Carbohydrates

AQA spec ref: 3.1.1 - 3.1.2 - Monomers and polymers; Carbohydrates

Carbohydrates are molecules containing carbon, hydrogen, and oxygen, with the general formula Cₙ(H₂O)ₙ. They serve as the primary energy source in cells, as energy stores, and as structural materials. Understanding their structure explains their function - a key principle AQA returns to repeatedly.

Monomers and Polymers

A monomer is a small, soluble molecule that can be joined to others of the same type to form a polymer. A polymer is a large molecule made of many repeating monomers. The monomer of carbohydrates is a monosaccharide.

Monomers are joined by condensation reactions: two monomers are joined with the simultaneous removal of a water molecule. The bond formed is called a glycosidic bond (in carbohydrates).

Polymers are broken down by hydrolysis reactions: a water molecule is added across a glycosidic bond, breaking it and releasing the monomers. This is the reverse of condensation.

These two reactions - condensation and hydrolysis - apply across all biological polymers: proteins (peptide bonds), nucleic acids (phosphodiester bonds), and lipids (ester bonds).

Monosaccharides

Monosaccharides are the simplest carbohydrates - single sugar units. They are:

• Sweet-tasting, crystalline solids
• Highly soluble in water
• Immediately available for use in respiration

The most important monosaccharide in AQA biology is glucose (C₆H₁₂O₆). Two isomers exist:

α-glucose: the -OH group on carbon 1 (C-1) points downward (below the plane of the ring). The ring structure (Haworth projection) is a hexagon with the formula:

• C-1: has -OH below and -H above
• C-6: has -CH₂OH group above the ring

β-glucose: the -OH group on C-1 points upward (above the plane of the ring). This single difference has enormous consequences - α and β glucose form completely different polymers with completely different properties.

Other monosaccharides:

• Fructose (C₆H₁₂O₆) - in fruit; combined with glucose to form sucrose
• Galactose (C₆H₁₂O₆) - combined with glucose to form lactose
• Ribose (C₅H₁₀O₅) - pentose sugar; component of RNA and ATP
• Deoxyribose (C₅H₁₀O₄) - component of DNA (one fewer -OH than ribose)

Disaccharides

When two monosaccharides are joined by a condensation reaction, a disaccharide is formed (and one water molecule is released). The bond formed is a glycosidic bond.

The "1,4" or "1,2" notation tells you which carbon atoms of each sugar are joined. In a 1,4 bond, C-1 of one sugar is bonded to C-4 of the next.

Disaccharides can be broken back to their monomers by hydrolysis with the appropriate enzyme (e.g. sucrase breaks sucrose; lactase breaks lactose).

Tests for Reducing Sugars and Non-Reducing Sugars

Benedict's test - detects reducing sugars (all monosaccharides + maltose + lactose):

1. Add Benedict's reagent (blue) to the sample
2. Heat in a water bath (~80°C) for 5 minutes
3. A brick-red / orange precipitate forms if a reducing sugar is present
4. The colour intensity can indicate approximate concentration: blue (negative) → green → yellow → orange → brick red (highest concentration)

Non-reducing sugars (e.g. sucrose) give a negative Benedict's test. To test for them:

1. First check with Benedict's - should be negative (blue)
2. Hydrolyse the sugar by boiling with dilute HCl (acid hydrolysis), then neutralise with NaHCO₃
3. Repeat Benedict's test - if now positive (brick red), a non-reducing sugar was present

This works because hydrolysis converts the disaccharide to its constituent monosaccharides, which are reducing sugars.

Polysaccharides

Polysaccharides are polymers of many monosaccharide monomers joined by glycosidic bonds. They are insoluble (important for storage - does not affect osmosis) and compact. The three main polysaccharides in AQA are starch, glycogen, and cellulose.

Starch - Energy Store in Plants

Starch is the main carbohydrate storage molecule in plants (in seeds, tubers, leaves). It is a mixture of two polymers:

Amylose - an unbranched chain of α-glucose monomers joined by α-1,4 glycosidic bonds. The chain coils into a helix due to the α-1,4 bonding angle. About 20 - 30% of starch.

Amylopectin - a branched chain of α-glucose monomers. The main chain uses α-1,4 glycosidic bonds; branches form at α-1,6 glycosidic bonds approximately every 24 - 30 glucose units. The branching increases the number of terminal ends, allowing faster enzymatic hydrolysis (more points for amylase to act on). About 70 - 80% of starch.

Why starch is a good storage molecule:

• Insoluble→no osmotic effect
• Compact (coiled helix)→stores a lot of glucose in a small space
• Can be hydrolysed quickly by amylase when glucose is needed
• Chemically inert→doesn't react with cell components

Glycogen - Energy Store in Animals

Glycogen is the equivalent of starch in animals and fungi. It is made of α-glucose monomers joined by α-1,4 glycosidic bonds (main chain) and α-1,6 glycosidic bonds (branches). It is more highly branched than amylopectin (branches every 8 - 12 glucose units), which means more free ends for rapid hydrolysis.

Why glycogen is suited to animal metabolism:

• Highly branched → many free ends → can be rapidly broken down by glycogen phosphorylase to release glucose during high-energy demand
• Insoluble→compact storage without osmotic effect
• Stored in liver (maintains blood glucose) and muscle cells (for local use)

Cellulose - Structural Material in Plants

Cellulose is made of β-glucose monomers joined by β-1,4 glycosidic bonds. The key structural consequence of the β-1,4 bond: each alternate glucose is rotated 180° relative to the previous one. This means the chains are straight and can lie parallel to each other.

Parallel chains are cross-linked by hydrogen bonds between -OH groups on adjacent chains. Many chains pack together to form microfibrils. Many microfibrils bundle together to form macrofibrils. These bundles give cellulose its enormous tensile strength.

Why the β configuration matters: α-1,4 bonds produce a helix (as in starch). β-1,4 bonds (with rotation) produce straight chains that can hydrogen-bond in bundles → structural rigidity.

Why animals cannot digest cellulose: humans lack cellulase - the enzyme to break β-1,4 glycosidic bonds. Ruminants (cows, sheep) have bacteria in their gut that produce cellulase. Cellulose forms dietary fibre in humans - it adds bulk to the gut and aids peristalsis.

Functions of the cell wall:

• Prevents osmotic lysis by resisting expansion when water enters
• Provides mechanical support to plant tissues
• Is fully permeable to water and small ions (unlike the cell membrane)

Iodine Test for Starch

Add iodine solution (I₂/KI) to the sample:

• Positive: blue-black colour - iodine molecules slot into the helical coils of amylose
• Negative: orange-brown (no starch)

This test is specific for starch (particularly amylose). It does not detect glycogen or cellulose.

Summary

• Condensation: monomers joined, water removed, glycosidic bond formed
• Hydrolysis: water added, glycosidic bond broken, monomers released
• α-glucose: -OH on C-1 points down. β-glucose: -OH on C-1 points up
• Disaccharides: maltose (α-glu+α-glu), sucrose (α-glu+fructose), lactose (glu+galactose)
• Starch: α-1,4 (amylose, unbranched helix) + α-1,4/1,6 (amylopectin, branched). Plant energy store. Insoluble, compact.
• Glycogen: α-1,4/1,6 glycosidic bonds. Highly branched. Animal/fungal energy store. Many free ends → rapid hydrolysis.
• Cellulose: β-1,4 glycosidic bonds. Alternating orientation → straight chains → H-bonds between chains → microfibrils → tensile strength. Plant cell wall.
• Benedict's test: reducing sugars → brick red; non-reducing → hydrolyse first, then test
• Iodine test: starch→blue-black

AQA Exam Tips

• α vs β glucose: the difference is only the orientation of the -OH on C-1. This one change causes amylose to form a helix (coils around itself) vs cellulose to form straight chains. AQA will give you a Haworth projection and ask you to identify α or β.
• Why starch is insoluble: polymers of glucose are too large to dissolve in water; also, the many -OH groups are tied up in H-bonds within the molecule. Insolubility is why starch doesn't affect osmosis - important in cells.
• Number of H-bonds in cellulose: many -OH groups on β-glucose chains form H-bonds with adjacent chains. These individually weak bonds are numerous → collectively very strong. This is why cellulose microfibrils have high tensile strength.
• Benedict's test - quantitative: the intensity of the red/orange colour is proportional to the concentration of reducing sugar. A colorimeter can be used for more precise quantification (measure absorbance at ~540 nm).
• Glycogen vs starch: both are α-glucose polymers with α-1,4 and α-1,6 bonds, but glycogen is more highly branched. More branches = more free ends = faster enzymatic release of glucose. This is why glycogen suits the rapid energy demands of animals.
• Non-reducing sugar test: must state: 1. Initial negative Benedict's (no reducing sugars). 2. Acid hydrolysis (boil with HCl). 3. Neutralise with NaHCO₃ (Benedict's only works in alkaline conditions). 4. Positive Benedict's → non-reducing sugar present.