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  • They are a broad group of molecules that includes fats, fatty acids, sterol, waxes, glycerides and phospholipids fats are a subgroup of lipids called triglycerides.
  • Cholesterol is an example of the type of lipids called sterol.
  • The main function of the lipids include energy storage, cell signalling and cell structure.


  • Constitutes 3 elements : carbon, hydrogen and oxygen.
  • 1gm gives 117kj of energy (or 4.1 kcal of energy).
  • They belong to three types : monosaccharides, disaccharides and polysaccharides.
  • Monosaccharides : these are the simplest form of carbohydrates, and cannot be broken down any further. Example : glucose and fructose.
  • Monosaccharides dissolve in water, taste sweet and are called sugars.
  • Used as an energy source and in biosynthesis.
  • Disaccharides : disaccharides are compounds made by two monosaccharides bond together. Example : sucrose and lactose used for carbohydrate transport.
  • Like monosaccharides, disaccharides dissolve in water.
  • Polysaccharides : these are compounds made by complex chains of monosaccharides. Example : cellulose and glucose.
  • Used for energy storage (glycogen) and for cell walls (cellulose).
  • Cellulose is the most abundant organic molecule on earth.
  • Spider silk is a polysaccharide.
  • During the process of digestion, all carbohydrates are broken down to monosaccharides.

Amino acids

  • They are molecules that contain an amine group and a carboxyl group.
  • Example: glycine, monosodium glutamate.
  • They are the building blocks of the proteins.
  • Applications include metabolism, drug therapy, flavour enhancement, manufacture of biodegradable plastics.


  • They are compounds made from amino acids.
  • Proteins are used as enzymes, in muscle formation, as cell cytoskeleton, cell signalling and immune response.
  • Made up of carbon, hydrogen, oxygen, nitrogen and sometimes sulphur.
  • Important for growth repair of the body. (75% of our body is proteins only).
  • Proteins are first broken into amino acids and then digested.
  • Protein energy malnutrition : 
  • Kwashiorkor : due to deficiency of protein.
  • Means neglected child ( when mother stops breastfeeding )
  • Marasmus : deficiency of proteins, carbohydrates and fats.

Nucleic acids

  • They are macromolecules formed by chains of nucleotides.
  • Common examples include DNA and RNA.
  • DNA (deoxyribonucleic acid)
  • Contains two strands of nucleotides arranged in a double helix structure.
  • In cells, DNA is organised into long structures called chromosomes.
  • Used primarily for long term storage of genetic information.
  • DNA was first isolated by swiss physician Friedrich Miescher in 1869.
  • The double helix structure was suggested by James Watson and Francis Crick.
  • RNA (ribonucleic acid)
  • Contains one strand of nucleic acids.
  • Less stable than DNA.
  • Used primarily for protein synthesis.
  • Messenger RNA carries information from DNA to the ribosomes. RNA translates the information in the mRNA.
  • RNA synthesis was discovered by Severo Ochoa.


  • Provides twice the energy of carbohydrates.
  • Acts as the reserve food Material because excess fat is stored in the liver and as adipose tissue.
  • An enzyme called lipase digests fats.
  • There are two types of fatty acids : saturated fatty acids : solids at room temperature, unsaturated fatty acids : liquids at room temperature.
  • Fatty acids are basic chemical units of fat.
  • Saturated fat is used by the liver to manufacture cholesterol.
  • Is considered the most dangerous kind of fat because it has shown to raise blood cholesterol levels, particularly the LDL.
  • Polyunsaturated fats : do not appear to raise blood cholesterol levels.
  • Monounsaturated fats : Do not seem to have any effect on blood cholesterol.
  • Trans fat : byproducts of hydrogenation, a chemical process used to change liquid unsaturated fat to a more solid fat. 
  • Total fat intake should be no more than 30 percent of your daily calorie intake.
  • Hydrogenation : process by which unsaturated fatty acids are converted into saturated fatty acids by the addition of hydrogen.

Also Read:

Reproductive System

Nervous System And Brain

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