Biological molecules Revision Notes

Biological molecules such as carbohydrates, fats, and proteins are fundamental to life. Each type of molecule is composed of specific chemical elements arranged in characteristic ways. Understanding which elements constitute these molecules helps explain their structure and function in living organisms.

The primary elements involved are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sometimes sulfur (S). These elements combine in different ratios and structures to form carbohydrates, fats, and proteins.

• Biological molecules are made of specific chemical elements.
• Carbon, hydrogen, and oxygen are common to all three types.
• Nitrogen and sulfur are unique to proteins.

Carbohydrates are organic molecules primarily composed of carbon, hydrogen, and oxygen. The atoms are usually found in a ratio close to 1:2:1 (C:H:O). For example, glucose, a simple sugar, has the formula C₆H₁₂O₆, which reflects this ratio.

This elemental composition allows carbohydrates to serve as energy sources and structural components in cells. The presence of oxygen and hydrogen in a 2:1 ratio is a defining feature of carbohydrates, distinguishing them from other biomolecules.

• Carbohydrates contain carbon, hydrogen, and oxygen.
• Typical ratio of hydrogen to oxygen is 2:1, similar to water.
• Example: Glucose (C₆H₁₂O₆)

Fats, also called lipids, are made of carbon, hydrogen, and oxygen, like carbohydrates, but with a different ratio. Fats have relatively fewer oxygen atoms compared to hydrogen and carbon, making them more energy-dense.

For instance, a common fat molecule (triglyceride) consists of glycerol and three fatty acid chains, all containing C, H, and O atoms. The lower oxygen content compared to carbohydrates affects their solubility and energy storage properties.

This difference in elemental composition is important for the biological roles of fats, such as long-term energy storage and insulation.

• Fats contain carbon, hydrogen, and oxygen.
• Lower proportion of oxygen compared to carbohydrates.
• Example: Triglycerides composed of glycerol and fatty acids.

Proteins are complex molecules made from amino acids. Unlike carbohydrates and fats, proteins contain nitrogen in addition to carbon, hydrogen, and oxygen. Some amino acids also contain sulfur.

The presence of nitrogen is a key feature that distinguishes proteins from carbohydrates and fats. Sulfur is found in certain amino acids like cysteine and methionine, contributing to protein structure through disulfide bonds.

This elemental composition enables proteins to perform diverse functions such as enzymes, structural components, and signalling molecules.

• Proteins contain carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur.
• Nitrogen is present in amino groups of amino acids.
• Sulfur occurs in some amino acids, affecting protein structure.

To summarise, the three main biological molecules differ in their elemental makeup: carbohydrates and fats contain carbon, hydrogen, and oxygen, but fats have less oxygen relative to hydrogen. Proteins include all these elements plus nitrogen and sometimes sulfur.

Recognising these differences is essential for understanding their biological roles and chemical behaviour.

• Carbohydrates: C, H, O (ratio ~1:2:1)
• Fats: C, H, O (less oxygen than carbohydrates)
• Proteins: C, H, O, N, (sometimes S)

Living organisms contain large molecules essential for structure and function. These large molecules, known as macromolecules, are formed by linking smaller molecules called monomers. Understanding how these macromolecules form helps explain their properties and roles in biology.

The three main types of large biological molecules covered here are carbohydrates (including starch, glycogen, and cellulose), proteins, and lipids (fats and oils). Each type is built from specific smaller molecules through chemical bonds forming polymers or complex structures.

• Macromolecules are large molecules made by joining smaller units.
• Carbohydrates, proteins, and lipids are key biological macromolecules.
• Each macromolecule type has characteristic building blocks.

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, often in the ratio 1:2:1. Glucose is a simple sugar (monosaccharide) and the fundamental building block for many carbohydrates.

When many glucose molecules join together, they form polysaccharides, which are large carbohydrate molecules. Three important polysaccharides are starch, glycogen, and cellulose, each formed by linking glucose units but differing in structure and function.

Starch is the main energy storage molecule in plants. It consists of glucose units linked mainly by alpha-1,4 glycosidic bonds, forming a helical structure. Glycogen is the equivalent energy storage molecule in animals and fungi, similar to starch but more highly branched, allowing rapid release of glucose when needed. Cellulose is a structural polysaccharide in plant cell walls, made of glucose units linked by beta-1,4 glycosidic bonds, forming straight chains that bundle into fibres providing strength.

• Glucose is the monomer for starch, glycogen, and cellulose.
• Starch stores energy in plants; glycogen stores energy in animals.
• Cellulose provides structural support in plant cell walls.
• Differences in bonding and structure lead to different properties and functions.

Proteins are large molecules made from smaller units called amino acids. There are 20 different amino acids commonly found in proteins, each with a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group).

Amino acids link together by peptide bonds formed in condensation reactions between the amino group of one amino acid and the carboxyl group of another. This forms a polypeptide chain, which folds into a specific three-dimensional shape to become a functional protein.

Proteins serve many roles including enzymes, structural components, transport molecules, and antibodies. The sequence and type of amino acids determine the protein’s properties and function.

• Proteins are polymers made from amino acid monomers.
• Peptide bonds link amino acids in a chain.
• Protein function depends on amino acid sequence and folding.
• Condensation reactions form peptide bonds, releasing water.

Fats and oils belong to a group of molecules called lipids. They are formed by combining glycerol, a three-carbon alcohol, with fatty acids, which are long hydrocarbon chains with a carboxyl group at one end.

A fat or oil molecule is called a triglyceride because it consists of one glycerol molecule bonded to three fatty acid molecules through ester bonds formed by condensation reactions.

The properties of fats and oils depend on the types of fatty acids present. Saturated fatty acids have no double bonds and tend to be solid at room temperature (fats), while unsaturated fatty acids contain one or more double bonds and are usually liquid at room temperature (oils).

• Triglycerides are formed from glycerol and three fatty acids.
• Ester bonds link fatty acids to glycerol via condensation reactions.
• Saturated fatty acids make fats solid; unsaturated fatty acids make oils liquid.
• Lipids are important for energy storage and insulation.

Large biological molecules are formed by chemically bonding smaller molecules in specific ways. This process creates polymers or complex molecules with unique properties essential for life.

Glucose units link to form polysaccharides like starch, glycogen, and cellulose, each with distinct functions based on their bonding and structure.

Amino acids join to form proteins, with peptide bonds creating polypeptide chains that fold into functional shapes. Glycerol and fatty acids combine to form triglycerides, important lipids for energy storage and cell membranes.

• Small molecules (monomers) join to form large biological molecules (polymers).
• The type of bond and structure determines the molecule’s function.
• Understanding these relationships explains biological roles and properties.

Starch is a polysaccharide composed of glucose units and is commonly found in plants as an energy store. To test for starch, iodine solution is used because it forms a complex with the helical structure of starch molecules, producing a distinctive colour change.

The test requires a few drops of iodine solution (a mixture of iodine and potassium iodide) added directly to the sample. If starch is present, the solution changes from yellow-brown to a blue-black colour. If starch is absent, the iodine remains yellow-brown.

For example, testing a potato extract with iodine will produce a blue-black colour, confirming starch presence. Testing a sugar solution will show no colour change, indicating no starch is present.

• Use iodine solution (iodine + potassium iodide).
• Add a few drops to the sample on a white tile or in a test tube.
• Blue-black colour indicates starch presence.
• Yellow-brown colour means starch is absent.

Reducing sugars, such as glucose and maltose, can donate electrons to other molecules and reduce copper(II) ions in Benedict’s solution to copper(I) oxide, which precipitates out of solution.

To perform the test, mix the sample with Benedict’s solution in a test tube and heat the mixture gently in a boiling water bath for about 5 minutes. The initial blue colour of Benedict’s solution changes depending on the amount of reducing sugar present: green (low concentration), yellow, orange, and brick-red (high concentration).

For example, a glucose solution heated with Benedict’s solution will produce a brick-red precipitate, while a sucrose solution (a non-reducing sugar) will not change colour unless hydrolysed first.

• Add equal volumes of sample and Benedict’s solution in a test tube.
• Heat in boiling water bath for 5 minutes.
• Colour change from blue to green, yellow, orange, or brick-red indicates reducing sugar.
• No colour change means no reducing sugar present.

Proteins contain peptide bonds that react with copper(II) ions in an alkaline solution to form a violet-coloured complex.

To test for proteins, add an equal volume of biuret reagent (a solution of sodium hydroxide and copper sulfate) to the sample. A colour change from blue to purple indicates the presence of protein. No colour change means protein is absent.

For example, testing milk with biuret reagent produces a purple colour, confirming protein presence, whereas distilled water remains blue.

• Mix sample with biuret reagent in equal volumes.
• Purple colour indicates protein presence.
• Blue colour means no protein detected.

Fats and oils are lipids that are insoluble in water but soluble in ethanol. The ethanol emulsion test detects lipids by dissolving the sample in ethanol and then adding water to precipitate the lipids as an emulsion.

To perform the test, shake the sample with ethanol to dissolve any fats or oils. Then add water and shake gently. A cloudy white emulsion forms if lipids are present, caused by tiny droplets of fat suspended in water.

For example, testing vegetable oil with ethanol and water produces a milky emulsion, while testing sugar solution does not produce any emulsion.

• Shake sample with ethanol to dissolve lipids.
• Add water and shake gently.
• Cloudy white emulsion indicates fats or oils.
• Clear solution means no lipids present.

Use clean apparatus to avoid contamination between tests. For Benedict’s test, ensure the water bath is at boiling temperature for consistent results. Handle reagents carefully: iodine can stain skin and clothes, and copper sulfate is harmful if ingested.

Repeat tests to confirm results and use controls (known positive and negative samples) to validate procedures. Record observations precisely, noting colour changes and precipitate formation.

Limitations include subjective colour interpretation in Benedict’s test; using a colorimeter can improve accuracy. The ethanol emulsion test may give false positives if detergents are present.

• Use clean test tubes and pipettes for each test.
• Heat Benedict’s test samples in boiling water bath safely.
• Wear gloves and eye protection when handling chemicals.
• Repeat tests and use controls for reliability.
• Be aware of limitations and possible false positives.