Hydrolysis of starch in the body

Question 1

Can humans hydrolyse cellulose? Explain your answer.

Answer 1

Humans cannot hydrolyse cellulose because the body does not contain the enzyme cellulase. Cellulase is required to break the C1 to C4 glycosidic links between the B-glucose monomers that make up cellulose.

Question 2

Describe the first step in the hydrolysis of starch.

Answer 2

The enzyme amylase, present in saliva, initially hydrolyses some of the starch molecules in the mouth to produce maltose disaccharides.

Question 3

Why does salivary amylase only hydrolyse a small percentage of starch?

Answer 3

The optimal pH for salivary amylase is approximately 6, meaning it is denatured upon reaching the acidic conditions of the stomach. Salivary amylase can therefore only hydrolyse starch for the short time it is in the mouth.

Question 4

Describe the second step in the hydrolysis of starch.

Answer 4

After travelling from the mouth and into the small intestine, the enzyme pancreatic amylase hydrolyses the remaining starch molecules to produce the disaccharides maltose, lactose, or fructose.

Question 5

Describe the third step in the hydrolysis of starch.

Answer 5

In the last part of the small intestine, the disaccharides are hydrolysed by their correpsonding enzyme to produce their constituent monosaccharides.

Question 6

Describe the fourth step in the hydrolysis of starch.

Answer 6

The monosaccharides pass through the intestine and enter the bloodstream for transport around the body.

Question 7

What property allows monosaccharides to enter the bloodstream?

Answer 7

Monosaccharides are able to enter the bloodstream because they are highly soluble molecules due to the presence of a polar hydroxyl group. This means monosaccharides can form hydrogen bonds with surrounding water molecules.

Question 8

What are the products of the hydrolysis of sucrose?

Answer 8

The hydrolysis of sucrose produces one glucose and one fructose molecule.

Question 9

What are the products of the hydrolysis of maltose?

Answer 9

The hydrolysis of maltose produces two glucose molecules.

Question 10

What are the products of the hydrolysis of lactose?

Answer 10

The hydrolysis of lactose produces one glucose and one galactose molecule.

Question 11

Explain why the hydrolysis of disaccharides has the same general formula, regardless of the type of disaccharides.

Answer 11

Disaccharides are structural isomers of each other, meaning they each contain the same type and number of atoms, and hence the same molecular formula, but vary in the orientation and sequencing of these atoms. No matter their type, all disaccharides have the molecular formula C12H22O11. Similarly, all monosaccharides are structural isomers with the molecular formula C6H12O6.

Question 12

Maltose is hydrolysed to glucose via the enzyme maltase. Write the equation for this reaction.

Answer 12

C12H22O11 (aq) + H2O (aq) => 2C6H12O6 (aq)

Question 13

What enzyme is required in the hydrolysis of lactose?

Answer 13

Lactase.

Question 14

What enzyme is required in the hydrolysis of sucrose?

Answer 14

Sucrase (invertase).

Question 15

What enzyme is required in the hydrolysis of maltose?

Answer 15

Maltase.

Question 16

What causes some individuals to develop lactose intolerance?

Answer 16

Lactose intolerance occurs when the human body possesses low levels of, or lacks entirely, the enzyme lactase. This means that lactose cannnot be hydrolysed to glucose and galactose for energy.

Question 17

In the absence of lactase, describe the digestion of lactose.

Answer 17

In the absence of lactase, the undigested lactose passes into the large intestine, where bacterial fermentation break down the lactose into small carboxylic acids. These carboxylic acids are then broken down into hydrogen, carbon dioxide and methane.

Question 18

What is the glycemic index and why is it considered useful for consumers?

Answer 18

The glycemic index is a figure indicating the rate at which a carbohydrate causes an increase in the level of glucose in the bloodstream. Optimal health requires a slow increase in glucose rather than fluctuations to extreme levels, so it is important that consumers can choose to consume foods with a low glycemic index.

Question 19

What is the glycemic index of glucose?

Answer 19

100.

Question 20

Explain, making reference to chemical structure, why glycemic index increases as the proportion of amylose decreases.

Answer 20

All starch molecules consist of amylose and amylopectin molecules. Amylose is digested more slowly than amylopectin because it consists of closely-packed linear chains of glucose monomers arranged in a helical structure, which is less available to the enzyme amylase. Conversely, amylopectin is a longer, more loosely-branched molecule than can be easily hydrolysed. As the proportion of amylose decreases and that of amylopectin increases, the starch is hyrolysed to glucose more quickly and the food is given a higher GI rating.

Question 21

“Foods that contain a greater amount of amylopectin typically have a lower glycemic index”. Is the above statement true or false?

Answer 21

False. Foods that contain a greater amount of amylopectin typically have a higher glycemic index because amylopectin can be easily hydrolysed due to its long and loosely-branched structure, meaning glucose is released into the bloodstream more quickly.

Question 22

State two limitations of the glycemic index in regards to its ability to indicate the ‘healthiness’ of a food.

Answer 22

1. The glycemic index does not take into account the ability of a food to produce the hormone insulin, which allows cells to absorb glucose from the bloodstream.
2. The glycemic index only measures the rate at which glucose is released into the bloodstream, and does not take into account any other important nutrients present in the food.

Question 23

Explain the difference between glycemic load and glycemic index.

Answer 23

While the glycemic index indicates how quickly (rate) carbohydrates increase glucose levels in the blood, glycemic load indicates by how much (quantity) glucose levels in the blood increase due to a carbohydrate.

Question 24

“Glycemic load depends on the amount of fibre in a food sample”. Is the above statement true or false?

Answer 24

True. Fibre is cellulose, which cannot be hydrolysed in the human body as the enzyme cellulase is not present. Fibre is not a source of available carbohydrate because it cannot release glucose, and therefore is not considered in the glycemic index or glycemic load of a food.