LIVER & GALLBLADDER

=Liver & Gallbladder= [image:http://i.imgur.com/FRw0WOu.png?2] Following a meal, excess energy can be stored as ''glycogen'' (a carbohydrate) in the liver. In-between meals the glycogen can be broken down into sugar to ensure a constant supply of energy is delivered to our body cells. In this way the liver acts as a short term store of energy. Once the glycogen reserves are exhausted the body will start to break down fat (long term energy storage). The liver also produces bile, a substance that aids the digestion of lipids (fats & oils). Bile is produced continuously by the liver, stored and concentrated in the gallbladder and when an organism eats, is released into the duodenum. Bile (also called gall) is an ''alkaline'', dark green to yellowish brown fluid that plays two key roles in digestion: • it helps ''neutralise excess stomach stomach acid'' before it enters the duodenum. • it acts as a ''emulsifier'' which helps to increase the surface area of lipid for digestion =Bile as an Emulsifier= [image:http://i.imgur.com/tGdxof2.png?1] Lipids such as fats and oils will not dissolve in water. When oil and water are shaken the oil becomes dispersed as tiny droplets, but ultimately the two liquids separate to form different layers. This is why lipids are said to be immiscible (non-mixable) in water. [image:http://i.imgur.com/XEo0edT.gif] Bile salts act as an ''emulsifier'' because they have a ''hydrophilic'' (water loving) head that is attracted to water molecules and a ''hydrophobic'' (water hating) tail that is attracted to lipid molecules. This causes the bile salts to accumulate around droplets of lipids with the hydrophobic sides facing towards the lipid and hydrophilic sides facing outwards. The resulting structure is called a ''micelle''. The hydrophilic sides are negatively charged, and this charge prevents fat droplets coated with bile from re-assembling into larger fat particles. This ensures that the lipid molecules remain dispersed throughout the water. The resulting mixture is called an ''emulsion'' -a stable mixture of two liquids that are normally immiscible (don't mix). This greatly increases the ''surface area'' of the lipid that is exposed to lipase enzymes, thereby increasing the rate of chemical digestion. The digram (left) shows how an emulsifier (blue) can interact with and disperse a lipid (red) by forming tiny micelles.