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Biological Molecules
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This is a document that consists of notes for the topic, Biological Molecules, as part of the AS/A level OCR Biology A course
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Molecule of water = 1 atom of Oxygen joined to 2 atoms of Hydrogen by shared electrons1. Because shared negative hydrogen electrons are pulled towards the oxygen atom,
the other side of each hydrogen atom is left with a slight positive charge
2. The unshared negative electrons on the oxygen atom give it a slight negative
charge - makes water a polar molecule (has 𝛿− charge on one side and 𝛿+ on
the other)
3. Slightly negatively-charged oxygen atoms attract the slightly positively-charged
hydrogen atoms of other water molecules
4. This is Hydrogen Bonding (weak intermolecular bonds)
Structure related to its properties and functions:
● Hydrogen bonds give water a high specific heat capacity (energy required to
raise the temp of 1 gram of a substance by 1°C)
○ Hydrogen bonds absorb a lot of energy - so high SHC
○ So water doesn’t experience rapid temp changes - good habitat
● Hydrogen bonds give water a high latent heat of evaporation
○ A lot of energy needed to break hydrogen bonds between H2O molecules
○ Useful for living organisms - water’s good for cooling things (sweat
when too hot - cools surface of skin)
● Water’s polarity makes it very cohesive (attraction between molecules of same
type)
○ Very cohesive because they’re polar
○ Helps water to flow (good for transporting substances)
● Water’s polarity makes it a good solvent
○ Water = polar so slightly positive end will be attracted to negative ion
(and the same for the other one)
○ Ions get surrounded = they dissolve
○ Important ions can dissolve in water in blood and then transported
● Ice = Less dense
○ Water molecules held further apart - each water molecule forms 4
hydrogen bonds to other water molecules, making a lattice shape
○ Forms insulating layer on top of water (fishes don’t freeze/still move
around)
Monomers = individual molecules that make up a polymer
Polymer = molecule made from monomers bonded together
Carbohydrates: First important biological molecule
● Monomers that make this are called monosaccharides
○ Example: Glucose: Monosaccharide with 6 carbon atoms (hexose)
○ 2 forms: alpha and beta
○ It’s structure is related to its function as the main energy source in animals
+ plants - structure makes it soluble - easily transported
, Example: Ribose: Monosaccharide with 5 carbon atoms
(pentose)
All carbohydrates made up of the same 3 chemical
elements: Carbon, Hydrogen, Oxygen
For every carbon atom = 2 hydrogen atoms and 1 oxygen
atom
Monosaccharides are joined together by glycosidic bonds
During synthesis, a hydrogen atom on one monosaccharide bonds to a hydroxyl (OH)
group on the other, releasing a water molecule = Condensation reaction
Hydrolysis = water molecule reacts with glycosidic bond, breaking it apart
When 2 monosaccharides join together = disaccharide
Example:
● 2 alpha glucose molecules are joined together by a glycosidic bond to form
maltose
● Alpha glucose and fructose joined together = sucrose
● Beta glucose and galactose = lactose
When more than 2 monosaccharides join together = polysaccharides
Example: Lots of alpha glucose molecules are joined together by glycosidic bonds to
form amylose
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