Digestion and Absorption Quiz

Digestion and Absorption

• Digestion is the process of breaking down food into molecules small enough to be absorbed.
• Absorption is the process of moving nutrients from the gastrointestinal (GI) tract into the bloodstream.
• Transport is the process of moving nutrients throughout the body using the circulatory and lymph systems.
• Elimination is the excretion of undigested food through feces

Digestion of Carbohydrates

• Carbohydrates broken down include:
  • Polysaccharides: starch, glycogen, and cellulose
  • Disaccharides: sucrose, maltose, and lactose
  • Monosaccharides: glucose and fructose
• Monosaccharides do not need digestion.
• Carbohydrate digestion begins in the mouth with the help of salivary amylase (ptyalin) which:
  • Hydrolyzes (1-4) glycosidic bonds in starch and glycogen
  • Produces dextrins and maltose
  • Requires Cl- ions for activation and a pH of 6.7
• Digestive action stops in the stomach due to its high acidity, but resumes in the small intestine.
• The small intestine has two stages of carbohydrate digestion:
  • Pancreatic Amylase: hydrolyzes dextrins to a mixture of maltose, isomaltose, and limit dextrin, requiring Cl- ions for activity with an optimal pH of 7.1
  • Intestinal Brush Border Enzymes: responsible for final carbohydrate digestion, including:
    • Lactase: hydrolyzes lactose into glucose and galactose.
    • Sucrase: hydrolyzes sucrose into glucose and fructose.
    • Maltase: hydrolyzes maltose into two glucose molecules.
    • Isomaltase: hydrolyzes isomaltose into two glucose molecules.

Why Cellulose is Not Digested by Humans?

• Cellulose is a polysaccharide found in plants containing (1-4) glycosidic bonds in its structure.
• Humans lack the enzyme needed to break these bonds.
• Cellulose provides fiber that aids in intestinal motility and stool softening.

Absorption of Carbohydrates

• The final products of carbohydrate digestion are glucose, fructose, and galactose.
• Three mechanisms are responsible for absorption of these sugars:
  • Passive Diffusion: relies on concentration gradients between intestinal lumen and mucosal cells.
  • Facilitated Diffusion: requires carrier proteins and is sensitive to concentration gradients.
  • Active Transport: requires energy, a transport protein, and sodium ions.
    • Glucose and galactose are absorbed via this mechanism using a sodium-dependent glucose transporter (SGLT-1).
    • This process is called cotransport or symport.
    • Energy is provided by ATP-linked sodium pumps.

Glucose Transporters

• There are 7 important glucose transporters (GLUT-1 to GLUT-7) for uptake of glucose into specific cells.
• GLUT-2 is found in the intestines, liver, and kidneys, and is a critical transporter for glucose uptake.
• GLUT-5 is found in the intestines and transports fructose.

Disorders of Digestion and Absorption of Carbohydrates

• Lactose Intolerance: deficiency of the enzyme lactase results in lactose accumulation in the gut.
  • Lactase hydrolyzes lactose into glucose and galactose.
  • Undigested lactose is fermented by bacteria in the large intestine, producing gases (H2 and CO2) and acids (acetic acid), causing:
    • Abdominal cramps
    • Flatulence
    • Diarrhea
    • Dehydration due to water drawn into the lumen
• Causes of Lactose Intolerance:
  • Congenital: present at birth
  • Acquired: develops later in life (primary or secondary)

Digestion of Lipids

• Digestion begins in the stomach where heat liquidizes lipids.
• Emulsification: dispersion of lipids into small droplets to reduce surface tension.
  • This process is essential for lipid digestion.
  • Emulsification is achieved by:
    • Bile salts
    • Peristaltic contractions
• Enzymatic Action on Lipids:
  • Lingual lipase and gastric lipase act on short-chain triglycerides (SCTs).
  • Pancreatic lipase, cholesteryl esterase, and phospholipase-A2 are essential enzymes for lipid digestion.
• Pancreatic Lipase: hydrolyzes fatty acids in the 1 and 3 positions of triglycerides, producing 2-monoacylglycerol (2-MAG) and two free fatty acids.
• Cholesteryl Esterase: hydrolyzes cholesteryl esters into cholesterol and free fatty acids.
• Phospholipase-A2: hydrolyzes fatty acids from the 2-position of glycerophospholipids, producing lysophospholipids.
• Orlistat: an anti-obesity drug, inhibits gastric and pancreatic lipases, reducing fat absorption and weight loss.

Absorption of Lipids

• Mixed Micelles: spherical particles with hydrophilic exteriors and hydrophobic interiors.
  • Formed by free fatty acids, 2-MAG, free cholesterol, lysophospholipids, and bile salts.
  • Help in absorbing lipids.
• Absorption:
  • All digested products passively diffuse into mucosal cells.
  • Short and medium-chain fatty acids do not require bile salts for absorption.
  • They are absorbed directly into intestinal cells and enter the portal blood.
  • They are transported to the liver bound to serum albumin.
• Fat-Soluble Vitamins (A, D, E, and K): packaged in micelles and absorbed.

Synthesis of Lipids in Intestinal Cells

• Long-chain fatty acids are activated by thiokinase to fatty acyl CoA.
• Acyl CoA combines with 2-MAG to form triglycerides.
• Absorbed cholesterol and lysophospholipids are reactivated to form cholesteryl esters and phospholipids.
• Free glycerol released in intestinal lumen is not reutilized but passes directly to the portal vein.
• Glycerol-3-phosphate, formed from glucose within intestinal cells, can be used for triglyceride synthesis.

Abnormalities in Lipid Digestion and Absorption

• Lipid Malabsorption: loss of lipid in the feces.
  • Can lead to loss of fat-soluble vitamins and essential fatty acids.
  • Occurs in conditions such as:
    • Bile salt deficiency: liver disease or bile duct obstruction.
    • Pancreatic enzyme deficiency: pancreatitis or cystic fibrosis.
    • Defective chylomicron synthesis.
• Steatorrhea: condition characterized by lipid loss in feces.

Digestion of Proteins

• Whole proteins are not absorbed.
• Pepsinogen, secreted by the stomach, is activated by its own acid to become the active enzyme pepsin.
• Pancreatic enzymes further break down proteins into peptides.
• Intestinal brush border enzymes break down peptides into amino acids, which are then absorbed.