Biochemistry Study Guide

Biochemistry exams reward students who can do two things at once: remember a lot of specific facts (which bond, which enzyme, which test) and explain why in terms of chemistry (shape, polarity, bonding, energy). This guide is organised as a checklist. Work through each section, tick off what you can explain without notes, and follow the links for anything that feels shaky. It covers the biochemistry typically found in upper high-school chemistry and biology courses and the first year of university.

How to use this guide

1. Read each checklist line as a question: “Can I explain this to a friend?”
2. Mark each item: ✔ confident, ~ partly, ✘ not yet.
3. Spend most of your revision time on the ~ and ✘ items.
4. Finish each section by writing out one diagram or equation from memory.

1. The chemistry of life: foundations

• Name the six most abundant elements in living things (C, H, O, N, P, S) and give a role for each (see elements in the human body).
• Explain why carbon’s four covalent bonds make it ideal for building large molecules (carbon).
• Describe condensation (joining monomers, releasing water) and hydrolysis (splitting with water).
• Explain why water is a good solvent for ions and polar molecules, and why hydrogen bonding gives water its high specific heat capacity.
• Distinguish polar/hydrophilic from non-polar/hydrophobic groups.

Key fact: the four major biomolecule classes are carbohydrates, lipids, proteins and nucleic acids (overview).

2. Carbohydrates

• Give the general formula Cₙ(H₂O)ₙ for simple sugars and name examples: glucose, fructose, galactose, ribose.
• Draw α- and β-glucose and state the difference (position of the –OH on carbon 1) (glucose structure).
• Describe the glycosidic bond and name three disaccharides and their monomers: maltose (glucose + glucose), sucrose (glucose + fructose), lactose (glucose + galactose) (disaccharides).
• Compare starch (amylose + amylopectin), glycogen and cellulose in structure and function (polysaccharides).
• Explain why cellulose is strong: straight β-glucose chains held together by hydrogen bonds.

Exam tip: when asked “why is starch a good storage molecule?”, give three reasons — insoluble (no osmotic effect), compact (coiled/branched), and easily hydrolysed to glucose.

3. Lipids

• Describe a triglyceride: glycerol + three fatty acids joined by ester bonds.
• Distinguish saturated and unsaturated fatty acids and explain why C=C double bonds (especially cis) lower melting point (fatty acids).
• Describe a phospholipid and explain how it forms a bilayer (membranes).
• Recognise the four-ring steroid skeleton in cholesterol and steroid hormones.

Key fact: fats release about 37 kJ g⁻¹ when oxidised, roughly twice as much as carbohydrates or proteins (about 17 kJ g⁻¹).

4. Proteins

• Draw the general structure of an amino acid (amine group, carboxyl group, H and R group on the α-carbon) (amino acids).
• Show how a peptide bond forms by condensation (peptide bonds).
• Explain the four levels of protein structure and the bonds that hold each (protein structure levels):
  • primary: sequence (peptide bonds)
  • secondary: α-helix and β-sheet (hydrogen bonds between backbone groups)
  • tertiary: 3D fold (hydrogen, ionic, disulfide bonds and hydrophobic interactions between R groups)
  • quaternary: more than one chain (e.g. haemoglobin has four)
• Explain denaturation by heat and pH, and why it’s usually irreversible (denaturation).
• Explain the zwitterion form of amino acids at physiological pH.

Exam tip: “disulfide bridge” is a covalent bond between two cysteine residues — don’t call it a hydrogen bond.

5. Enzymes

• Define an enzyme as a biological catalyst that lowers activation energy (enzymes explained).
• Compare the lock-and-key and induced-fit models (comparison).
• Sketch and explain graphs of rate against temperature, pH, substrate concentration and enzyme concentration (factors).
• Distinguish competitive and non-competitive inhibition, including their effect on Vmax and Km (inhibition).
• Explain Km and Vmax in the Michaelis–Menten model (enzyme kinetics).
• State the difference between a cofactor and a coenzyme (cofactors).

Exam tip: the rate falls above the optimum temperature because the active site changes shape (denaturation), not because the enzyme is “killed” — enzymes aren’t alive.

6. Energy: ATP and respiration

• Describe ATP and explain why its hydrolysis releases energy that cells can use (ATP).
• Write the overall equation for aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
• Outline the four stages and where each happens:
  • glycolysis — cytoplasm, glucose → 2 pyruvate, net 2 ATP
  • link reaction — mitochondrial matrix, pyruvate → acetyl-CoA + CO₂
  • Krebs cycle — matrix, releases CO₂, makes NADH and FADH₂
  • electron transport chain — inner membrane, most ATP made, O₂ is the final electron acceptor
• Compare aerobic and anaerobic respiration in humans (lactate) and yeast (ethanol + CO₂) (comparison).
• Explain respiration as a series of redox reactions (redox).

Key fact: aerobic respiration makes roughly 30–32 ATP per glucose (older textbooks say 36–38; check which figure your course uses).

7. Photosynthesis

• Write the overall equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light, chlorophyll).
• Describe the light-dependent reactions (thylakoid membranes: water split, O₂ released, ATP and NADPH made) (details).
• Describe the Calvin cycle (stroma: CO₂ fixed by the enzyme Rubisco, sugar made using ATP and NADPH) (Calvin cycle).
• Explain why chlorophyll looks green and name the metal ion at its centre (Mg²⁺).
• Compare photosynthesis with respiration (comparison).

8. Nucleic acids

• Describe a nucleotide: phosphate + pentose sugar + nitrogenous base (nucleic acids).
• Describe the DNA double helix: sugar–phosphate backbone, antiparallel strands, bases paired by hydrogen bonds (DNA structure).
• State the base-pairing rules and number of hydrogen bonds: A–T (2), G–C (3) (base pairing).
• Compare DNA and RNA: sugar, bases (T vs U), strands (DNA vs RNA).
• Outline semi-conservative replication and protein synthesis (replication, protein synthesis).

9. Practical skills and food tests

• Starch: iodine solution, orange-brown → blue-black (starch test).
• Reducing sugars: Benedict’s reagent, heat, blue → green/yellow/orange/brick-red (reducing sugars).
• Protein: biuret reagent, blue → purple/lilac (biuret test).
• Lipids: emulsion test with ethanol then water, cloudy white emulsion.
• Plan an enzyme experiment with a controlled variable, a range and repeats (enzyme experiment).

Exam tip: a non-reducing sugar (e.g. sucrose) gives a negative Benedict’s test until it’s hydrolysed by boiling with dilute acid and then neutralised.

Ten quick-fire checks

Answer these in one line each; answers are below.

1. Bond joining amino acids?
2. Bond joining monosaccharides?
3. Bond joining glycerol to fatty acids?
4. Final electron acceptor in aerobic respiration?
5. Where does glycolysis happen?
6. Which base is in RNA but not DNA?
7. Metal ion in haemoglobin’s haem group?
8. What does Rubisco fix?
9. Why do cis double bonds lower a fat’s melting point?
10. What does a competitive inhibitor compete for?

Answers: 1 peptide; 2 glycosidic; 3 ester; 4 oxygen; 5 cytoplasm; 6 uracil; 7 Fe²⁺ (iron); 8 carbon dioxide; 9 kinks stop chains packing closely, so intermolecular forces are weaker; 10 the active site.

Exam technique

• Use the right bond names. Examiners look for “hydrogen bond”, “ionic bond”, “disulfide bridge”, “ester bond” — not “bonds break”.
• Link structure to function in every “explain” answer: “Glycogen is highly branched, so it has many ends where enzymes can add or remove glucose quickly.”
• Graphs: describe the trend with numbers from the graph, then explain it with chemistry.
• Equations: balance respiration and photosynthesis equations and include state symbols if asked.

Key takeaways

• Master the four biomolecules, their monomers and the bonds that link them.
• Explain enzyme behaviour in terms of shape, collisions and activation energy.
• Know where each stage of respiration and photosynthesis happens and what goes in and out.
• Learn the food tests with their exact colour changes.

For a lighter start, read what is biochemistry?, then test yourself with the myths in biochemistry misconceptions.