Human Digestion Processes Quiz

Course Information

• Course title: Coordinated Course in Physiology and Biochemistry
• Course code: DIET 311
• Instructor: Dr. Richard Doe

Learning Objectives

• Students will demonstrate understanding of nutrient absorption mechanisms throughout the alimentary tract.
• Students will understand metabolic rate and factors regulating it.
• Students will understand neuro-endocrine factors that regulate energy balance in individuals.

Digestive System and Body Metabolism

• Digestion: breakdown of ingested food
• Absorption: passage of nutrients into the bloodstream
• Metabolism: production of cellular energy (ATP) and other chemical and energy transformations within the body

Review of the Alimentary Tract

• Salivary Glands: secrete lubricating fluid containing carbohydrate-digesting enzymes
• Oral Cavity, Teeth, Tongue: mechanically process, moisten, and mix food with saliva
• Liver: secretes bile for lipid digestion; stores nutrients
• Gallbladder: stores and concentrates bile
• Large Intestine: absorbs water and compacts indigestible materials for elimination
• Pharynx: propels materials into the esophagus
• Esophagus: transports materials to the stomach
• Stomach: chemically breaks down materials using acids and enzymes; mechanically processes through muscular contractions
• Pancreas: secretes buffers and digestive enzymes; also secretes hormones
• Small Intestine: site of enzymatic digestion and absorption of water, organic substrates, vitamins, and ions

Functions of the Digestive System

• Ingestion
• Mechanical Processing
• Digestion
• Secretion
• Absorption
• Excretion

Digestion

• Degradation of complex food stuffs through digestive enzymes.
• Three main categories of energy rich foodstuffs: carbohydrates, proteins and lipids

Absorption

• Absorbing digestible units formed from digestion process
• Absorbed along with water, vitamins and electrolytes
• Absorption into blood and lymph through the lumen of the GI tract

Digestion (cellular mechanism)

• Luminal digestion: enzymes secreted within the GI tract from salivary glands, stomach, pancreas
• Membrane or contact digestion: hydrolytic enzymes synthesized by enterocytes and inserted into the brush border membranes; important part of microvilli
• Cytoplasmic digestion: digestive enzymes located in the cytoplasm

Processing and Absorption of Nutrients

• Disassembles organic food into smaller fragments
• Hydrolyzes carbohydrates, proteins, lipids, and nucleic acids for absorption

Absorption Mechanism of Monosaccharides

• Glucose and galactose: SGLT1 (secondary active, Na-dependent transport)
• Fructose: GLUT5 (facilitated, Na-independent transport)

Digestion and Absorption of Carbohydrates

• Digestion starts at the mouth.
• Salivary and pancreatic enzymes digest to disaccharides and trisaccharides
• Brush border enzymes digest into monosaccharides.
• Absorption happens across the intestinal epithelia

Carbohydrate Digestion and Absorption

• Components of carbohydrate diet: monosaccharides (glucose, fructose, sorbitol), disaccharides (sucrose, lactose, maltose), oligosaccharides/polysaccharides (starch, amylose, amylopectin, glycogen).
• Non-digestible carbohydrates: dietary fibers (cellulose, B-1,4 linked glucose polymers)

Digestive Enzymes for Carbohydrate Digestion

• Luminal digestive enzymes – salivary and pancreatic amylases.
• Brush border enzymes – sucrase, lactase, isomaltase, maltase, glucoamylase

Luminal Digestive Enzymes for Carbohydrate Digestion

• Salivary and pancreatic amylase: cleaves a-1,4 glycosidic bond of amylose and amylopectin; creates maltose, maltotriose, and a-limit dextrins

Brush Border Enzymes (digest disaccharides and oligosaccharides):

• Sucrase: digests sucrose to glucose and fructose
• Lactase: digests lactose to glucose and galactose.
• Isomaltase: di/tri saccharide digestion; digestion of a1,6 glucosidic linkages into glucose, maltose, and oligosaccharides
• Maltase: digests maltose to glucose
• Glucoamylase: digests maltooligosaccharides to glucose

Digestion and Absorption of Proteins

• The low pH destroys tertiary and quaternary structures of proteins.
• digestive enzymes (pepsin, trypsin, chymotrypsin, elastase): involved.
• Absorbed into bloodstream as amino acids

Proteolytic Digestive Enzymes

• Gastric secretion
• Pancreatic secretion
• Brush border enzymes
• Cytoplasmic enzymes

Endopeptidases (hydrolyze internal peptide bonds)

• Trypsin
• Chymotrypsin
• Elastase
• Pepsin

Exopeptidases (hydrolyze external peptide bonds)

• Carboxypeptidase A
• Carboxypeptidase B
• Aminopeptidase

Protein Digestion

• Gastric proteolysis: pepsin is activated by low pH from pepsinogen (an inactive proenzyme), and acts as an endopeptidase (cleaving internal peptide bonds),
• Small intestine: pancreatic proteases; secreted as inactive proenzymes; activation by enterokinase (enteropeptidase); conversion to active enzymes; hydrolysis to amino acids and small peptides (dipeptides, tripeptides, tetrapeptides)

Protein Absorption

• Absorption of products (amino acids, di/tripeptides) by multiple transport proteins across epithelial cells.
• 7 membrane bound amino acid transporters (some Na+-dependent) within the intestinal epithelial cell brush border
• Absorption of larger peptides by peptide transporters, and proteins by immunoglobulin absorption

Amino Acid Transport

• 5 classes of amino acid transporters (basolateral membrane)
• 2 Na-dependent
• 3 Na-independent
• Transported to the portal blood

Lipid Digestion and Absorption

• Lipid digestion utilizes lingual and pancreatic lipases.
• Bile salts improve chemical digestion.
• Lipid-bile salt complexes called micelles are formed.
• Micelles diffuse into intestinal epithelia.
• Lipids enter the blood as chylomicrons

Lipids in the GI tract

• Exogenous (diet): triglycerides, phospholipids, sterols (e.g. cholesterol), sterol esters
• Endogenous (bile, desquamated intestinal epithelial cells)

Digestion of Lipids

• Lingual lipase (initiates in the stomach, optimal pH=4, break down mainly medium chain triglycerides)
• Gastric lipase (secreted by chief cells)
• Pancreatic lipase (glycerol ester hydrolase, triglycerides)
• Pancreatic phospholipase A2 (phospholipids)
• Pancreatic cholesterol esterase (cholesterol esters)

Mechanism of Lipid Absorption

• Intestinal villi are coated with a layer of unstirred water.
• Reducing absorption of poorly water-soluble lipids

Emulsification

• Lipids are emulsified by bile acids in the small intestine to form small droplets. It's important to ensure access and efficiency of enzymes for digesting lipids. Bile salts are conjugated, polar, and water soluble agents.

Micelle Formation and Lipid Absorption

• At a certain concentration, bile salts aggregate into micelles that incorporate lipid digestion products
• Lipids become water-soluble because of micelle solubilization.
• Diffusion across the water layer by enterocytes (mostly in the jejunum)
• Enhanced by Na+-dependent long-chain fatty acid transport protein (MVM-FABP=microvillous membrane fatty acid-binding protein ) and cholesterol transport protein

In the Enterocytes

Lipid Transport into the Lymphatic Vessels

• Lipids are bound by cytosolic lipid transport proteins and transported to the smooth endoplasmic reticulum
• Triglycerides are reassembled from fatty acids and monoglycerides
• Triglycerides, lecithin, cholesterol, and cholesterol esters are packaged into lipoproteins called chylomicrons.
• Short to medium-chain fatty acids are absorbed directly into the bloodstream and transported bound to serum albumin via exocytosis

Absorption of Bile Acids

• Conjugated bile acids are absorbed by Na+-dependent secondary active transport in the terminal ileum
• Unconjugated bile acids are absorbed by diffusion
• Recirculated to the liver via portal circulation and reused in bile production

Intestinal Secretion

• 1500 ml/day
• Mucus, electrolytes, water (Composition)

The Liver

• Performs metabolic and hematological regulation
• Produces bile
• Histological organization: lobules containing single-cell-thick plates of hepatocytes; lobules united to form common hepatic duct
• Liver duct joining cystic ducts to form common bile ducts

Anatomy of the Liver

• Caudate lobe, left hepatic vein, inferior vena cava, left lobe (anterior surface) , right lobe
• Hepatic portal vein, hepatic artery proper, round ligament, falciform ligament, gallbladder (posterior surface)

Liver Histology

• Kupffer cells, hepatocytes, sinusoids, bile canaliculi, hepatic artery proper, bile duct, hepatic portal vein

The Gallbladder

• Stores and concentrates bile
• Between meals
  • (A) Fundus, Body, Neck, Cystic Duct, Liver, Duodenum, Pancreas
  • (B) Intestinal Lumen Pancreatic Duct

Functions of the Liver

• Energy metabolism and substrate interconversion
• Synthetic function
• Transport and storage
• Protective and clearance function

Bile Secretion

• Bile: digestive/absorptive function of the liver.
• Components: bile salts (conjugates of bile acids), bile pigments (e.g. bilirubin), cholesterol, phospholipids (lecithins), proteins, electrolytes (similar to plasma, isotonic with plasma)
• Secretion of bile: 600-1200 ml/day

Bile Functions

• Bile salts (conjugates of bile acids): crucial for lipid absorption in the small intestine, emulsifying lipids, forming mixed micelles for efficient lipid absorption
• Bile acids are derived from cholesterol; responsible for cholesterol excretion
• Bilirubin (hemoglobin degradation product), excretion
• Enterohepatic circulation: bile acids are actively absorbed and recirculated

Control of Digestive System

• Movement of materials along the digestive tract controlled by neural mechanisms, parasympathetic and local reflexes, hormonal mechanisms, and local mechanisms
• Neural and hormonal mechanisms coordinate these glands; GI activity stimulated by parasympathetic innervation inhibited by sympathetic innervation
• Reflexes coordinate stomach and intestines (enterogastric, gastroenteric, gastroileal reflexes

Regulation of Digestive Activities

• CNS, long reflex, myenteric plexus, short reflex, stretch receptors, chemoreceptors, buffers, acids, enzymes released, secretory cells, peristalsis, segmentation, enteroendocrine cells, and hormones, circulation

Phases of Gastric Secretion

• Cephalic phase: sight, smell, taste, or thought of food, increase gastric activity
• Gastric phase: stomach distension (stretch receptors), increase gastric activity
• Intestinal phase: duodenum (presence of partially digested food), decrease gastric activity

The Pancreas

• Pancreatic duct penetrates duodenal wall
• Endocrine functions: insulin, glucagon, somatostatin
• Exocrine functions: majority of pancreatic secretions; pancreatic juice secreted into the small intestine (carbohydrases, lipases, nucleases, proteolytic enzymes)

The Pancreas (Secretory Functions)

• Endocrine pancreatic secretion (hormones: insulin, glucagon, somatostatin) regulates metabolism
• Exocrine pancreatic secretion (aqueous component, enzyme component)

Digestive Functions (Pancreas)

• Production and secretion of digestive enzymes
• Neutralization of acidic chyme, maintaining proper pH for pancreatic enzymes

Protective Functions (Pancreas)

• Neutralization of acidic chyme, protecting intestinal mucosa from acid damage

Pancreatic Enzymes (Specific Hydrolytic Activity)

• Proteolytic (Endopeptidases: Trypsin(ogen), Chymotrypsin(ogen), (Pro)elastase; Exopeptidases: (Pro)carboxypeptidase A/B, (Pro)aminopeptidase)
• Amylolytic (a-Amylase)
• Lipases (Lipase)
• (Pro)phospholipase A2
• Carboxylester Hydrolase (cholesterol esterase)
• Nucleolytic (Ribonuclease, Deoxyribonuclease)

Enzyme Activation

• Proteolytic enzymes secreted in inactive zymogen form.
• Enteropeptidase (enterokinase) activates trypsinogen to trypsin, then to other proteolytic enzymes.
• Trypsin inhibitor prevents premature activation in pancreatic ducts
• alpha-amylase is secreted in active form

Cellular Mechanism of Pancreatic Secretion

• Carbonic anhydrase reaction which produces H2CO3
• Na+/H+ exchange and H+/K+-ATPase to eliminate H+
• Cl-/HCO3- exchange secretes bicarbonate into the duct lumen.
• Electrogenic Cl- channels recycle Cl− back into lumen.
• Acid tide: net H+ release into the blood stream.

Test your knowledge on the stages of digestion and the role of various enzymes and nutrients in the human digestive system. This quiz covers key concepts related to digestion, absorption, and lipid transport essential for understanding human nutrition.