanical digestion to mix food with gastric juice and (2) the chemical digestion of proteins and fats. Gastric juice contains (1) hydrochloric acid (HCl) to denature proteins and activate pepsin, (2) pepsin an enzyme to digest protein, (3) gastric lipase an enzyme to digest fat and (4) mucus to protect the stomach lining. Chyme is semi-solid product of mechanical and chemical digestion in the stomach. From the stomach, chyme is slowly released through the pyloric sphincter to the small intestine. Chemical digestion continues in the small intestine using pancreatic enzymes and bile. Absorption is the process of taking molecules across a cell membrane and into cells of the body. A small amount of absorption occurs in the stomach. Most absorption of nutrients occurs in the small intestines. The lining of the GI tract has special structures (villi) to facilitate absorption. Villi are folds in the lining that are in close contact with nutrient molecules. The brush border is composed of microvilli, which greatly increase the surface area. Undigested food components move through a sphincter called the ileocecal valve to the large (colon) intestine. Material is stored in the colon up to hours prior to elimination. Water and some nutrients are absorbed. The lining of the stomach is designed to cope with hydrochloric acid but other regions of the GI tract are not. The entire process of digestion and absorption is regulated by negative feedback systems. REMEMBER! Each area of the body must maintain homeostasis and part of homeostasis involves keeping an appropriate ph level. Loss of appropriate ph means denaturation of proteins will occur and enzymes will be destroyed. 20

4 Buffers include a special class of proteins that help maintain correct ph even when the environment is changing. The ph Scale is a type of indicator system. It allows us to measure current ph and track changes over time. Metabolism: Ultimately all living things get their energy from the sun. Plants use a process called photosynthesis to take in the sun s visible light rays and transform them into a form of energy that a cell can use: chemical bond energy. Equation for photosynthesis: 12H 2 O + 6CO 2 C 6 H 12 O 6 + 6O 2 + 6H 2 O Enzymes, ATP, sunlight and protein pigments are all required in order to produce these products. This reaction is aerobic. The process of metabolism deals with all chemical reactions that process any nutrients. Some of these reactions will be anabolic and others will be catabolic. Anabolic (Anabolism) Catabolic (Catabolism) Reactions in living systems transfer energy from storage or from a less useful form to a more useful form, such as ATP. In order for any reaction to occur: Correct reactants must be present and in close proximity. Appropriate enzyme(s) must be available. Additional energy, such as ATP, must be available if necessary. There are always numerous steps in metabolic reactions. Why? To control the release of energy (especially heat: to avoid denaturation of proteins such as enzymes) and to create entry and exit points for materials to join or leave the reaction (such as in gluconeogenesis: the alternative reaction to glycolysis when glucose is absent/insufficient). Enzymes are required for all metabolic reactions and they often require assistance, which includes the use of Coenzymes Cofactors 21

5 The process of cell respiration is the core of a cell s metabolic process. Cell respiration begins when food enters the cytoplasm of a cell. The process of cell respiration: Is the reverse of photosynthesis. Is aerobic overall, even though the first of the three sets of reactions is anaerobic. Is catabolic overall, even though some steps may be anabolic. Releases discrete units of energy. Uses various energy molecules including ATP and enzymes. Uses special energy transport molecules such as NAD, which contains niacin/b 3 and FAD, which contains riboflavin/b 2. Consists of three sets of reactions: glycolysis, TCA Cycle and ETC/ETS. Occurs in the cytoplasm. Glycolysis Changes glucose to pyruvate (pyruvic acid). It is an anaerobic process. It is catabolic overall. The fate of the pyruvate formed by glycolysis depends upon the availability of oxygen. Under normal circumstances, enough oxygen is available and pyruvate can be modified (the Co-A step of the reaction) to enter the cell organelles known as mitochondria (sing: mitochondrion). When oxygen is unavailable or not enough is available then the pyruvate is processed via an alternative pathway in the cytoplasm. This alternative pathway is called fermentation. Products formed from fermentation vary depending upon the organism. We form lactate (lactic acid). Fermentation for us is a temporary, fail-safe system and cannot be maintained long term. TCA (Kreb s or Tricarboxylic Acid Cycle) Occurs in the matrix of each mitochondrion. It is an aerobic and catabolic process. It produces discrete packets of energy. It uses transport molecules, such as NAD and FAD to move energy. It transfers the energy to the mitochondrial membranes. Electron Transport System Also known as the cytochrome system. Located within the mitochondrial membrane. Consists of stepping stones made of cytochromes and others molecules. 22

6 Channels energy released from Krebs Cycle into the space between the layers of the mitochondrial membrane. Fermentation alternative pathway when oxygen is lacking If not enough oxygen is available you have this fail-safe mechanism that shunts the pyruvate formed from glycolysis into a special anaerobic pathway located in the cytoplasm. This reaction occurs only in the cytoplasm and produces energy by processing pyruvate into lactate (lactic acid), which is then stored away. This process is a lifesaver for humans not a permanent fix to an oxygen-supply problem. It can only be temporary. What if you have plenty of oxygen, but lack glucose? How can you perform cell respiration? Wouldn t a fail-safe for fad diet survival be nice? Here it is: Gluconeogenesis alternative pathway when glucose is lacking An alternative method of continuing cell respiration when no/not enough glucose is present. Uses amino acids, glycerol, fats and fatty acids to do cell respiration by using alternative entry points and special enzymes for forward/backward reactions. Increases the amount of by-products such as ketone bodies from the metabolism of amino acids and fatty acids. Can use some amino acids to make glucose and others just to make energy. Digestive Disorders: Heartburn GERD ( Reflux ) Ulcers Food Allergies Diarrhea Constipation Discussion Topics/Questions: If you swallow chewing gum, will it stay in your stomach for seven years? p. 152 Colon Cleansing: beware of parasites p. 157 Probiotics p. 158 When you mix it up: combining foods ( Fit for Life ) p