Edexcel AS Biology Unit 1 Topic 2.1

• Fluid Mosaic Model

• Plasma membrane is made up of:
  • Cholesterol
  • Proteins 
    • Peripheral: attached to surface of lipid bilayer
    • Integral: embedded in lipid bilayer
  • Phospholipids
  • Polysaccharides

• Made up of:
  • Two non-polar, hydrocarbon fatty acid tails
  • Glycerol group
  • Phosphate group

• The head: 
  • Made from a glycerol group, which is attached to an ionised phosphate group
  • HYDROPHILIC
• The tails:
  • HYDROPHOBIC

• Cholesterol:
  • Disturbs the close-packing of the phospholipids, increasing the flexibility of the membrane

• On outer surface of cell, antenna-like carbohydrate molecules form complexes with some of the membrane proteins (forming glycoproteins) and lipids (glycolipids). The functions of these may be:
  • Cell-cell recognition
  • Receptor sites for chemical signals
  • involved in the binding of cells into tissues

The free passage of molecules from a region of their high concentration to a region of low concentration.

• A substance that is otherwise unable to diffuse across the plasma membrane does so due to its effect on particular molecules present in the membrane
• These molecules form into pores big enough for diffusion
• They close up when the substance is no longer present

• Occurs against a concentration gradient
• Requires energy from ATP
• Involves 'pumps' (carrier proteins)

• Occurs through the movement of vesicles of matter across the membrane by cytosis
• Uptake = endocytosis and export= exocytosis
• Uses energy from metabolism (ATP)
• Transports large particles and macromolecules

• The surface area available for gas exchange: the greater the repiratory surface's surface area, the greater the rate of diffusion
• The difference in concentration: the greater the gradient in concentrationacross the respiratory surface, the greater the rate of diffusion
• The length of the diffusion path: the shorter the diffusion path, the greater the rate of diffusion

• Have:
  •  a very large area in a small space
  • a ventilation mehcnaism to maintain the supply of air or oxygenated water
• May have:
  • an internal transport system to delvier oxygen to tissues
  • a respiratory pigment to improve the efficiency of transport
• E.g
  • Gills (compact but with large surface area)
  • Lungs (large surface area)
  • Tracheae (internal tubes that divide and reach the cells of the body) (present in many insects)

• Larger animals generally have specialised organs for gas exchange
• They provide a large, thin surface are for gas exchange
• Conditions for diffusion are also often improved by 3 refinements:
  • a ventilation mechanism: pumping mechanism that moves water or air over the gills or into and out of the lungs or tubes, which maintains the concentration gradient for diffusion
  • a blood circulatory system: accelerates the removal of dissolved oxygen from the respiratory surface as soon as it has diffused in, maintaining the concentration gradient
  • a respiratory pigment: increases the gas-carrying ability of the blood- e.g. haemoglobin

• Diaphragm: contracts and flattens, pushing down on abdomen
• External Intercostal Muscles: contract, moving rib cage in and out
• Internal Intercostal Muscles: relax
• Thorax: volume increases, air pressure falls below atmospheric pressure
• Air flow: in

• Diaphragm: relax, pressure from abdominal contents pushes diaphragm back up into dome shape
• External Intercostal Muscles: relax
• Internal Intercostal Muscles: contract, moving rib cage down and in
• Thorax: volume decreases, the air pressure rises above atmospheric pressure
• Air flow: out