Punjab PSC Exam: Biology Notes

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The 7 Basic Food Substances

All the food we eat is made up of the following 7 basic substances:

  • Carbohydrates
  • Fats
  • Proteins
  • Vitamins
  • Minerals
  • Fibre
  • Water

Lets discuss one by one

Carbohydrates: Carbohydrates are organic substances made up of carbon, hydrogen and oxygen. They are very important because they provide energy for the body. There are 3 types of carbohydrates: Sugars, starch, and cellulose.


  • Glucose (C6H12O6)
  • Fructose (sugars in fruit)
  • Sucrose (table sugar)
  • Lactose (found in milk)
  • Maltose (found in barley grains)

Starch: Found in bread, potatoes, rice, cereals etc. Plants store food as starch.

Cellulose: Found in all unrefined plant food. An important source of fibre.

Carbohydrates are all made up of molecules of glucose bonded (joined) together. The simplest form of carbohydrate is glucose. Two molecules of glucose joined together with a bond, form maltose, lactose and sucrose sugars. Starch, cellulose and glycogen are formed when 3 or more glucose molecules are joined together with bonds.

Glucose՚s molecule is represented by a hexagon: A single sugar molecule is called a monosaccharide. Examples of monosaccharides are glucose and fructose.

Sucrose, maltose and lactose are all disaccharides because they have 2 sugar molecules bonded together.

Starch, cellulose and glycogen are all polysaccharides because they are made up of 3 or more sugar molecules bonded together.

Carbohydrates are found in cereals, pasta, bread, fruit, potatoes sugary food such as ice cream etc.

Glucose՚s chemical formula is the following: C6H12O6.

Plants store food as starch, while animals store food as glycogen. Both glycogen and starch are polysaccharides. Polysaccharides are NOT sweet but ARE insoluble.


Fats are organic substances. Lipids are fats in a liquid state. Fats are useful for our body, because they:

  • provide energy
  • can be stored for later use
  • build up cell membranes
  • layers serve as an insulating layers under mammal՚s skins
  • oils on the surface of the skin makes the skin waterproof

Fat is found in vegetable oil, milk, fried foods, eggs, beef etc.

The simplest fat molecule is made up of 1 molecule of glycerol and 3 fatty acids bonded together.


Proteins are organic substances made up of hydrogen, carbon and oxygen, nitrogen and sometimes they contain sulphur. Proteins are needed by the body to grow and repair tissues (a cellular structure) , they are components of cell membranes, are used to produce enzymes (biological catalysts) and hormones.

The simplest possible protein is an amino acid, thus proteins are made up of amino acids, which can be represented as any form of shape (circle, rectangle, square) .

Amino acids are joined together by peptide bonds. When 2 amino acids connected together with a peptide bond, a dipeptide forms. When 3 or more amino acids are joined together, a polypeptide is formed.

Amino Acid Dipeptide Polypeptide: When proteins are heated, they are denatured; they change shape, its properties and functions are destroyed. Food rich in proteins are milk, meat, eggs, nuts, fish etc.


Water is vital for animals and almost all living organisms. It makes up to one third of the human body mass. Water is an inorganic substance with the chemical formula H2O.

Water is important for animals because it gives support to aquatic animals, gametes (sex cells like sperms and eggs) travel in a watery medium, sweating has a cooling effect on the body, and urine and tears are mostly made up from water. There is water even in the joints, so that reduces friction when bones move. Even blood is partially made up of water.

Water is also needed by plants, to make leaves turgid, guard cells move by osmosis and water takes part in the chemical reaction in which plants make there food (by photosynthesis) . Some seeds germinate with the help of water.


Many minerals are important for our body. There are other trace elements not listed in

the table which are useful for other bodily functions.

Mineral Found: Use in the body: Deficiency disease

Calcium: Milk, cheese, fish, mineral water, Developing bones and maintaining their rigidity. Forms intracellular cement and the cell membranes, and in regulating nervous excitability and muscular contraction. Deficiency can cause Rickets

Iron: Tomatoes, liver, kidneys. Part of haemoglobin in red blood cells. Deficiency causes anaemia headaches, tiredness, and lethargy

Phosphorous: Many foods, e. g. Milk, Important for bones and teeth. Deficiency causes Osteomalacia (rickets)

Sodium: Salt, many foods. Present in extra cellular fluid, and regulates it. Deficency causes Cramps

Iodine: Sea food, drinking water Needed to synthesize hormones of the thyroid gland. Deficency causes Goitre

Fluorine: Water, toothpaste. Builds a layer above enamel. Deficency causes tooth decay

Magnesium: Most foods Important for metabolism. Tremors and convulsions

Vitamins: Vitamins are very, very important for the body, but only in small quantities.


Fibre is mainly cellulose from plant cell walls. Humans cannot digest fibre, but it is

important because it helps food to pass from the gut, and prevents constipation.

Food rich in fibre are whole meal bread, bran, cereals, fresh fruit and vegetables.

Food Tests

Test for Starch: With Iodine solution. If result is positive, a blue-black precipitate forms.

Test for Glucose: With Benedict՚s Solution and the mixture is heated. If the result is positive, an orange brown solution forms.

Test for Proteins: With Copper Sulphate and Sodium hydroxide. A purple colour forms if the tested food contains proteins.

Test for Fats: With Ethanol (alcohol) A milky white solution forms in presence of fat.

Test for Vitamin C: With DCPIP. A blue to a colorless liquid forms in presence of vitamin C.