2.2 Carbohydrates & Lipids

The recommended daily intake of lipids should make up 25% of your total energy intake. The recommended energy intake for 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.

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 a lower energy content than traditional 'high-fat' foods. Fig. 1 shows the structure of a fat substitute known as PGME.

Fig. 1

Based on the information in Fig. 1, suggest possible reasons why PGME cannot be digested in the stomach by lipase like regular triglycerides.

Phospholipids are a different type of lipid and they form the main component of cell surface membranes. 

Explain how the structure of a phospholipid molecule enables it to form a cell surface membrane.

Fig. 1 shows the structures of two monosaccharides, ∝-glucose and fructose. These combine to form the disaccharide sucrose.

Fig. 1

Draw the structure of sucrose, including any by-products produced

[2]

Label the bond between the two monosaccharides

[2]

Participants and coaches involved in competitive sports have taken part in research into the optimum formulation of sports drinks. One such study is set out below.

Eight trained, competitive male cyclists took part in a study in which they fasted for 12 hours.  They were then given either water, glucose solution, or a 2:1 glucose: fructose beverage immediately prior to competing in a 10-mile time-trial race. The results are shown in Table 1.

Table 1

Calculate the percentage improvement in finishing time from using the  2:1 glucose: fructose mixture, to using glucose solution on its own.

[2]

High-fructose corn syrup (HFCS) is a widely used material in the food industry. Fructose is discerned as sweeter than glucose by human taste buds; this gives food manufacturers the opportunity to maintain food’s sweetness by using less carbohydrate material. Fructose is not metabolised in all cells (as glucose is) but is metabolised in the liver to form glycogen and can be subsequently converted to fat. Health campaigners have claimed that increased use of HFCS promotes obesity.

Suggest why humans have a mechanism for metabolising fructose.

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 Fig. 1. 

Fig. 1

Suggest why the lipase solution that she used had to be freshly prepared.

[1]

Suggest why whole milk was used, rather than semi-skimmed milk.

[1]

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

Fig. 2

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

Fig. 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.  

Fig. 3

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

Fig. 1 shows the components of a triglyceride.

Draw out the molecular structure of the triglyceride 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. 

Fig. 1

Triglycerides are not referred to as polymers.

State why this is the case.

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.

With reference to their structure, suggest a possible function of chylomicrons.

Name the monomers from which a sucrose molecule is made.

A teacher produces a dilution series of a sucrose solution in order to plot a calibration curve. The teacher starts with a stock solution of sucrose of a concentration of 0.75 mol dm^-3 and distilled water with which they make a series of dilutions from 0.1 to 0.5 mol dm^-3.

The teacher hydrolysed the sucrose by adding hydrochloric acid before performing Benedict's test on the five sucrose dilutions ranging from 0.1 mol dm^-3 to 0.5 mol dm^-3. A sample of each solution was placed in a colorimeter and the light absorbance was measured. Table 1 shows the light absorbance for each sample.

Table 1

Calculate the percentage increase in light absorbance between a sucrose concentration of 0.1 mol dm^-3 and 0.5 mol dm^-3.

Explain why it was necessary to hydrolyse the sucrose before adding Benedict's reagent.

Sucralose is a popular artificial sweetener that is often used to replace sucrose in our diets. Fig.1 compares the structure of sucrose and sucralose.

Fig. 1

Based on the information in Fig.1, discuss the main differences in structure between sucrose and sucralose.

Cod liver oil is often used as a dietary supplement due to the fact that it is very high in an unsaturated fatty acid called Omega-3.

Define the term 'unsaturated fatty acid'.

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

Fig. 1

Using the data in Fig. 1, describe the results obtained from the study.

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

Explain how a triglyceride forms.

Discuss the reasons why triglycerides make good energy stores.

Proteins are macromolecules composed of many amino acids.

Two amino acids are represented in the diagram in Fig.1.

Complete the diagram to show how the two amino acids react together to form a dipeptide.

[2]

Amylose and cellulose are polysaccharides.

Fig. 2 shows the structure of part of a cellulose molecule.

With reference to Fig. 2, state how the structure of a cellulose molecule differs from the structure of an amylose molecule.

Cellulose is the main component of plant cell walls.

Explain why cellulose is suitable as a component of plant cell walls.

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

Fig. 1

Identify structures A-C in Fig. 1.

State what the ‘R’ stands for in structure B in Fig. 1.

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

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

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