How are the four main groups of biological molecules built, and how does structure fit function?
Carbohydrates, lipids and proteins: their monomers, the condensation and hydrolysis reactions that join and break them, the bonds formed, and how molecular structure relates to biological function.
A focused CCEA A-Level Biology answer on carbohydrates, lipids and proteins, covering their monomers, condensation and hydrolysis reactions, the bonds formed, and how structure relates to function.
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What this dot point is asking
CCEA wants you to know the monomers of carbohydrates, lipids and proteins, the condensation reactions that join them and the hydrolysis reactions that break them, the bonds formed in each case, and how the structure of each molecule suits its biological role.
Carbohydrates
Monosaccharides such as glucose are the monomers. Two monosaccharides join by a glycosidic bond to form a disaccharide (for example glucose plus fructose gives sucrose). Many monosaccharides form polysaccharides: starch (amylose and amylopectin) and glycogen are compact, branched energy stores; cellulose is made of beta-glucose chains held by hydrogen bonds into strong fibres for plant cell walls.
Lipids
A triglyceride is made from one glycerol and three fatty acids, joined by ester bonds in condensation reactions. Triglycerides are good energy stores because they are insoluble and release a lot of energy per gram. A phospholipid has one fatty acid replaced by a phosphate group, giving a hydrophilic head and hydrophobic tails, which is why phospholipids form bilayers in membranes.
Proteins
The shape of the tertiary structure determines a protein's function, which is why a specific enzyme has a specific active site. Globular proteins (such as enzymes and haemoglobin) fold into compact, roughly spherical shapes that are soluble and metabolically active, while fibrous proteins (such as collagen) form long, insoluble strands suited to structural roles. The bonds that hold the tertiary structure together (hydrogen, ionic, disulfide and hydrophobic interactions) form between the R-groups of the amino acids, so the primary sequence ultimately determines the final 3D shape.
Examples in context
Example 1. Starch versus cellulose in plants. A potato cell stores energy as starch, a polymer of alpha-glucose. Because the alpha-glucose units all point the same way, amylose coils into a compact helix that is easy to pack and to hydrolyse when energy is needed. The same plant builds its cell walls from cellulose, a polymer of beta-glucose. To form glycosidic bonds between beta-glucose units, every alternate molecule must be flipped 180 degrees, giving long straight chains. Many chains hydrogen-bond side by side into strong microfibrils, giving the wall its tensile strength. This shows how a small difference in monomer (alpha versus beta-glucose) produces molecules with completely different functions, storage versus support.
Example 2. Haemoglobin and quaternary structure. Haemoglobin in red blood cells is a globular protein with quaternary structure: four polypeptide chains, each holding an iron-containing haem group. The folded tertiary structure of each chain creates a pocket that binds oxygen reversibly, and the four subunits interact so that binding of one oxygen molecule makes the others bind more easily (cooperative binding). This illustrates how the levels of protein structure together produce a molecule precisely shaped for transporting oxygen around the body.
Try this
Q1. Name the bond formed when two amino acids join, and state the type of reaction. [2 marks]
- Cue. A peptide bond, formed by a condensation reaction.
Q2. Explain why phospholipids form a bilayer in water. [2 marks]
- Cue. Hydrophilic phosphate heads face the water; hydrophobic fatty acid tails face inwards away from water.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20206 marksDescribe the structure of a triglyceride and explain how its structure makes it suited to its role as an energy store.Show worked answer →
A 6-mark answer needs the structure, the bonds, and the link to function.
Structure: a triglyceride is made from one glycerol molecule and three fatty acids. Each fatty acid joins to the glycerol by an ester bond, formed in a condensation reaction that releases one water molecule per bond (three waters in total).
Fatty acids may be saturated (no carbon to carbon double bonds) or unsaturated (one or more double bonds, which give kinks and lower melting points).
Link to function: triglycerides are good long-term energy stores because they are insoluble in water, so they do not affect the water potential of the cell and cause no osmotic effects. They have a high proportion of carbon to hydrogen bonds and little oxygen, so they release roughly twice as much energy per gram as carbohydrate when respired. They also yield metabolic water on oxidation, useful in animals such as desert mammals.
Markers reward glycerol plus three fatty acids, ester bonds by condensation, insolubility, and the high energy yield per gram.
CCEA 20184 marksExplain the difference between condensation and hydrolysis reactions, using the formation and breakdown of a disaccharide as an example.Show worked answer →
Condensation joins molecules and removes water; hydrolysis breaks them and adds water.
In condensation, two monosaccharides (for example glucose and fructose) join when a glycosidic bond forms between them, and one water molecule is removed. This produces the disaccharide sucrose.
In hydrolysis, the glycosidic bond is broken by adding a water molecule, splitting sucrose back into glucose and fructose. This is what happens during digestion of the disaccharide.
Markers reward water removed in condensation, water added in hydrolysis, the glycosidic bond named, and a correct worked example.
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Sources & how we know this
- CCEA GCE Biology specification — CCEA (2016)