Digestion

The digestion and absorption of lipids involves:

• emulsification by bile salts

• micelles.

Explain each of the following.

The role of micelles in the absorption of lipids

Describe and explain the arrangement of phospholipid molecules in the cell-surface membrane.

Describe the processes of facilitated diffusion and active transport.

Facilitated diffusion

What are microvilli?

Vitamin A is a fat-soluble substance.

Micelles are involved in the process of vitamin A absorption.

Describe the process of vitamin A absorption into cells lining the ileum.

Some hospital patients suffer from diarrhoea caused by infection with the bacterium Clostridium difficile. The C. difficile bacteria release toxins. These toxins cause the diarrhoea.

The toxins damage the cells lining the ileum, causing them to lose their microvilli. The damage to the cells reduces the absorption of the products of digestion and reduces the absorption of water, resulting in diarrhoea.

Explain why the damage to the cells lining the ileum reduces absorption of the products of digestion and why this reduces absorption of water.

Not all patients in hospital with C. difficile develop diarrhoea.

Scientists measured the anti-toxin antibody concentration in hospital patients with and without C. difficile infection.

They measured the anti-toxin antibody concentration four times:

• on admission to hospital (day 0)

• on day 3

• on day 6

• on the day the patient left the hospital.

Figure 4 shows the scientists’ results.

The scientists suggest that the anti-toxin antibody could be given to some patients as a form of passive immunity.

Use Figure 4 to suggest how this passive immunity would work and which patients should be offered this anti-toxin antibody.

To be used as passive immunity treatment, the anti-toxin antibody would be injected. If it was given by mouth, it would be digested.

Describe how the anti-toxin antibody would be digested.

Describe the transport of carbohydrate in plants.

Compare and contrast the structure of starch and the structure of cellulose.

Describe the complete digestion of starch by a mammal.

The action of endopeptidases and exopeptidases can increase the rate of protein digestion. Describe how.

As humans age, there is a decrease in body protein.

Give the name of one body protein that could have resulted in:

reduced muscle power

Scientists investigated the effect of two types of dietary protein on the ability of old men to produce body proteins.

Table 2 shows information about the two types of dietary protein investigated.

Figure 4 shows the percentage of protein absorbed that becomes body protein in old men following a meal of casein or whey.

A statistical test confirmed that the difference between the results shown in Figure 4 was significant.

Suggest which type of dietary protein would be better for old men to eat to cause a net gain of body proteins. Use the information provided to explain your answer.

Figure 1 shows a cell from the lining of the ileum specialised for absorption of products of digestion.

SGLT1 is a carrier protein found in the cell-surface membrane of this cell, it transports glucose and sodium ions (Na⁺) into the cell.

The action of the carrier protein X in Figure 1 is linked to a membrane-bound ATP hydrolase enzyme.

Explain the function of this ATP hydrolase.

The movement of Na⁺ out of the cell allows the absorption of glucose into the cell lining the ileum.

Explain how.

Describe and explain two features you would expect to find in a cell specialised for absorption.

Figure 2 is a diagram of one SGLT1 carrier protein.

Figure 2

Draw phospholipids on Figure 2 to show how the carrier protein, SGLT1, would fit into the cell-surface membrane.

Do not draw more than eight phospholipids.

Figure 2 shows the SGLT1 polypeptide with NH₂ at one end and COOH at the other end.

Describe how amino acids join to form a polypeptide so there is always NH₂ at one end and COOH at the other end.

You may use a diagram in your answer.

Space for diagram:

To study lipid digestion, a scientist placed a tube into the gut of a healthy 20-year-old man. The end of the tube passed through the stomach but did not reach as far as the ileum.

The scientist fed the man a meal containing triglycerides through the tube. The scientist also used the tube to remove samples from the man’s gut at intervals after the meal.

The scientist measured the type of lipid found in the samples. Some of her results are shown in Table 1.

Use your knowledge of lipid digestion to explain the differences in the results for samples A and B shown in Table 1.

You should assume that no absorption had occurred.

After collecting the samples, the scientist immediately heated them to 70 °C for 10 minutes.

Explain why.

Describe the role of micelles in the absorption of fats into the cells lining the ileum.

Describe how a non-competitive inhibitor can reduce the rate of an enzyme-controlled reaction.

Pectin is a substance found in some fruit and vegetables.

A scientist investigated the effect of pectin on the hydrolysis of lipids by a lipase enzyme.

His results are shown in Figure 1.

The scientist concluded that pectin is a non-competitive inhibitor of the lipase enzyme.

Use Figure 1 to explain why the scientist concluded that pectin is a non-competitive inhibitor.

The scientist also found that pectin stops the action of bile salts. He prepared two suspensions:

• suspension A – lipid and bile salts

• suspension B – lipid, bile salts and pectin.

He did not add lipase to either suspension.

He observed samples from the suspensions using an optical microscope. Figure 2 shows what he saw in a typical sample from each suspension.

Calculate the maximum length of the large lipid droplet marked X in Figure 2.

Using a ruler with millimetre intervals always includes an uncertainty in the measurement. Use the uncertainty in your measurement to determine the uncertainty of your calculated maximum length.

You can assume there is no uncertainty in the magnification.

No large lipid droplets are visible with the optical microscope in the samples from suspension A.

Explain why.

Bacteria are often used in industry as a source of enzymes. One reason is because bacteria divide rapidly, producing a large number of them in a short time.

Describe how bacteria divide.

Washing powders often contain enzymes from bacteria. These enzymes include proteases that hydrolyse proteins in clothing stains.

Figure 1 shows the effect of temperature on a protease that could be used in washing powder.

Explain the shape of the curves at 50 C and 60 C.

Some proteases are secreted as extracellular enzymes by bacteria.

Suggest one advantage to a bacterium of secreting an extracellular protease in its natural environment.

Explain your answer.

Mammals have some cells that produce extracellular proteases. They also have cells with membrane-bound dipeptidases.

Describe the action of these membrane-bound dipeptidases and explain their importance.