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Plasma Membranes


Fluid-Mosaic Model


Facillitated Diffusion


Water Potential

Active Transport

The Absorption of Glucose from the Small Intestine

The chemical digestion of carbohydrates results in the production of monosaccharides such as glucose. These need to be absorbed by the small intestine and passed into the bloodstream for use by the body. The process of diffusion alone would not result in all of the glucose present in the small intestine being absorbed as an equilibrium would be reached and any remaining glucose would pass out of the body in the faeces. Our digestive systems have evolved to absorb all of the glucose produced.

Glucose is therefore absorbed by the small intestine using an active process. It is considered an active process because ATP is required for it to happen. However it uses the ATP indirectly as it is the movement of sodium ions which actually powers the movement of glucose into the cells. It is also an example of CO-TRANSPORT because two molecules (glucose AND sodium) are involved.



  1. Sodium ions are actively transported out of the epithelial cell into the blood by the sodium potassium ATPase. This protein pump is present in the membrane of all eukaryotic cells.
  2. Sodium ions are now at a lower concentration in the epithelial cell than in the lumen of the small intestine.
  3. Sodium ions now diffuse down their concentration gradient through a co-transport protein present in the plasma membrane of the epithelial cell. The energy released as the sodium ions move down their concentration gradient allows glucose molecules to pass through the co-transporter too despite the epithelial cell having a higher concentration of glucose than the lumen of the small intestine.
  4. The glucose now passes into the blood via facilitated diffusion.


Prokaryotic Organisms - Bacteria

Vibrio Cholera