AS Biology: Absorption in the Small Intestine

In digestion carbohydrates are digested to form soluble molecules; glucose, fructose and galactose. These are all digested in a similar way. Glucose will be used as the example.

Glucose is absorbed through the walls of the intestine. The walls if the intestine are folded to form villi. 

Villi have thin walls lined with epithelial cells. On the other side is a rich network of blood capillaries. They are positioned on the interface between the lumen of the intestine and the blood and tissues (inside the body). They are a specialised exchanged system adapted for the efficient aborption of the products of digestion:

- increase the surface area

- thin walled: reducing the diffusion pathway

- able to move: maintain a diffusion gradient

- supplied with blood vessels: move absorbed molecules: maintain diff. gradient

- Carbohydrates are continually being digested

- Higher concentration of glucose in the small intestine than in the blood (concentration gradient)

- Causing glucose to diffuse into the blood

- Blood is constantly being circulated, glucose therefore is being moved to be used up by respiring cells

- Helps maintain a steep concentration gradient between the lumen of the intestine and the blood

- Villi contain muscles which are constantly contracting and relaxing, mixing the contents of the intestine

- Ensures that the glucose that has diffused into the blood, adjacent to the villi, is constantly replaced by newly digested glucose

- Helping to maintain the concentration gradient

Diffusion results in an equal concentration of glucose in the lumen of the intestine and the blood. Need another method to ensure all the glucose in the intestine is transported into the blood. Done by Active Transport.

- Sodium ions = actively transported = out of epithlial cells = into the blood     (using one type of protein-carrier molecule found in the cell surface membrane of  the epithelial cells) 

- This results in a higher concentration of sodium ions in the lumen of the intestine  than in the epithelial cells, this causes:

- Sodium ions + glucose = to diffuse = out of the lumen = into the epithelial cells       (using the potassium-sodium pump and another type of carrier)

- Glucose = facilitated diffusion = out of epithelial cells = into the blood (using another carrier)            

- Remember, the sodium ions are moving down their concentration gradient, while glucose is moving UP. It is the sodium ion concent. gradient that powers the glucose (notATP)