Biology 1.5 - Lipids

What does hydrolysis of triglycerides produce?

--> 3 fatty acids and a molecule of glycerol.

State and explain the 4 roles of lipids.

--> A source of energy: When oxidised, lipids provide more than twice the energy as the same mass of carbohydrate and release valuable water. --> Waterproofing: Lipids are insoluble in water and are therefore very useful for waterproofing. Both plants and insects have waxy, lipid cuticles that conserve water, while mammals produce an oily secretion from the sebaceous glands in the skin. --> Insulation: Fats are slow conductors of heat and when stored beneath the body surface help to retain body heat. They also act as electrical insulators in the myelin sheath around nerve cells. --> Protection: Fat is often stored around delicate organs, such as the kidney.

Where would you be most likely to find adipose tissue in the body?

--> Around delicate organs, as fats are good at cushioning and protecting things from impacts. --> Under the skin, this is to prevent heat loss, as fats are good insulators.

Why are triglycerides so good at storing energy?

--> Because they have a high ratio of carbon hydrogen bonds to carbon atoms. --> They also have a low mass to energy ratio, this also makes them a very good storage molecule, as more energy can be stored in a small volume.

What very useful substance do triglycerides release when they are broken down?

--> Because they have a high ratio of hydrogen to oxygen atoms, triglycerides release water when oxidised. --> This is very useful to animals that live in hot locations, like camels, where water is scarce. The hump of a camel is made up of fat, and is used to make water when needed.

Why might the high energy density of triglycerides be useful to an animal?

--> Because they have to move around, unlike plants, it is important that they don't have too much mass. --> So having a very large amount of energy stored in a small mass is particularly useful to animals, hence why they use triglycerides and fats to store energy in the long term, rather than starch.

What is the difference between cis and trans double bonding in fatty acids?

--> Cis double bonding is where the carbon atoms are both on the same side of the double bond. This creates the biggest kink in the chain and means that the fatty acids cannot line up properly, causing . them to have lower melting temperatures and to exist as oils at room temperature. --> Trans double bonding is where the carbon atoms are on opposite sides to each other, this produces a straight chain and means that these molecules have a higher melting temperature, causing them to be solid fats at room temperature.

What is glycerol?

--> Glycerol is a 3 carbon alcohol with the formula C3H8O3.

How do triglycerides form?

--> One molecule of glycerol and 3 fatty acid molecules come together and undergo a condensation reaction. --> This causes a molecule of water to be released and an ester bond to be formed between the oxygen making up the COOH group and the OH group on the glycerol molecule. --> This happens a total of 3 times and 3 molecules of water are released.

What are triglycerides made up of?

--> One molecule of glycerol and 3 fatty acids. --> These fatty acids are bonded with ester bonds to the glycerol, these bonds are formed in a condensation reaction.

How is the structure of phospholipids related to their properties?

--> Phospholipids are polar molecules. This means that in an aqueous environment, phospholipids form a bilayer within cell-surface membranes. As a result, a hydrophobic barrier is formed between the inside and outside of a cell. --> The phospholipid structure allows them to form glycolipids by combining with carbohydrates within the cell-surface membrane. These glycolipids are important in cell recognition.

Describe phospholipids

--> Phospholipids are similar to triglycerides, however one of the fatty acid molecules is replaced with a phosphate molecule. --> Whereas fatty acid molecules repel water (they are hydrophobic), phosphate molecules attract water (they are hydrophilic). Therefore, a phospholipid is made up of two parts, a polar hydrophilic 'head', which interacts with water and doesn't interact with fat; and a fatty acid tail, which is hydrophobic and therefore doesn't interact with water, but does readily mix with fat. This is because it is non-polar. This means that, when phospholipids are placed in water, they will position themselves so that the heads are as close to the water as possible and the tails are as far away from the water as possible.

What is the difference between saturated, monounsaturated and polyunsaturated fatty acids?

--> Saturated fatty acids have no carbon-carbon double bonds in the chain. --> Monounsaturated fatty acids have 1 carbon-carbon double bond and polyunsaturated have more than 1.

How do different triglycerides have different properties?

--> The glycerol part of the triglyceride is the only part that stays the same, the 3 fatty acids that join onto it can be any one of 70 different fatty acids. --> This means that there are 100s of possible triglycerides that can be made, each of them having different properties.

What is the structure of fatty acids?

--> These are long chain hydrocarbons with the carboxyl COOH group at the end.

Comment on the solubility of triglycerides.

--> Triglycerides are insoluble in water, this means that they do not affect the water potential of cells and therefore do not damage tissues due to osmotic gain or loss of water.

Where are lipids found in the body?

Adipose tissue, this is what the body uses to store energy.

What is a molecule called when it has both hydrophilic and hydrophobic sections?

Amphipathic.

What elements do all lipids contain?

Carbon, hydrogen and oxygen.

What bonds keep triglycerides together?

Ester bonds, between the glycerol and fatty acid sections.

What solvents are lipids soluble in?

Organic solvents, like acetone and ethanol.

Where do triglycerides get their name from?

The fact that they have 3 fatty acids combined with glycerol.

How soluble are lipids in water?

They are not soluble in water, this is because they are non-polar molecules and therefore cannot form the hydrogen bonds with water molecules that allow them to dissolve.

What is the test for lipids?

This test is known as the emulsion test and is as follows: --> Take a completely dry and grease-free test tube. --> To 2 cm^3 of the sample being tested, add 5 cm^3 of ethanol. --> Shake the tube thoroughly to dissolve any lipid in the sample. --> Add 5 cm^3 and shake gently. ---> A cloudy white colour indicates the presence of a lipid. --> As a control, repeat the procedures using water instead of the sample, the final solution should remain clear. The cloudy colour is due to any lipid in the sample being finely dispersed in the water to form an emulsion. Light passing through the emulsion is refracted as it passes between the water and lipid droplets, making it appear cloudy.