Biology Unit 1 (AQA)

Covering Topic 2 Onwards

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Enzymes and Digestion

Major parts of the digestive system:

  • Oesophagus - carries food from the mouth to the stomach, adapted for transport, made up of a thick muscular wall.
  • Stomach - a muscular sac with an inner layer that produces enzymes which digest protein. Other glands produce mucus which protect the stomach from it's own enzymes.
  • Small inestine - long muscular tube. Food is further digested by ezymes that are produced in its walls and by more glands. The inner wall of the small inestine is folded into villi which gives them a greater surface area. On the villi are microvilli which greaten the surface area even more, making them more adapted to digestion into the blood stream.
  • Large intestine - absorbs water, making the food drier and forming faeces.
  • Rectum - faeces are stored before being removed via the anus by the process of; egestion.
  • Salivary glands - secrete amylase into the mouth, which breaks starch into maltose.
  • Pancrease - produces pancreatic juice, proteases to digest protein, lipase to digest lipids and amylase to digest starch.
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Digestion takes place in two main ways, physical and chemical digestion.

Physical breakdown - large food needs to broken into smaller pieces by chewing and by being churned up by muscles in the stomach wall. Smaller pieces of food makes it easier for ingestion but also provides a large surface area for chemical digestion.

Chemical digestion - also involves large > small. Enzymes function by hydrolysis, the splitting of molecules by adding water to the chemical bonds. Each enzyme is specific to a molecule due to the shape of the active site, therefore more than one enzyme is needed to break down a piece of food. Carbohydrases break down carbohydrate to monosaccharides, lipases break down lipids into glycerol and fatty acids, proteases break down proteins into amino acids. Once broken down, the new molecules are absorbed intol the blood via the small intestine. They are then carried to different parts of the body and are often bult into large molecules, these molcules are incorporated into body tissues and used in processes of the body; this is called assimilation.

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Carbohydrates - monosaccharides

Monosaccharides are sweet-tasing soluble substances with the general formula (CH2O)n. Monosaccharide + Monosaccharide = Disaccharide

Testing for reducing sugars (Benedict's test)

  • Add 2cm3 of the food sample to a test tube. It not in liquid form, grind it with water.
  • Add an equal volume of Benedict's reagent (Copper(II) Sulfate)
  • Heat the mixture in a gently boiling water bath for 5 minutes.

Benedict's reagent is blue, a food subtance with a high concentration of sugar turns orange-brown from the blue.

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Carbohydrates - Disaccharides and polysaccharides

Monosaccharides + Monosaccharides = Disaccharides

Glucose + Glucose = Maltose

Glucose + Fructose = Sucrose

Glucose + Galactose = Lactose

When the monosaccharides join together, water is removed and is called a condensation reaction. The bond that is formed between is called a glycosidic bond. When water is added under the correct conditions, the bond is broken and the addition of water causes the monosaccharides to be formed again, this is called hydrolysis.

Testing for non-reducing sugars

Some disaccharides, such as sucrose, are non-reducing because they do not change the colour of Benedict's when they are heated alone with it. In order to detect a non-reducing sugar, it must be broken into its respective monosaccharides by hydrolysis. The process is as follows (cont.)

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Carbohydrates - Di and polysaccharides (cont.)

Testing for non-reducing sugars

The process is as follows:

  • If the sample is not in liquid form, it must first be ground with water.
  • Add 2cm3 of the food sample to 2cm3 of Benedict's reagent to a test tube.
  • Place the test tube in a gently boiling water bath for 5 minutes. If the bendict's reagent does not change colour, stays blue, then a reducing sugar is not present.
  • Add another 2cm3 of the food sample to 2cm3 of dilute hydrochloric acid in a test tube and place in water bath for 5 minutes. The dilute HCl will hydrloyse any disaccharide present into its respective monosaccharides.
  • Add sodium hydrogencarbonate solution to the test tube in order to neutralise the HCl as benedict's will not work in acidic conditions. Test with a pH meter to check that the solution is alkaline.
  • Re-test the resulting solution by heating it with 2cm3 of benedict's reagent in a water bath for 5 minutes.
  • If a non-reducing sugar is present in the original sample, the benedict's solution will now turn orange-brown.
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Polysaccharides are polymers, formed by combining together many monosaccharide molecules. The monosaccharides are joined by glycosidic bonds that were formed via condensation reactions. As polysaccharides are very large molecules, they are insoluble, making them good for storage within cells. When they are hydrolysed, polysaccharides break down into disaccharides or monosaccharides. Some polysaccharides, such as cellulose, are not used for storage but give structural support to plant cells. Starch is a polysaccharide. Starch is easily detected by its ability to change colour of iodine in the potassium iodide solution from yellow to blue/black. The test is carried out at room temperature, the test is as follows:

Testing for starch

  • Add 2cm3 of the sample into a test tube
  • Add two drops of iodine solution and stir
  • The presence of starch is indicated by a blue/black coloration.
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Enzymes are specific to one molecule so therefore it takes more than one to break down a particular food substance. These enzymes are produced at different points throughout the digestive system in order to break them down into their monomers. In humans, the following process is true of starch digestion:

  • Food is eaten and physically broken down by the teeth into smaller pieces enabling a larger surface area for enzymes later on.
  • The act of chewing causes saliva to enter the mouth from salivary glands and is mixed with the food through chewing.
  • Saliva contains salivary amylase. This enzyme starts hydrolysing any starch in the food to maltose. It also contains mineral salts which help to maintain the neutral pH, the optimum pH for salivary amylase to work at.
  • Food is swallowed and enters the stomach, where it is of an acidic pH. The acid denatures the amylase and prevents futher hydrolysis of starch.
  • The food passes into the small intestine where it mixes with pancreatic juice from the pancreas. This contains pancreatic amylase. This continues the hydrolysis of any remaining starch into maltose. Alkaline salts are produced by the pancrease and the intestinal wall to maintain a neutral pH so that amylase can function.v
  • Muscles in the intestine wall push the food along. Its epithelial lining produces maltase. The maltase hydrolyses maltose into glucose, which is abosrbed into the blood stream.
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Disaccharide digestion

As well as starch/maltose digestion there are two other main disaccharides in our diets which need to be broken down; sucrose and lactose.


In natural foods, sucrose is usually contained within cells and therefore must be physically broken down by the teeth in order to release it. The sucrose passes through the stomach into the small intestine, whose epithelial lining produces the enzyme sucrase. Sucrase hydrolyses the single glycosidic bond in the sucrose molecule to produce the two monosaccharides that make up sucrose, glucose and fructose.


Lactose is the sugar present in milk and in dairy products such as cheese and yoghurt. The small intestine's epithelial cells produce lactase. Lactase hydrolyses the glycosidic bond that links the glucose and galactose monosaccharides.

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Lactose intolerance

Milk is the only food of young babies and therefore produce high levels of lactase. As milk forms a much smaller part of the diet in adults, the production of lactase becomes less and less. In some people, this reduction becomes so great that they end up producing little or no lactase.

As lactase is not being produced, the undigested lactose is able to enter the large intestine where microorganisms break it down giving rise to a large volume of gas. This results in nausea, diarrhoea and cramps. Some people with this condition cannot consume milk (or lactose) at all, while others can drink or eat only a little. Lactose intolerance can be avoided by not eating or drinking foods or drinks with lactose in but the main problem with not drinking or eating dairy products is that sufficient calcium, in the absence of milk, is not taken into the body. This can be resolved by taking foods rich in calcium or by adding the enzyme lactase to milk before drinking it.

In young babies, however, their sole food is milk so if a baby becomes lactose intolerant, which is rare, then it can be very serious. These babies need to be fed special non-milk food that is rich in calcium and vitamin D that natural milk provides.

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