Molecular Biology (SL) — IB Biology SL SL Study Guide

For: IB Biology SL candidates sitting IB Biology SL.

Covers: All core SL Molecular Biology content including polarity-derived properties of water, structure and function of carbohydrates, lipids and proteins, DNA structure and semi-conservative replication, and basic enzyme kinetics.

You should already know: IGCSE Biology, basic chemistry.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the IB Biology SL style for educational use. They are not reproductions of past IBO papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official IBO mark schemes for grading conventions.

1. What Is Molecular Biology (SL)?

Molecular biology is the study of biological activity at the molecular level, focusing on the structure, function, and interactions of the four core classes of biomolecules (carbohydrates, lipids, proteins, nucleic acids) that underpin all cellular and organismal processes. It is the first core topic in the IB Biology SL syllabus, and its principles form the foundation for every subsequent topic, from cell biology and metabolism to genetics, evolution, and ecology. It is sometimes referred to as "biological chemistry", but differs from general chemistry by focusing explicitly on molecular processes unique to living systems. Examiners frequently test links between molecular structure and biological function, so memorization of properties alone will not earn full marks — you must always connect structure to role.

2. Water — properties from polarity

Water’s unique biological properties stem entirely from its polar covalent structure and the hydrogen bonds that form between adjacent water molecules. The oxygen atom in a $H_{2}O$ molecule is significantly more electronegative than hydrogen, so it pulls shared electrons toward itself, creating a partial negative charge on the oxygen end and partial positive charges on the two hydrogen ends. These partial charges allow weak hydrogen bonds (around 1/20 the strength of a covalent bond) to form between the positive hydrogen of one water molecule and the negative oxygen of another.

Key properties and their biological significance include:

1. Cohesion: Attraction between identical water molecules via hydrogen bonds, creating high surface tension and allowing water to form continuous, unbroken columns. This enables transpiration pull, the process that moves water from plant roots to leaves via xylem tissue.
2. Adhesion: Attraction between water molecules and other charged or polar surfaces. Combined with cohesion, this enables capillary action, which moves water through small spaces in soil and plant cell walls.
3. Thermal properties: Water has a very high specific heat capacity ($4200Jk{g}^{-1}°C^{-1}$), meaning it absorbs large amounts of heat before changing temperature, creating stable aquatic habitats and stable internal temperatures in organisms. It also has a high latent heat of vaporization, so evaporating water (e.g. sweat) removes large amounts of heat from the body for cooling.
4. Solvent properties: Water dissolves all polar or charged solutes (e.g. ions, glucose, amino acids), acting as a transport medium for nutrients and waste in blood, cytoplasm, and xylem/phloem.

Worked Example

Explain why small aquatic insects can walk on the surface of still water (2 marks). Answer: (1) Cohesion between adjacent water molecules via hydrogen bonds creates high surface tension at the water-air interface. (2) This tension is strong enough to support the low mass of small insects without breaking, allowing them to walk on the surface.

3. Carbohydrates, lipids, proteins

All three biomolecules are organic (carbon-containing) compounds built from smaller monomers via condensation reactions (which release one $H_{2}O$ molecule per bond formed) and broken down via hydrolysis reactions (which consume one $H_{2}O$ molecule per bond broken).

Carbohydrates

Carbohydrates have a general formula of $C_n(H_{2}O{)}_n$, and are used for energy storage, energy release, and structural support:

• Monosaccharides: Single sugar monomers (e.g. glucose, ribose, fructose) that are the primary fuel for cellular respiration.
• Disaccharides: Two monosaccharides linked by a glycosidic bond (e.g. sucrose = glucose + fructose, lactose = glucose + galactose).
• Polysaccharides: Long chains of monosaccharides:
• Starch: Alpha-glucose chains, energy storage in plant cells
• Glycogen: Highly branched alpha-glucose chains, short-term energy storage in animal liver and muscle cells
• Cellulose: Beta-glucose chains, strong structural component of plant cell walls

Lipids

Lipids are non-polar, hydrophobic molecules used for long-term energy storage, insulation, buoyancy, and membrane structure:

• Triglycerides: 3 fatty acid chains linked to a glycerol molecule via ester bonds. They store $37kJ{g}^{-1}$ of energy, more than double the $17kJ{g}^{-1}$ stored in carbohydrates. Saturated fatty acids have no double bonds between carbon atoms (solid at room temperature, mostly animal origin), while unsaturated fatty acids have one or more double bonds (liquid at room temperature, mostly plant origin).
• Phospholipids: Amphipathic molecules with a hydrophilic phosphate head and two hydrophobic fatty acid tails. They spontaneously form bilayers in aqueous environments, forming the core of all cell membranes.

Proteins

Proteins are built from 20 distinct amino acid monomers, each with an amine group, carboxyl group, and variable R group that determines its chemical properties:

• Primary structure: Linear sequence of amino acids linked by peptide bonds
• Secondary structure: Local folding into alpha-helices or beta-pleated sheets via hydrogen bonds between the peptide backbone
• Tertiary structure: 3D global shape of the polypeptide, formed via R-group interactions (hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions)
• Quaternary structure: Structure formed when multiple polypeptide chains bind together (e.g. hemoglobin, antibodies)

Protein shape determines function: globular proteins (e.g. enzymes) are soluble and have specific binding sites, while fibrous proteins (e.g. collagen, keratin) are strong and insoluble for structural roles.

Worked Example

Explain why triglycerides are preferred over glycogen for long-term energy storage in migratory birds (2 marks). Answer: (1) Triglycerides store more than twice as much energy per gram as glycogen, so birds can carry more energy without adding excess body mass that would impede flight. (2) Triglycerides are non-polar and do not associate with water, so they do not add extra water mass to the body, unlike hydrophilic glycogen which binds to water molecules.

4. DNA structure and replication

Deoxyribonucleic acid (DNA) is the nucleic acid that stores genetic information in all living organisms.

DNA Structure

DNA is built from nucleotide monomers, each containing:

1. A deoxyribose 5-carbon sugar
2. A phosphate group
3. One of four nitrogenous bases: adenine (A), cytosine (C), guanine (G), thymine (T)

Nucleotides are linked by phosphodiester bonds between the phosphate group of one nucleotide and the 3' carbon of the deoxyribose sugar of the next, forming a sugar-phosphate backbone. DNA forms a double helix of two antiparallel strands (one runs 5' to 3', the other runs 3' to 5') held together by hydrogen bonds between complementary base pairs: A pairs with T via 2 hydrogen bonds, C pairs with G via 3 hydrogen bonds.

Semi-Conservative Replication

DNA replication produces two identical copies of the original DNA molecule, each with one original parent strand and one newly synthesized daughter strand:

1. Helicase: Unwinds the DNA double helix and breaks hydrogen bonds between base pairs, separating the two strands to form a replication fork.
2. DNA polymerase: Binds to the separated parent strands, adds free complementary nucleotides to the 3' end of the growing daughter strand following base pairing rules, and proofreads for mismatched nucleotides to reduce mutation rate.

Worked Example

A parent DNA strand has the sequence 5'-ATG CAA GCT-3'. What is the sequence of the complementary daughter strand, written in 5' to 3' orientation (2 marks)? Answer: First write the complementary 3' to 5' sequence via base pairing: 3'-TAC GTT CGA-5'. Flip the orientation to get the 5' to 3' sequence: 5'-AGC TTG CAT-3'. Examiners regularly deduct marks for missing orientation labels or writing sequences in the wrong direction.

5. Enzymes — basic kinetics

Enzymes are globular proteins that act as biological catalysts: they lower the activation energy of chemical reactions without being consumed or altered in the reaction, so they can be reused repeatedly.

Core Enzyme Properties

• Active site: A specific groove on the enzyme surface that binds only to a specific substrate molecule. The induced fit model describes how the active site changes shape slightly to bind more tightly to the substrate after initial contact, catalyzing the reaction.
• Denaturation: Permanent structural change to the enzyme’s tertiary structure caused by extreme temperature or pH, which breaks the weak bonds holding the active site shape, so the substrate can no longer bind and the enzyme loses function.

Factors Affecting Enzyme Activity

1. Temperature: At low temperatures, low kinetic energy leads to few collisions between enzyme and substrate, so reaction rate is low. Rate increases with temperature up to the enzyme’s optimum temperature, after which high thermal energy causes denaturation and rate drops rapidly to 0.
2. pH: Each enzyme has an optimum pH where its active site has the correct charge. Deviations from optimum pH alter R-group charges, causing denaturation and reduced rate.
3. Substrate concentration: Reaction rate increases linearly with increasing substrate concentration at low concentrations, as more active sites are occupied. Rate plateaus at the maximum rate ($V_{max}$) when all active sites are saturated (no free active sites left to bind additional substrate).

Worked Example

Pepsin is a protein-digesting enzyme in the human stomach, which has a pH of ~2. Explain why pepsin is inactive when it enters the small intestine, which has a pH of ~7.5 (2 marks). Answer: (1) Pepsin’s optimum pH is ~2, matching the stomach environment. (2) The neutral pH of the small intestine alters the charge of R groups in pepsin’s active site, breaking tertiary structure, denaturing the enzyme, and preventing substrate binding.

6. Common Pitfalls (and how to avoid them)

• Wrong move: Describing cohesion and adhesion interchangeably, or failing to link them to hydrogen bonds. Why it happens: Both properties involve hydrogen bonds, so students mix up their definitions. Correct move: Explicitly state cohesion = attraction between water molecules, adhesion = attraction between water and other surfaces, and always link both to hydrogen bonding in exam answers to earn full marks.
• Wrong move: Stating that saturated fatty acids have double bonds. Why it happens: Students confuse saturated (no double bonds, fully saturated with hydrogen atoms) and unsaturated (has double bonds) definitions. Correct move: Remember "saturated = straight, solid at room temp, no double bonds; unsaturated = kinked, liquid at room temp, has double bonds" to avoid mix-ups.
• Wrong move: Drawing DNA strands as parallel, or failing to label 5' and 3' ends. Why it happens: Students forget the antiparallel structure of DNA. Correct move: Always label 5' and 3' ends when drawing or writing DNA sequences, and flip the direction of complementary strands. Examiners deduct up to 1 mark per answer for missing orientation labels.
• Wrong move: Describing enzyme denaturation as the enzyme "dying". Why it happens: Students anthropomorphize enzymes, treating them as living structures. Correct move: Denaturation is a structural change to the enzyme’s tertiary structure that destroys the active site. Never refer to enzymes as "dead" in exam answers, as this will earn zero marks for that point.
• Wrong move: Claiming enzymes alter the overall energy of reactants or products. Why it happens: Students confuse activation energy reduction with overall reaction energy change. Correct move: Enzymes only lower the activation energy required for a reaction to proceed; they do not change the overall free energy of the reaction, and are not consumed in the process.

7. Practice Questions (IB Biology SL Style)

Question 1 (2 marks)

State two properties of water that make it an effective transport medium in multicellular organisms, and explain one property. Worked Solution: Award 1 mark for each correct property, 1 mark for correct explanation, maximum 2:

• Property 1: Cohesion, property 2: Solvent ability (1 mark for both, or either)
• Explanation for cohesion: Hydrogen bonds between water molecules allow it to form a continuous unbroken column, so it can be pulled up plant xylem via transpiration, or flow through animal blood vessels. OR Explanation for solvent ability: Water is polar, so it dissolves all charged/polar solutes including glucose, ions, and amino acids, allowing them to be transported in blood or plant sap.

Question 2 (3 marks)

Compare the structure and function of triglycerides and phospholipids. Worked Solution: Award 1 mark per correct comparison point, maximum 3:

1. Triglycerides have 3 fatty acid chains bound to glycerol, while phospholipids have 2 fatty acid chains and one phosphate group bound to glycerol.
2. Triglycerides are entirely hydrophobic, while phospholipids are amphipathic, with a hydrophilic phosphate head and hydrophobic fatty acid tails.
3. Triglycerides function primarily in long-term energy storage, insulation and buoyancy, while phospholipids are the core structural component of all cell membranes.

Question 3 (3 marks)

Outline why the replication of DNA is described as semi-conservative, and name one enzyme involved in the process. Worked Solution: Award 1 mark per correct point, maximum 3:

1. Semi-conservative replication means that every new DNA molecule produced contains one original, conserved parent strand, and one newly synthesized daughter strand.
2. This is because the original parent strands act as templates for the synthesis of new complementary strands via complementary base pairing.
3. Named enzyme: Helicase or DNA polymerase.

Question 4 (4 marks)

Sketch a graph of enzyme reaction rate against temperature, and explain the shape of the curve above the optimum temperature. Worked Solution: Award 1 mark for correct sketch: Curve rises from 0°C to optimum temperature, then drops steeply back to 0 rate at high temperature. 3 marks for explanation:

1. Above the optimum temperature, the thermal energy of the enzyme molecules increases to the point that it breaks the weak hydrogen and ionic bonds holding the enzyme’s tertiary structure in place.
2. This causes the shape of the enzyme’s active site to change permanently (denaturation).
3. The substrate can no longer bind to the altered active site, so the reaction rate drops rapidly to zero as all enzymes become denatured.

8. Quick Reference Cheatsheet

9. What's Next

Mastery of molecular biology is non-negotiable for success in IB Biology SL, as it forms the foundation for every subsequent topic in the syllabus. Your understanding of phospholipid amphipathicity directly supports your study of cell membrane structure and transport in the next topic, enzyme kinetics underpins all metabolism content including cellular respiration and photosynthesis, and DNA structure and replication is the basis for all genetics, evolution, and biotechnology units you will cover later. Spending time memorizing structure-function links now will save you hours of re-learning when you move to these more advanced topics.

If you struggle with any of the concepts covered in this guide, or want extra practice problems tailored to your weak spots, you can ask Ollie, our AI tutor, at any time for personalized explanations and feedback. You can also find more full-length IB Biology SL practice exams, mark schemes, and topic-specific quizzes on the homepage to test your mastery before your official exams.