Digestive system

Why do we need a digestive system? The digestive system uses mechanical and chemical methods to break complex molecules down into simpler molecules that can be absorbed into the blood. Proteins (meat, diary, nuts) into Amino Acids Carbohydrates (starch) into Simple Sugars Lipids into fatty Acids and Glycerol The process takes time, and therefore the tube is long and convoluted to allow the food processing to occur.

First site of digestion - mouth ( Biology text page ) Mechanical breakdown begins in the mouth by chewing (Mastication) which increases the surface area of the food Chemical breakdown of Starch by the enzyme Salivary Amylase. Maltose (glucose-glucose) is the product of this digestion.

Food is then swallowed as a moist ball called a bolus The epiglottis ensures food doesn’t go down our windpipe( trachea)

How is the food moved along the digestive tract? ( Biology text page ) Peristalsis is the muscular contractions that propel the food down the esophageus and along the digestive tract.

Second site of digestion- stomach ( Biology text page ) The stomach is a muscular sac with sphincters at each end. (Cardiac and Pyloric) The stomach lining secretes about 2 litres of gastric juices per day. This Gastric juice contains hydrochloric acid, pepsinogen, and mucus Secretions are controlled by nervous stimuli (eg smells, thoughts/patterns of behaviour) and endocrinal signals ( hormones).

Stomach wall contains Goblet cells They secretes hydrochloric acid and pepsinogen. Hydrochloric Acid (HCl) lowers pH of the stomach to 2 / 3, which kills most bacteria and activates the enzyme pepsinogen to pepsin. Pepsin is a type of protease (protein enzyme) that breaks down proteins into peptides (small chains of amino acids The Mucus ensures that the Hydrochloric Acid does not burn a hole in the stomach lining, ie stopping ulcer forming

Stomach in Summary ( Biology text page ) Protein digestion - pepsin begins the process of protein digestion cleaving large proteins into shorter amino acid chains called peptides. Storage – It allows a meal to be consumed and the materials released slowly into the duodenum for further digestion( small intestine). Food may be churned in the stomach for up to four hours before moving onto the duodenum. Acidic Environment to assist in activating enzymes and killing bacteria( Mucus is produced to stop corrosion of the lining Limited absorption - water, electrolytes, and fat soluble molecules including alcohol

The Small Intestine ( Biology text page ) Small intestine is a coiled tube over 6 meters long. Made up of three sections 1.Duodenum 2.Jejunum 3.Ileum

Third Site of digestion - Duodenum ( Biology text page ) The pancreas secretes juices containing bicarbonate to neutralizing the stomach acid. Brunner's glands also produce alkaline secretions to counteract the effects of gastric acids that reach the duodenum. Epithelial cells of the duodenum secrete a watery mucus. The liver produces bile, which is stored in the gall bladder before entering the bile duct into the duodenum.

Electron micrograph of the Villi ( Biology text page )

Cross-section of the ileum

Structures and function of a Villus

Summary of Digestion in the Duodenum Protein digestion completed with the enzyme Trypsin working in an alkaline environment Carbohydrate digestion completed with the enzyme amylase and maltase reducing disaccharides to glucose Bile (from the liver) emulsifies (breaks into small particles) lipids (fats), therefore allowing lipases to digest fats(lipids)

Hormonewhere foundactionresultshow stopped Gastrinstomach cells presence of food in stomach stimulates stomach glands to produce more hydrochloric acid stomach produces more acid to aid in the digestion and breakdown of food when acid reaches a pH of 1.5, the stomach cells stop producing gastrin Secretinduodenal cells when chyme enters the duodenum, secretin enters the blood and tells the pancreas to release bicarbonate to reduce the acid pH of intestines is increased so other enzymes can work and absorption can take place. when stomach acid has been sufficiently neutralized, secretin is no longer secreted by the duodenal walls Cholecystokinin (CCK) intestinal cells presence of fats in the intestines stimulates the gall bladder to contract, releasing bile into the small intestine bile mixes with chyme and emulsifies fats so they can be more easily absorbed when fat no longer present in the intestine, production of CCK stops Gastric- inhibitory Peptide intestinal cells presence of fats in the small intestine triggers release of hormone Gastric-inhibitory peptide slows GI tract motility and inhibits gastric acid production. Digestion and absorption of fats takes longer than carbohydrates or proteins when fat no longer present in the intestine, production of gastric- inhibitory peptide stops Hormones involved in digestion

The large Intestine - Colon Secretions in the large intestine are an alkaline mucus that protects epithelial tissues and neutralizes acids produced by bacterial metabolism. Water, salts, and vitamins are absorbed, the remaining contents in the lumen form feces (mostly cellulose, bacteria, bilirubin). Bacteria in the large intestine, such as E. coli, produce vitamins (including vitamin K) that are absorbed.

What you need to know about the digestive system List all parts (in order) of the human digestive system List the auxiliary organs that contribute one or more substances to the digestive process. Describe the cross-sectional structure of the small intestine and explain how its structure is related to its function. Describe –Protein digestion and absorption –Carbohydrates digestion and absorption –Lipid digestion and absorption

We are what we eat !!!