Carbohydrates and lipids

1. Overview

Carbohydrates and lipids are both built from small units and shaped to fit a biological job.

Carbohydrates are polymers. Monosaccharides such as alpha- and beta-glucose link through glycosidic bonds (made by condensation, broken by hydrolysis) to form the polysaccharides starch, glycogen and cellulose. Their different structures suit them to energy storage (starch, glycogen) or structural support (cellulose).

Lipids are not polymers. A triglyceride forms when glycerol joins three fatty acids by ester bonds, giving a non-polar, hydrophobic energy store. A phospholipid has one fatty acid replaced by a phosphate-containing head, which lets it build the bilayer of cell membranes.

In every case the shape and chemistry of the molecule explain what it does, from compact storage to membrane formation.

Key Definitions

• Polysaccharide: a large polymer made of many monosaccharide units joined by glycosidic bonds.
• Glycosidic bond: the covalent bond formed between two monosaccharides in a condensation reaction, with the loss of a water molecule.
• Hydrolysis: the breaking of a bond, such as a glycosidic or ester bond, by the addition of a water molecule.
• Reducing sugar: a sugar with a free (reactive) aldehyde or ketone group that acts as a reducing agent and gives a positive result with Benedict's reagent.
• Non-reducing sugar: a sugar with no free aldehyde or ketone group, so it must first be hydrolysed before it gives a positive Benedict's test.
• Triglyceride: a lipid formed from one molecule of glycerol joined to three fatty acids by ester bonds.
• Ester bond: the covalent bond formed between the carboxyl group of a fatty acid and a hydroxyl group of glycerol in a condensation reaction.
• Phospholipid: a lipid with a hydrophilic phosphate-containing head and two hydrophobic fatty acid tails.
• Amphipathic: describing a molecule, such as a phospholipid, that has both a hydrophilic (water-loving) region and a hydrophobic (water-hating) region.

Content

Breaking glycosidic bonds by hydrolysis

A glycosidic bond is made when two monosaccharides join in a condensation reaction, releasing one water molecule. The reverse process, hydrolysis, breaks the bond by adding a water molecule across it: the bond splits and the components of water (an H and an OH) attach to the two ends.

Hydrolysis releases the individual sugar units from disaccharides and polysaccharides. In cells it is catalysed by specific enzymes (for example, amylase hydrolyses starch). In the laboratory, boiling with dilute acid achieves the same result.

Reducing and non-reducing sugar tests

A reducing sugar has a free (reactive) aldehyde or ketone group that acts as a reducing agent. When heated with Benedict's reagent (blue copper(II) ions), this group reduces the copper, producing a brick-red precipitate of copper(I) oxide. The colour change runs through green, yellow and orange to red as the amount of reducing sugar increases. All monosaccharides (such as glucose) and some disaccharides (such as maltose) are reducing sugars.

A non-reducing sugar such as sucrose has no free aldehyde or ketone group, because the reactive groups are tied up in the glycosidic bond, so on its own it gives a negative (stays blue) Benedict's test. To test for it, follow three steps in order:

1. Hydrolyse the sample by boiling with dilute hydrochloric acid, breaking the glycosidic bond to release reducing monosaccharides.
2. Neutralise the mixture (usually with sodium hydrogencarbonate), because Benedict's reagent only works in alkaline conditions.
3. Repeat the Benedict's test. A colour change now shows that a non-reducing sugar was present.

Starch: amylose and amylopectin

Starch is the storage carbohydrate of plants and is a mixture of two polymers of alpha-glucose:

Feature	Amylose	Amylopectin
Branching	Unbranched	Branched
Glycosidic bonds	1,4 only	1,4 along chains, 1,6 at branch points
Shape	Coils into a compact helix held by hydrogen bonds	Bushy, with many branch ends
Consequence	Space-efficient store	Many ends let enzymes release glucose quickly

Starch is well suited to storage because it is compact, insoluble (so it does not affect the water potential of the cell or move out by osmosis), and too large to diffuse out of the cell. When energy is needed, hydrolysis releases glucose for respiration.

Glycogen

Glycogen is the storage carbohydrate of animals and fungi, stored mainly in liver and muscle cells. It is also a polymer of alpha-glucose with 1,4 and 1,6 glycosidic bonds, but it is more highly branched than amylopectin.

The greater number of branches gives many more free ends where enzymes can act, so glucose can be hydrolysed and released rapidly to meet the high metabolic demands of active animals. Like starch, glycogen is compact and insoluble, so it is a good store that does not disturb water potential.

Cellulose and the plant cell wall

Cellulose is a structural polysaccharide and a polymer of beta-glucose joined by 1,4 glycosidic bonds. Because adjacent beta-glucose molecules are alternately inverted (rotated 180 degrees) so that the bond can form, cellulose chains are straight and unbranched rather than coiled.

These straight chains lie parallel and are held together by many hydrogen bonds between the chains. Each hydrogen bond on its own is weak, but because there are so many of them between adjacent chains, their combined effect is strong; it is the sheer number of hydrogen bonds, not any single strong bond, that provides the strength. Large numbers of cross-linked chains group into microfibrils, which bundle into stronger macrofibrils (fibres). This arrangement gives the chains high tensile strength (resistance to being pulled apart).

In the plant cell wall, macrofibrils are laid down in layers running in different directions, embedded in a matrix, producing a strong mesh. Two key points about the wall:

• It is fully (freely) permeable to water and dissolved solutes, so it does not control what enters the cell.
• Its role is mechanical: the strong cellulose fibres resist the outward turgor pressure when the cell takes up water and becomes turgid, preventing the cell from bursting and providing support to the plant.

Triglycerides: structure

Triglycerides are the main storage lipids. Each is formed from one molecule of glycerol (a 3-carbon alcohol with three hydroxyl groups) joined to three fatty acids. A condensation reaction between the carboxyl group of each fatty acid and a hydroxyl group of glycerol forms an ester bond, releasing three water molecules in total.

A fatty acid has a carboxyl group attached to a long hydrocarbon tail. The tails come in two types:

Type	Carbon-to-carbon double bonds	Shape of tail	State at room temperature
Saturated	None	Straight, pack closely	Usually solid (fats)
Unsaturated	One or more	Kinked, cannot pack tightly	Usually liquid (oils)

Triglycerides are non-polar, hydrophobic molecules because the long hydrocarbon tails do not interact with water.

Triglycerides: functions

The structure of triglycerides suits several roles:

• Energy storage: the hydrocarbon tails are highly reduced (rich in C-H bonds), so triglycerides release about twice as much energy per gram as carbohydrates when respired, making them an energy-dense store.
• Compact, anhydrous storage: being insoluble and hydrophobic, they store more energy per unit mass than hydrated glycogen and do not affect water potential.
• Thermal insulation: fat stored under the skin reduces heat loss.
• Protection: fat around organs such as the kidneys acts as a cushion.
• Metabolic water: respiring stored fat releases water, useful for animals in dry habitats.

Phospholipids

A phospholipid is similar to a triglyceride but with one fatty acid replaced by a phosphate-containing group. This gives the molecule two contrasting parts:

• A hydrophilic (polar) phosphate head that is attracted to water.
• Two hydrophobic (non-polar) fatty acid tails that are repelled by water.

A molecule with both a water-loving and a water-hating end is amphipathic. In water, phospholipids automatically arrange so the heads face the water and the tails point away from it, forming a bilayer. This behaviour is the basis of the cell surface membrane, where two layers of phospholipids form a stable barrier with the hydrophobic tails in the middle.

Worked example

Exam-style question: A student tests a solution and finds it gives a negative result with Benedict's reagent. After boiling a fresh sample with dilute hydrochloric acid, neutralising it, and repeating the Benedict's test, a brick-red precipitate forms. Identify the type of sugar present and explain these results. [3]

Model answer:

• The solution contains a non-reducing sugar (such as sucrose).
• It gave a negative first test because it has no free aldehyde or ketone group to reduce the copper(II) ions in Benedict's reagent.
• Boiling with acid hydrolysed the glycosidic bond, releasing reducing monosaccharides, which then reduced the Benedict's reagent to give the brick-red precipitate (the acid was neutralised first because Benedict's test needs alkaline conditions).

Worked example

Exam-style question: A short polysaccharide is built by joining 120 alpha-glucose monomers into a single unbranched chain. The chain is then completely hydrolysed back to individual glucose units. (a) State how many water molecules are released when the chain is formed. (b) State how many water molecules are used when the chain is fully hydrolysed. (c) Explain why glycogen, which is highly branched, can release glucose faster than this unbranched chain. [4]

Model answer:

• (a) Each glycosidic bond is made by a condensation reaction that releases one water molecule. A chain of $n$ units has $n-1$ bonds, so $120 - 1 = \textbf{119}$ water molecules are released.
• (b) Hydrolysis is the reverse: one water molecule is added to break each bond, so 119 water molecules are used.
• (c) Glycogen has many branch points (1,6 glycosidic bonds), giving many free ends where enzymes can act at the same time, so glucose is hydrolysed and released rapidly; an unbranched chain has only one or two ends, so release is slower.

Key Equations

This topic is mainly qualitative and structural, so there are no equations to learn. Remember the stoichiometry instead:

• Forming one triglyceride releases three water molecules (one per ester bond).
• For a polysaccharide of $n$ glucose units joined in one chain, the number of glycosidic bonds, and so the number of water molecules released on formation, is:

$$\text{water molecules released} = n - 1$$

The same number of water molecules is used to fully hydrolyse the chain.

Common Mistakes to Avoid

• Describing starch or glycogen as affecting the "concentration of water" in a cell. Use the correct term water potential: these stores are insoluble, so they do not lower the cell's water potential or cause water to move in by osmosis.
• Calling the cellulose cell wall "slightly permeable" or "partially permeable". The wall is fully (freely) permeable to water and solutes; it does not control entry. Its job is to provide tensile strength that resists the outward turgor pressure.
• Defining a reducing sugar vaguely. Say specifically that it has a free aldehyde or ketone group that acts as a reducing agent; a non-reducing sugar lacks this free group because it is locked in the glycosidic bond.
• Mixing up alpha- and beta-glucose. Starch and glycogen are polymers of alpha-glucose (coiled/branched); cellulose is a polymer of beta-glucose (straight chains with alternate units inverted).
• Saying cellulose chains coil like amylose. Cellulose chains are straight because of the beta-1,4 bonds, which lets them line up and hydrogen-bond into strong microfibrils.
• Writing that triglycerides or fats are polymers. Lipids are not polymers and have no single repeating monomer; a triglyceride is just glycerol plus three fatty acids.
• Confusing ester bonds with glycosidic bonds. Ester bonds join fatty acids to glycerol (lipids); glycosidic bonds join sugars (carbohydrates).
• Forgetting why phospholipids form a bilayer. State that they are amphipathic (hydrophilic head, hydrophobic tails), so in water the heads face outward and the tails face inward.

Exam Tips

• In the non-reducing sugar test, always include all three steps in order: hydrolyse with acid, neutralise, then repeat Benedict's (drop any of these and you lose marks).
• Quote the colour change for a positive Benedict's test (blue to brick-red, through green, yellow and orange for increasing amounts) rather than just "it turns red".
• Watch the command word: describe the structure of starch, glycogen or cellulose means state the monomer, the bond types (1,4 / 1,6) and the shape only; explain or relate structure to function also demands the consequence (such as compact/insoluble/many ends, leading to a faster rate), so add that linking clause or you lose the "explain" marks.
• When asked to relate structure to function, write linked sentences: name the structural feature, then state the consequence (for example, "glycogen is highly branched, so it has many ends for rapid hydrolysis of glucose").
• Be precise about bonds: 1,4 along chains and 1,6 at branch points; ester bonds in lipids; glycosidic bonds in carbohydrates.
• For energy questions, say triglycerides release about twice the energy per gram of carbohydrate because the tails contain many C-H bonds.
• Use hydrophilic/hydrophobic and polar/non-polar correctly when describing phospholipids, and link these properties to the formation of a bilayer in membranes.