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Notes on Biomolecules: The Molecules of Life

Living systems are composed of organic chemicals. The complex organic molecules which form the basis of life i.e. which build up living organisms and are required for their growth and maintenance are called bio molecules.

  • 99% of the mass of most cells is H, O, N, and C.
  • Bio molecules are complex, but are made up of simpler components (Amino acids,  simple sugars ,fatty acids and nucleotides )
  • ProteinsCarbohydratesLipids and Nucleic acids  are the most abundant bio molecules 

Difference between organic or inorganic molecules?
  Organic molecules are complex molecules containing carbon atoms; they are found in living or once living thing. All other molecules are inorganic such as water (H2O), carbon dioxide (CO2), molecular oxygen (O2) & nitrogen. Both types of molecules are essential. Organic molecules that are responsible for the incredible diversity of living things.
 Why COis considered as inorganic molecule?
  Organic compounds are actually compounds containing carbon covalently bonded with hydrogen. Many compounds that contain carbon are considered inorganic, for example - carbon monoxide, carbon dioxide, carbonates, cyanides, carbides, carbonic acid, and thiocyanates. Also there is that one thing about carbon being covalently bonded with hydrogen in organic compounds. Due to that criteria carbon dioxide is considered inorganic. But actually there is no clear reason.
Biodiversity is a function of the capacity of carbon atoms to form large & complex molecules. Each carbon atom can form as many as 4 covalent bonds & this result in organic molecules with large chains, rings, & interconnected rings of carbon atoms to which other elements are bonded.

  Organic molecules are grouped into 2 main categories

Organic molecules

What property unites H, O, C and N and renders these atoms so appropriate to the chemistry of life?
Answer: Their ability to form covalent bonds by electron-pair sharing

What are the bond energies of covalent bonds?

Bond    Energy kJ/mol

H-H                436
C-H                414
C-C                343
          C-O                351

H, O, C and N make up 99 + % of atoms in the human body

Elements Present in Human Body
% Weight
Hydrogen (H)
Carbon (C)
Oxygen (O)
Nitrogen (N)
Sulphur (S)
Sodium (Na)
Calcium (Ca)
Magnesium (Mg)
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