[3] Lipids

What elements do lipids contain?

Carbon, hydrogen and oxygen;

How does the proportion of oxygen to carbon and hydrogen in lipids compare with carbohydrates?

The proportion of oxygen to carbon and hydrogen is smaller in lipids than in carbohydrates;

What is the solubility of lipids in water?

Lipids are insoluble in water;

What are lipids soluble in?

Organic solvents such as alcohols and acetone;

What are the two main groups of lipids?

Triglycerides (fats and oils) and phospholipids;

What is the role of phospholipids in cell-surface membranes?

They contribute to membrane flexibility and the transfer of lipid-soluble substances across membranes;

How much energy do lipids provide compared with the same mass of carbohydrate?

When oxidised, lipids provide more than twice the energy of the same mass of carbohydrate;

What else is released when lipids are oxidised?

Water;

Why are lipids useful for waterproofing?

Lipids are insoluble in water, making them useful for waterproofing;

How do plants and insects use lipids for waterproofing?

They have waxy, lipid cuticles that conserve water;

How do mammals use lipids for waterproofing?

Sebaceous glands in the skin produce an oily secretion;

Why are fats useful for thermal insulation?

Fats are slow conductors of heat and, when stored beneath the body surface, help retain body heat;

How do lipids provide electrical insulation?

They act as electrical insulators in the myelin sheath around nerve cells;

How do lipids provide protection?

Fat is often stored around delicate organs, such as the kidneys;

At what temperature are fats solid?

Fats are solid at room temperature (10–20°C);

At what temperature are oils liquid?

Oils are liquid at room temperature (10–20°C);

What is a triglyceride made from?

Three fatty acids combined with glycerol;

What does the 'tri' in triglyceride refer to?

The three fatty acids;

What type of bond forms between a fatty acid and glycerol?

An ester bond;

What type of reaction forms an ester bond in a triglyceride?

A condensation reaction;

What does hydrolysis of a triglyceride produce?

Glycerol and three fatty acids;

How many different fatty acids are there?

There are over 70 different fatty acids;

What functional group do all fatty acids contain?

A carboxyl (-COOH) group;

What is attached to the carboxyl group of a fatty acid?

A hydrocarbon chain;

What is a saturated fatty acid?

A fatty acid whose hydrocarbon chain has no carbon-carbon double bonds;

Why is a fatty acid with no carbon-carbon double bonds called saturated?

All carbon atoms are linked to the maximum possible number of hydrogen atoms;

What is a monounsaturated fatty acid?

A fatty acid with a single carbon-carbon double bond;

What is a polyunsaturated fatty acid?

A fatty acid with more than one carbon-carbon double bond;

Why are triglycerides an excellent source of energy?

They have a high ratio of energy-storing carbon-hydrogen bonds to carbon atoms;

Why are triglycerides good storage molecules?

They have a low mass-to-energy ratio, so much energy can be stored in a small volume;

Why do triglycerides not affect water potential?

They are large and non-polar, making them insoluble in water;

Why do triglycerides release water when oxidised?

They have a high ratio of hydrogen to oxygen atoms, so oxidation releases water molecules;

Why is the water released by lipid oxidation useful to desert animals?

It provides an important source of water where environmental water may be limited;

How does a phospholipid differ from a triglyceride?

One fatty acid is replaced by a phosphate molecule;

What is the structure of a phospholipid?

A hydrophilic phosphate head and a hydrophobic tail containing two fatty acids;

What does hydrophobic mean?

Repels water;

What part of a phospholipid is hydrophobic?

The fatty acid tails;

What does hydrophilic mean?

Attracts/interacts with water;

What part of a phospholipid is hydrophilic?

The phosphate head;

Why are phospholipids polar molecules?

They have two ends that behave differently: a hydrophilic phosphate head and hydrophobic fatty acid tails;

How do phospholipids orientate themselves in water?

Hydrophilic heads are close to water while hydrophobic tails are away from water;

What structure do phospholipids form in an aqueous environment?

A bilayer;

Where is the phospholipid bilayer found?

Within cell-surface membranes;

What barrier is formed by the phospholipid bilayer?

A hydrophobic barrier between the inside and outside of the cell;

What is the role of the hydrophilic phosphate heads?

They help hold the phospholipids at the surface of the cell-surface membrane;

What are glycolipids?

Molecules formed when phospholipids combine with carbohydrates within the cell-surface membrane;

What is the role of glycolipids?

They are important in cell recognition;

What test is used to identify lipids?

The emulsion test;

What should the test tube be like before carrying out the emulsion test?

Completely dry and grease-free;

How much sample is used in the emulsion test?

2 cm³;

How much ethanol is added to the sample in the emulsion test?

5 cm³ of ethanol;

Why is ethanol added in the emulsion test?

To dissolve any lipid in the sample;

What is done after adding ethanol?

The tube is shaken thoroughly;

What is added after the ethanol?

5 cm³ of water;

What is the positive result of the emulsion test?

A milky-white emulsion indicates the presence of a lipid;

What is the negative result of the emulsion test?

The final solution remains clear;

What control is used in the emulsion test?

Repeat the procedure using water instead of the sample;

What should the control show in the emulsion test?

The final solution should remain clear;

Why does the lipid sample become cloudy in the emulsion test?

Any lipid is finely dispersed in the water to form an emulsion;

Why does the emulsion appear cloudy?

Light is refracted as it passes from oil droplets to water droplets, making it appear cloudy;

Why are triglycerides suitable for energy storage?

They have a high ratio of energy-storing carbon-hydrogen bonds, a low mass-to-energy ratio, and are insoluble in water so they do not affect water potential;

Why do phospholipids form a bilayer?

Hydrophilic phosphate heads interact with water while hydrophobic fatty acid tails are positioned away from water;

How does the structure of phospholipids create a hydrophobic barrier?

The hydrophobic fatty acid tails form a region between the aqueous environments inside and outside the cell;