Module 2: Biological Molecules OCR notes

  • Created by: SamThiru
  • Created on: 05-04-18 16:02

Chapter 3: Biological Molecules

Biological Elements

·Cells are composed of molecules which are made up of atoms.

·Different types of atoms are made up of elements which are distinguished by the number of protons in the nucleus.

·Atoms connect with each other by forming bonds, and 2 or more elements bonded forms a molecule.

®    Carbon can form 4 bonds.

®    Nitrogen can form 3 bonds.

®    Oxygen can form 2 bonds.

®    Hydrogen can form 1 bond.

·When an atom has a different number of electrons to protons, an ion is formed. When ions are in solution they are called electrolytes.

·Elements are present in each biological molecule:

®  Carbohydrates - C, H & O

®  Lipids - C, H & O

®  Proteins C, H, O, N & S

®  Nucleic Acids C, H, O, N & P

·Biological molecules are often made up of polymers which are long chain molecules made by linking monomers in a repeating pattern.



·Water is composed of oxygen and hydrogen which is covalently bonded.

·The electrons are not shared equally in the covalent bond, so there would a slight negative charge on the oxygen and a slight positive charge on the hydrogen.

·The molecule is said to be polar and so the oppositely charged parts of the molecule are attracted to each other; the bonds between the molecules are called hydrogen bonds.

·Hydrogen bonds are relatively weak, but since there are many of these bonds, they are strong altogether.

®  Water has an unusually high boiling point due to the hydrogen bonds. It takes a lot of energy to increase the temperature of water and cause it to evaporate.

®  Water unusually is less dense in its solid form (ice) ; this is because the hydrogen bonds fix the position of the polar molecules slightly further apart in the liquid state. (tetrahedral shape)

®  Water is cohesive and adhesive since the molecules are attracted to each other and other materials.

They are attracted more to each other rather than air causing surface tension.






·     Since water is a polar molecule, it can be used as a solvent in which many solutes such as proteins can be dissolved. It acts as a medium where chemical reactions can take place and can transport solutes into and out of cells.

·     Within living things, water acts as a transport medium; the cohesion between molecules means that when the water is transported, the molecules will stick together. The cohesion and adhesion  results in capillary action - water can rise against the force of gravity.

·     It acts as a coolant, which helps buffer temperature changes during chemical reactions. It helps maintain constant temperature due to its high specific heat capacity - useful for enzymes.



·     Carbohydrates are formed with the elements


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