Digestion and the small intestines

Absorption of Glucose in the intestines: 

• When Carbohydrates are first broken down there is a higher concentration of glucose in the small intestine than there is in the blood. Therefor glucose moves across the epithelial cells of the small intestine, into the blood, by a process called diffusion. When the concentration in the lumen becomes lower than in the blood, there is no longer a concentration gradient, therefore diffusion stops. 
• The remaining glucose is absorbed by active transport with sodium ions:
• Sodium ions are actively transported out of the epithelial cells into the blood, by the sodium potassium pump, creating a concentration gradient, theres a higher concentration of sodium ions in the lumen than the cells.
• This concentration gradient causes sodium ions to diffuse into the cell from the lumen, this is done by sodium-glucose co-transporter proteins. The co-transporter carries glucose into the cell with the sodium, the increase of glucose in the cell increases the concentration.
• The glucose then diffuses down its concentration gradient into the blood, through a protein channel, via facilitated diffusion.

How is the small intestine adapted to its digestive and absorptive functions:

• Large surface area provided by villi and microvilli, 
• Thin epithelium gives a short diffusion pathway, 
• The dense capillary network for absorbing amino acids and sugars and the lacteal for the absorption of digested fats; which ensures a steep concentration is maintained,
• Many mitochondria in the epithelial cells provide ATP/ energy for active transport,
• Carrier proteins (in membranes) provide a path for polar molecules to pass through the membrane,
• Enzymes built into the epithelial membrane make it more likely for enzyme substrate complexes to form and ensure products for absorption are released close to the channel and carrier proteins, 

Digestion of Carbohydrates and Disaccharides:

Food moves through the digestive system by peristalsis. 

Mouth = digestion of carbohydrates,

Stomach = digestion of protein,

Duodenum = most digestion, receives pancreatic juice from the pancreatic duct, and bile from the gall bladder (produced in the liver.)

Ileum = most absorption, 

Colon = absorption of water and minerals,