Lipids: emulusion test + triglycerides + phospholipids

Question 1

Describe the emulsion test for lipids/ fats
(Include the outcome for a negative and positive test)

Answer 1

• take 2cm^3 of test sample and add about 5cm^3 of ethanol (ethanol is highly flammable)
• shake the test tube thoroughly to dissolve any liquid in the sample
• add 5cm^3 of water and shake gently

Negative test: clear
Positive test: forms a milky white emulsion

Question 2

State what forms triglycerides

Answer 2

One glycerol and three fatty acids

Question 3

Describe the process on how a triglyceride is formed

Answer 3

• triglycerides are formed by one glycerol and three fatty acids via a condensation reaction
• the OH group of each fatty acid is taken and bonds onto the hydrogen of the glycerol and are released as water
• the carbon atoms from each of the three fatty acids then bond onto the oxygen of the glycerol and forms 3 ester bonds
• a triglyceride is formed

Question 4

Describe the structure of saturated fatty acids and where it can be found

Answer 4

• every carbon is bonded to as many hydrogen atoms as possible - no more can be added, hence saturated (with hydrogen)
• carbon chain is straight with no kinks
• triglycerides of saturated fatty acids pack together to form solid fat at room temperature
• mainly found in animal fats from meat and dairy products

Question 5

Describe the structure of unsaturated fatty acids and where they can be found

Answer 5

• triglycerides consisting of ‘kinky’ unsaturated fatty acids do not pack easily together and form liquid oils at room temperature
• the more double bonds, the more kinks there will be
• not every carbon atom is bonded to as many hydrogen atoms as it could be, hence unsaturated (with hydrogen) and so there are double bonds present
• double bonds introduce a definite ‘kink’ in the carbon atom chain
• mainly found in vegetable oil, nuts and fish

Question 6

What is the function of triglycerides and how does their structure help them to carry out their function well?

Answer 6

Triglycerides are mainly used as energy storage molecules. They are good for this because:
- the long hydrocarbon tails of the fatty acids release loads of energy when they are broken down
- they are insoluble so they won’t affect the water potential of the cell
- they clamp together as insoluble droplets in cells

Question 7

Name two examples of lipids

Answer 7

Triglycerides and Phospholipids

Question 8

What bonds are formed during the condensation of triglycerides

Answer 8

Ester bonds

Question 9

Examples of differences between Phospholipids and triglycerides

Answer 9

• Triglycerides consist of three fatty acids, whilst Phospholipids only consist of two fatty acids (one saturated and one unsaturated)
• Phospholipids contain a phosphate molecule, whilst triglycerides don’t
• Phospholipids are known as a polar molecule as it consists of a hydrophilic head and hydrophobic tail, whilst triglycerides are only hydrophobic

Question 10

What is the main function of Phospholipids?

Answer 10

They form membranes

Question 11

Describe the structure of Phospholipids

Answer 11

Phospholipids are formed via a condensation reaction between one phosphate molecule, a glycerol molecule and 2 fatty acids
- where ester bonds are formed
- water is released

Question 12

Which direction does the hydrophilic head of a phospholipid point towards?

Answer 12

• hydrophilic heads point towards extra cellular fluid and water cytoplasm

Question 13

Which direction does the hydrophobic fatty acid tail in Phospholipids point?

Answer 13

• fatty acid tails points inwards forming a bilayer
• this middle section is impermeable to water
• only lipid soluble molecules can diffuse across it