Carbohydrates | GCE A-level Biology

Table of Contents

The simplest carbohydrates
Examples of hexose sugars: glucose, fructose, galactose
Molecules often have the form of a ring, made up of some C atoms and one O atom
Glucose molecules has 2 forms: α-glucose(the hydroxyl OH group of the Carbon 1 is above the plane of the carbon ring) and β-glucose

Two monosaccharides join together in a condensation reaction to form a disaccharide.
The two molecules are held together by a covalent bond called a glycosidic bond
A disaccharide can be split via hydrolysis (addition of water) into its monosaccharides.

mainly for storing energy (starch and glycogen) or for forming strong structures (cellulose)

Is a polymer of alpha glucose.
It is an energy store in plant.
It has two components which are amylase and amylopectin.
Both are polymers of alpha glucose
Amylases have a straight chain and the glucose residues are joined together by 1,4 glycosidic bonds.
Amylase forms 20% of starch
These bonds cause the chains to coil helically into more compact shape.
Amylopectin is also more compact as it has many branches formed by 1,4 and 1,6 glycosidic bonds.
Coiled chains (may) contain 1 500 monomers with branches every ten (10) units
Forms 80% of Starch
As suspension of amylase in water gives a blue black colour with iodine (potassium iodide) and amylopectin give a red violet colour .

act as storage polysaccharides in animals and fungi
Made of α-glucose molecules linked together by glycosidic bonds
Most of the bonds are α 1-4 links (C1 on one glucose + C4 on the next)
There are some 1-6 links, which form branches in the chain.
The bonds can be hydrolyzed by carbohydrase enzymes to form monosaccharides, used in respiration.
The branches increase the rate of hydrolysis

Made of many β glucose molecules, linked by β 1-4 links.
Adjacent glucose molecules in the chain are upside-down to one another.
The chain is straight (not spiraling).
there are hydrogen bonds between chains forming very strong microfibrils
Microfibrils then bond together to form microfibrils which are woven in layers to form structures, such as cell walls
as a result, cell wall will not break easily if the plant cell absorbs water; difficult to digest (few organisms have enzyme that can break the β 1-4 bonds).
Its tensile strength helps plant cells in osmosis

	Starch	Glycogen	Cellulose
Name of monomer	alpha glucose	alpha glucose	beta glucose
bond between monomers	1,4 glycosidic 1,4 + 1,6 glycosidic	1,4 + 1,6 glycosidic	1,4 glycosidic
Characteristics of chains	Coiled unbranched + long branched	Short branched chains	Straight long chains