How do the structures of carbohydrates, lipids, proteins and nucleic acids determine their biological functions?
Describe the structure of carbohydrates, lipids, proteins and nucleic acids and relate structure to function
A focused answer to the H2 Biology Cell Biology outcome on biological molecules. The monomers and polymers of carbohydrates, lipids, proteins and nucleic acids, the condensation and hydrolysis reactions that join and split them, and how structure fits function.
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What this dot point is asking
SEAB wants you to describe the structure of the four classes of biological macromolecule (carbohydrates, lipids, proteins and nucleic acids), to explain how their monomers join by condensation and split by hydrolysis, and crucially to relate each structure to its function. This molecular vocabulary underpins membranes, enzymes, respiration and genetics.
The answer
Carbohydrates
The monomer is a monosaccharide such as glucose. Two monosaccharides join by a condensation reaction (releasing water) through a glycosidic bond to form a disaccharide; many join to form a polysaccharide.
- Starch and glycogen are storage polysaccharides of alpha-glucose. Their coiled, branched shape is compact and quickly hydrolysed for energy.
- Cellulose is a structural polysaccharide of beta-glucose. Its straight chains hydrogen-bond into strong microfibrils, ideal for the plant cell wall.
Lipids
Lipids are not polymers. A triglyceride is one glycerol joined to three fatty acids by ester bonds (formed by condensation). Triglycerides are excellent energy stores because their long hydrocarbon chains are highly reduced, releasing much energy on oxidation.
A phospholipid replaces one fatty acid with a phosphate group, giving a hydrophilic head and two hydrophobic tails. This amphipathic structure is the basis of the membrane bilayer.
Proteins
The monomer is an amino acid; amino acids join by peptide bonds (condensation) to form polypeptides. Structure is described at four levels: primary (the sequence), secondary (alpha helices and beta pleated sheets held by hydrogen bonds), tertiary (the overall three-dimensional fold), and quaternary (two or more polypeptide chains together). The precise shape determines function, as in enzymes and antibodies.
Nucleic acids
The monomer is a nucleotide (a pentose sugar, a phosphate, and a nitrogenous base). Nucleotides join by phosphodiester bonds. DNA stores genetic information as a double helix of two antiparallel strands held by complementary base pairing; RNA is usually single-stranded and carries or interprets that information.
Examples in context
Example 1. Energy density of fat versus carbohydrate. Triglycerides store roughly twice the energy per gram of carbohydrate because their hydrocarbon tails are more highly reduced, so more energy is released when they are oxidised. This is why long-term energy reserves in animals are stored as fat.
Example 2. Haemoglobin and quaternary structure. Haemoglobin is four polypeptide subunits, each holding a haem group. This quaternary structure allows cooperative oxygen binding, where binding at one subunit raises the affinity of the others, giving the sigmoidal oxygen dissociation curve.
Try this
Q1. Name the bond formed when two amino acids undergo a condensation reaction, and state what else is produced. [2 marks]
- Cue. A peptide bond is formed, and a molecule of water is released.
Q2. Explain why cellulose is described as a structural polysaccharide. [2 marks]
- Cue. Its straight beta-glucose chains form hydrogen bonds between adjacent chains, producing strong microfibrils that give the plant cell wall mechanical strength.
Q3. Distinguish between a triglyceride and a phospholipid in terms of structure. [2 marks]
- Cue. A triglyceride has glycerol joined to three fatty acids; a phospholipid has glycerol joined to two fatty acids and one phosphate-containing group, making it amphipathic.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksCompare the structure of starch and cellulose, and explain how the difference in their structure suits each polysaccharide to its biological role.Show worked answer →
The answer should contrast the bonding and then link each structure to a role.
Both are polymers of glucose joined by glycosidic bonds, but starch is built from alpha-glucose linked by 1,4 (and in amylopectin 1,6) bonds, while cellulose is built from beta-glucose linked by 1,4 bonds in which alternate units are flipped 180 degrees.
In starch the alpha linkage produces a coiled, branched molecule that is compact and easily hydrolysed, which suits it to energy storage: it holds many glucose units in a small space and can be mobilised when needed.
In cellulose the beta linkage produces straight, unbranched chains that lie alongside one another and form many hydrogen bonds between chains, creating strong microfibrils. This suits cellulose to a structural role as the main component of the plant cell wall.
Markers reward the alpha versus beta distinction, the resulting shape (coiled and branched versus straight chains with hydrogen bonding), and a correct functional link for each.
Original3 marksExplain why a phospholipid molecule forms a bilayer when surrounded by water, with reference to its structure.Show worked answer →
The key is the molecule's two contrasting ends.
A phospholipid has a hydrophilic (water-attracting) phosphate-containing head and two hydrophobic (water-repelling) fatty acid tails, making it amphipathic.
In water the hydrophilic heads orient outwards to face the aqueous surroundings on both sides, while the hydrophobic tails turn inwards away from water, associating with one another.
This arrangement, repeated over a sheet, produces a bilayer two molecules thick with the tails forming a non-polar interior. Markers reward the terms hydrophilic and hydrophobic (or amphipathic), the correct orientation of heads and tails relative to water, and the conclusion that this gives a stable bilayer.
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