Carbohydrates

Monosaccharides

monomers from which larger carbohydrates are made

Monosaccharide Examples

-glucose

-galactose

-fructose

Glycosidic bonds form when -

a condensation reaction occurs between two monosaccharides

Disaccharides

formed by condensation of two monosaccharides

maltose

condensation of two glucose molecules

sucrose

condensation of a glucose & fructose molecule

lactose

condensation of glucose & galactose molecule

a-glucose (isomer of glucose)

b-glucose (isomer of glucose)

polysaccharides →

formed by condensation of many glucose units

condensation of a-glucose →

glycogen & starch

condensation of b-glucose →

cellulose

monosaccharide attributes

-sweet tasting

-soluble

-(CH₂O)_n

glucose use -

main substrate for respiration

Glycogen structure -

many molecules of alpha glucose joined together by 1,4 & 1,6 glycosidic bonds

glycogen structure - large no. of side branches

energy can be released quickly → enzymes can act simultaneously on branches

glycogen structure - large but compact molecule

maximises amount of energy it can store

glycogen structure - insoluble

doesn’t affect water potential of cells & cannot diffuse out of cells

Starch →

-stores energy in plants

-mixture of 2 polysaccharides (amylose & amylopectin)

Amylose

unbranched chain of glucose molecules joined by 1,4 glycosidic bonds →coiled structure, compact molecule →stores a lot of energy

Amylopectin

branched & made up of glucose molecules joined by 1,4 and 1,6 glycosidic bonds →side branches = acted upon simultaneosly by enzymes →broken down to release energy

Starch Properties - insoluble

doesn’t affect cell water potential

Starch Properties - compact

a lot of energy can be stored in a small space & when hydrolysed released alpha glucose can be transported easily

Cellulose - component of cell walls

composed of long & unbranched chains of b-glucose, joined by glycosidic bonds

Microfibrils

strong threads made of long cellulose chains running parallel to eachother →joined together by hydrogen bonds forming strong cross linkages

Cellulose Function

stops cell wall bursting under osmotic pressure →exerts inward pressure that stops influx of water →cell stays turgid & rigid →maximises surface area of plants for photosynthesis

Benedicts Reagant

test for reducing sugars →sugars that can donate an electron to Benedicts

Reducing Sugars =

all monosaccharides & some disaccharides (maltose)

Benedicts reagant =

alkaline solution of Copper (II) Sulfate

Benedicts Reagant Reaction

addition of a reducing sugar + heat →insoluble red precipitate (copper (I) oxide)

Benedicts Test

1. add 2cm³ of food sample to be tested (liquid)

2. add 2cm³ of Benedict’s Reagant

3. heat mixture gently in water bath for 5 minutes

4. brick red = positive

non-reducing sugars

polysaccharides & some disaccharides

Non-reducing Sugar test

1. 2cm³ of food sample (liquid) & 2cm³ of Benedicts placed in hot water bath for 5 minutes

2. No colour change (blue →brick red) leads to

3. add 2cm³ of same food sample & 2cm³ of dilute HCl

4. place test tube in water bath for 5 minutes (dilute HCl will hydrolyse disaccharides + polysaccharides to their constitutent monosaccharides)

5. add sodium hydrogencarbonate to neutralise test tube (benedicts X work in acidic conditions) & use pH paper to check solution is neutral

6. retest solution by adding 2cm³ Benedict’s Reagant and placing in water bath for 5 minutes

7. positive = blue benedicts →brick red

Starch Test

iodine/potassium iodide = orange/brown →blue/black