Lipids

Question 1

What are the main properties of lipids?

Answer 1

• They contain carbon, hydrogen and oxygen
• The proportion of carbon : oxygen and hydrogen is smaller than in carbohydrates - They are non-polar (triglycerides)
• They are insoluble in water but soluble in organic solvents (e.g. alcohols, acetone)

Question 2

What are the main functions of lipids?

Answer 2

• Storage of energy for long term use
• Hormonal roles
• Insulation - thermal and electrical
• Protection of internal organs
• Structural components of cells

Remember SHIPS

Question 3

What are the 2 main types of lipids?

Answer 3

• Triglycerides
• Phospholipids

Question 4

What is meant by a macromolecule?

Answer 4

Lipids are examples of macromolecules - these are large molecules composed of several smaller ones that are not necessarily polymers.

Question 5

How do triglycerides form?

Answer 5

They form when condensation reactions occur between 1 glycerol and 3 fatty acids.

The hydroxyl groups of the glycerol combine with the carboxyl groups of the fatty acids to form an ester linkage.

Question 6

How do ester bonds form?

Answer 6

An ester bond forms when a hydroxyl group (-OH) group from the glycerol bonds with the carboxyl (-COOH) group of the fatty acids.

Question 7

Why is esterification a condensation reaction?

Answer 7

A hydrogen from the glycerol combines with an OH from the fatty acid, making water.

This happens three times to form a triglyceride, meaning 3 water molecules are formed and released. This makes esterification a condensation reaction.

Question 8

How are triglycerides stored in animals and plants?

Answer 8

They are stored as fats (solid) in animals and as oils (liquid) in plants.

Question 9

What are fatty acids?

Answer 9

Fatty acids are long hydrocarbon chains which may differ in length and in the number of double bonds.

Question 10

What is meant by a saturated fatty acid?

Answer 10

A fatty acid that possesses no double bonds.

This means it has the maximum amount of H atoms.

Question 11

What are the properties of saturated fatty acids?

Answer 11

• They are usually solid at room temperature
• They originate from animal sources (fats)
• They are linear in structure

Question 12

What is meant by unsaturated fatty acids?

Answer 12

Fatty acids with double bonds.

They can either be monounsaturated (1 double bond) or polyunsaturated (more than 1 double bond).

Question 13

What are the properties of unsaturated fatty acids?

Answer 13

• They are bent in structure (known as kinks)
• They are usually liquid (oils) at room temperature
• They originate from plant sources

Question 14

How is a phospholipid structured?

Answer 14

Phospholipids consist of 1 glycerol, 2 fatty acids and a phosphate group (PO4^3-)

Question 15

How is the phosphate group bonded to the glycerol in a phospholipid?

Answer 15

They are joined by a phosphoester bond.

Question 16

What is meant by the “polar head” of a phospholipid?

Answer 16

The glycerol and the phosphate group.

Question 17

What is the key property of the polar head of a phospholipid?

Answer 17

It is hydrophilic, meaning it attracts water.

Question 18

What is the key property of the fatty acid tails in a phospholipid?

Answer 18

They are hydrophobic, meaning they repel water.

Question 19

What is meant by “amphipathic” and why is this significant?

Answer 19

Phospholipids are amphipathic which means they have both hydrophilic and hydrophobic parts.

This allows them to form monolayers or bilayers in water.

Question 20

How do phospholipids form a bilayer in water?

Answer 20

The fatty acid tails face inwards towards each other, and the polar heads face outwards, dissolving in the aqueous intra and extracellular fluids.

Question 21

Why is it important that phospholipids can form bilayers in water?

Answer 21

• The hydrophilic heads can be used to hold at the surface of the cell surface membrane
• Their structure allows them to form glycolipids with carbohydrates which are important on the plasma membrane for cell recognition.
• They are ideal for forming plasma membranes as they enable integration of other molecules and help regulate the movement of substances into and out of the cell.

Question 22

What is the third key type of lipid?

Answer 22

Sterols (steroid alcohols) including cholesterol.

Cholesterol is manufactured primarily in the liver and intestines.

Question 23

How is cholesterol structured?

Answer 23

It has a 4 carbon ring structure with a hydroxyl group at one end.

It forms a small, thin molecule that fits into the lipid bilayer, providing strength and stability.

Question 24

What is the function of cholesterol?

Answer 24

It is important in the formation and fluidity of cell surface membranes and in the production of certain hormones (e.g. testosterone).

It is also needed to make vitamin D.

Question 25

What needs to happen to transport cholesterol into the bloodstream?

Answer 25

Cholesterol is insoluble - meaning it needs to combined with proteins to form soluble lipoproteins.

Question 26

What are the 2 main lipoproteins and what are their functions?

Answer 26

• Low-density lipoproteins (LDL) - carry cholesterol from the liver to cells.
• High-density lipoproteins (HDL) - carry excess cholesterol back to the liver.

Question 27

How are HDLs structured?

Answer 27

They are made when triglycerides combine with cholesterol and proteins/

They have a greater percentage of proteins and less cholesterol compared to LDLs, hence their higher density.

Question 28

How do HDLs lower blood cholesterol levels?

Answer 28

They scavenge excess cholesterol in the body tissues and carry it back to the liver where it is broken down.

This lowers blood cholesterol levels and helps remove fatty plaques of atherosclerosis.

Question 29

Why are LDLs considered “bad”?

Answer 29

They are associated with the formation of atherosclerotic plaques.

It is desirable to maintain a higher HDL:LDL ratio in the blood.

Question 30

What dietary factors increase blood cholesterol levels?

Answer 30

High intakes of certain types of fat affect blood cholesterol levels.

• Saturated fats increase LDL levels
• Trans fats are worse - they increase LDL levels and decrease HDL levels
• Unsaturated fats increase HDL levels
• Monounsaturated fats - help remove LDLs from the blood
• Polyunsaturated fats - increase activity of LDL receptor sites to actively remove LDLs.

Question 31

Why do we not see false positives due to monosaccharides, disaccharides, starch, proteins and nucleic acids?

Answer 31

Monosaccharides and disaccharides are soluble in water, meaning they will not form a precipitate.

Starch, proteins and nucleic acids are all insoluble in organic substances such as ethanol, meaning they will not be extracted from the sample.

Question 32

How can this test be made semi-quantitative and fully quantitative?

Answer 32

Semi-quantitative results can be achieved by:

• ensuring an equal mass / volume of each sample is used - ensures comparability of result
• the height of the emulsion could be measured to identify the quantity of lipid present

The emulsion test could be made fully quantitative by using a colorimeter to measure the density of the emulsion, indicating the quantity of lipid present.