Examples of Carbohydrates

Biology: Cambridge International A Level, topic 2.2.3

Starch

Starch is a polysaccharide formed by condensation reactions of alpha-glucose molecules.

Functions of starch

Starch is the main energy storage material in plants.
Starch is stored in the seeds of plants.
Starch is broken down into glucose by plants when they need more energy.
Starch can act as a source of food for humans and animals.

Features of starch

Starch does NOT change the water potential in a cell because it's insoluble in water.
Starch is made up of amylopectin and amylose, which are both alpha-glucose polysaccharides.

Amylopectin

Amylopectin is a highly branched chain of alpha-glucose monomers.
The branched structure means that enzymes can easily access the glycosidic bonds and so the glucose molecules can be quickly released when needed.

Amylose

Amylose is a linear chain of alpha-glucose monomers.
Because of the way the monomers are joined, the glucose chains have a helical structure.
This means that the amylose strands can pack closely together, making this form of starch particularly good for storage.

Iodine test

A common test for the presence of starch, particularly in food testing, is to add iodine.
Iodine is used commonly as a name for a solution of potassium iodide.
Step 1: Place a small sample into the dimple of a spotting tile or to a boiling tube.
Step 2: Add a few drops of iodine and observe any colour change.
If starch is present, the solution will change colour from orange → blue-black.

Glycogen

Glycogen is formed by condensation reactions of alpha-glucose molecules and is the main energy storage material in animals. It can be thought of as the animal equivalent of starch.

Function of glycogen

Glycogen is a highly branched molecule, similar to amylopectin (a form of starch).
When animals need to release energy, glycogen's highly branched structure means that glucose can be released fast.

Glycogen in the human body

Glycogen is a good storage molecule because it is so compact.
When blood glucose levels decrease, glycogen is broken down to release glucose. This is called glycogenolysis.

Cellulose

Cellulose is the major component of cell walls in plants.

Structure of cellulose

Cellulose is a long chain of beta-glucose.
Beta-glucose molecules are linked by glycosidic bonds to form linear cellulose chains that are unbranched.
Microfibrils are strong fibres that are made of many cellulose chains that are held together by hydrogen bonds.

Function of cellulose

Cellulose is the most abundant natural polymer.
The cell walls of plant cells are mostly made of cellulose.
These cell walls offer structural support because of the strength of the microfibril fibres that they are made of.

Digestion of cellulose

Cellulose cannot be broken down by human digestive enzymes.
Herbivores, such as cows and horses, are able to digest plant material rich in cellulose.

Test yourself on Examples of Carbohydrates

Which of the following is true of starch?
Where is starch primarily stored in plants?
If starch is present, what colour change is seen during the iodine test?
Which of the following is true of glycogen?
When blood glucose levels decrease, glycogen is broken down to release glucose. What is this process called?

Practise this topic for free