Biological molecules AQA AS Biology PART 1 OF 8 TOPICS: Carbohydrates

An AQA summary based on the specification at your hands and in black and white so that you can personalise it to whatever your heart desires!

This document is a quick summary on all that you need to know about carbohydrates including monosaccharides, disaccharides, polysaccharides and tests for reducing sugars, non-reducing sugars and starch.

NB: Dont get confused if a glucosidic bond is referred to as 1,4 alpha glycosidic bond for example. This means that the glycosidic bond is between carbon number 1 of one glucose molecule and carbon number 4 on another glucose molecule with both glucose molecules being alpha.


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Biological molecules (AQA AS Biology) PART 1 of 8 TOPICS
Monosaccharides are the monomers from which larger carbohydrates are made. These include
glucose, fructose and galactose. Only the
structure of glucose needs to be known for the
whole of a level which is illustrated as follows
on either side. Notice the OH group and the
hydrogen are on opposite sides on carbon
number 1 (there is no particular way of
counting the carbons however exam questions will refer to this particular
carbon as carbon 1. Number 4 is always the carbon opposite carbon number 4).
Two monosaccharides form a disaccharide in any combination where glucose is always one of the
reactants -
These reactions are called condensation reactions in which a water molecule is removed to form a
glycosidic bond. To get back to the monosaccharides, hydrolysis takes place.
Glucose + Glucose = Maltose
Glucose + Fructose = Sucrose
Glucose + Galactose = Lactose
Polysaccharides are made by the condensation reactions of many monosaccharides. Glycogen and
starch are made from many alpha glucose molecules and cellulose is formed from many beta glucose
molecules. As part of AQA, you need to know the characteristics of these three polysaccharides:
Starch: Is insoluble so it does not draw water in by osmosis. Starch will not diffuse easily out and is
stored in a tight place because it is compact due to the coils it has. It can easily be hydrolysed into
alpha glucose molecules which can used in respiration. Starch is found in plants.
Glycogen: Is shorter and is more branched than starch. It is more readily hydrolysed than starch.
Cellulose: Is made up of many straight unbranched chains of beta glucose molecules that run parallel
to one another. Hydrogen bonds link these chains together which gives cellulose its strength which we
are familiar with when talking about cell walls.
Knowing the tests for the sugars and for starch has to be known for the exam:
Reducing sugars (includes glucose, fructose, galactose, maltose and lactose):
Add Benedict's reagent and heat

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Blue to brick red precipitate
Non-reducing sugar (sucrose):
Add HCl (hydrochloric acid) and boil
Add NaOH (sodium hydroxide) to neutralise
Add Benedict's reagent
Blue to brick red precipitate
Add Iodine Potassium solution
Blue-black colourisation…read more


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