Biological molecules: Lipids

Question 1

What characteristics do all lipids share?

Answer 1

• They all contain carbon and oxygen
• the proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates
• they are non polar due to the large % of carbons and hydrogens making them hydrophobic
• they are soluble in organic solvents like alcohols and acetones

Question 2

What are the main groups of lipids?

Answer 2

Triglycerides (fats and oils) and phospholipids

Question 3

What are the roles of lipids and explain each one

Answer 3

• phospholipids contribute to the flexibility of cell membranes and the transfer of lipids soluble substances across them
• they’re a source of energy as when oxidised, lipids provide over twice the energy as the same mass of carbohydrate and release valuable water
• they are useful for waterproofing as they are hydrophilic. Plants have a waxy lipid cuticle that conserves water and mammals produce an oily secretion from the sebaceous glands in the skin
• provide insulation as fats are slow conductors of heat and when stored beneath the body surface help to retain body heat and act as an insulator in the myelin sheath around nerve cells
• provide protection as fat is stored around delicate organs like the kidneys and the heart

Question 4

What’s the difference between fats and oils?

Answer 4

Fats are solid at room temp but oils are liquid

Question 5

What are triglycerides made up of?

Answer 5

3 fatty acids bonded to glycerol

Question 6

How is a triglyceride formed?

Answer 6

Through the condensation reaction of three fatty acids and a glycerol

Question 7

In a triglyceride what is the bond called between each fatty acid and OH group from the glycerol?

Answer 7

An Ester bond

Question 8

What is the structure of a glycol and fatty acid molecule?

Answer 8

(Check biology camera album)

Question 9

What does the fact that the glycerol molecule in a triglyceride is the same mean about the difference in properties of fats and oils?

Answer 9

The differences in the properties of different fats and oils come from the variation of fatty acids

Question 10

How many different fatty acids are there and what do they all have?

Answer 10

70 different fatty acids and they all have a carboxyl (-COOH) group with a hydrocarbon chain attached.

Question 11

When is a fatty acid saturated?

Answer 11

If the hydrocarbon chain has no carbon - carbon double bonds as all the carbon atoms are linked to he maximum possible number of hydrogen atoms.

Question 12

When is a fatty acid mono-unsaturated?

Answer 12

If here is one double bond present in the hydrocarbon chain

Question 13

When is a fatty acid polyunsaturated?

Answer 13

If there is 2 or more double bonds in the hydrocarbon chain

Question 14

What are the four ways the structure of triglycerides relate to their properties?

Answer 14

• they have a high ratio of energy storing carbon - hydrogen bonds to carbon atoms making them an excellent source of energy
• have a low mass - energy ratio making them good storage molecules as much energy can be stored in a small storage volume so benefits animals as it reduces the mass they have to carry to move around
• they are insoluble in water as they are large non - polar molecules and so doesn’t affect osmosis in cells or the water potential of them
• As they have a high ratio of hydrogen to oxygen atoms, they release water when oxidised and so provide an important source of water especially for organisms living in dry deserts

Question 15

What are the properties of saturated fatty acids?

Answer 15

• have no double bonds
• form linear hydrocarbon chains
• solid at room temp
• high melting points
• pack closely together

Question 16

What are the properties of unsaturated fatty acids?

Answer 16

• contain c=c
• double bond in the carbons allow the hydrocarbon chains to Bend and not allowing the fatty acid molecules packing closely together
• there are fewer interactions between fatty acid chains so weaker interactions
• liquid at room temp
• Low melting point

Question 17

What are the two parts that make up a phospholipid?

Answer 17

• The hydrophilic head made of phosphate which interacts with water and not fat
• the hydrophobic tail which orients its self away from water but mixes readily with fat

Question 18

Why are phospholipids polar?

Answer 18

Because they are molecules that have 2 ends that behave differently

Question 19

What is the structure of a phospholipid?

Answer 19

(Check biology album in camera)

Question 20

What are phospholipids made out of?

Answer 20

Glycerol, 2 fatty acids and a phosphate group

Question 21

What are the three ways the structure of a phospholipid relates to its properties?

Answer 21

• phospholipids are polar molecules having a hydrophilic phosphate head and a hydrophobic tale of two fatty acids so in an aqueous environment, phospholipid molecules form a bilayer, within cell surface membranes meaning a hydrophobic barrier is formed between the inside and outside of a cell
• the hydrophilic heads of a phospholipid molecules help to hold at the surface of the cell surface membrane
• the phospholipid structure allows them to form glycolipids (important in cell recognition) by combining with carbohydrates within the cell surface membrane

Question 22

Describe the steps involved to carry out the emulsion test

Answer 22

1. Take a completely dry and grease free tube
2. Take 2cm^3 of the sample being tested and add 5cm^3 of ethanol
3. Shake the tube thoroughly to dissolve any lipid in the sample
4. Add 5cm^3 of water and shake gently
5. A cloudy white colour indicates the presence of a lipid
6. As a control repeat the procedure using water instead of the sample and the solution should remain clear

Question 23

In the emulsion test, what causes the cloudy white colour?

Answer 23

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