Carbohydrates

What are carbohydrates?

Starter:Write down as many things as you can remember about carbohydrates…

… or glucose in particular

What are they?

• Organic compounds which comprise of only C, H and O

• Carbo-Hydr-ate

Types of Carbohydrate

• Monosaccharides

• Disaccharides

• Polysaccharides

Monosaccharides

• Simplest – ‘Single sugars’• Same no. of C as O atoms

e.g. Glucose is C6H12O6

• Have the general formula (CH2O)n

Where n is any number between 3 and 7

• White crystalline solids• Dissolve in water to form sweet tasting

solutions

The structure of glucose

• α-glucose

You only need to be able to draw α-glucose in its simplified form….

• β-glucose

The structure of glucose

• α-glucose and β-glucose are ISOMERS

Monosaccharides form Disaccharides

+

H2O

Maltose

• The bond formed is a glycosidic bond

• A condensation reaction occurs to join the 2 monosaccharides

Monosaccharides form Disaccharides

• α 1-4 glycosidic bonds

Disaccharides hydrolyse to form Monosaccharides

α-glucose α-glucose

H2O

Disaccharides form Polysaccharides

Starch

Starch• The α-helical structure of starch makes it good

for storage (it’s compact!)• Starch is insoluble due to its structure• It is therefore the main plant storage sugar

Starch

Made of two alpha glucose polymerAmyloseSpiral structure with 1,4 glycosidic bondAmylopectinBranched structure with 1,4 and some 1,6 glycosidic bond

Glycogen

Insoluble compact store of glucose in animalAlpha glucose with 1,4 and 1,6glycosidic bondBranched structure

Cellulose• Polymer of β-glucose• Each monomer is

inverted.• Has consequences for

its properties

Cellulose• Forms chains which run parallel with hydrogen

bonds between the chains to form microfibrils

• Microfibrils are strong• Being fibrous, cellulose is structurally

important in plant cell walls

Glucose is not the only monosaccharide...

• Fructose:

Glucose and fructosecombine to form sucrose (a disaccharide)

Glucose is not the only monosaccharide...

• Galactose

Galactose Glucose

Glucose and galactose combine to form Lactose

Summary

• α-glucose + α-glucose = maltose• The polymer of α-glucose is starch• The polymer of β-glucose is cellulose• Glucose + fructose = sucrose• Glucose + galactose = lactose

Carbohydrates Questionsanswer in full sentences please

1. What is the function of the following carbohydrates in living things: glycogen, starch, cellulose, sucrose?

2. What are the 2 structural isomers of glucose called? Describe in words why they are different. Draw the 2 structures.

Biochemical Test for a Monosaccharide

• All monosaccharides and some disaccharides are Reducing Sugars

i.e. They readily reduce other chemicals when in solution

• We test for reducing sugars using the

Benedict’s Test

Benedicts Test

• 2cm³ of food sample (in liquid form)• 2cm³ of Benedict’s Reagent • Heat mixture in gentle boiling water bath for

5minsConc. of reducing sugar Colour of solution &

precipitate

None Stays blue

Very Low Green

Low Yellow

Medium Brown/orange

High Brick red

• In order to detect a reducing sugar, it must first be ___________into its monosaccharide components:– First test with Benedict’s reagent– If no colour change:• Add 2cm³ of food sample to 2cm³ of dilute HCl• Place in gently boiling water bath for 5 mins• Slowly add some sodium hydrogen carbonate solution

until neutralized • Re-test using Benedict’s reagent

hydrolysed

If I am the Answer, what is the Question?

Monosaccharide

Disaccharide

Polysaccharide

Glycosidic bond

Condensation reaction

Hydrolysis reaction

Isomer

Carbohydrate

(CH2O) n

Benedict’s Test

Maltose

Starch