Carbohydrate

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January 1999

B io Factsheet
Example glucose fructose galactose

Number 39

Carbohydrates: Revision Summary
Carbohydrates contain 3 elements: Carbon (C), Hydrogen (H) and Oxygen (O). Thus, if we remove water from carbohydrates, all that remains is carbon. Carbohydrates can be divided into 3 categories; monosaccharides, disaccharides and polysaccharides (Table 1). Table 1. Structure of monosaccharides, disaccharides and polysaccharides Category Monosaccharide (made of 1 sugar molecule)

Site
fruit fruit, nectar milk
H

Structure
OH

OH

H

α-glucose Disaccharide (made of 2 monosaccharides joined together) Polysaccharide (made of many monosaccharides joined together) maltose = α-glucose + α-glucose sucrose = glucose + fructose lactose = glucose + galactose germinating seeds phloem tissue, fruit milk

β-glucose

fructose

O

maltose starch glycogen cellulose chitin = = = = polymer of glucose polymer of α-glucose polymer of β-glucose polymer of glucosamine (glucose with an amino acid attached) chloroplast stroma muscle cells plant cell wall exoskelteton of arthropods O O

O O

cellulose

Monosaccharides and Disaccharides
Monosaccharides and disaccharides are sugars. They all have the basic formula (CH2O)n and can be classified according to how many carbon atoms they contain. 3C = triose sugars e.g. glyceraldehyde C3H5O2 5C = pentose sugars e.g. ribose C5H10O5 6C = hexose sugars e.g. glucose C6H12O6

Formation of disaccharides - typical exam questions
Common exam questions include: 1. Name the reaction involved when a disaccharide is formed 2. Name the type of bond formed 3. Show, by drawing a diagram, how a disaccharide is formed Questions 1 and 2 are very simple - Disaccharides form in a condensation reaction which forms a glycosidic bond. The only way to get Question 3 correct is to practice! Fig 1 shows how maltose and sucrose are formed from their monosaccharides.

Fig 1. Formation of maltose and sucrose from their monosaccharides CH2OH O H H H CH2OH O H HO CH2OH O O H HO HOH2C H

α-glucose

+
HO HO HO H H CH2OH O H H H H OH HO H OH

α-glucose

H

glucose
HO H OH

+
HO H HO H HO HO CH2OH H

fructose

OH

OH

condensation reaction

CH2OH O H H

maltose
H HO HO O α1-4 glycosidic bond OH H OH

+ H2O

sucrose
HO H O HO

OH

1

H

H CH2OH

H

H

OH

condensation reaction
O H HOH2C O

H

+ H2O
HO CH 2OH

OH

H

Carbohydrates
Polysaccharides

Bio Factsheet

Polysaccharides are polymers i.e. they are made up of many repeating units. Three polysaccharides which commonly feature in exam questions are starch, cellulose and glycogen. By far the most common question asks "How is the structure of polysaccharides related to their function?". It should be noted that all 3 have the advantage that they are insoluble in water and therefore have no osmotic effect i.e. effect on water potential and are unable to diffuse out of the cell. More specific features are summarised in Table 2. Table 2. Structure:Function of polysaccharides Polysaccharide Structure Structure:Function Insoluble in water, therefore good storage compound e.g. in stroma of chloroplasts The helix forms a compact shape which allows tight packing and is therefore an excellent storage molecule.

Starch – Made of two polymers of α-glucose; amylose and amylopectin Main storage polysaccharide amylose – a chain of glucose molecules joined by α-1,4-glycosidic bonds which, in plants. by hydrogen bonding, form a helix. It is this helix which holds and forms a complex with iodine when we test for starch CH2OH O H H H O HO HO H HO H H O H HO H OH H OH O CH2OH CH2OH O O H H H H H H H OH O HO H O H HO H HO H O H HO HO H OH H O H HO H OH HO H HO H O H O HO H HO H OH H H H OH O

amylopectin – glucose molecules joined by α-1,4-glycosidic bonds but after every 25 glucose molecules adjacent chains are connected by α-1,6-glycosidic bonds i.e. amylopectin is branched. CH2OH CH2OH O O H...
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