Mechanism of Absorption in the Small Intestine


  • Very short
  • Shaped like a C
  • Neutralizes stomach acids
  • Responsible for most of the small intestine's chemical digestion
  • Absorbs Fe+


  • Located after the jejunum
  • Responsible for absorbing vitamin B12 and leftover bile salts


  • Villi made up of epithelial cells
  • Absorbative cells absorb nutrients
  • Endocrine cells secrete hormones
  • Goblet cells secrete mucous

Form of Epithelial Cells

  • Different membranes on cell
  • Apical membrane faces lumen and Basolateral Membrane faces other cells and towards the capillaries and lacteal.
  • Apical membrane has brush surface, covered in microvilli

Digestion of Carbohydrates

Starch and glycogen broken down by amylase into maltose, sucrose and Lactose. 

These molecules are then broken down further by enzymes Maltase, Sucrase and Lactase into glucose galactose and fructose

Glucose and Galactose transport (apical)

SGLT (Sodium Glucose Linked Transport)

Glucose and galactose transport (Basolateral)

Monomers travel out basolateral membrane through GLUT2 through facilitated diffusion


Fructose passes through apical membrane through GLUT5  and out a GLUT2.


Monomers then enter capillaries through simple diffusion

Proteins and Peptides digestion

  • Broken down by pepsin in stomach (endopeptidase)
  • Broken down by exopeptidases in duodenum
  • "Cut" amino acids off, leaving free amino acids and smaller peptides

Absorption of di/tripeptides and free amino acids

  • Dipeptides and Tripeptides are transported by an H+ cotransporter
  • Amino acids are transported by a Na+ cotransporter
  • Peptidases break down some dipeptides and tripeptides inside the cell
  • They the diffuse into capillaries

Absorption of small peptides

Small peptides are absorbed and transported by transcytosis, a combination of endocytosis, vesicular transport and exocytosis

Proscess of Digestion of Fats

  1. Fat enters small intestine as non water soluble droplets
  2. Bile salts from the liver coat fat droplets. 
  3. Colipase breaks some of the bile off, allowing lipase to break the droplets into cholesterol monoglycerides and fatty acids which are then stored in micelles. 

Process of absorption of fats

  1. a) Monoglycerides and fatty acids move out of micelles and pass through apical membrane through simple diffusion. Cholesterol is transported into cell through a cell transporter
  2. Monoglycerides and fatty acids then move to the smooth endoplasmic reticulum where they are synthesized into triglycerides.
  3. Tryglicerides combine with cholesterol (and some proteins in the cell already) to form chylomicrons
  4. Chylomicrons enter the golgi apparatus where they are packaged in vesicles and transported to the basolateral membrane, where they exit the cell through exocytosis.
  5. Chylomicrons will "attempt" to enter the capillaries, but are too large to diffuse through its basement membrane, so they diffuse into the lacteals.

Vitamins, minerals, water and ions

  • Fat soluble vitamins are absorbed with fats
  • Water soluble vitamins are absorbed through mediated transport (except vitamin B12)
  • Ions each have there own transport proteins (Na+ has three)
  • Water is absorbed wherever osmotic gradients allow
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Mechanisms of Absorption in the Small Intestine

by brewkelly


Public - 4/14/16, 5:10 PM