Biology revision

Lactose intolerance - The inability to digest the lactose found in milk and other dairy products into its constituent monosaccharides, glucose and galactose.

·      Lactose is a disaccharide and as such cannot pass through the plasma membrane of the cells of the gut epithelium.

·       They must first be hydrolysed by enzymes bound to the membrane of microvilli. 

Digestion of lactose is carried out by the enzyme lactase present in all young mammals 

·      The monosaccharides produced are transported across the plasma membranes of the gut epithelial cells into the bloodstream and then metabolised by tissues to yield ATP or are stored as glycogen in the liver and muscle for subsequent use. (Sodium ions move out of the base of the epithelial cell by active transport which establishes a concentration gradient between the lumen of the gut and the epithelial cell. Glucose and Galactose are co-transported into the epithelial cells with sodium)

L·     Lactose therefore passes unchanged into the colon bacteria living here switch their metabolism and begin to ferment lactose,

·      producing lactic acids and large amounts of gases including carbon dioxide and methane.

·      These lower the water potential of the lumen of the gut, water enter the gut via osmosis from the blood.

1.    

1.  This gas causes bloating,

2.    crampsflatulencediarrhoea,vomitting general discomfort acidic faces due to the gases, watery due to osmosis and acidic due to the lactic acid and also contain lactose.

4.    

5.   

6

Lactase splits lactose into β-glucose and galactose

 Lactose intolerant person lacks lactase → therefore lactose is neither digested nor absorbed

High levels of soluble lactose remain in small intestine

 Supports large populations of bacteria / ferment lactose producing gas / causing discomfort

 Water potential becomes more negative / H2O moves into small intestine / not reabsorbed