Biological Molecules: Carbohydrates and Lipids

Carbohydrates and lipids are two groups of biological molecules built mainly from carbon, hydrogen and oxygen: carbohydrates (sugars, starch, glycogen, cellulose) supply energy and structure, and lipids (fats, oils, phospholipids) store energy and form membranes. In brief: glucose is a monosaccharide, starch and glycogen are energy-storage polysaccharides made of many glucose units, cellulose is a structural polysaccharide in plant cell walls, and a fat is made of one glycerol and three fatty acids.

These molecules make up a large share of food and of the living body. This guide explains monosaccharides, disaccharides and polysaccharides, compares starch, glycogen and cellulose, describes fatty acids and triglycerides, shows how food tests identify them, works through a structure example and corrects common mistakes.

What are biological molecules?

Biological molecules, or biomolecules, are the organic compounds made by living things. The four main groups are carbohydrates, lipids, proteins and nucleic acids. Many are large molecules (polymers) made by joining many small units (monomers) in condensation reactions, which release a molecule of water. They are broken down by hydrolysis, which uses water.

What are carbohydrates?

Carbohydrates contain carbon, hydrogen and oxygen, and the hydrogen and oxygen are usually in the ratio 2:1, as in water. The general formula is (CH₂O)ₙ.

Monosaccharides

Monosaccharides are single sugar units. Glucose (C₆H₁₂O₆) is the most important, as it is the main fuel for respiration. Fructose, found in fruit, and galactose, found in milk sugar, have the same formula but different structures. Glucose exists as two forms, alpha and beta, which differ in the position of one hydroxyl group on carbon 1. Monosaccharides are soluble, sweet-tasting and are reducing sugars.

Disaccharides

Two monosaccharides join by a condensation reaction to make a disaccharide. The bond is a glycosidic bond.

• Glucose + glucose → maltose (malt sugar).
• Glucose + fructose → sucrose (table sugar).
• Glucose + galactose → lactose (milk sugar).

Polysaccharides

Polysaccharides are long chains of monosaccharides. They are large, insoluble and not sweet. Insolubility makes them useful for storage, because they do not affect the water potential of cells.

How do starch, glycogen and cellulose differ?

Starch is the energy store of plants. It is a mixture of amylose, a long unbranched coiled chain of alpha-glucose, and amylopectin, a branched chain. Glycogen is the energy store of animals and fungi. It is similar to amylopectin, but is more highly branched, so enzymes can release glucose quickly from many ends. It is stored mainly in the liver and muscles.

Cellulose is a structural polysaccharide in plant cell walls. It is made of beta-glucose units, in which every second glucose is flipped. The chains are straight and lie side by side, held together by hydrogen bonds, forming strong fibers called microfibrils. Most animals cannot digest cellulose, because they lack the enzyme that breaks beta bonds, so it acts as dietary fiber.

What are lipids?

Lipids are a varied group of molecules that are insoluble in water but soluble in organic solvents such as ethanol. They contain carbon, hydrogen and oxygen, with much less oxygen than carbohydrates, which makes them richer in energy.

Triglycerides (fats and oils)

A triglyceride has one molecule of glycerol joined to three fatty acids by ester bonds, formed by condensation reactions that release three molecules of water. A fatty acid has a hydrocarbon chain with a carboxyl group (–COOH) at one end.

• Saturated fatty acids have no double bonds between carbon atoms. The chains are straight and pack closely, so saturated fats such as butter are usually solid at room temperature.
• Unsaturated fatty acids have one or more double bonds. The chains are kinked, so they pack less tightly, and the fats, such as olive oil, are liquid at room temperature.

Triglycerides store energy, giving about twice as much energy per gram as carbohydrates. They also insulate and protect organs, and fat is a source of metabolic water, as in camels’ humps.

Phospholipids

A phospholipid has glycerol joined to two fatty acids and a phosphate group. The head is hydrophilic and the tails are hydrophobic. In water, they form bilayers, which is the basis of cell membranes. Other lipids include steroids, such as cholesterol and sex hormones, and waxes.

Carbohydrates and lipids compared

How can you test for them?

• Starch: add iodine solution. A blue-black color shows starch.
• Reducing sugars (glucose, maltose): heat with Benedict’s solution. The color changes from blue through green, yellow and orange to brick red as the amount of sugar increases. Sucrose is not a reducing sugar. It must first be hydrolyzed with dilute acid and then neutralized.
• Lipids (emulsion test): dissolve the sample in ethanol and pour the solution into water. A cloudy white emulsion forms if lipid is present.

Worked example: building a disaccharide and a triglyceride

Use atom counts to check the condensation reactions.

1. Maltose: two glucose molecules have a total of 2 × C₆H₁₂O₆ = C₁₂H₂₄O₁₂. One water molecule (H₂O) is removed in the condensation reaction, leaving C₁₂H₂₂O₁₁. This is the formula of maltose.
2. Triglyceride: glycerol is C₃H₈O₃. Three molecules of palmitic acid (C₁₆H₃₂O₂) add up to C₄₈H₉₆O₆. The total is C₅₁H₁₀₄O₉. Three molecules of water (H₆O₃) are removed, giving C₅₁H₉₈O₆. This is the formula of tripalmitin, a triglyceride.
3. Check the number of water molecules: a disaccharide forms one glycosidic bond, so one water is released. A triglyceride forms three ester bonds, so three are released. In general, joining n units in a chain releases n − 1 water molecules.

The calculation confirms that condensation reactions conserve atoms, with water as the only by-product. Reversing the process, hydrolysis adds the water back, which is how digestive enzymes break food down.

How were these molecules discovered?

The structure of fats was described by the French chemist Michel Eugène Chevreul in the early nineteenth century, who showed that fats are made of glycerol and fatty acids. Emil Fischer worked out the structures of many sugars in the 1880s and 1890s and received the Nobel Prize in Chemistry in 1902. Cellulose was first isolated by Anselme Payen in 1838.

Which mistakes are common?

Mistake 1: Saying that all carbohydrates are sugars

Starch, glycogen and cellulose are carbohydrates but are not sugars. They are insoluble polysaccharides.

Mistake 2: Mixing up the structure of starch and cellulose

Starch uses alpha-glucose and is coiled or branched. Cellulose uses beta-glucose and forms straight chains.

Mistake 3: Saying that fats are polymers like starch

Triglycerides are not polymers of repeating units. They are made of a few smaller molecules.

Mistake 4: Forgetting that sucrose does not give a positive Benedict’s test

It is a non-reducing sugar, so a negative result does not prove the absence of sugar.