Digestive System: Digestion and Absorption of Specific Food Groups

Learning objectives

• Identify the digestive or gastrointestinal tract and the accessory organs
• Describe the process of chemical digestion in the mouth, stomach and the small intestine
• Name the different groups of food that undergo digestion
• Characterise the food groups in relation to their digestive enzymes
• Describe the process of absorption of specific food groups; carbohydrates and proteins in the small intestine
• Describe the breakdown and transport of fats in the small intestine and into the lymphatic vessels
• Describe blood glucose regulation by insulin and glucagon respectively
• Describe lactose intolerance in a person
• Identify disorders that can result from a dysfunctional absorption

Digestive system: overview

• The digestive system involves successive stages:
  • Ingestion
  • Mechanical breakdown
  • Digestion (chemical breakdown)
  • Absorption/Assimilation
  • Elimination/Defecation
• Two main components:
  • Gastrointestinal (GI) tract: where digestion and absorption take place
  • Accessory organs: secrete fluids/enzymes to aid digestion (mainly salivary glands, liver and pancreas)

GI tract vs accessory organs

• GI tract: continuous chain of organs from mouth to anus; lined with smooth muscle; movement via peristalsis (waves of contraction against the wall)
• Accessory organs: liver, gallbladder, pancreas, salivary glands; secrete enzymes and fluids that aid digestion

Parts of the alimentary canal (takes $24-48$ hours)

• Mouth, Pharynx, Esophagus, Stomach, Small Intestine, Large Intestine, Rectum, Anus

Major organic compounds digestible by the body

• Carbohydrates → glucose
• Proteins → amino acids
• Fats → fatty acids + glycerol
• Nucleic acids → nucleotides
• These are the 4 major types of organic compounds

Digestive processes: digestion and peristalsis

• Digestion: the process of breaking down food into smaller, simpler components for absorption
• Carbohydrates → glucose; Proteins → amino acids; Fats → fatty acids + glycerol; Nucleic acids → nucleotides
• Peristalsis: smooth muscle waves that propel food along the tract; driven by the esophageal reflex in the esophagus

Oral cavity and esophagus: initial digestion and transport

• Digestion begins in the oral cavity and oesophagus with two processes:
  • Physical digestion: grinding of large food chunks into smaller particles
  • Chemical digestion: enzymes break down large polymeric biomolecules into monomers/oligomers
• Chemical digestion is essential for breaking food into particles that can be absorbed later in the small and large intestine
• Saliva contains amylase which begins starch digestion to maltose and dextrin
• Esophagus: bolus moved to the pharynx by the tongue; mucus glands secrete mucus to moisten and lubricate bolus; peristaltic contractions push bolus to stomach

Stomach: chemical and mechanical digestion

• Gastric juice contains pepsin (protease) and HCl to digest protein
• Mechanical churning (muscular contractions) forms chyme, a semi-liquid mass of digested food
• Chyme is stored in the stomach and released slowly into the duodenum (first part of the small intestine)
• The stomach and intestines are lined by epithelium with an invaginated surface to greatly increase absorptive area

Gastric glands and cell types

• Gastric glands contain four main cell types:
  • Mucous cells: mucus production
  • Parietal cells: secrete HCl; acid activates pepsin and kills swallowed microorganisms
  • Chief cells: secrete pepsinogen (converted to pepsin in acidic environment)
  • Enteroendocrine G cells: secrete gastrin, which stimulates acid secretion
• Combination of mucus, HCl and enzymes is called gastric juice

Duodenum: chyme entry and secretions from accessory organs

• Duodenum receives:
  • Bile (produced by the liver, stored in the gallbladder): emulsifies fats to aid lipase action
  • Pancreatic juice (from the pancreas): mixture of proteases, lipases and amylase; plays a major role in digestion of proteins and fats

Small intestine: digestion and absorption powerhouse

• Major site of digestion and absorption; chyme moves slowly to allow thorough digestion and absorption
• The small intestine produces its own enzymes: peptidases, sucrase, lactase, maltase
• Total absorptive surface area ≈ $250 \,m^2$ (roughly the size of a basketball court)
• Lining consists of enterocytes arranged in villi, each cell with microvilli (brush border)

Large intestine: absorption and gut flora

• Converts digested leftovers into feces
• Gut flora (colon bacteria) can decompose substances not digested by humans and synthesize various vitamins absorbed through the colon walls
• Fecal matter is stored in the rectum until defecation

Secretions into the digestive tract

• Substances secreted: mucus, bile salts, bilirubin, HCl, sodium bicarbonate (
  NaHCO3
  )
• These substances facilitate breakdown and digestion of food

Digestive enzymes: overview

• Enzymes and their general reactions:
  • Amylase: starch → simple sugars; produced by salivary glands, pancreas, and small intestine
  • Protease: proteins → amino acids; produced by stomach (pepsin) and by pancreas & small intestine
  • Lipase: lipids → fatty acids + glycerol; produced by pancreas and small intestine
• Enzyme sources and functions summarize where digestion occurs

Functional table: GI organs and enzyme roles (summary)

• Mouth and pharynx: exocrine secretions (mucus, amylase) to start digestion and aid swallowing
• Esophagus: mucus to lubricate; peristalsis moves bolus to stomach
• Stomach: HCl, pepsin; protein digestion begins
• Pancreas: pancreatic enzymes (proteases, lipases, amylase) and bicarbonate; neutralizes chyme
• Liver and Gallbladder: bile salts for fat emulsification; bile also has non-digestive roles
• Small intestine: enzymes for digestion; absorption of nutrients; bicarbonate neutralizes stomach acid
• Large intestine: mucus; absorption of water and electrolytes; fecal formation

Carbohydrate digestion and absorption

• Carbohydrates are the first biomolecule class digested chemically in the GI tract
• Salivary amylase begins breaking down amylose (starch) into maltose and dextrin in the mouth
• In the stomach, carbohydrate digestion effectively stops; resumes in the small intestine
• In the small intestine:
  • Pancreatic amylase continues breakdown of starch and glycogen into disaccharides and trisaccharides
  • Brush-border enzymes (lactase, sucrase, maltase) break disaccharides into monosaccharides and are absorbed by the intestinal epithelium
• Polysaccharide hydrolysis specifics:
  • Amylase acts on internal α-1,4-glycosidic linkages
  • Isomaltase acts on α-1,6-glycosidic bonds
  • α-glucosidases and isomaltase are attached to the brush-border membrane of enterocytes
• Absorption of monosaccharides:
  • Glucose and galactose: secondary active transport with Na+ via SGLT1
  • Fructose: facilitated diffusion via GLUT5
  • Monosaccharides exit enterocytes into blood via basolateral GLUT2 and diffusion
• Most ingested carbohydrate is digested/absorbed within the first 20% of the small intestine
• Glycemic index notes:
  • Simple carbohydrates and potato starch: high GI (fast absorption, large insulin spike)
  • Beans/legumes and mixed foods: lower GI (slower absorption)
• Glycemic regulation and insulin/glucagon:
  • Insulin promotes glucose uptake and storage; loss of insulin causes Type 1 diabetes; insulin resistance contributes to Type 2 diabetes
  • Blood glucose regulation is governed by insulin and glucagon actions; fasting glucose typically between $70 \, ext{mg}/100\,\text{mL}$ and $100 \,\text{mg}/100\,\text{mL}$

Absorption mechanics for carbohydrates (brush-border and transporters)

• Monosaccharide transport pathways:
  • Glucose and galactose: SGLT1 (Na+-coupled transporter) in the enterocyte apical membrane; Na+ gradient drives uptake
  • Fructose: GLUT5 (facilitated diffusion) on the apical membrane
  • After entry, monosaccharides exit enterocytes into blood via GLUT2 on the basolateral membrane
• Preliminary digestion occurs in the gut lumen; final stage takes place on the mucosal surface

Digestion of starch and disaccharides (illustrative steps)

• Amylose and amylopectin are degraded: lumen digestion by amylase to limit dextrins and maltose
• Brush-border enzymes (lactase, maltase, sucrase) produce glucose, galactose, and fructose for absorption
• Lactose digestion requires lactase to yield glucose + galactose (absorbed via SGLT1)

Glycemic index and human energy regulation (summary)

• Glycemic index influences postprandial blood glucose and insulin response
• Insulin promotes glucose uptake by muscle and liver; glucagon stimulates glycogen breakdown and gluconeogenesis during fasting
• Normal fasting blood glucose range: $ext{roughly } 70-100 \,\frac{\text{mg}}{100\,\text{mL}}$

Digestion of protein: from stomach to small intestine

• Protein digestion begins in the stomach:
  • Neural and gastric stimuli increase gastric juice production
  • HCl denatures proteins; low pH activates pepsin from pepsinogen
  • Pepsin begins proteolysis to peptide fragments
• In the small intestine, pancreatic proteases (trypsin and chymotrypsin) continue protein digestion; carboxypeptidase and aminopeptidase finish
• The result: free amino acids and di-/tripeptides are absorbed by enterocytes

Protein digestion mechanics and absorption details

• Peptidases at brush border hydrolyze peptides to free amino acids
• Absorption mechanisms:
  • Free amino acids: secondary active transport coupled to Na+ gradient
  • Di-/tri-peptides: co-transport with H+ ions; within the enterocyte, di-/tri-peptides are hydrolyzed to amino acids
  • Some intact peptides may cross via transcytosis (less common)
• Dipeptides and tri/tetrapeptides mainly absorbed and then cleaved to amino acids inside the cell or after release into blood
• Enzymes involved:
  • Trypsin and chymotrypsin (pancreatic)
  • Carboxypeptidase (brush border or pancreatic)
  • Aminopeptidase (brush border)
  • Enterokinase activates pancreatic proteases

Fat digestion and absorption (major pathway)

• Most dietary fat is in the form of triacylglycerols (TAGs)
• Fat digestion occurs mainly in the small intestine; fat is poorly soluble in water, so emulsification is required
• Emulsification:
  • Bile salts/phospholipids emulsify large fat droplets into smaller droplets, increasing surface area for lipase action
  • Emulsion droplets are stabilized and form micelles with bile salts
• Pancreatic lipase: hydrolyzes TAGs to free fatty acids and monoglycerides at the lipid droplet surface
• Absorption:
  • Short- and medium-chain fatty acids and glycerol are absorbed directly into the bloodstream via capillaries
  • Long-chain fatty acids and monoglycerides form new TAGs and are packaged into chylomicrons, which enter lacteals (lymphatic vessels)

Fat absorption pathway: step-by-step

• Emulsification by bile salts forms emulsion droplets
• Pancreatic lipase digests TAGs to monoglycerides and free fatty acids
• Fatty acids and mono-/glycerides diffuse into enterocytes and are re-esterified to TAGs
• TAGs are packaged with cholesterol and apolipoproteins into chylomicrons
• Chylomicrons exit enterocytes into lacteals (lymph), eventually entering the bloodstream
• Short- and medium-chain fatty acids absorbed into blood; long-chain fatty acids travel via the lymphatic system

Storage and utilization of fats

• In adipose tissue, lipoprotein lipase cleaves TAGs to free fatty acids and glycerol for uptake into adipocytes and storage as TG
• During exercise, fats can be mobilized and used by muscle cells (via fatty acids bound to albumin in the blood)

Stomach protection and mucus layer

• The stomach wall is protected by a mucus layer that is slightly alkaline
• Mucus neutralizes hydrogen ions near the epithelium
• Tight junctions limit diffusion of H+ into tissue
• Epithelial cells are rapidly replaced every few days by new cells from gastric pits

Lactose intolerance

• Lactose is the major carbohydrate in milk and must be digested by lactase to yield glucose and galactose
• Lactase is located on the luminal plasma membranes of intestinal epithelial cells
• Lactose intolerance occurs when lactase activity is insufficient
• Consequences: unabsorbed lactose draws water into the intestine (osmotic effect) and bacteria ferment lactose in the colon, producing gas and short-chain fatty acids, leading to diarrhea and abdominal pain
• Management: consume lactose-free milk or take lactase supplements with dairy products

Summary: connecting back to the learning objectives

• GI tract and accessory organs identified; digestion and absorption processes summarized for carbohydrates, proteins, and fats
• Enzyme groups and their target food groups explained; brush-border and luminal enzymes highlighted
• Absorption mechanisms outlined for carbohydrates (SGLT1, GLUT family) and for amino acids/peptides
• Fat digestion and lymphatic transport via chylomicrons described; storage and mobilization of fats touched upon
• Blood glucose regulation by insulin and glucagon explained; fasting glucose ranges noted
• Lactose intolerance defined, mechanism explained, and management options given
• Disorders arising from dysfunctional absorption discussed (e.g., electrolyte/acid-base disturbances due to GI losses)

Quick reference numbers and constants (LaTeX)

• Transit time in alimentary canal: $24-48 \,\text{hours}$
• Surface area of the small intestine: $A \approx 250 \,\text{m}^2$
• Normal fasting blood glucose range: $\text{approx } 70-100 \,\frac{\text{mg}}{100 \,\text{mL}}$
• Daily GI tract fluid movement (approximate): $10 \,\text{L}$ of fluids pass through the GI tract per day
• Absorption locations:
  • Carbohydrates: glucose/galactose via SGLT1; fructose via GLUT5; exit via GLUT2
  • Proteins: amino acids via Na+-dependent transporters; di-/tri-peptides via H+-dependent transporters; some peptides via transcytosis
  • Fats: long-chain fatty acids + monoglycerides form chylomicrons for lymphatic transport; short/medium chains via portal blood

References within this material

• General GI physiology and specific enzyme roles as outlined by Dr. Mark I.R. Petalcorin and colleagues in the provided transcript