AQA A-Level Biology: Topic 1 Biological Molecules

Biological molecules are the building materials of every living thing. Topic 1 of the AQA A-Level Biology specification asks you to understand what these molecules are made of, how they are built and broken down, and how their structure links to what they do in the body. This guide explains the content so the ideas make sense, rather than asking you to memorise them cold.

Monomers and polymers

Most large biological molecules are polymers: long chains made by joining many smaller, repeating units called monomers. Three of the most important groups in biology follow this pattern.

• Carbohydrates are polymers made from monosaccharides such as glucose.
• Proteins are polymers made from amino acids.
• Nucleic acids (DNA and RNA) are polymers made from nucleotides.

Monomers are joined together in a condensation reaction. Two monomers form a chemical bond between them, and a molecule of water is released each time a bond forms. The reverse process, hydrolysis, breaks a bond by adding a molecule of water. The same two reactions build and break carbohydrates, proteins and nucleic acids, so once you understand them in one place you understand them everywhere.

Carbohydrates

Carbohydrates are made from carbon, hydrogen and oxygen, and they are the body's most accessible source of energy.

Monosaccharides

Monosaccharides are the single sugar units, the monomers of carbohydrates. Glucose is the most important because it is the main substrate for respiration. Glucose exists in two slightly different ring forms, alpha glucose and beta glucose, which differ only in the arrangement of groups on one carbon. That small difference has large consequences later, because it changes the shape of the polymers glucose can form.

Disaccharides

When two monosaccharides join by a condensation reaction they form a disaccharide and a glycosidic bond links them. Common examples are maltose (glucose and glucose), sucrose (glucose and fructose) and lactose (glucose and galactose).

Polysaccharides

Joining many monosaccharides gives a polysaccharide. The three you need to understand are storage and structural molecules, and their jobs come directly from their structure.

Starch is not a single molecule but a mixture of two. Amylose is an unbranched chain that coils into a tight helix, which makes it compact. Amylopectin is a branched chain, with side branches coming off the main chain. Glycogen, the equivalent store in animals, is built like amylopectin but is even more highly branched.

Branching matters because each branch ends in a glucose unit that can be removed. A highly branched molecule therefore has many ends, so glucose can be released quickly when it is needed, which suits glycogen to animals with high energy demands. Both starch and glycogen are also compact and insoluble, so they make good energy stores: they do not affect the water potential of the cell and can be broken down quickly when energy is needed.

Cellulose is different again. Because it is built from beta glucose, the chains are forced to lie straight rather than coil. Many straight chains are then linked side by side by hydrogen bonds into strong fibres, which give plant cell walls their strength.

Testing for sugars and starch

Reducing sugars, such as glucose and maltose, are tested with Benedict's reagent. The sample is heated with the blue reagent, and if a reducing sugar is present the colour changes. The final colour shows roughly how much sugar there is, running from blue when there is none, through green, yellow and orange, to brick red at a high concentration.

A non-reducing sugar, such as sucrose, does not react with Benedict's on its own. It is first boiled with acid to hydrolyse it into its reducing monosaccharides, then neutralised and tested with Benedict's as before.

Starch is tested with iodine solution (iodine dissolved in potassium iodide), which turns from orange-brown to blue-black if starch is present.

Lipids

Lipids are a varied group that includes triglycerides and phospholipids. Unlike carbohydrates and proteins they are not polymers, but they are still made by condensation reactions.

Triglycerides

A triglyceride is made from one molecule of glycerol and three fatty acids, joined by ester bonds formed in condensation reactions. Triglycerides are excellent energy stores because they carry a large amount of chemical energy for their mass and are insoluble in water.

Phospholipids

A phospholipid is similar, but one of the fatty acids is replaced by a phosphate group. This gives the molecule two parts with opposite behaviour: a hydrophilic phosphate head that mixes with water, and two hydrophobic fatty acid tails that do not. Because of this, phospholipids arrange themselves into a bilayer in water, which is the basis of every cell membrane.

Testing for lipids

Lipids are tested with the emulsion test. The sample is shaken with ethanol and then mixed with water, and if lipid is present a milky-white emulsion forms.

Proteins

Proteins do more different jobs than any other group of molecules, from enzymes to antibodies to muscle fibres. Their variety comes from the way a simple set of monomers can be arranged.

Amino acids

The monomers of proteins are amino acids. There are twenty that occur in living things, and they all share the same general structure: a central carbon joined to an amino group, a carboxyl group, a hydrogen atom and a variable R group. Only the R group differs between amino acids, and it is this variation that gives proteins their range.

Two amino acids join by a condensation reaction to form a peptide bond, releasing water. Many amino acids joined this way form a polypeptide.

The four levels of protein structure

A protein's job depends on its three-dimensional shape, and that shape is built up in stages.

The key idea is that the primary sequence determines everything above it. Change one amino acid and you can change the folding, and a protein with the wrong shape usually cannot do its job.

Testing for proteins

Proteins are tested with the Biuret test. Biuret reagent is pale blue and turns purple if protein is present.

Nucleic acids: DNA and RNA

Nucleic acids carry the genetic information that tells a cell how to build its proteins. The two you need are DNA and RNA. Both are polymers, and their monomer is the nucleotide.

Nucleotides

A nucleotide is made of three parts joined together: a pentose (five carbon) sugar, a phosphate group, and a nitrogen-containing base. Nucleotides join in condensation reactions, linking the phosphate of one to the sugar of the next. This builds a sugar-phosphate backbone, and the bonds along it are called phosphodiester bonds.

DNA structure

DNA is made of two polynucleotide strands twisted into a double helix. The two strands are held together by hydrogen bonds between their bases, and the pairing is specific. This is complementary base pairing: adenine always pairs with thymine, and cytosine always pairs with guanine. The two strands run in opposite directions, described as antiparallel, and the sugar in DNA is deoxyribose.

Because the order of bases acts as a code and the molecule as a whole is stable, DNA is well suited to storing genetic information.

RNA structure

RNA is usually a single polynucleotide strand and is shorter than DNA. Its sugar is ribose, and the base uracil replaces thymine. RNA carries information from DNA to the sites of protein synthesis: messenger RNA (mRNA) carries the code, and transfer RNA (tRNA) brings the amino acids.

ATP

ATP (adenosine triphosphate) is the molecule cells use as their immediate source of energy. It is a nucleotide made of the base adenine, the sugar ribose, and three phosphate groups.

Energy is released when ATP is hydrolysed to ADP (adenosine diphosphate) and a single phosphate group. This small, quick reaction releases a manageable amount of energy exactly where it is needed, which is why cells use ATP rather than releasing energy from glucose all at once. ATP is then resynthesised from ADP and phosphate during respiration, so it is constantly recycled.

Water

Water makes up most of every cell, and life depends on its properties. Almost all of them come from one feature: water is a dipolar molecule. The oxygen atom is slightly negative and the hydrogen atoms are slightly positive, so water molecules attract one another and form hydrogen bonds.

One direct consequence is that water is a metabolite: it is a reactant in condensation and hydrolysis reactions, so it takes part in building and breaking many of the molecules in this topic. The rest of its useful behaviour comes from the hydrogen bonds between molecules. | Property | Why water has it | Why it matters in biology | | --- | --- | --- | | Good solvent | It is polar, so it surrounds charged and polar particles | Metabolic reactions happen in solution, and dissolved substances can be transported | | High specific heat capacity | Hydrogen bonds absorb a lot of energy before the temperature rises | Buffers temperature changes, giving cells and organisms a stable environment | | High latent heat of vaporisation | A lot of energy is needed to break the hydrogen bonds and evaporate water | Evaporation removes a lot of heat for little water lost, so it cools effectively | | Strong cohesion | Hydrogen bonds hold neighbouring molecules together | Columns of water are pulled up through plants, and surface tension forms where water meets air |

Inorganic ions

Inorganic ions are charged particles that do not contain carbon, dissolved in the cytoplasm and in body fluids. Some are needed in large amounts and others in tiny amounts, and each one has a specific role that comes from its properties.

• Hydrogen ions determine pH, which in turn affects the shape of enzymes and the rate of enzyme-controlled reactions.
• Iron ions are part of haemoglobin, where they bind oxygen so it can be carried around the body.
• Sodium ions are needed for the co-transport of glucose and amino acids across cell membranes.
• Phosphate ions are part of DNA, RNA, ATP and phospholipids, and are central to storing and transferring energy.

How this fits together

Almost everything in Topic 1 comes back to one theme: structure determines function. The same handful of reactions build very different molecules, and the shape of each molecule explains what it does. Hold onto that idea and the detail becomes much easier to place.

If you want to go deeper on the proteins that run the body's reactions, read the companion guide on enzymes.