The Human Digestive System (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Large insoluble molecules, such as starch and proteins, are made of long chains of smaller molecules joined together by chemical bonds

• These bonds must be broken down during digestion

The purpose of digestion is to break large, insoluble molecules into small, soluble molecules so they can be absorbed into the bloodstream

• The blood then transports these molecules to body cells

• Cells use these molecules for respiration or to build new large molecules needed for growth, repair and energy storage

Enzymes are biological catalysts that speed up chemical reactions without being used up or changed

• There are three main types of digestive enzymes: carbohydrases, proteases and lipases

Starch is a carbohydrate made of long chains of glucose molecules

Carbohydrases break down carbohydrates to simple sugars.

• Amylase is a carbohydrase which breaks down starch into maltose, which is then broken down into glucose by the enzyme maltase

Glucose is water-soluble and can be absorbed into the blood

Cells use glucose during respiration to release energy

Amylase is made in the salivary glands, the pancreas and the small intestine 

Proteins are long chains of amino acids

Proteases are a group of enzymes that break down proteins into amino acids in the stomach and small intestine

Cells use amino acids to make new proteins

• New proteins are needed for growth and repair

Excess amino acids cannot be stored

• The liver breaks down excess amino acids to form urea

• Urea is carried by the blood to the kidneys

• The kidneys excrete urea in urine

Protein digestion takes place in the stomach and small intestine, with proteases made in the stomach (pepsin), pancreas and small intestine

Lipids include fats and oils

Lipase enzymes digest lipids into glycerol and fatty acids

Cells can reform fats from glycerol and fatty acids

• Fats are stored as an energy reserve

• Cells break down fats and use them in respiration when needed

Lipase enzymes are produced in the pancreas and secreted into the duodenum