Topic 2 Unit 1 Snab Edexcel

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  • Created by: Tamsin
  • Created on: 12-05-10 16:03

Properties of Gas Exchange Surfaces

  • Large surface area of the alveoli
  • Numerous capillaries around the alveoli
  • Thin walls of the alveoli and capillaries meaning a short distance between the alveolar air and blood in the capillaries

Diffusion needs to be rapid due to the body’s demand for oxygen.

  • Surface area: as the surface area increases the rate of diffusion increases
  • Concentration gradient: the greater the concentration gradient the faster diffusion occurs
  • Thickness of the gas exchange surface: the thicker the surface the slower the diffusion

Sticky mucus means that mucus will block airways preventing ventilation of the alveoli below the blockage. Blockages allow air to enter but not to exit therefore the lung tissue is being stretched damaging lung elasticity. CF suffers find exercise difficult due to oxygen not diffusing quick enough and therefore slowing down the process of aerobic respiration needed to produce energy.

Amino Acid Structure

Primary Structure

Two amino acids join to make a dipeptide and then the dipeptides link together to make polypeptides. The sequence of amino acids is the primary structure.

Secondary Structures

The chain will fold into either a α-helix or β-pleated sheet. Within the helix, hydrogen bonds form between the C=O of the carboxylic acid and the –NH of the amine group. This stabilises the shape. The chains could also link together in parallel forming a β-pleated sheet arrangement. In a structure of a protein there can be a selection of both arrangements.

Tertiary and Quaternary Structures

The polypeptide chains then bend to produce 3D-shapes. The chemical bonds between R-groups keep the structure together. An R-group is polar when the sharing of electrons is not even. Polar R-groups attract other polar molecules, like water. Hydrophilic and hydrophobic molecules determine the shape and arrangement of the shape.

A protein may be made up of several polypeptide chains. For example haemoglobin is made up of 4 polypeptide chains, this is known as the quaternary structure. Single chain proteins will stop at the tertiary stage.

Conjugated Proteins

Some proteins have another chemical group associated with their polypeptide chain. E.g.: myoglobin is associated with an iron containing group.

Globular and Fibrous Proteins

Globular: polypeptide chain is folded into a spherical shape. These proteins are soluble due to the hydrophilic side chains. Enzymes are globular proteins and their 3D-shape means that they can form enzyme-substrate complexes and catalyse reactions within cells. The 3D-shape also means that they can bind to other molecules.

Fibrous: Do not fold into a ball but remain in chains. Several polypeptide chains can be linked to add strength. These are insoluble molecules. Examples are: keratin: skin, hair, collagen in the skin, tendons, bones, cartilage and blood vessel walls.

Cell Membrane Structure

The cell membrane is a phosolipid bilayer. It is made up of 2 fatty acids and a phosphate.

The phosphate head is polar therefore hydrophilic and the tail are non-polar therefore hydrophobic. When added to water phospholipids arrange themselves to avoid contact with the water. They either form on the surface with tails




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Jack Butler


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Mubarak Mohamoud


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