Enzymes and Metabolism

Enzymes and Metabolism

UNIT B Chapter 9: Digestive System Section 9.1 9.1 The Digestive Tract The digestive system is involved in the ingestion and digestion of food and elimination of indigestible material. Digestion takes place within the digestive tract, which begins with the mouth and ends with the anus. Digestion involves mechanical and chemical digestion. o Mechanical digestion: chewing of food, and churning and mixing of food in the stomach o Chemical digestion: enzymes break macromolecules down into small organic molecules TO PREVIOUS SLIDE

ABSORPTION: the passage of digested nutrients from the gut lumen into the blood or lymph, which distributes them through the body. ELIMINATION: the expulsion of indigestible residues from the body. UNIT B Chapter 9: Digestive System Figure 9.1 The human digestive tract. The upper part of the tract includes the mouth, pharynx, esophagus, stomach, and small intestine. The large intestine consists of the cecum, the colon (ascending, transverse, descending, and sigmoid colons), the rectum,

and the anus. Note also the location of the accessory organs of digestion: the pancreas, the liver, and the gall bladder. TO PREVIOUS SLIDE Section 9.1 UNIT B Chapter 9: Digestive System Section 9.1 The Mouth The mouth takes food into the body.

Teeth: involved in chewing food Tongue: composed of skeletal muscle and involved in forming the bolus (a mass of food that is ready for swallowing) Roof of the mouth: composed of a hard palate and a soft palate; prevents ingested food from entering the nasal cavity Tonsils: contain lymphoid tissue that protect against infections Salivary glands: produce saliva to keep the mouth moist; saliva contains an enzyme that digests starch TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System Section 9.1 The Pharynx

The pharynx is a passageway that receives air from the nasal cavities and food from the mouth. Swallowing (a reflex action) occurs in the pharynx. The soft palate moves back to close off the nasopharynx The trachea moves up under the epiglottis to cover the glottis (the opening to the larynx (voice box)) During swallowing, food enters the esophagus because the air passages are blocked TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System Figure 9.2 Swallowing. When food is swallowed, the soft palate closes off

the nasopharynx and the epiglottis covers the glottis, forcing the bolus to pass down the esophagus. Therefore, a person does not breathe while swallowing. TO PREVIOUS SLIDE Section 9.1 UNIT B Chapter 9: Digestive System TO PREVIOUS SLIDE

Section 9.1 UNIT B Chapter 9: Digestive System The Esophagus The esophagus is a long muscular tube that moves food from the mouth to the stomach by peristalsis. Peristalsis (rhythmic muscular contractions) pushes food along the esophagus and through the digestive tract to the stomach. TO PREVIOUS

SLIDE Section 9.1 UNIT B Chapter 9: Digestive System Section 9.1 The Stomach The stomach is an organ that receives food from the esophagus, mechanically and chemically digests food, and moves food into the small intestine. TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System

Section 9.1 Structure and Function of the Stomach The human stomach has thick walls with folds (called rugae) that allow it to expand and fill with food. Lining of the stomach: has gastric glands that secrete gastric juice containing mucus and digestive enzymes Wall of the stomach: o Three muscle layers (longitudinal, circular, oblique) o Responsible for moving and churning food (mechanical digestion), and mixing food with gastric juice to break it down (chemical digestion) TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System

Section 9.1 Alcohol and other liquids are absorbed in the stomach, but most solid food is not. When food leaves the stomach, it is a thick, soupy liquid called chyme. o Chyme enters the small intestine by way of the pyloric sphincter. TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System Figure 9.3 Anatomy of the stomach. a. The stomach has a thick wall

with folds that allow it to expand and fill with food. b. The mucosa contains gastric glands, which secrete gastric juice containing mucus and digestive enzymes. TO PREVIOUS SLIDE Section 9.1 UNIT B Chapter 9: Digestive System The Small Intestine The small intestine receives chyme from the stomach and

completes the digestion of food. Macromolecules are broken down into nutrients, which are absorbed in the small intestine and pass into the blood. TO PREVIOUS SLIDE Section 9.1 UNIT B Chapter 9: Digestive System Section 9.1 Structure and Function of the Small Intestine The small intestine is composed of three parts. Duodenum: upper part of the small intestine o Contains a bile duct that delivers bile from the liver and

pancreatic juice from the pancreas Enzymes in pancreatic juice complete food digestion Jejunum: middle part of small intestine Ileum: lower part of small intestine o Contains lymphoid tissues involved in immune response to intestinal pathogens TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System Section 9.1 Structure and Function of the Small Intestine The wall of the small intestine contains villi, which increase the surface area to improve the absorption of nutrients.

Villi: fingerlike projections that contain microvilli o Microvilli increase the surface area of the villus for the absorption of nutrients o Each villus contains blood capillaries and a small lymphatic capillary called a lacteal o Nutrients are absorbed into the blood capillaries and the lacteals, which carry them to body cells TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System Section 9.1 Figure 9.4 Anatomy of the small intestine. The wall of the small intestine has folds that bear fingerlike projections called villi. Microvilli, which project from the villi, absorb the products of

digestion into the blood capillaries and the lacteals of the villi. TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System Section 9.1 Regulation of Digestive Secretions Digestive secretions are controlled by the nervous system and hormones. After eating a meal: The stomach produces the hormone gastrin o Gastrin: stimulates the gastric glands to secrete more gastric juice The duodenal wall produces the hormones secretin and CCK

o Secretin and CCK: stimulate the pancreas to secrete pancreatic juice and the gall bladder to secrete bile TO PREVIOUS SLIDE Hormones that Control Digestion: Hormone Released by What Part/ in response to what? upper part of stomach/in response to protein in the stomach Acts on What Part?

Gastric juice secreting cells at top of stomach SECRETIN Small intestine/Acid chyme from stomach Pancreas and parietal cells of stomach CCK (Cholecysto kinin)

Small intestine/Acid chyme in stomach Pancreas and Liver (gall bladder) GASTRIN What does it do? Causes secretion of gastric juices Also increases the churning action of the stomach Is inhibited by the presence of HCl,

Secretin, and GIP negative feedback loop Causes pancreas to release NaHCO3 and pancreatic enzymes Also inhibits gastric acid secretion by the stomach Causes liver to secrete bile and pancreas to secrete pancreatic juice. Also acts as a hunger supressant UNIT B Chapter 9: Digestive System Figure 9.5 Hormonal control of digestive gland secretions.

Gastrin (blue), produced by the lower part of the stomach, enters the bloodstream and thereafter stimulates the stomach to produce more digestive juices. Secretin (green) and CCK (purple), produced by the duodenal wall, stimulate the pancreas to secrete its digestive juices and the gall bladder to release bile. TO PREVIOUS SLIDE Section 9.1

UNIT B Chapter 9: Digestive System Section 9.1 The Large Intestine The large intestine absorbs water, salts, and some vitamins. It also stores indigestible material until it is eliminated as feces. TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System Section 9.1 Structure and Function of the Large Intestine The large intestine includes the

cecum, colon, rectum, and anal canal. Cecum: a small pouch that forms the first part of the large intestine o Contains the appendix or vermiform appendix, which may play a role in fighting infection TO PREVIOUS SLIDE Figure 9.6 Junction of the small intestine and the large intestine. The cecum is the blind end of the large intestine. The appendix is attached to the cecum.

UNIT B Chapter 9: Digestive System Section 9.1 Structure and Function of the Large Intestine Colon: includes the ascending, transverse, descending, and sigmoid colon Rectum: the last part of the large intestine; opens at the anus Anus: rectum opening; site of defecation (expulsion of feces) TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System Section 9.1

Defecation Reflex Eliminating feces from the body is a way the digestive system maintains homeostasis Feces are about three-quarters water and one-quarter solids o Solids: bacteria, fibre, and other indigestible materials o Bacteria break down some indigestible material, and produce some vitamins that our bodies can absorb o Water that is unsafe for drinking has a high number of coliform (intestinal) bacteria, indicating that a significant amount of feces has entered the water TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System

Section 9.2 9.2 Accessory Organs of Digestion The accessory digestive organs are the pancreas, liver, and gall bladder. Figure 9.8 Liver, gall bladder, and pancreas. a. The liver makes bile, which is stored in the gall bladder and sent (black arrow) to the small intestine by way of the common bile duct. The pancreas produces digestive enzymes that are sent (black arrows) to the small intestine by way of

the pancreatic duct. TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System The Pancreas The pancreas is an organ that has both endocrine and exocrine functions. Exocrine function of the pancreas o Pancreatic cells produce pancreatic juice (to neutralize stomach acid) and digestive enzymes Endocrine function of the pancreas o Pancreas secretes insulin and glucagon, hormones that regulate

blood glucose (sugar) levels TO PREVIOUS SLIDE Section 9.2 UNIT B Chapter 9: Digestive System Section 9.2 Structure and Function of the Pancreas The pancreas contains pancreatic islets (islets of Langerhans), which are clusters of at least three types of endocrine cells: Alpha cells: produce glucagon Beta cells: produce insulin Delta cells: produce somatostatin

TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System Section 9.2 Structure and Function of the Pancreas Insulin Hormone secreted when blood glucose level is high Stimulates the uptake of glucose by cells (liver, muscle, adipose tissue) to lower blood glucose Glucagon Hormone secreted when blood glucose level is low Stimulates the liver to break glycogen down into glucose to increase blood glucose

Stimulates adipose tissue to break fat down to glycerol and fatty acids (to make glucose) TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System The Liver The liver is the largest gland in the body. The liver has many functions, including: Detoxifying blood Making plasma proteins

Maintaining blood glucose levels Producing bile, which contains bile salts that emulsify fat in the small intestine Producing urea, a nitrogenous waste product from the breakdown of amino acids TO PREVIOUS SLIDE Section 9.2 UNIT B Chapter 9: Digestive System Section 9.2 Structure and Function of the Liver The liver contains about 100 000 lobules that serve as its

structural and functional units. Three structures are located between the lobules: Bile duct: takes bile away from the liver Hepatic artery branch: brings oxygen-rich blood to the liver Hepatic portal vein: transports nutrients from the intestines TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System Figure 9.8 b. A hepatic lobule. The liver contains over 100 000 lobules, each lobule composed of many cells that perform the various functions of the liver. They remove and add materials to the blood and deposit bile in a duct. TO PREVIOUS SLIDE

Section 9.2 UNIT B Chapter 9: Digestive System Figure 9.10 Hepatic portal system. The hepatic portal vein takes the products of digestion from the digestive system to the liver, where they are processed before entering a hepatic vein. TO PREVIOUS SLIDE Section 9.2

UNIT B Chapter 9: Digestive System The Gall Bladder The gall bladder is a muscular sac attached to the surface of the liver. Excess bile from the liver is stored in the gall bladder Bile leaves the gall bladder and proceeds to the duodenum via the common bile duct o Bile emulsifies fat to prepare it for further breakdown by digestive enzymes TO PREVIOUS SLIDE

Section 9.2 UNIT B Chapter 9: Digestive System Section 9.3 9.3 Digestive Enzymes Digestive enzymes help break down the major components of food: carbohydrates, proteins, nucleic acids, and fats. TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System Section 9.3

Carbohydrate Digestion by Enzymes The digestion of starch (a carbohydrate) begins in the mouth. Salivary amylase (produced by the salivary glands) digests starch into maltose (a disaccharide) Pancreatic amylase (produced by the pancreas) and maltase (produced by the small intestine) then convert maltose in the small intestine to glucose (a monosaccharide). Glucose can be absorbed by the small intestine. TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System Figure 9.11 Digestion and absorption of nutrients. a. The breakdown of

carbohydrates, such as starch, involves amylase enzymes. TO PREVIOUS SLIDE Section 9.3 UNIT B Chapter 9: Digestive System Section 9.3 Carbohydrate Digestion by Enzymes Other disaccharides, such as lactose, have their own enzyme that digests them in the small intestine. Lactase is an enzyme that digests lactose, a sugar found in milk.

o Individuals who have a lactase deficiency often have symptoms of lactose intolerance (diarrhea, gas, cramps) caused by the fermentation of non-digested lactose by intestinal bacteria TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System Section 9.3 Protein Digestion by Enzymes The digestion of proteins begins in the stomach. Pepsin is an enzyme produced by gastric glands that acts on proteins to produce peptides.

Trypsin (produced by the pancreas) and peptidases (produced in the small intestine) break down peptides into amino acids TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System Figure 9.11 Digestion and absorption of nutrients. b. Protein digestion involves the action of protease enzymes. TO PREVIOUS SLIDE Section 9.3

UNIT B Chapter 9: Digestive System Section 9.3 Fat Digestion by Enzymes Lipase (produced by the pancreas) acts in the small intestine and digests fat molecules in the fat droplets after they have been emulsified by bile salts Glycerol and fatty acids enter the cells of the villi, where they are rejoined and repackaged as lipoprotein droplets (chylomicrons) before entering the lacteals TO PREVIOUS SLIDE UNIT B Chapter 9: Digestive System

Figure 9.11 Digestion and absorption of nutrients. c. For fat digestion, bile salts emulsify the fats so that lipase enzymes can digest the particles. TO PREVIOUS SLIDE Section 9.3 UNIT B Chapter 9: Digestive System Section 9.3 Regulation of Digestive Enzymes

Enzymes function best at an optimum temperature and pH that helps maintain the proper shape to fit their substrate. Since the digestive system is maintained at a constant 37C, enzymatic activity is largely controlled by pH o The pH of the stomach is between 1 and 2 but can increase to around 7.4 to 7.8 when sodium bicarbonate in pancreatic juice is released from the pancreas o This increase in pH occurs after chyme enters the duodenum, and allows different digestive enzymes to be active depending on the pH TO PREVIOUS SLIDE

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