Hydrolysis of Carbohydrates Chemistry Tutorial

Key Concepts

• Carbohydrates are also known as sugars or saccharides.
• Hydrolysis is a reaction with water.

  Hydrolysis reactions are also referred to as hydrolytic reactions.

• Acid hydrolysis is a reaction with acidified water (acidic conditions).
• Disaccharides can be hydrolysed under acidic conditions.

  1 disaccharide molecule

  H2O, H+ → heat

  2 monosaccharide molecules

• Polysaccharides can be hydrolysed under acidic conditions.

  1 polysaccharide molecule

  H2O, H+ → heat

  many monosaccharide molecules

• Acid hydrolysis of disaccharides and polysaccharides produces monosaccharides by breaking the glycosidic links (ether bonds) between monomer units in the structure of the molecule.
• Human beings can digest disaccharides and the polysaccharide known as starch by hydrolysis using enzymes in enzyme catalysed hydrolysis reactions.

  starch

  H2O, enzyme →

  many units of glucose

• The ability of humans to digest the disaccharide lactose, "milk sugar", decreases as we develop due to a decline in the activity of the enzyme lactase.

  Lactose intolerance results from low levels of the enzyme lactose and hence incomplete hydrolysis of lactose.

• Human beings cannot digest the polysaccharide known as cellulose because we lack an appropriate enzyme to hydrolyse cellulose.

  Cellulose acts as dietary fibre in our diet.

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Hydrolysis of Disaccharides

Sucrose, table sugar, is an example of a disaccharide.
It is produced by the condensation reaction between the monosaccharides glucose and fructose as shown below:

glucose	+	fructose	→	sucrose	+ water
	+		→		+ H2O(l)

Notice the -C-O-C- (glycosidic link or ether bond) between the glucose unit and the fructose unit in a molecule of sucrose.

The reverse reaction is the hydrolysis of sucrose to produce glucose and fructose:

sucrose	H2O →	glucose	+	fructose
	H2O →		+

Notice that a water molecule has effectively been added across the glycosidic link resulting in the formation of two monosaccharides; glucose and fructose.

In the laboratory we would use heat and acidic conditions to hydrolyse sucrose, but in your body you use an enzyme, sucrase, in an enzyme catalysed hydrolysis reaction.

Lactose, "milk sugar", is also a disaccharide that can undergo hydrolysis:

lactose	H2O →	glucose	+	galactose
	H2O →		+

Notice that a water molecule has effectively been added across the glycosidic link resulting in the formation of two monosaccharides; glucose and galactose.

In the laboratory we would use heat and acidic conditions to hydrolyse lactose, but in your body you use an enzyme, lactase, in an enzyme catalysed hydrolysis reaction.

For every 1 molecule of disaccharide that undergoes hydrolysis, 2 molecules of monosachharide are produced.

Hydrolysis of Polysaccharides

Amylose is a component of starch.
It is a polymer made up of many glucose units, that is, it is a polysaccharide.
The skeletal structure below shows 3 glucose units, but the square brackets with the subscript n means that there are n repeating units in the chain:

Notice the -C-O-C- (glycosidic links or ether bonds) between each glucose unit in the structure of amylose.

If we hydrolyse amylose so that we add water across all these glycosidic links (ether bonds) we end up with n+2 glucose molecules:

amylose	H2O →	(n+2)glucose
	H2O →	(n+2)

Note that if the hydrolysis of amylose does not go to completion you will have a mixture containing molecules of various chain lengths.
Maltose, for example, is a disaccharide made up of 2 units of glucose, and is the major disaccharide product of the digestion of starch in humans.
The disaccharide maltose can be broken down into glucose in a hydrolysis reaction catalysed by the enzyme maltase in your body.

In the laboratory we would hydrolyse amylose using heat and acidic conditions, but in your body you use an enzyme, amylase found in saliva, in an enzyme catalysed hydrolysis reaction.

Digestion of Carbohydrates in the Human Body

You eat carbohydrates such as sugars and starches to give you energy.

The digestion of carbohydrates by enzyme catalysed hydrolysis begins in your mouth and continues in your stomach and small intestine.

The final product of the digestion of carbohydrates are monosaccharides such as glucose and fructose.

Glucose is used by your cells during respiration to produce energy.

Any excess glucose that is not immediately required for cell respiration will be stored.

Some of the excess will be stored as fat in adipose tissue.

Some will undergo condensation polymerisation to produce glycogen which is stored in the liver.
When required, this glycogen undergoes enzyme catalysed hydrolysis to produce the glucose required for cell respiration.