Lipids

Question 1

What three elements do lipids contain

Answer 1

Carbon, oxygen and hydrogen

Question 2

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

Answer 2

It is smaller in lipids than in carbohydrates.

Question 3

What are the two main groups of lipids

Answer 3

Triglycerides and phospholipids.

Question 4

Summarise how soluble lipids are

Answer 4

Insoluble in water but soluble in organic solvents such as alcohols and acetone.

Question 5

List the five key roles of lipids

Answer 5

1) Cell membranes
2) Source of energy
3) Waterproofing
4) Insulation
5) Protection

Question 6

Describe the role of lipids in cell membranes

Answer 6

Lipids are used in both cell-surface membranes and membranes around organelles.
Phospholipids contribute to the flexibility of membranes and the transport of lipid soluble substances across them.

Question 7

Describe the role of lipids as a source of energy

Answer 7

When oxidised, lipids provide over twice the amount of energy as the same mass of carbohydrate and release valuable water.

Question 8

Describe the role of lipids as waterproofing

Answer 8

Lipids are insoluble in water to they are useful as waterproofing.
Plants and insects have waxy lipid cuticles that conserve water.
Mammals produce an oily secretion from the sebaceous glands in the skin.

Question 9

Describe the role of lipids as insulation

Answer 9

Fats are slow conductors of heat so when stored beneath the body surface help to retain body heat.
They also act as electrical insulators in the myelin sheath around nerve cells.

Question 10

Describe the role of lipids as protection

Answer 10

Fat is often stored around delicate organs such as the kidney for extra protection.

Question 11

What is the difference between fats and oils

Answer 11

Fats are solids at room temperature (10-20 degrees Celsius) whereas oils are liquids.

Question 12

Describe the structure of a triglyceride

Answer 12

• Triglycerides have three fatty acids combined with glycerol.
• Each fatty acid forms an ester bond with glycerol in a condensation reaction.
• As the glycerol molecule in all triglycerides is the same, the differences in properties come from the fatty acids.
• There are over 70 different fatty acids and each contain the carboxyl (-COOH) group with a hydrocarbon chain attached.
• If the hydrocarbon contains no double bonds, then the triglyceride is saturated.
• If the carbon chain contains one double bond, the triglyceride is mono-unsaturated
• If the carbons chain contains multiple double bonds, then the triglyceride is polyunsaturated.

Question 13

What is a saturated triglyceride

Answer 13

A triglyceride that contains no double carbon bonds within its carbon chains (found in the fatty acids molecules)

Question 14

What is a mono-unsaturated triglyceride

Answer 14

A triglyceride that contains one double carbon bond in its carbons chains (found in the fatty acid molecules)

Question 15

What is a polyunsaturated triglyceride

Answer 15

A triglyceride that contains multiple double carbon bonds within its carbon chains (found in the fatty acid molecules)

Question 16

Describe how the structure of triglycerides relates to their properties

Answer 16

• Triglycerides have a high ratio of energy storing carbon-hydrogen bonds to carbon atoms and are therefore an excellent source of energy.
• Triglycerides have low mass to energy ratio which makes them good storage molecules as a lot of energy can be stored in a small volume. This is especially beneficial to animals as it reduces the mass they have to carry as they move around.
• Being large, non-polar molecules, triglycerides are insoluble in water. As a result, their storage does not affect osmosis in cells or the water potential of them.
• As they have a high ratio of hydrogen to oxygen atoms, triglycerides release water when oxidised and therefore provide an important source of water, especially for organisms living in dry deserts.

Question 17

Describe the structure of phospholipids

Answer 17

-Phospholipids consist of one glycerol, two fatty acids and a phosphate group.
- Whereas fatty acid molecules repel water (are hydrophobic), phosphate molecules attract water (are hydrophilic).
- This means that phospholipids are made up of two parts- a hydrophilic ‘head’ that interacts with water (is attracted to it) but not fat and a hydrophobic ‘tail’ that orients itself away from water but mixes readily with fat.
- This makes phospholipids polar as they have two different ends that behave in different ways.

Question 18

What does hydrolysis of a triglyceride produce

Answer 18

Three fatty acids and a glycerol

Question 19

Describe how the structure of phospholipids relates to their properties

Answer 19

• Phospholipids are polar molecules (having a hydrophilic phosphate head and hydrophobic tail of two fatty acids). This means that in an aqueous environment phospholipid molecules form a bilayer within cell surface membranes. As a result, a hydrophobic barrier is formed between the inside and the outside of a cell.
• The hydrophilic phosphate heads of phospholipid molecules help to hold at the surface of the cell-surface membrane.
• The phospholipid structure allows them to form glycolipids by combining with carbohydrates within the cell surface membrane. These glycolipids are important in cell recognition.