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Food and Its Importance | Grade 7 Science Lesson

We need food to

  • Repair worn out tissues
  • Maintain a healthy body
  • Grow new cells and tissues, and
  • Provide us with energy for our daily activities like walking.

The food we eat may be grouped as follows:

  • Proteins
  • Carbohydrates
  • Fats
  • Fibre
  • Vitamins
  • Mineral salts and
  • Water.

All of these foods, except water and fibre, are called nutrients. Proteins, carbohydrates and fats are the three main types of food that supply the body with energy. Different food substances have different roles in the body.


Proteins are one of the basic building blocks of the human body, making up about 16% of tour total body mass. Proteins provide us with energy, build new cells for body growth and repair worn-out tissues. Proteins can be found in food such as meat, fish, milk, eggs, nut, cheese and beans.

Deficiency of proteins may lead to a disease known as kwashiorkor, which causes swollen abdomens.

Test for proteins

The Biuret test is used to detect the presence of proteins. A few drops of Biuret solution are added to the food sample containing proteins, shaking after each drop. Proteins are present if the colour of the sample changes to purple.


Carbohydrates are the principal source of energy in most humans. It provides between 40 – 80% of the total energy intake in different populations. Sugars and starch are examples of carbohydrates. There are many kinds of sugars such as glucose and sucrose. Starch is present in plant food such as potatoes, tapioca and rice.

Importance of carbohydrates

Carbohydrates are an immediate source of energy. They are found mainly in food like rice, bread, noodles and potatoes.

Test for starch

A few drops of iodine solution are added to the food sample containing starch. Starch is present when the colour of iodine turns from brown to blue-black. It is absent when the colour remains unchanged.

Test for glucose

Benedict’s solution is used to test for the presence of glucose in food samples. The food samples must be in solution form. An equal volume of Benedict’s solution is added to the solution. The mixture is then warmed in a beaker of hot water. Glucose is present if the colour of the sample changes to an orange or red precipitate. It is absent when the colour remains unchanged.


Fats provide almost twice as much energy as proteins and carbohydrates. They are important for good health. Fats provide us with energy and warmth. Fat can be found in food such as oil, butter and nuts.

Test for fats

The grease spot test is used to detect the presence of fats. When a drop of food sample is placed or smeared on a piece of filter paper and allowed to dry, a translucent spot is formed. This spot is known as the grease spot and it indicates the presence of fat.


Fibre is obtained from plant materials. Fibre cannot be digested and will be passed out of the body. Fibre absorbs water, making the waste matter soft, preventing constipation. Constipation is the result of eating too little fibre and causes faeces to become dry and hard, making it difficult to be expelled from the body.

Fibre keeps our digestive system healthy by providing bulk to faeces and helping to prevent constipation. A variety of beans, fruits, vegetables and whole grain products such as whole meal breads, oats and cereals contain fibre.


Vitamins are naturally occurring chemicals in minute quantities and can be found in most of the foods we consume. It has a variety of functions in our body. Nonetheless, we only require a small amount to keep us healthy.

Viramin A is needed for healthy skin and eyes. Deficiency in vitamin A causes night blindness. Some yellow or orange fruits such as mangoes and papayas as well as vegetables such as spinach, broccoli, tomatoes and carrots contain vitamin A.

Vitamin B is needed for a healthy nervous system. Deficiency of vitamin B causes beri-beri, a disease that leads to the loss of function in the muscles. Fish, potatoes and vegetables such as broccoli and spinach contain vitamin B.

Vitamin C is needed for healthy blood, skin and gums. It increases the body’s natural defenses against diseases. Deficiency of vitamin C leads to a disease called scurvy, which is characterized by swollen and bleeding gums, muscle weakness and joint pains. Citrus fruits such as oranges, lemons, kiwis and limes contain vitamin C.

Deficiency of vitamin D causes weak and brittle bones leading to a bone disease known as rickets. Eggs, cheese and some fish such as tuna contain vitamin D.

Mineral salts

Mineral salts such as calcium and iron are required only in small amounts to keep us healthy and prevent diseases.

Calcium is essential to keep bones and teeth strong and healthy. A deficiency in calcium leads to rickets and osteoporosis. Dairy products such as milk, cheese and yoghurt as well as dark green leafy vegetables contain calcium.

Iron is fundamental in the formation of healthy red blood cells. A deficiency in iron can lead to a blood disease called anaemia. Liver, meat, soybeans and dark green leafy vegetables such as spinach contain iron.

Iodin is essential for normal growth. It is important for the formation of hormones in the thyroid glands. A deficiency of iodine leads to goitre. Seafoods such as crabs and seaweeds contain iodine.

A deficiency of fluoride leads to an increased risk of dental cavities. Fish, tea and most types of vegetables contain fluoride.


Water is essential to life. Humans can live for several weeks without food, but only a few days without water. Water is important as water carries substances around the body and helps to remove wastes. Every day, you lose water through your breath, sweat, urine and faeces. For your body to function properly, you must make sure that you consume enough water. Water prevents constipation, maintains normal body temperature and prevents dehydration. Beverages and foods such as watermelons, cucumbers and radishes contain water.

Composition of foods

You can easily check the types of nutrients you are taking in by looking at the food labels on a food package. By reading the food labels, you can find out information about all the ingredients that make up a food product, and the amount of energy, protein, fat, dietary fibre and nutrients provided by the food products.

Balanced diet

A balanced diet contains the correct proportion of proteins, carbohydrates, fats, vitamins, mineral salts, fibre and water. Each individual has different nutritional needs. A balanced diet for one person may not be balanced for another. Following the ‘healthy plate’ or the ‘food pyramid’ are two main ways to ensure that your meal is a balanced diet.

‘Healthy plate’

A ‘healthy plate’ comprises the following:

  • One serving of fruit
  • A glass of water
  • A glass of milk
  • Half a plate of vegetables
  • A quarter plate of carbohydrates
  • A quarter plate of proteins

‘Food Pyramid’

The ‘food Pyramid’ divides foods into groups showing the correct proportions of the food groups to eat. According to the ‘Food Pyramid’ we should reduce the intake of fatty, oily, salty and sugary foods. We should take a moderate amount of proteins and dairy products. Complex carbohydrates also should be taken accordingly.

Eating junk food such as fast foods or foods high in salt, oil, or sugar may have harmful effects on health.

  • An unbalanced diet

A diet consisting of too much or too little of a particular type of food is an unbalanced diet. It does not supply the right amounts of nutrients needed and often leads to malnutrition. The effects of an unbalanced diet include:

  • Obesity
  • Diabetes
  • Being underweight
  • Developing coronary heart diseases


When a person regularly eats too much of carbohydrates and fats and does not exercise enough, he or she will become overweight. If this continues, it may lead to obesity. Obesity causes major health problems such as heart diseases, stroke and diabetes. It also puts a strain on the joints in the body.


Diabetes is a disease caused by high levels of sugar in the blood. This may lead to many other conditions such as kidney damage and blindness. Diabetes can affect people of any age. There is no cure for it. An unbalanced diet or a diet with high sugar content can increase the risk of developing diabetes.

Being underweight

When the amount of food that a person takes in is much less than what is required daily, she or she may weigh less than the expected weight of a healthy person. When a person is underweight, he or she may have poor physical stamina and a weak immune system leaving them open to infection.

Coronary heart diseases

A diet that contains a large amount of fats may lead to coronary heart disease. An unhealthy diet that contains large amounts of fats or processed foods may cause weight gain. This can cause stress on the heart as it needs to pump blood harder to the larger body.