Biological Molecules: Lipids (AQA A Level Biology)

Avocados are high in an unsaturated fatty acid called Omega-3.

Define the term ‘unsaturated fatty acid’.

Figure 1 shows the data obtained by scientists investigating the effect of omega-3 consumption on the relative risk of coronary heart disease.

Figure 1

Using the data in Figure 1, evaluate whether coronary heart disease can be prevented by increasing omega-3 intake.

Fatty acids such as omega-3 join with other biological molecules to form triglycerides.

State all the constituent molecules that are required to form a triglyceride.

Name the type of bond that is formed in the production of a triglyceride molecule.

Many of the foods we eat contain lipids. For example, nuts have a high lipid content.

Name two groups of lipids found in nuts.

Name and describe a test that can be carried out on nuts to show they contain lipids.

Lipase is an enzyme that breaks down lipids. Figure 1 shows an experiment set up by a teacher to investigate the effect of lipase concentration on the hydrolysis of lipids.

The pH of beaker B is measured for 3 minutes at the start of the experiment. Beaker A containing lipase solution is added to beaker B. The data logger recorded the change in pH over the next 5 minutes.

 Figure 1

Predict the change in pH after the lipase has been added. Explain your answer.

State two variables that must be controlled to accurately and reliably carry out the experiment in Figure 1. 

Figure 1 shows three structures (A, B and C) that are found in phospholipid molecules.

Figure 1         

Give the letter and name of the structures found in phospholipids.

State what is meant by the ‘R’ seen in structure B in Figure 1?

Hexanoic acid is a saturated fatty acid consisting of six carbon atoms.      

Use the correct components from Figure 1 to draw the structure of a triglyceride formed from solely hexanoic acid and label the bond joining separate molecules.

In the formation of a phospholipid, identify how many water molecules are produced and/or used up.

The government’s recommended daily reference intake of lipids should make up 25% of your total energy intake. The recommended energy intake for most adult men is 8100 kJ day^–1. The energy content of lipids is 38 kJ g^–1. Calculate the recommended lipid intake per day for adult men. Give your answer in grams and show your working.

Triglycerides are the main form of dietary lipids ingested by humans. They are digested by the enzyme lipase in the stomach.

Describe a biochemical test that could be performed to determine whether a sample of food contained triglycerides.

Using your knowledge of the structure of triglycerides, explain why the solution goes cloudy when water is added.

A student is writing an essay on lipids. Within the essay the following statements are made:

• Triglycerides are polymers

• Triglycerides and phospholipids are structurally different

• Phospholipids can only contain saturated fats

State which of the statements, if any, are incorrect and explain your answer.

Lipids are high energy foods because of the number of carbon atoms in the fatty acid chains. Synthetically made fat substitutes are used by food manufacturers to make ‘low-fat’ foods with lower calorific values. Figure 1 shows the structure of a fat substitute known as PGME.

Figure 1

Unlike naturally occurring fats, PGME cannot be digested in the stomach by lipase. Explain why.

Phospholipids are a naturally occurring lipid and are the main constituent of cell membranes. Using your knowledge of the structure of phospholipids, explain how they form a cell membrane.

PGME, shown in Figure 1, is a lipid. Despite being a lipid, it cannot cross the cell-surface membranes of cells lining the small intestine.

Suggest why it cannot cross cell-surface membranes.

The fluidity of a membrane is determined by intermolecular forces between fatty acid chains. The more intermolecular forces, the less fluid the membrane.

Figure 2 shows three possible fatty acid chains found in phospholipids.

 Figure 2

If three membranes - A, B and C- were made from the Figure 2 fatty acids A, B and C respectively, suggest which membrane would be the most fluid and which would be the least fluid. Give a reason for your answer.

Figure 1 shows the components of a triglyceride. Draw out the molecular structure of thetriglyceride that forms from these two components. Show clearly and label the molecular structure of the bond(s) that form(s) between the molecules, and any by-products formed. 

Describe a biochemical test that will identify this triglyceride in a liquid sample and how a positive result would be recognised.

Triglycerides contain repeated fatty acid molecules, however they are not referred to as polymers. Explain why.  

The food manufacturing industry tends to favour the use of saturated fats in processed foods. Healthy-eating campaigners prefer the use of unsaturated fats wherever possible. Use your knowledge of lipids to analyse the two sides of this debate.

Doctors monitored the diets and lifestyles of 7,877 coronary heart disease (CHD) sufferers over a 20-year period. At a review point in the study, the doctors collected data on the typical dietary composition of each sufferer. The data were gathered into 4 groups. The results are shown in Table 1.

Table 1

The doctors made an assessment of the likelihood of members of each group suffering from ongoing symptoms of CHD within the next 10 years. This data is represented in Figure 1.

Figure 1

Deduce which of the following dietary changes would make a stronger impact on the risk of recurrence of CHD:

1. Switching from an ‘unhealthy’ carbohydrate intake to a ‘healthy’ one

2. Switching from an ‘unhealthy’ fat intake to a ‘healthy’ one

Give evidence from Table 1 and Figure 1 to support your answer.  

Using data from Table 1 and Figure 1, evaluate the findings of this study as a basis for doctors to offer medical lifestyle advice to patients wishing to minimise CHD symptoms for the rest of their lives.

One subset of polyunsaturated fatty acids is omega-3 fatty acids. These are found in food products like oily fish, nuts, seeds and certain plant oils. Omega-3 fatty acids have been associated with good heart health, with more research being done to determine whether they can alleviate other conditions. Figure 2 shows the results of a study into the possible link between omega-3 fatty acids and asthma in adults.  

A reporter claimed that eating an omega-3 rich diet would reduce the incidence of asthma in adults. Evaluate the reporter’s claim.  

Figure 3 shows the molecular structures of three omega-3 fatty acids found in natural foodstuffs. Identify the common feature that is believed to make them a healthy part of a person’s diet.    

A scientist set up an experiment to examine the effect of temperature on the hydrolysis of lipids by the enzyme lipase. She used 100 cm³ of whole cow’s milk in a beaker, immersing the beaker in a water bath at a range of different temperatures. During that time, she set up a pH probe and data logger.  After 10 minutes, she added 10 cm³ of freshly prepared lipase solution and mixed the beaker’s contents, and continued to monitor pH. The apparatus is shown in Figure 1. 

Figure 1

Suggest why 

(i) The lipase solution that she used had to be freshly prepared

(ii) Whole milk was used, rather than semi-skimmed milk 

Figure 2 shows the pH readings starting from the time at which lipase was added to the milk.

Figure 2

Explain the shapes of the curves at 5°C, 35°C and 55°C in Figure 2.

Figure 3 is a sketched copy of the curve at 35°C from this experiment. Sketch and explain the position of the curve that would be obtained with a solution containing phospholipids only (also at 35°C) , rather than using whole milk.  

Figure 3

Outline the three main biochemical outcomes for triglycerides that are ingested as part of an animal’s diet. 

The recommended energy intake for adult males in the UK is 8,400 kJ day^-1. Given that 30% of a typical day’s energy intake comes from fat, calculate the energy content of fat, assuming that an adult male consumes 55 g of fat in one day. State your answer to the nearest whole number and suggest the appropriate units.

Chylomicrons are molecular assemblies that include phosphate ‘heads’ of phospholipids and proteins on their surface. Within the interior of the assembly are triglycerides, lipoproteins and cholesterol. In relation to their structure, suggest a function for chylomicrons in the metabolism of lipids.

Bile does not contain lipase enzymes but it plays a key role prior to the digestion of lipids. Describe and explain the role played by bile and its components in the processing of lipids.

A phospholipid molecule has a different structure to that of a triglyceride. Describe the difference between the two types of molecule.  

Describe how the amphipathic nature of a phospholipid’s structure allows phospholipids to perform their function.

Figure 1 shows the structure of cholesterol.

Figure 1

Suggest which two features of the structure of cholesterol make it effective in performing its role in relation to membrane fluidity.

Describe and explain the structure and functions of micelles.

The lipid composition of red blood cell membranes is approximately 24% cholesterol. Phagocytes (a kind of white blood cell) contain around 11% cholesterol. Explain this difference.