Human Biology: Digestive System

Human Biology: Digestive SystemLesson 2: Chemical Digestion and Absorption

Todays ObjectivesAnalyse the functional inter-relationships of the structures of the digestive system, including:Identify the pancreas as the source gland for insulin, and describe the function of insulin in maintaining blood sugar levelsExplain the role of bile in the emulsification of fatsDescribe the functions of anaerobic bacteria in the colonDescribe how the small intestine is specialized for chemical and physical digestion and absorptionDescribe the structure of the villus, including microvilli, and explain the functions of the capillaries and lacteals within it

Todays ObjectivesDescribe the components, pH, and digestive actions of salivary, gastric, pancreatic, and intestinal juices, including:Relate the following digestive enzymes to their glandular sources and describe the digestive reactions they promote: salivary amylase, pancreatic amylase, proteases (pepsin, pepsinogen, trypsin), lipase, peptidase, maltase, nucleaseDescribe the role of water as a component of digestive juicesDescribe the role of sodium bicarbonate in pancreatic juiceDescribe the role of hydrochloric acid in gastric juiceDescribe the role of mucus in gastric juiceDescribe the importance of the pH level of various regions of the digestive tractReviewLast day we learned the major structures of the digestive tract, and the difference between chemical and physical digestionChemical digestion occurs when special digestive enzymes are used to break down the molecules in foodChemical digestion of carbohydrates begins in the mouthChemical digestion of proteins begins in the stomachChemical digestion of fats begins in the duodenum (small intestine)

Chemical Digestion: CarbsDigestion occurs as a result of hydrolysisSalivary Amylase: Enzymes in salivaActs on starch to break it into many molecules of maltoseMaltose is later broken down in the system to glucose

AmylaseSTARCH + WATER -----------> MALTOSE

Chemical Digestion: CarbsPancreatic Amylase: Also acts on starch to convert it to maltoseOccurs in the duodenum but produced by the pancreasMaltase: Converts maltose to glucoseProduced in the small intestine maltaseMALTOSE + WATER ----------------> 2 GLUCOSE

Chemical Digestion: ProteinProteases: Break down proteins to peptidesTwo types of protease:Pepsin: Produced by the gastric glands of the stomachTrypsin: Produced by the pancreas

Peptidases: Break down peptides into amino acidsProduced by the small intestine Pepsin/TrypsinProtein + Water ----------------> Peptides PeptidasesPeptides + Water ----------------> amino acidsChemical Digestion: FatsBile: Breaks down fat into fat droplets in the duodenumProduced by the liverStored in the gall bladderBile is not an enzyme

Lipase: Breaks down fat droplets into glycerol and 3 fatty acidsProduced by the pancreas BileFat -----------> Fat Droplets LipaseFat droplets + Water --------------> Glycerol + 3 Fatty AcidsEmulsificationEmulsification is the process that breaks down fats into fat dropletsA person who has had his gall bladder removed will have trouble digesting fatty foodsThe gall bladder stores bile for use at the proper time during the digestive process

EmulsificationEmulsifiers (such as bile) can cause fats to mix with waterThey contain molecules with a nonpolar and a polar endThe molecules position themselves in the fat droplet so that their nonpolar ends point inward into the droplet, and the polar ends point outwardNow the droplets can disperse in water

Digestive actions of Gastric, Pancreatic, and Intestinal JuicesPancreatic Juice: pancreatic amylase, trypsin, lipase, and sodium bicarbonateFormed in the pancreasSecreted into the duodenum via the pancreatic ductGastric juice: hydrochloric acidFormed in the stomach HCl changes pepsinogen into pepsin for digestion of proteinHCl (hydrochloric acid) + pepsinogen ----> PepsinDigestive actions of Gastric, Pancreatic, and Intestinal JuicesIntestinal Juice: maltase, peptidaseFormed in the small intestineMaltase breaks down maltose, peptidase breaks down peptidesNuclease:Formed in the small intestine, PancreasNuclease breaks down RNA and DNA into nucleotides

Control of Gastric (stomach) SecretionsThe following occurs especially after eating a protein-rich mealGastrin: a hormone produced in the lower part of the stomachGastrin enters the bloodstream and later stimulates gastric glands in the upper part of the stomach to produce pepsinogen and HClPepsinogen and HCl react with each other to produce pepsinControl of Gastric (stomach) SecretionsHCl can burn the lining of the stomach, so mucous is produced to protect the stomach liningIf a portion of the stomach does get burned, it is called an ulcer

Control of Intestinal SecretionsThe duodenal wall produces hormones, the most important of which are secretin and cholecystokinin (CCK), in response to the presence of acid chymeSecretin stimulates the release of pancreatic juice from the pancreasCCK stimulates the release of bile from the gall bladder

Control of Intestinal SecretionsAcid, especially HCl, stimulates the release of secretin, while partially digested protein and fat stimulate the release of CCKThese hormones then enter the bloodstream

The Role of InsulinInsulin: A hormone produced by the pancreasSecreted when blood sugar concentration is highCauses liver and muscles to take up and store excess glucose as glycogenAlso promotes synthesis of protein and fatsAs a result, insulin lowers blood sugar levelLow Blood Sugar High Blood Sugar INSULIN GLUCAGON The Role of GlucagonGlucagon: Another pancreatic hormoneSecreted when blood sugar concentration is lowCauses liver and muscles to break down glycogen into glucoseStops protein and fat synthesisAs a result, glucagon raises blood sugar levelPancreasThe Pancreas is called both an Exocrine and an Endocrine organExocrine: produces some enzymatic substancesEndocrine: produces hormones