The digestive system and food tests – GCSE Combined Science Biology

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Digestion is how your body breaks food down into pieces small enough to use. It happens along a long tube called the gut, with help from enzymes. Enzymes are chemicals that speed up reactions.

It matters because the food you eat is made of large molecules. Your blood cannot take those in. Until food is digested, you cannot use it to grow, repair your body or get energy.

By the end you will be able to follow food through the gut. You will name the main enzymes and what they do. You will also describe the food tests and what bile does.

Why we digest food

Starch, proteins and fats are large molecules. They are too big to pass through the wall of the gut into the blood. Think of trying to post a whole pizza through a letterbox. You need to cut it into small pieces first.

Digestion breaks the large molecules into small soluble molecules. Soluble means they dissolve. These small molecules can be absorbed into the blood.

The body uses them to build new carbohydrates, lipids and proteins. They can also be respired to release energy. Enzymes in the digestive system speed up this breakdown.

Following the food

Food starts in the mouth. The salivary glands add saliva, which contains an enzyme. Food then goes down the oesophagus, the tube that leads to the stomach.

The stomach churns the food. It also makes acid and protease, an enzyme that digests protein.

The liver, gall bladder and pancreas also add chemicals to the food. The liver makes bile, which is stored in the gall bladder. The pancreas makes enzymes that go into the small intestine.

The digestive system shows how living things are organised. Cells that are alike form a tissue. Different tissues form an organ, and several organs working together form an organ system. The stomach is an organ made of muscular tissue, which churns the food, glandular tissue, which makes digestive juices with enzymes and acid, and epithelial tissue, which covers the inside and the outside. The digestive system is the organ system that digests and absorbs food.

The intestines

After the stomach, food passes into the small intestine. This is where most digestion happens. It is also where the soluble products of digestion are absorbed into the blood.

The small intestine is lined with villi, which are tiny folds. They give a large surface area, so more food can be absorbed quickly. They have thin walls and a rich supply of blood vessels.

What is left goes into the large intestine. Here water is absorbed from the undigested food.

The three main enzymes

Amylase breaks down starch into sugars. It is made in the salivary glands, the pancreas and the small intestine.

Protease breaks down proteins into amino acids. It is made in the stomach, the pancreas and the small intestine. Lipase breaks down lipids, which are fats and oils, into fatty acids and glycerol. It is made in the pancreas and the small intestine.

The ending of each name tells you the job. Amylase works on starch, protease on protein and lipase on lipids.

Bile

Bile is made in the liver and stored in the gall bladder. It is released into the small intestine. Bile is not an enzyme.

Bile is alkaline, so it neutralises the hydrochloric acid that comes from the stomach. This gives the right pH for the enzymes in the small intestine.

Bile also emulsifies fats. It breaks big drops of fat into many tiny droplets. This gives a larger surface area for lipase to work on, so the fat is digested faster.

Food tests

For starch, add iodine solution. It turns from orange-brown to blue-black. For sugar, add Benedict’s solution and heat it in a water bath. It turns from blue to green, yellow or brick-red.

For protein, add Biuret solution. It turns from blue to purple. For lipid, shake the food with ethanol, then pour the mixture into water. A cloudy white emulsion shows lipid is present.

First crush the food and mix it with distilled water. Wear eye protection, because the reagents can irritate the eyes.

Recap

Digestion breaks large molecules into small soluble ones. Most digestion and absorption happens in the small intestine.

Amylase: starch to sugars. Protease: protein to amino acids. Lipase: lipid to fatty acids and glycerol.

Bile neutralises acid and emulsifies fat. Iodine tests for starch, Benedict’s for sugar, Biuret for protein and ethanol for lipid.