Digestion and absorption

• Define digestion

  Digestion is the hydrolysis of larger insoluble molecules into smaller soluble molecules to be absorbed across cell surface membranes
• Explain digestion in the three parts of the small intestine

  Duodenum: Pancreatic enzymes mix with food

  Jejunum: Chemical digestion occurs

  Ileum: Smaller molecules absorbed across ileum wall into the blood
• Explain the digestion of sucrose

  Membrane-bound sucrase in ileum wall hydrolyses the glycosidic bond, releasing glucose and fructose
• Explain the digestion of lactose

  Membrane-bound lactase in ileum wall hydrolyses the glycosidic bond, releasing glucose and galactose
• Describe carbohydrate digestion

  -Salivary amylase secreted into the mouth hydrolyses glycosidic bonds, releasing maltose

  -Pancreatic amylase secreted into the duodenum hydrolyses the remaining starch to maltose

  -Membrane-bound maltase in the ileum wall hydrolyses maltose, releasing glucose
• Describe lipid digestion

  -Bile secreted into the duodenum emulsifies large lipid globules into small lipid droplets, increasing SA

  -Pancreatic lipase secreted into the duodenum hydrolyses ester bonds, releasing fatty acids, glycerol and some monoglycerides
• Describe protein digestion

  -Gastric endopeptidases secreted into the stomach hydrolyses peptide bonds in the middle of a polypeptide, releasing smaller polypeptides

  -Pancreatic endopeptidases secreted into the duodenum hydrolyses the remaining large polypeptides to smaller ones

  -Pancreatic exopeptidases secreted into the duodenum hydrolyses the smaller polypeptides, releasing dipeptidases and amino acids

  -Membrane-bound dipeptidases in the ileum wall hydrolyses remaining dipeptides, releasing amino acids
• Describe absorption by the ileum

  -The wall is folded into villi = large SA

  -Each villus is made of a single layer of epithelial cells with a capillary behind = short diffusion pathway

  -Muscle contraction in the ileum wall and circulation in the capillary = large concentration gradient
• What do fatty acids, glycerol, and monoglycerides combine with to form micelles in the ileum?

  Bile
• Why are micelles important for the absorption of lipids in the ileum?

  They make fatty acids, glycerol, and monoglycerides soluble for transport to the cell surface membrane
• What happens to bile after it helps form micelles in the ileum?

  Bile detaches, allowing fatty acids, glycerol, and monoglycerides to diffuse into epithelial cells
• After diffusion into epithelial cells, where do fatty acids, glycerol, and monoglycerides move to?

  They move to the smooth endoplasmic reticulum (ER)
• What do fatty acids, glycerol, and monoglycerides form in the smooth ER?

  Triglycerides
• What happens to triglycerides after they are formed in the smooth ER?

  They move to the Golgi apparatus
• What are triglycerides combined with in the Golgi apparatus to form chylomicrons?

  Proteins and cholesterol
• What are chylomicrons packaged into after they are formed?

  Vesicles
• What happens to chylomicrons after they are packaged into vesicles?

  They fuse with the cell surface membrane
• How are chylomicrons released from the epithelial cells?

  By exocytosis
• Where do chylomicrons enter after being released from the epithelial cells?

  The lacteal, a branch of the lymphatic system
• What is the process of lipid absorption in the ileum?

  1. Fatty acids, glycerol, and monoglycerides combine with bile to form micelles.
  2. Micelles transport lipids to the cell surface membrane.
  3. Bile detaches, allowing diffusion into epithelial cells.
  4. Lipids move to the smooth ER and form triglycerides.
  5. Triglycerides move to the Golgi to form chylomicrons with proteins and cholesterol.
  6. Chylomicrons are packaged into vesicles and released by exocytosis into the lacteal.