F212 Biological molecules

F212 Biological molecules

describe how hydrogen bonding occurs between water molecules, and relate this and other properties of water, to the roles of water in living organisms

Þ    Water molecules consist of one atom of oxygen joined to two atoms of hydrogen by shared electrons- bonds between atoms are strong

Þ    Water molecules are polar- negative charge on one side and positive charge on the other side (negative oxygen and positive hydrogen)  causes attraction between water molecules called hydrogen bonding

Þ    Hydrogen bond (a covalent bond): a weak electrostatic interaction that can occur wherever molecules contain a slightly negatively charged atom bonded to slightly positively charged hydrogen atoms

Uses in living organisms

Þ    High specific heat capacity (high thermal stability): hydrogen bonds between water molecules can absorb a lot of energy, so water takes a lot of energy to heat up. Useful because it stops rapid temperature changes (less energy available for increasing temperature), allowing organisms to keep their temperature fairly stable

Þ    High latent heat of evaporation: it takes a lot of heat energy to break the hydrogen bonds so a lot of energy is used up when water evaporates. Useful because acts as major cooling mechanism; when water evaporates from sweat it absorbs heat energy from the body

Þ    Cohesion: water molecules are very cohesive (stick together) because of their polarity, making it flow easily and good for transporting substances

Þ    Solvent properties: molecules with charges and polar molecules are easily dissolved in water because of the polarity of water molecules. Ions will be completely surrounded by water molecules- positive end to negative ion, negative end to positive ion

Þ    Cooling allows maximum number of hydrogen bonds to form, water molecules space out to allow this

Þ    Water expands as it freezes, ice is less dense than water

Þ    Water is a transport medium for food particles and minerals, medium for gametes to swim

Þ    Water has high viscosity- cohesive

Þ    Water is transparent to light so photosynthesis is possible in shallow water

Þ    Water is adhesive to xylem walls, allowing it to travel up

*describe, with the aid of diagrams, the structure of an amino acid

Þ    All amino acids have the same general structure: a carboxyl group (COOH) and an amino group (NH₂) attached to a carbon atom.

Þ    The variable group on all amino acids is known as R

Þ    Glycine is the smallest amino acid, the R group is H

Þ    Humans have about 20 different amino acids

*describe, with the aid of diagrams, the formation and breakage of peptide bonds in the synthesis and hydrolysis of dipeptides and polypeptides

Þ    Amino acids (monomers of proteins) are joined together by peptide bonds (a covalent bond)

Þ    ‘Essential amino acids’ are those that cannot be made by the body

Þ    Dipeptides are formed when two amino acids join together

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