Carbohydrate digestion

You should know: Starch, the role of salivary and

pancreatic amylases and of maltase located in the

intestinal epithelium

• Disaccharides, sucrase and lactase.

• Lactose intolerance

What sort of sugar is the monomer for starch?

What type of bond links the sugar molecules?

What is the name of this molecule?

Stages in carbohydrate digestion (i) In the mouth Starch → maltose via enzyme amylase. Stomach acidic pH 2 which denatures

amylase. pancreatic amylase which continues

hydrolysis of starch → maltose. Maltase in epithelial lining of small

intestine.Maltose → α glucose

Disaccharide digestion Epithelial cells in lining of small intestine

produce enzyme sucrase.Sucrose (disaccharide) → glucose + fructose (2 monosaccharides).

Epithelial lining produce enzyme lactase.

Lactose (disaccharide) → glucose + galactose (2 monosaccharides

Lactose Intolerance Babies produce lots of lactase. Amount diminishes as we

get older. Lactase is found in the microvilli of the epithelial cells

lining the small intestine. Some people produce little or no lactase. Nowadays we eat lots of milk products. Some cannot

digest it all. Undigested lactose reaches the large intestine where

micro-organisms break it down and produce loads of gas. Result = bloating, nausea, diarrhoea and cramps. Lactose intolerant individuals need to avoid foods with

lactose in them.

Causes of lactose intolerance PrimaryUsually inherited = fault in gene that codes for lactase production.

SecondaryPerson can’t produce enough lactase due to damage to small intestine through injury or disease. E.g. coeliac disease, inflammatory bowel disease or Crohn’s diseases.

Tests for lactose intolerance Patient fasts before test. Then drink a lactose sample. A blood sample is taken every 2 hours

and the level of glucose in the blood is measured.

If lactose is being broken down, then glucose level should rise. If intolerant, then will stay low.

Hydrogen breath test Patient drinks fluid containing lactose.

Patients breath measured for hydrogen. Normal breath has little. If lactose intolerant then will have

higher levels because bacteria in large intestine are digesting the lactose and producing large amounts of hydrogen.

Stool Acidity test Measure level of acidity in stools. If

lactose intolerant then fermented in large intestine by bacteria to produce lactic acid and other fatty acids.