These revision notes provide an in depth summary of this specific chapter of AQA Biology in the A Level Specification -
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- Are particular groups of chemicals that are found in living organisms
- Their study is known as molecular biology
Bonding and the formation of molecules
- Atoms combine with one another in a number of different ways
Covalent bonding:
- Atoms share a pair of electrons in their outer shell – outer shell of both atoms filled making a
more stable compound, known as a molecule
Ionic bonding:
- Ions with opposite charges attract one another – this electrostatic attraction is ionic bonding
- +ve Sodium ion attracted to –ve Chlorine ion to form sodium chloride
- Ionic bonds are weaker than covalent bonds
Hydrogen bonding:
- Electrons within a molecule aren’t evenly distributed – spend more time at one position
- This region is more –ve charged than rest of the molecule
- Molecule with uneven distribution of charge is said to be polarised (it’s a polar molecule)
- Negative region of one polarised molecule and the positively charged region of another
attract to one another
- A weak electrostatic bond is formed and although individually each bond is weak, collectively
they form important forces that alter the physical properties of molecules
o Especially true for water
Polymerisation and the formation of macromolecules
- Monomers can be linked to form long chains (polymers) made by polymerisation
- The monomers of a polymer are usually based on carbon
- Many, such as polyesters, are industrially produced with others such as polysaccharides
being made naturally by living organisms
- Basic sub-unit of a polysaccharide is a monosaccharide (single sugar) e.g. glucose
- Polynucleotides are formed from mononucleotide sub-units
- Polypeptides formed by linking together peptides that have amino acids as their basic sub-
unit
Condensation and hydrolysis reactions
- Formation of new polymers is by a process known as a
condensation reaction where water is produced
- Therefore, the formation of a polypeptide from an amino
and that of the polysaccharide starch from the
monosaccharide glucose are both condensation reactions
- Polymers can be broken down by addition of water by hydrolysis
o Water molecules break the bonds that link the sub-units of a polymer – splitting the
molecule into its constituent parts
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