Biological Molecules - Lipids

Question 1

Describe the solubility of lipids.

Answer 1

Lipids are insoluble in water as they are hydrophobic and non-polar, but soluble in other lipids or organic solvents, e.g. ethanol.

Question 2

What elements do lipids contain?

Answer 2

carbon, hydrogen, oxygen (and phosphorus if it’s a phospholipid)

Question 3

What are the two main types of lipids?

Answer 3

triglycerides (fats and oils) and phospholipids

Question 4

What are triglycerides made up of?

Answer 4

triglycerides are formed by the condensation of one molecule of glycerol and three molecules of fatty acid

Question 5

What do fatty acid molecules have?

Answer 5

They have long hydrocarbon chains which are hydrophobic, i.e. repel water. Therefore, they make lipids insoluble in water.

Question 6

How do fatty acids vary in structure?

Answer 6

All fatty acids have the same structure, but vary in hydrocarbon length.

Question 7

How do you form triglycerides?

Answer 7

1. Triglycerides are formed by a condensation reaction between hydroxyl groups on glycerol and carboxyl groups on fatty acids.
2. An ester bond is formed - this process is called esterification.
3. Overall, three ester bonds form and three molecules of water form.

Question 8

What does breaking the ester bonds do?

Answer 8

Releases the three fatty acids and one glycerol molecule in a hydrolysis reaction.

Question 9

What can fatty acids either be?

Answer 9

The R-group of a fatty acid (the hydrocarbon tails) may be saturated or unsaturated.

Question 10

What are the properties of saturated fatty acids?

Answer 10

• no double c-c bonds
• all carbons are joined to hydrogen and so is ‘saturated’ with hydrogen
• mostly solid at room temperature
• higher melting point

Question 11

What are the properties of unsaturated fatty acids?

Answer 11

• at least one double c-c bond causing a kink in the chain
• mostly liquid at room temperature (oils)
• lower melting point

Question 12

What is a cis configuration?

Answer 12

Unsaturated fats have a cis configuration. Therefore they are bent (i.e. have a kink), and cannot pack tightly together (hence, liquid at rtp).

Question 13

What is a trans configuration?

Answer 13

• Trans configuration have a straight chain, like saturated fatty acids.
• They are less common in nature.
• Synthetically, hydrogenation is used to create saturated trans fatty acids.

Question 14

What are the functions of triglycerides?

Answer 14

• Energy source -> Triglycerides have a higher proportion of hydrogen than proteins and carbohydrates. This can be broken down easily to release energy (ATP).
• Heat insulation -> Adipose tissue (fat tissue) are slow conductors of heat (good insulation)
• Shock absorption -> Protection for delicate organs (e.g. kidneys).
• Buoyancy -> Fat is less dense than water so aquatic animals stay afloat.
• Energy store -> Excess fat is stored in the form of adipose tissue. Insoluble in water so doesn’t affect water potential of the cell.

Question 15

What do phospholipids form?

Answer 15

Cell membranes in the phospholipid bilayer.

Question 16

What do phospholipids contain?

Answer 16

One glycerol molecule, one phosphate-containing group and two fatty acid chains.

Question 17

What is the difference between the phosphate group and the fatty acid tails?

Answer 17

The phosphate group is negatively charged (hydrophilic and attracts water).

The fatty acid tails are non-polar (hydrophobic and repel water).

Question 18

What are phospholipids said to be?

Answer 18

Amphipathic - have both hydrophilic and hydrophobic parts.

Question 19

How does an ester bond form?

Answer 19

The ester bond forms in a condensation reaction between the phosphate group/fatty acid chain and glycerol.

Question 20

What is cholesterol?

Answer 20

• Small size and flattened shape.
• Hydrophobic molecule and can pass through phospholipid bilayer.
• Binds to hydrophobic tails of phospholipids -> Regulates the fluidity of the membrane and prevents the membrane becoming too stiff.
• Controls permeability.

Question 21

Describe the emulsion test for lipids.

Answer 21

1. Take a completely dry and grease-free test tube.
2. To 2cm3 of the sample being tested, add 5cm3 of ethanol.
3. Shake the tube thoroughly to dissolve any lipid in the sample.
4. Add 5cm3 of water and shake gently.
5. A milky-white emulsion indicates the presence of a lipid. The higher the lipid concentration, the more white the sample will appear.
6. As a control, repeat the procedures using water instead of the sample; the final solution should remain clear.

Question 22

How are esters made?

Answer 22

Esters are made by esterification (when an acid and an alcohol react in a condensation reaction to form ester and water). All lipids are esters.

Question 23

What are the properties of triglycerides?

Answer 23

• The most common lipids are triglycerides.
• These have three fatty acids attached to the glycerol.
• The more unsaturated the fatty acids are, the lower the lipids melting point.
• They are non-polar, hydrophobic and therefore insoluble in water. Form droplets in cytoplasm.
• They are less dense than water.

Question 24

What characteristics do lipids share?

Answer 24

• They contain carbon, hydrogen and oxygen.
• The proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates.
• They are insoluble in water.
• They are soluble in organic solvents such as alcohols and acetone.

Question 25

What is the difference between fats and oils?

Answer 25

• Fats are generally made of saturated fatty acids, while oils are made of unsaturated ones.
• Fats are solid at room temperature, whereas oils are liquid.

Question 26

What are the roles of lipids?

Answer 26

• Cell membranes: (Cell-surface membranes and membranes around organelles). Phospholipids contribute to the flexibility of membranes and the transfer of lipid-soluble substances across them.
• 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 therefore useful as a 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.

Question 27

What is monounsaturated and polyunsaturated?

Answer 27

If there is a single double bond, it is monounsaturated. If more than one double bond is present, it is polyunsaturated.

Question 28

Why are triglycerides good sources of energy?

Answer 28

Triglycerides have a high ratio of energy-storing carbon-hydrogen bonds to carbon atoms and are therefore an excellent source of energy.

Question 29

Why are triglycerides good storage molecules?

Answer 29

Triglycerides have low mass to energy ratio, making them good storage molecules because much 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.

Question 30

Why are triglycerides insoluble in water?

Answer 30

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.

Question 31

Why do triglycerides provide an important source of water?

Answer 31

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 32

What two parts are phospholipids made of?

Answer 32

a hydrophilic ‘head’ - which interacts with water (is attracted to it) but not with fat

a hydrophobic ‘tail’ - which orients itself away from water but mixes readily with fat

Question 33

Are the phosphate molecules or the fatty acids polar?

Answer 33

The phosphate molecule is polar. This means that when these polar phospholipid molecules are placed in water they position themselves so that the hydrophilic heads are as close to the water as possible and the hydrophobic tails are as far away from the water as possible.

Question 34

Why are phospholipids polar molecules?

Answer 34

Phospholipids are polar molecules, having a hydrophilic phosphate head and a 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 outside of a cell.

Question 35

What does the phospholipid structure allow?

Answer 35

The phospholipid structure allows them to form glycolipids by combining with carbohydrates within the cell-surface membrane. These glycolipids are important in cell recognition.

Question 36

What turns the sample of lipid a cloudy colour?

Answer 36

The cloudy colour is due to any lipid in the sample being finely dispersed in the water to form an emulsion. Light passing through this emulsion is refracted as it passes from oil droplets to water droplets, making it appear cloudy.

Question 37

Give examples of the types lipids.

Answer 37

• triglycerides
• phospholipids
• waxes
• steroids