(23.10.11) Mechanisms of Digestion and Absorption & Processing of Nutrients

Describe the general processes of digestion and absorption.

• Digestion break down ingested foods into their chemical building blocks 

• Only these molecules are small enough to be absorbed across wall of small intestine 

Define Digestion 

Catabolic process that breaks macromolecules down into monomers small enough for absorption 

1. Intrinsic and accessory gland enzymes are involved in digestion 

2. Enzymes carry out hydrolysis → whereby water is added to break chemical bonds 

This type of chemical reaction typically occurs as nutrients are digested in the body.

Hydrolysis → refers to the enzymatic breakdown of large polymers into monomers while adding the parts of a water molecule to the broken bonds

T/F: Most digestion occurs in the stomach 

→ FALSE 

Most digestion occurs in the SMALL INTESTINE, and is accomplished by enzymes secreted to the lumen that hydrolyze larger molecules into monomers 

Absorption of nutrients primarily occurs in the __________.

→ Small intestine

Define Absorption  

Process of moving substances from lumen of gut into body

1. Tight junctions ensure molecules must pass through epithelial cell rather than between them 

2. Lipid molecules can be absorbed passively through membrane, but polar molecules are absorbed by active transport 

3. Most nutrients are absorbed before chyme reaches ileum 

List the order of Absorption of Molecules in the Intestine

1. ENTERING epithelial cells across the apical membrane 

2. In the lumen 

3. Across the basolateral face of the interstitial fluid 

4. DIFFUSE into capillaries or lacteals

How are carbohydrates absorbed?

ONLY monosaccharides can be absorbed

List the START & END Products of Carbohydrate Digestion

1. BEGINS with the mouth as salivary amylase splits Polysaccharides into oligosaccharides

   • Starch

   • Glycogen

2. Intestine, pancreatic amylase breaks down Polysaccharides → oligosaccharides and disaccharides

   • Lactose

   • Maltose

   • Sucrose

3. Brush border enzymes break oligo- and disaccharides → Monosaccharides

   • Glucose

   • Frutose

   • Galactose

4. Monosaccharides exit acrosss the basolateral membrane by facilitated difussion

SUMMARY Carbohydrate digestion and absorption in the small intestine

1. Pancreatic amylase breaks down starch and glycogen (Polysaccharides) → oligosaccharides and disaccharides  

2. Brush border enzymes break oligosaccharides and disaccharides → monosaccharides

3. Monosaccharides (glucose and galactose) are cotransported across the apical membrane of the absorptive epithelial cell 

   • This active transport uses the Na+ concentration gradient established by the Na⁺-K⁺ ATPase (pump) in the basolateral membrane

4. Monosaccharides exit across the basolateral membrane by facilitated diffusion and enter the capillary via intercellular clefts 

Explain Effect and Cause of Lactose intolerance 

• EFFECT 

  • Any lactose eaten remains undigested and creates an osmotic gradient in intestine that prevents water from being absorbed → resulting in diarrhea 

• CAUSE 

  • Deficient amounts of lactase and cannot consume lactose

List the START & END Products of Protein Digestion

1. BEGINS in stomach when pepsinogen is converted to pepsin (@ pH 1.5-2.5) 

   • Becomes inactive in high pH of duodenum 

   • Large polypeptides

2. Small intestine, pancreatic protease break down Proteins and protein fragments into smaller pieces and some individual amino acids 

   • Small polypeptides

   • Small peptides 

3. Brush border enzymes break oligo- and dipeptides into amino acids 

   • Amino acids (some dipeptides and tripeptides) 

4. Amino acids are co-transpoted across the apical membrane of the absorptive epithelial cell

5. Amino acids exit across the basolateral membrane via facilitated diffusion 

How is Protein digestion different from other macromolecule digestion?

Proteins digested into amino acids in the GI tract include not only dietary proteins but also enzyme proteins secreted into the GI tract lumen 

Which of the following digestive processes could be affected by a patient's taking a large amount of antacids?

→ Protein digestion

• Protein digestion is partially accomplished by pepsin, an enzyme that works best in a pH range of 1.5 to 2.5

Which chemical activates the transformation of trypsinogen to trypsin?

Enteropeptidase → a protein produced by and bound to the membranes of intestinal cells. Contact with this protein converts trypsinogen to trypsin, the active form of the enzyme

SUMMARY Protein Digestion and absorption in the small intestine

1. Pancreatic proteases break down proteins and protein fragments into smaller pieces and some individual amino acids

   • Trypsin & Chymotrypsin cleave protein into smaller peptides 

   • Carboxypeptidase takes off the amino acid at a time from end

2. Brush border enzymes break protein fragments into amino acids

   • Amino-peptidases, Carboxypeptidases, and Dipeptidases break oligopeptides and dipeptides into amino acids

3. Amino acids are cotransported across the apical membrane of the absorptive epithelial cell

   • This active transport uses the Na+ concentration gradient established by the Na+-K+ ATPase (pump) in the basolateral membrane

4. Amnio acids exit across the basolateral membrane via facilaited diffusion and enter the capillary via intercellular clefts 

Explain Cause and Effectof Unabsorbed Protein

• CAUSE

  • Intact proteins are taken up by intestinal epithelial cells by endocytosis and are released into body

  • Most common in newborn infants because of immaturity of their intestinal mucosa 

• EFFECT 

  • May result in food allergies as immune system “sees” intact proteins as antigenic and mounts an attack → Allergies usually disappear as mucosa matures

List the START & END Products of Lipid Digestion

1. BEGINS unemulsified triglycerides

2. Need pre-treatment with bile salts (from liver) that break large fat globules into smaller ones

   • Emulsification → triglycerides and their breadown products are insoluble in wate

3. Pancreatic lipases hydrolyze triglycerides, yielding monoglycerides and free fatty acids 

4. Monoglycerides and fatty acids combine with bile salts and lecithin to form micelles @ the plasma membrane, fatty acids and monoglycerides diffuse into epithelial cells of the mucosa

5. Within the epithelial cells, fatty acids and monoglycerides are converted back into triglycerides, then combined with lecithin, cholesterol, and other lipids, to form Chylomicrons → which are exocytosed to the interstitial space, and taken into lacteals for transport 

SUMMARY Lipids Digestion and absorption in the small intestine

1. Emulsification → Bile salts in the duodénum break fat globules into smaller fat droplets (increasing the surface area avialable to lipase enzyme) 

2. Digestion → Pancreatic lipases hydrolyze triglycerides, yielding monoglycerides and free fatty acids 

3. Micelle formation → Free fatty acids and monoglycerides assemble with bile salts, forming micelles 

   • Micelles ferry their contents to enterocytes

4. Diffusion → Fatty acids and monoglycerides diffuse from micelles into enterocytes 

5. Chylomicron formation → Fatty acids and monoglycerides are recombined and packaged with other fatty substances and proteins to form chylomicrons 

6. Chylomicron transport → Chylomicrons are extruded from enterocytes by exocytosis, enter lacteals and are carried away from the intestine in lymph 

SUMMARY by which Breakdown Products of Foodstuffs are Absorbed (TRANSPORTED) in the Small Intestine 

1. Carbohydrates 

   • Salivary amylase digest polysaccharides (starch) in the mouth

   • Pancreatic amylase digest polysaccharides in the small intestine

   • Fructose molecules → are absorbed by facilitated diffusion into intestinal epithelial cells 

   • Glucose and galactose → are shuttled via secondary active transport with Na+ into intestinal epithelial cells

2. Proteins 

   • Pepsin digests proteins in the stomach

   • Trypsin and Chymotrypsin digests proteins in the small intestine

   • Amino acids → are shuttled via secondary active transport with Na+ into intestinal epithelial cells

3. Lipase

   • Gastric lipase digests lipids in the stomach 

   • Pancreatic lipase digests lipase in the small intestine 

   • Fatty acid and glycerides → must combine with bile salts to form → micelle → are absorbed by simple diffusion into intestinal epithelial cells

Where are the enzymes for digestion of disaccharides and small polypeptides located?

→ Brush border

Vitamin Absorption in the Small Intestine

1. Fat-soluble vitamins (A, D, E, and K) are carried by micelle; diffuse into absorptive cells 

2. Water-soluble vitamins (C and B) are absorbed by diffusion or by passive or active transporters

3. Vitamin B12 (large, charged molecule) bind with intrinsic factor and is absorbed by endocytosis 

How are vitamins A, D, E, and K absorbed by the body?

→ by being incorporated into micelles

• Fat-soluble vitamins are incorporated into micelles when fats are present.

Vitamin Absorption in the Large Intestine

1. Vitamin K and B vitamins bacterial metabolism are absorbed

Electrolytes Absorption

Most ions are transported activity along length of small intestine

1. Iron and calcium are absorbed in duodenum 

2. Na+ absorption is coupled with active absorption of glucose and amino acids 

Usually amount in intestine is amount absorbed 

Explain how Iron & Calcium Absorption is Related to Need

• Ionic iron is stored in mucosal cells with ferritin 

  • When needed, transported in blood by transferrin 

• Ca²⁺ absorption is regulated by vitamin D and parathyroid hormone (PTH) 

Water Absorption

• 9 L water → most from GI tract secretions, enter small intestine 

  • 95% is absorbed in the small intestine by osmosis 

  • Most of rest is absorbed in large intestine 

• Net osmosis occurs if concentration gradient is established by active transport of solutes 

  • Water uptake is coupled with solute uptake 

Explain Cause and Effect of Malabsorption 

• CAUSE 

  • ANYTHING that interferes with delivery of bile or pancreatic juice 

• EFFECT 

  • Damaged intestinal mucosa (EX: bacterial infection or some antibiotics) 

Explain Cause, Effect, and Treatment of Gluten-Sensitive Enteropathy  

• CAUSE 

  • Common malabsorption disease

  • Immune reaction to gluten

• EFFECT 

  • Gluten causes immune cell damage to intestinal vili and brush border

• TREATMENT

  • Eliminate gluten from diet