The human digestive system: Carbohydrases break down carbohydrates to simple sugars. Amylase is a carbohydrase which breaks down starch.

The idea

Digestion converts large, insoluble food molecules into small, soluble molecules that can be absorbed into the blood. Carbohydrases are enzymes that catalyse the hydrolysis of carbohydrates into simple sugars. For example, starch is a polysaccharide made of many glucose units joined by glycosidic bonds; the carbohydrase amylase catalyses its breakdown into maltose and eventually glucose. Amylase is produced in the salivary glands and pancreas and acts in the mouth and small intestine. In the laboratory, amylase activity can be followed by testing starch with iodine solution, which turns from blue-black to brown as starch disappears, or by testing for reducing sugar with Benedict's solution.

What earns the marks

• Carbohydrases are enzymes that catalyse the breakdown of carbohydrates into simple sugars.
• Simple sugars are small, soluble molecules that can be absorbed across the wall of the small intestine.
• Amylase is a specific carbohydrase that catalyses the breakdown of starch.
• Starch is a polysaccharide; its hydrolysis produces smaller sugars such as maltose and glucose.
• Amylase is produced by the salivary glands and the pancreas and works in the mouth and small intestine.
• The breakdown of starch by amylase can be detected using iodine solution, which changes from blue-black to brown as starch is used up.

Common mistakes and how to put them right

• Writing that carbohydrases 'make' carbohydrates rather than break them down.

  Put it right by stating that carbohydrases catalyse hydrolysis of carbohydrates into simple sugars.

• Confusing amylase with protease or lipase.

  Put it right by linking amylase specifically to starch and carbohydrases generally to carbohydrates.

• Stating that starch is a simple sugar.

  Put it right by describing starch as a polysaccharide and glucose as a simple sugar.

Exam tips

• Name the substrate and product for each enzyme you describe, for example starch to maltose and glucose for amylase.
• Use the word 'catalyse' or 'break down' rather than vague words such as 'digest' on their own.
• When describing a test, state the starting colour and the final colour, for example iodine solution changes from blue-black to brown as starch is broken down.