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1: Biochemistry of macromolecules and metabolic pathways

Carbohydrates are a key group of biological molecules – about 10% of all the organic matter of a cell is made up of carbohydrates. This topic guide looks at their basic chemical structures and functions, as well as carbohydrates in the environment.

The three carbohydrates of interest that will be covered in this topic guide are starch, glycogen and cellulose. These are needed as an energy source, an energy store and as a major structural component in plants, respectively.

On successful completion of this topic you will: • understand the chemical principles that apply to the structures of

biological building block molecules (LO1) • understand the structures of biological macromolecules and the

relationships to biological functions (LO2).

To achieve a Pass in this unit you need to show that you can: • explain the principal properties and classification of monosaccharides,

aldoses and ketoses (1.2) • review the major features of storage and structural polysaccharides (2.3).

Carbohydrates1.5

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1.5: Carbohydrates

1 Monosaccharides Before you startIf you find some parts of this unit challenging, remember you are working at a higher level than you may be used to. In this unit it is important that you fully understand the following themes and topics before you begin:

• structure and function of biological molecules • enzyme structure and function • aerobic respiration.

If you need to check your understanding of proteins, carbohydrates, lipids and nucleic acids, Unit 2 Module 1 of OCR AS Biology (P. Kennedy and F. Sochacki, 2008), offers a good introduction to the topic.

If you need to check your understanding of aerobic respiration and the stages of glycolysis, link reaction, the Krebs cycle and the electron transport chain, you may find Unit 1 Module 4 of OCR A2 Biology (S. Hocking, 2008) useful.

A carbohydrate is an organic compound that contains carbon, hydrogen and oxygen. The simplest unit of a carbohydrate is a monosaccharide. If two monosaccharides join together because of a condensation reaction, it creates a disaccharide. A water molecule is removed and a new covalent bond is formed between the two monosaccharides called a glycosidic bond.

Monosaccharides are grouped depending on the number of carbon atoms they have:

• three carbon atoms present are classified as triose sugars • five carbon atoms present are classified as pentose sugars • six carbon atoms present are classified as hexose sugars.

Aldoses and ketosesMonosaccharides with one aldehyde group (–CH=O) (see Figure 1.5.1) per molecule are called aldose sugars. A common example is glucose. The chemical formula takes the form C

n(H

2O)

n.

Monosaccharides with one ketone group (see Figure 1.5.2) per molecule are known as ketose sugars. A common example is fructose found in fruits. The chemical formula takes the form C

n(H

2O)

n.

The structures of an aldose and a ketose are shown in Figure 1.5.3.

H O

C

C

H OH

CH OH

H

CH OH

C O

CH OH

H

H

An aldose A ketose

Key termsMonosaccharide: A single organic unit containing carbon, hydrogen and oxygen.

Disaccharide: Two single monosaccharide units bonded together by a glycosidic bond.

Glycosidic bond: A bond formed between two monosaccharides when a condensation reaction takes place to remove water.

Triose: Monosaccharide with three carbon atoms.

Pentose: Monosaccharide with five carbon atoms present.

Hexose: Monosaccharide with six carbon atoms present.

Aldehyde: An organic compound with the structure R–CHO consisting of a carbon double bond oxygen bonded to a hydrogen and an R group.

Aldose: Monosaccharide with an aldehyde carbonyl group.

Ketone: An organic compound with the structure RC(=O)R. R and R can be a variety of carbon-containing groups. There is a C=O bonded to two other carbon atoms.

Ketose: Monosaccharide with a ketone carbonyl group.

O

HR

CR R’

O

Figure 1.5.1: Chemical structure of an aldehyde group.

Figure 1.5.2: Chemical structure of a ketone group.

Figure 1.5.3: Structure of aldose and ketose sugar.

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1: Biochemistry of macromolecules and metabolic pathways

1.5: Carbohydrates

GlucoseGlucose (D-glucose, dextrose) is the main source of energy for living organisms. It is a hexose sugar with the formula C

6H

12O

6. Hexoses form cyclic sugars (see

Figure 1.5.4). There are two forms of cyclic sugars, α-glucose and ß-glucose, and when a ring structure forms, both form in different ways. In α-glucose the C

1 atom

has an –OH group below the ring and in ß-glucose the –OH group is above C1 (see

Figure 1.5.5).

HO

H

CH2OH

H

H

H

OHOH OH

OH

1

23

4

5

6

OHO

H

CH2OH

H

H

H

HOH OH

OH

1

23

4

5

6

Alpha-glucose Beta-glucose

Reducing sugarsGlucose, fructose and galactose are sometimes referred to as reducing sugars as they have the ability to reduce other compounds by losing electrons to other compounds. It is the presence of the aldehyde group in aldose sugars at the terminal end of the chain structure that enables reduction to occur – ketose sugars would therefore not behave in this way, except fructose because of the position of the ketone group.

A standard test for the presence of reducing sugars involves heating the sample with Benedict’s solution. The hydrogen (H) group of the aldehyde loses its electrons to copper II ions (Cu2+) in solution and reduces the copper, while the hydrogen is oxidised, becoming H+. The double bond breaks and the oxygen bonds to the copper, producing copper oxide.

Benedict’s solution can be used for medical purposes. The presence of a reducing sugar in urine is an indication of diabetes mellitus; further tests should be carried out in order to ascertain which sugar is present as it is only glucose that is indicative of diabetes.

Optical isomerismMonosaccharides can form optical isomers and can be mirror images of each other. A chiral carbon, a carbon that has four groups attached (see Figure 1.5.6), can identify an optically-active organic compound. The attached groups could be a single H (hydrogen) atom, a functional group or a chain of one or more other carbons.

Each carbon atom in a monosaccharide, except the first and the last that support a hydroxyl group, is chiral. This is important because a number of isomeric forms exist all with the same chemical formula. Galactose and glucose are both aldohexoses, but have different chemical and physical properties – look at Figure 1.5.7 and carefully examine their structures.

H

H

OH

C

C

H

H

H OH

C C

C C

OH H OH

OH

HO

1

23

4

5

6

Figure 1.5.5: Chemical structure of α-glucose and ß-glucose.

Figure 1.5.4: Chemical structure of a hexose sugar.

Chiral centre

Y C

X

Z

W

Figure 1.5.6: Structure of a chiral carbon.

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1.5: Carbohydrates

D-Glucose D-Galactose

C

C

C

C

C

C

C

C

C

C

C

C

O

O

O

O

O

OO

O

O

O

O

O

H

H

HH

H

H

H

HH

HH

H H

HH

HH

HH

H

HH H

H

1

2

3

4

5

6

1

2

3

4

5

6

Monosaccharides are grouped in the D-family according to the positioning of the –OH group. The –OH group should be to the right of the chiral carbon that is the furthest from the aldehyde group. For example, if the aldehyde is attached to the C

1 carbon atom, the carbon furthest away with an –OH group will be the C

5 carbon

atom. Both glucose and galactose are part of the D-family because their –OH groups are positioned to the right of C

5 carbon atom (see Figure 1.5.7).

2 PolysaccharidesStarchStarch is a mixture of two different compounds, amylose and amylopectin. Amylose is a polymer made of glucose monomers joined by α-1,4-glycosidic links that bond on a slight angle, creating a spiral-shaped molecule. This polymer is unbranched, whereas amylopectin has both an α-1,4-glycosidic link and an α-1,6-glycosidic link, and this creates branching. The basic structure of starch is shown in Figure 1.5.9.

Starch is an important storage chemical in plants; it is compact, insoluble and readily broken down into glucose to release energy. Amylase is a digestive enzyme found in saliva and it breaks starch into smaller molecules (see Figure 1.5.8).

Figure 1.5.7: Structural difference between D-glucose and D-galactose.

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1.5: Carbohydrates

OH HH

H OH

OH

OO

CH2OH

OH HH

H OH

OHOO

CH2OH

OH HH

H OH

OHO

CH2OH

OH HH

H OH

OHO

CH2

OH HH

H OH

OHO

CH2OH

Starch

OH OHH

H OH

OH

CH2OH

HHO

Glucose

Amylase

GlycogenGlycogen is structurally similar to amylopectin but with more branches. This is an advantage for this storage carbohydrate because it provides more ends for glucose molecules to be added to or removed from for efficient storage. The basic structure of glycogen is shown in Figure 1.5.9.

We store glycogen in the liver and the muscles so that glycogen is easily accessible when glucose is needed. Glycogen is easily broken down to glucose when the body’s glucose supply is depleted.

CelluloseCellulose is the most abundant structural polysaccharide in nature; it gives strength to plant cell walls. Cellulose chains are made from ß-glucose molecules arranged in many ß-1,4-glycosidic links and this produces unbranched chains. The cellulose molecules are arranged straight next to others and form hydrogen bonds along the complete length. This produces microfilaments bundled into microfibrils and, again, into macrofibrils. The basic structure of cellulose is shown in Figure 1.5.9.

Humans are unable to digest cellulose. However, it is necessary to stop problems like constipation as it provides bulk for peristalsis. Cellulose is a very strong, useful component that we also use to produce paper and cotton.

Figure 1.5.8: Enzymatic hydrolysis of starch.

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1.5: Carbohydrates

Starch

Cellulose

Glycogen

O O O O O

O

O

O O O

O O O O O

Further readingBoyle, M. & Senior, K. (2008) Biology, 3rd Edition, HarperCollins

Campbell, M.K. & Farrell, S.O. (2011) Biochemistry, Cengage Learning

Kennedy, P., Sochacki, F. & Hocking, S. (2008) OCR Biology AS, Heinemann (Pearson Education Limited)

Kennedy, P., Sochacki, F., Winterbottom, M. & Hocking, S. (2008) OCR Biology A2, Heinemann (Pearson Education Limited)

Moran, L., Horton, R., Scrimgeour, G., Perry, M. & Rawn, D. (2011) Principles of Biochemistry (International Edition), 5th Edition, Pearson

AcknowledgementsThe publisher would like to thank the following for their kind permission to reproduce their photographs:

Getty Images: Martin McCarthy / E+

All other images © Pearson Education

In some instances we have been unable to trace the owners of copyright material, and we would appreciate any information that would enable us to do so.

Figure 1.5.9: Basic structures of starch, cellulose and glycogen.

Take it furtherFind out about the polysaccharide chitin. 1 Draw the structure of chitin.2 Where is chitin found?3 Which other element is found in

chitin?4 Describe the formation of chitin.