Food And Digestion


Why We Need Food?

We all enjoy and eat food but why do we need it and what does our body do with it? Below are some of the main reasons why we need food.

For energy - You need food to work your muscles and other body organs. Food is the body’s fuel that keeps it going, without it the body would not work.

For growth and repair - In order for your body to grow you need to make new cells. New cells are also needed to replace old or damaged cells. Your body makes these new cells from the chemicals in your food.

To stay healthy - There are lots of reactions taking place in the cells of your body to keep you healthy and keep your body working. Certain chemicals are needed for these reactions which can only be taken in from the food you eat.

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A Balanced Diet

In order to maintain healthy, our body must have a healthy and balanced diet. This means we must eat sufficient food and also eat a variety of foods, this way our body gets all the different nutrients it needs.

There are seven nutrition groups found in the food we eat, these are:

  • Carbohydrates
  • Proteins
  • Fat
  • Fibre
  • Vitamins
  • Minerals
  • Water

A balanced diet is made up of all of the above nutrients.

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We need carbohydrates to give us energy. Starch and sugars such as glucose, sucrose and lactose are all carbohydrates. Starch is a large molecule made up of lots of smaller glucose molecules joined together. Plants turn their glucose into starch and store it for when they need it.  Glycogen is a large carbohydrate molecule like starch. It is also made up of glucose molecules joined together. Animals store glucose as glycogen in our liver and muscles, our body turns glycogen into glucose when we need it for energy.

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Your body is made up of millions of cells. These cells are mostly made up of protein. Our body needs protein to make new cells during growth and to replace old or damaged cells. Proteins are made up of lots of smaller amino acids joined together; there are about 20 different types of amino acids. The foods below are rich in protein.

Children especially need a lot of protein in their diets as they are growing, however, there are many children around the world whose diets are deficient in protein, and as a result, they suffer from illness. Kwashiorkor is a protein deficiency disorder in many developing countries. This occurs because people’s diets mainly consist of starchy vegetables and in particular they do not get enough animal protein which contains the essential amino acids required by the body.

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Fats are made up of 3 fatty acids joined to a single glycerol molecule.


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We need fats to give us energy. In fact, fats contain more energy than carbohydrates. Our body uses fat as a store of energy. Fat is stored under the skin and around the heart and kidneys. Fats are also needed for warmth (insulation) as they reduce heat loss from the body.

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However, fats can also be bad for us. There are two types of fats, saturated fat and unsaturated fat. The fat that comes from animals is saturated fat and that which comes from plants is unsaturated. Cholesterol is a fatty deposit which is made in the liver and found in blood. The level of cholesterol in the blood is influenced by the amount and type of fat in our diet. Saturated fats increase blood cholesterol levels and unsaturated fats reduce blood cholesterol levels. If we eat too much saturated fat, levels of cholesterol increase and this cholesterol will begin to stick to the lining of our arteries and over time build up. This makes the arteries narrower and our heart has to work harder to push the blood through the narrow vessels. This, in turn, can lead to an increase in the risk of heart disease and heart attacks.

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Fibre or roughage comes from plants, it is essentially the cellulose from the plant cell walls. Foods that are high in fibre include bran cereals, sweetcorn and celery.

Fibre cannot actually be digested, however, it is an important part of our diet for various reasons:

  • As it remains undigested it passes through the entire gut from mouth to anus and thus keeps food moving smoothly through our system.
  • It prevents constipation.
  • The fibre absorbs poisonous waste from the digesting food.
  • High fibre diets are believed to reduce the risk of heart disease, bowel cancer and cholesterol in the body.
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Vitamins and Minerals

We only need a small number of vitamins and minerals in our diet but these are essential for good health. If they are missing we can become ill. Deficiency diseases are caused when the body does not have enough of a certain type of vitamin or mineral e.g. anaemia is caused by lack of the mineral iron. These deficiencies are easily cured by eating the right kinds of food. Vitamins and minerals can be found in fruit, vegetables and cereals.

Salt is sodium chloride and is required in small amounts in our diet. On average an adult needs 6 grams per day, however, many people are consuming 60% more than this. Salt is naturally found in many types of food but particularly high levels can be found in processed food such as biscuits, crisps and cereals. Too much salt in the diet is linked to heart disease, high blood pressure and strokes.

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Water makes up roughly 65% of our body weight. We take in water when we eat and drink. Water is important because:

  • Chemical reactions in our cells take place in water.
  • Waste products are passed out of our bodies in the water.
  • Our blood transports substances that are dissolved in water
  • Water is in sweat that cools us down
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In order for the food that we have eaten to be useful to our body, it needs to be broken down into small molecules which can then easily be absorbed. Large food molecules such as starch, protein and fat are insoluble and cannot dissolve, thus they are unable to pass through the gut wall. On the other hand, small food molecules such as glucose, amino acids, fatty acids and glycerol are soluble and can dissolve, thus they are able to pass through the gut wall.


Digestion is the break down of large, insoluble food molecules into small, soluble food molecules so that they can be absorbed into the bloodstream.

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The Digestive System

The digestive system also is known as the gut is essentially a long tube about 9 metres long. It starts at the mouth and ends at the anus. There are many different parts to the digestive system but each part has its specific function.

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Oral Cavity (Mouth) - Teeth

Food enters the mouth and digestion begins with the teeth breaking down the food into smaller pieces. This serves 2 purposes:

  • Makes the food easier to swallow
  • Gives food a large surface area for enzymes to work on

A full set of adult teeth consists of 32 teeth. But not all teeth are the same and are shaped and designed for different jobs. There are four different types of human teeth:

  • Incisors are used for cutting and biting.
  • Canines are pointed and used for piercing and tearing.
  • Pre-molars are used for grinding and crushing.
  • Molars are like pre-molars and used for crushing and chewing.
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Salivary glands

The salivary glands secrete saliva which mixes with the chewed food. Saliva has two functions:

  1. Saliva contains the enzyme amylase which is a carbohydrase and breaks down starch into sugar.
  2. Saliva also contains mucus which lubricates the food and helps it pass down the oesophagus.
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Pharynx & Oesophagus - Swallowing

Swallowing is a reflex reaction and happens without us thinking about it. Before swallowing the tongue rolls the food into a softball and pushes it to the back of the mouth. The food pushes the soft palate upwards which blocks the upper pharynx and stops food going into the nasal cavity. Voluntary muscles in the face, neck and tongue push the food through the pharynx. As the food is swallowed it passes over the epiglottis which covers the opening of the respiratory system and prevents food entering it. Food passes the epiglottis and into the oesophagus which connects the pharynx to the stomach. Now the food enters the oesophagus and is called a bolus. The oesophagus has circular muscles in the wall. These muscles contract behind the bolus to push it along and the muscles in front of the food relax. This way food passes along the oesophagus to the stomach. This movement is known as peristalsis.

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The bolus enters the stomach. The stomach cells make gastric juices which mix with the food.

  • The gastric juices contain a protease enzyme called pepsin which breaks down proteins into amino acids.
  • The juices also contain hydrochloric acid, this is because pepsin works best in an acidic environment of pH 2.
  • The acid in the juices also kills any germs.

The muscular walls of the stomach churn the food and mix it well with the secretions. After 2-3 hours of churning the food is a thick liquid called chyme.

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Small Intestine

The small intestine is about 6 metres long. Chyme leaves the stomach via the pyloric sphincter and enters the small intestine.  The small intestine consists of three parts, the duodenum, jejunum and ileum.

In addition to digestion, the small intestine has another important job and this is ABSORPTION. The small intestine is ideal for absorption because it has:

  • A thin lining
  • Plentiful blood supply
  • Very large surface area

The surface area of the small intestine is around 9 square metres! This is possible because the small intestine is very long (around 6 metres) and it is lined with tiny finger-like projections called villi. Each villus, in turn, is covered with even smaller microvilli. The villi have very thin walls and a good blood supply which means that the digested food can be easily absorbed from the gut into the blood. There are millions of villi which provide a massive surface area to maximise the rate of absorption.

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Small Intestine - 2

In the small intestine, 3 important juices are added to the food.

1. BILE: The liver produces bile which is stored in the gall bladder and enters the small intestine via the bile duct.  Bile has 2 important functions:

  • Bile is alkaline and neutralises the acid which was added to the food in the stomach.  This provides the best pH for the enzymes in the small intestine to work at.
  • Bile emulsifies fats, that is it breaks large molecules of fat into smaller droplets which increases the surface area of fats for the enzyme lipase to work on.

2. PANCREATIC JUICE: The pancreas produces pancreatic juices which contain carbohydrases, proteases and lipases.  These enzymes empty into the duodenum to further continue digesting the food.
3. INTESTINAL JUICE:  The glands in the wall of the small intestine produce intestinal juice.  This also contains carbohydrases, proteases and lipases.  These enzymes complete the digestion of the food.

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Large Intestine

Any indigestible or non absorbed chyme passes into the large intestine. Excess water and salts are absorbed and the remaining chyme is converted into faeces. The faeces are stored in the rectum until it is excreted through the anal canal.

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Enzymes and Digestion

As we know enzymes are important molecules, they are particularly important in digestion as they help break down large, insoluble food molecules into small, soluble molecules which can be easily absorbed.  Enzymes work best at their optimum pH.  So for example if the stomach does not produce enough HCl, the enzyme, pepsin, will not work properly, (remember the optimum pH for pepsin is 2).

There are 3 main types of enzymes in our gut:  Protease, Carbohydrase, and Lipase.


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Protease enzymes are secreted by the stomach, pancreas and small intestine and their job is to digest proteins.  An example of a protease is pepsin which is secreted in the stomach. Proteins are long chains of amino acids, and protease enzymes break them into peptides (smaller chains of amino acids molecules) and eventually into individual amino acids, which are small and easily absorbed in the small intestine.

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Carbohydrase enzymes are secreted by the mouth, pancreas and small intestine.  The carbohydrase enzyme, amylase is secreted by the mouth and found in saliva. It starts to work as soon as we begin to chew our food.  Amylase digests long, complex starch (polysaccharide) molecules, into smaller, simpler maltose (disaccharide) molecules. As maltose is a disaccharide it still needs further digestion before it can be absorbed.  The enzyme maltase breaks it down into glucose. 

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Lipase is secreted by the pancreas and the walls of the small intestine. It digests complex fat (or lipid) molecules into simple, soluble fatty acid and glycerol molecules.

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