Lipids

Lipids are a varied group of substances that share thes echaracteristics: 

• Contain: CARBON, OXYGEN, HYDROGEN
• The proportion of O2 to CO2 and H2 is smaller than in some carbohydrates
• They are insoluble in water
• They are soluble in organic solvents eg. alcohols and acetone

The main groups of lipids are TRIGLYCERIDES (fats and oilds), PHOSPHOLIPIDS and WAXES

ROLE: 

• Main role is in PLASMA MEMBRANES - phospholipids contribute to the flexibility of membranes and transfer of lipid-soluble substances across them 
• ENERGY SOURCE - when OXIDISED, lipids provide more than 2x energy as same mass of carbohydrate
• WATERPROOFING - Insoluble in water so useful as waterproofing. Used by both plants and animals - waxy cuticles that conserve water, while mammals produce oil secreation
• INSULATION - Fats are slow conductors of heat and when stored under body surface help retain body temperature
• PROTECTION - Fat is often stored around delicate organs, such as the kidneys

Each fatty acid forms a bond with glycerol in a CONDENSATION REACTION

Phospholipids are similar to lipids EXCEPT that one of the FATTY ACIDS IS REPLACED BY A PHOSPHATE MOLECULE

The phosphate molecule is ATTRACTED to water (HYDROPHILIC) where as fatty acids REPEL WATER. 

A phosophate molecule therefore has 2 parts:

• HYDROPHILIC HEAD - interacts with water (attracted to it) but NOT with fat

• HYDROPHOBIC TAIL - repels water, doesnt interact with it but DOES with fat instead

Molecules that are like this, have two poles are POLAR

Test is called the EMULSION TEST

1. Take a completly dry and fat free tube

2. Add 2cm3 of the sample being tested to 5cm3 of ethanol or alcohol

3. Shake the tube thoroughly to dissolve any lipid in the sample

4. Add 5cm3 of water and shake gently

5. A cloudy-white colour indicates the presence of a lipid 

6. As a control, repeat prodecures using water instead of the sample, final soloution should remain clear

The colour is due to any lipid being in the sample being finely dispersed in the water to form an emulsion and light being refracted as it passes through the solutuion, making it appear cloudy

This is the plasma membrane that surrounds cells and forms the boundary between cell cytoplasm and the environment and controls the movements in and out of the cell. Structure: 

PHOSPHOLIPIDS:

• One layer pointing HYDROPHILIC heads INWARDS - reacting with water in the cell cytoplasm
• One layer pointing HYDROPHILIC heads OUTWARDS - reacting with water outside the cell
• Tails of both the layers (HYDROPHOBIC) into the centre of the membrane - no water

PROTIENS:

• EXTRINSIC - Either side of the surface of the bilayer - PARTLY EMBEDDED, never extended all the way. Act to give mechanical support to the membrane or as cell receptor for molecules
• INTRINSIC - Span across bilayer completly. Some act as carriers to transport water soluble materials across the membrane, other act as enzymes

Function: 

• Provide structural support; act as carriers; allow active transport across membrane
• Act as receptors eg. hormones; help cells adhere together

It is known as the Fluid-Mosaic Model because:

FLUID: Individual phospholipid molecules can move relative to one another. This gives the membrane a flexible structure that is constantly changing shape

MOSAIC: Proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as the stones of tiles in a MOSAIC