AS Biology: Lipids

Energy

• Lipids are an important source of energy in animals as they are also energy stores.
• They are well suited to this function because they are compact and insoluble.
• They are found as lipid droplets in the cytoplasm.
• When lipids are oxidised to release energy what is released. - metabolic water and is useful to organisms especially those that live in very dry conditions

In mammals such of the body lipid is found under the skin in adipose tissues where it prevents excessive heat loss. Lipids in plant seeds and fruits also provides thermal insulation against cold environmental conditions and also prevents moisture loss. 

Lipids also provides electrical insulations around neurones. Subcutaneous fat is also found around delicate body organs and gives protection against mechanical damage. It also gives buoyancy to some organisms. Some hormones are also lipids as well as all biological membranes.

Structure

Simple lipids are made up of glycerols and fatty acids

Fatty acids

• A fatty acid consists of a carboxyl group attached to a hydrocarbon chain. A fatty acids that contains the maximum number of hydrogen atoms that can be attached to the carbon atoms is called saturated fatty acid.
• Fatty acids that contain a double bond connecting two carbon atom are called unsaturated fatty acids because they do not contain the maximum number of hydrogen atoms. 
• Polyunsaturated is when there is more than one double bond present.

Double bonds

• The C=C bond changes the shape of the chain. It makes the liquid more fluid.
• Lipids with many unsaturated fatty acids are often oils. Those with mainly saturated fatty acids are more likely to be fats.

Triglycerides

• The most common lipid are known as fats and oils. Animals are usually fats and plants are usually oils. 
• Triglycerides are made up of three fatty acids join to one glycerol. if they are solid at room temperature they are fats and if they are liquid at room temperature they are oils.
• They are a good source of energy because they have a lot of bonds that could be broen down to release energy via respiration.
• They are good energy stores as they can hold a lot of energy in a small space.
• being hydrophobic means they also don't affect the water potential.

Phospholipids

• Phosphate molecules attract water (hydrophilic)
• It consists of a hydrophilic head which interacts with water and hydrophobic tail which orients itself away from water but mixes with lipid.
• When phospholipids are suspended in water they can form a variety of structures. 
• Phospholipids are the main components of membranes. They forma double membrane around the cell due to the hydrophobic interactions.

Cholesterol

• Cholesterol is a type of lipid but it isn't formed from fatty acids and glycerol.
• It regulates the stability and fluidity of membranes by sitting between phospholipids fatty acids tails as it is also hydrophobic.
• some hormones are made from cholesterol including oestrogen and testosterone.
• The lipid nature of these hormones allows them to pass through the phospholipid bilayer to reach cell contents. Vitamin D is also made from it.
• Too much cholesterol can be deposited in the wall of blood vessels causing atherosclerosis.
• In bile, produced by the liver, stored in the gall bladder, cholesterol can stick together forming gall stones.