Digestion and Absorption

• How are carbohydrates digestion and absorbed?

  Carbohydrates are ingested. They are digested in the small intestine via salivary amylase and pancreatic enzymes. They are absorbed in the small intestine. Undigested fibers are released in the colon.
• How are proteins digested and absorbed?

  Protein digestion begins in the stomach with stomach acid and pepsin and continues with pancreatic proteases. It is then absorbed in the small intestine and excreted via the colon.
• How are fats digested and absrobed?

  Fat digestion begins in the small intestine with the help of bile from the liver and pancreatic lipase. Fats are absorbed in the small intestine in the form of micelles.
• What role do microvilli have in determining the rate at which nutrients are absorbed?

  Microvilli significantly increase the surface area for absorption by creating tiny finger-like projections on the intestinal lining, allowing for more efficient uptake of nutrients like glucose, amino acids, and electrolytes.
• What role does the unstirred layer have in determining the rate at which nutrients are absorbed?

  The unstirred layer acts as a diffusion barrier, impacting the rate of absorption by controlling the concentration gradient near the intestinal wall.
• What role do tight junctions have in determining the rate at which nutrients are absorbed?

  Tight junctions between intestinal epithelial cells regulate the paracellular movement of molecules, ensuring selective absorption and preventing leakage of larger substances like proteins, thereby influencing the absorption rate of water and small ions.
• What are the different chemical classes of carbohydrates entering the duodenum?

  Primarily complex carbohydrates (polysaccharides) like partially digested starch (dextrins and oligosaccharides)
• What pancreatic secretions help digest carbohydrates?

  Pancreatic amylase breaks down complex carbs into smaller oligosaccharides and disaccharides like maltose
• What brush border enzymes help break down carbohydrates?

  Sucrase: breaks down sucrose into glucose and fructose. Lactase breaks down lactose into glucose and galactose. Maltase breaks down maltose into glucose
• What apical membrane transport system is used for carbohydrates?

  A sodium-glucose cotransporter (SGLT1) actively transports glucose into the intestinal epithelial cell by coupling their movement with sodium transport.
• What is the basolateral membrane transport system for carbohydrates?

  Fructose is transported across the basolateral membrane via facilitated diffusion through a specific transporter
• What chemical classes of proteins enter the duodenum?

  Larger polypeptides are the primary form of proteins entering the duodenum, partially digested by stomach pepsin
• What pancreatic secretions help digest proteins?

  Trypsin: a key protease that cleaves peptide bonds at the carboxy side of lysine and arginine residues. Chymotrypsin: another protease that cleaves peptide bonds at the carboxyl side of aromatic amino acids such as phenylalanine, tyrosine, and tryptophan. Carboxypeptidase: an exopeptidase that removes amino acids from the carboxyl terminus of peptides
• What brush border enzymes help digest proteins?

  Aminopeptidase: an exopeptidase that removes amino acids from the amino terminus of peptides. Dipeptidase: breaks down dipeptides into individual amino acids.
• What apical membrane transport system is used for proteins?

  Most amino acids are transported across the apical membrane via sodium-dependent co-transports, where the movement of sodium down its concentration gradient drives the uptake of amino acids. Dipeptides and tripeptides are transported across the apical membrane by specific peptide transporters.
• What basolateral membrane transport system is used for proteins?

  Once inside the enterocyte, amino acids are transported across the basolateral membrane into the bloodstream via facilitated diffusion through specific amino acid transporters
• What chemical classes of lipids enter the duodenum?

  Primarily triglycerides with smaller amounts of phospholipids and cholesterol.
• What pancreatic secretions help digest lipids?

  Pancreatic lipase is the main enzyme responsible for triglyceride digestion and breaking it down into fatty acids and monoglycerides. Colipase is a protein co-enzyme that is also secreted by the pancreas and helps anchor pancreatic lipase to the surface of lipid droplets, enhancing its activity.
• What is the function of bile salts in the digestion of lipids?

  Bile salts are amphipathic molecules secreted by the liver and stored in the gallbladder, which emulsify lipids into smaller droplets, creating a larger surface area for lipase to act upon.
• What is the function of micelles in the digestion of lipids?

  Micelles are small spherical structures formed by bile salts with a hydrophobic core that encapsulates the digested lipid products facilitating their transport through the aqueous environment of the intestinal lumen to the intestinal epithelial cells.
• What apical membrane transport system is used for lipids?

  Micelles containing digested lipids diffuse to the brush border of the enterocyte and release their contents.
• What basolateral membrane transport system is used for lipids?

  Within the enterocyte, fatty acids are re-esterified to form triglycerides, which are then packaged with phospholipids and cholesterol into lipoproteins called chylomicrons. Chylomicrons are then secreted from the enterocyte into the lymphatic system via exocytosis through the basolateral membrane. 
• What role does the endoplasmic reticulum play in processing lipids that are absorbed across the apical membrane of enterocytes?

  The endoplasmic reticulum (ER) acts as the site where these absorbed fatty acids are re-synthesized into triglycerides, packaged with apolipoproteins, and assembled into chylomicrons.   
• What is the difference between the secondary active transport of amino acids vs that of dipeptides and tripeptides?

  While amino acid transport typically relies on a sodium ion (Na+) gradient, the transport of dipeptides and tripeptides primarily utilizes a proton (H+) gradient, meaning that protons are co-transported with the peptides across the membrane to drive the process
• Define steatorrhea.

  Steatorrhea means there is excess fat in the stool.
• What causes steatorrhea?

  Its a symptom of fat malabsorption which means the digestive system is having trouble breaking down and absorbing fats.
• What is the consequence of a deficiency in the enzyme lactase?

  An absence or deficiency in the enzyme lactase will cause lactose intolerance. It causes an accumulation of lactose in the intestinal lumen, creating an increased luminal osmolarity that causes fluid accumulation and results in watery diarrhea.