Molecules (Human Biology OCR AS level)

proteins, carbohydrates, lipids, structure of haemoglobin.

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Amino acids.

The basic structure of an amino acid contains:

  • A central carbon
  • An R group
  • A hydrogen group
  • A amino group (NH2)
  • A carboxyl group (COOH)

When two amino acids join together by a condensation reaction (loss of water), they form a peptide bond.  The OH molecule from carboxyl group is lost and H atom from the amino group is lost.  The molecule is now called a dipeptide.

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Haemoglobin.

Haemoglobin is a globular protein which means that it is spherical in shape and has biochemical functions and it is soluble in water.  To form this protein, 4 different stages take place.

  • The primary structure - sequence and number of amino acids in the polypeptide chain.  Haemoglobin has 4 polypeptide chains: 2 alpha chains and 2 beta chains.
  • The secondary structure - the chains twist and coil and weak hydrogen bonds form between different peptide bonds in different parts of the chain. 
  • The tertiary structure - 3D forming of the haemoglobin and the polypeptide chains fold up again.  Bonds form between the R groups: weak hydrogen bonds, strong disulphide bonds, hydrophobic bonds and ionic bonds. 
  • The quaternary structure - there is a haem group nestling inside each polypeptide chain and the haem group is a ring of atoms with an iron ion in the centre and the non-protein part of the molecule is called the prosthetic group. 
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Role of haemoglobin carrying oxygen.

The haemoglobin molecule is made of 4 polypeptide chains

Each has a haem group attached, called the prosthetic group

In the centre of the haem group is the iron ion associating with one oxygen molecule.

BUT for haemoglobin, it can carry 4 oxygen molecules because there are 4 polypeptide chains so 4 haem groups

As the first haem group combines to an oxygen molecule, the shape of the protein changes exposing the next haem group making it easier to pick up another oxygen molecule. 

This is known as conformational change.

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Properties of water.

These properties make water an ideal transport medium:

  • Cohesive properties - not easily compressed - excellent medium for support - protects fetus from mechanical damage.
  • Solvent properties - polar molecules - transport medium - e.g blood and lymph.
  • Transparency - allows light to penetrate - aquatic plants can photosynthesise.
  • High Specific Heat Capacity (HSHC) - lot of heat needed to change temperature - thermostable - less likely to be damaged.
  • High Latent Heat of Eaporation (HLHE) - lot of heat needed to change to water vapour - body can evaporate by sweating - maintains constant body temp.
  • Lubricant properties - cohesive and adhesive forces - minimises friction between rib cage and lungs during breathing. 
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