IB Biology Molecular Biology: Biomolecules, Enzymes, and DNA

Molecular Biology is where students often know the vocabulary but lose the mechanism. You might remember "condensation", "active site", "semi-conservative replication", and "transcription", but IB Biology marks usually come from explaining what is joined, what is broken, what binds, or what sequence is copied.

This guide turns the Chapter 2 notes into a revision path for biomolecules, enzymes, DNA, RNA, respiration, and photosynthesis.

Quick Answer

For IB Biology molecular biology revision, focus on these links:

• carbohydrates, lipids, proteins, and nucleic acids differ in elements, building blocks, bonds, and functions
• condensation joins molecules and releases water; hydrolysis breaks bonds using water
• protein shape controls function, and denaturation disrupts that shape
• enzymes lower activation energy by binding substrates at active sites
• DNA stores information; RNA helps transfer and express it
• DNA replication is semi-conservative because each new DNA molecule contains one original strand and one new strand
• transcription produces mRNA from DNA; translation uses mRNA to build a polypeptide
• respiration releases ATP from glucose; photosynthesis stores light energy in organic molecules

Core Concept That Gets Marks

Molecular Biology answers often need exact action verbs:

Condensation and Hydrolysis

Condensation and hydrolysis are opposite reaction types, but students often reverse water's role.

Worked Example 1: Condensation vs Hydrolysis

Question: Explain how two monosaccharides can form a disaccharide and how the disaccharide can be broken down again.

Markscheme-style answer: Two monosaccharides join by a condensation reaction, forming a glycosidic bond and releasing water. The disaccharide can be broken by hydrolysis, where water is used to split the glycosidic bond.

Why this scores: It names the bond, the reaction type, and the role of water in both directions.

The Four Main Biomolecule Groups

For exam answers, add structure-function links. Lipids are often hydrophobic because they are mostly non-polar. Nucleic acids store information because base sequences can vary.

Weak Answer vs Mark-Worthy Answer

Proteins, Shape, and Enzymes

Proteins are chains of amino acids joined by peptide bonds. Their function depends on structure.

Enzymes are usually globular proteins that catalyse reactions by lowering activation energy. The active site is specific to the substrate. In induced fit, the enzyme changes shape slightly as the substrate binds.

Factors affecting enzyme activity:

• temperature increases kinetic energy until denaturation reduces activity
• pH changes can alter bonding and active-site shape
• substrate concentration increases rate until active sites are saturated

Worked Example 2: Enzyme Temperature Graph

Question: Explain why enzyme activity increases with temperature at first but decreases after the optimum.

Markscheme-style answer: At first, higher temperature gives enzyme and substrate molecules more kinetic energy, so collisions occur more often and more enzyme-substrate complexes form. Above the optimum, heat disrupts bonds maintaining the enzyme's tertiary structure, changing the active-site shape and reducing substrate binding.

Why this scores: It explains both sides of the graph and links denaturation to active-site shape.

DNA, RNA, and Protein Synthesis

DNA and RNA are nucleic acids made from nucleotides. Each nucleotide contains a base, sugar, and phosphate group. If you need more detail on bases, strands, and bonding, use the dedicated A1.2 nucleic acids revision page before doing mixed molecular-biology questions.

Transcription makes an mRNA copy from a DNA template. Translation uses the mRNA sequence at a ribosome to build a polypeptide.

DNA replication is semi-conservative. Each new DNA molecule contains one original strand and one newly synthesised strand. DNA helicase unwinds and separates strands; DNA polymerase adds complementary nucleotides.

PCR, Respiration, and Photosynthesis

PCR amplifies a specific DNA segment:

1. Denaturation: heat separates DNA strands.
2. Annealing: primers bind to target sequences.
3. Extension: Taq polymerase builds new strands.

Cell respiration releases usable energy from glucose, usually as ATP. Photosynthesis stores light energy in organic molecules. Their summary equations are related, but do not write that photosynthesis is simply respiration in reverse; the pathways, locations, and enzyme-controlled steps differ.

Question-Type Breakdown for IB Biology Molecular Biology

Exam-Ready Mini Checklist

Before moving on, check:

• Did I know which reaction releases water and which uses water?
• Did I link enzyme function to active-site shape and activation energy?
• Did I explain denaturation without saying enzymes "die"?
• Did I distinguish DNA from RNA using sugar, base, strand number, and role?
• Did I separate transcription from translation?
• Did I describe PCR as amplification of a specific DNA segment?
• Did I avoid saying photosynthesis is exactly respiration backwards?

Use A1.2 Nucleic acids questions for targeted DNA/RNA practice, then broaden into the IB Biology question bank. Put repeated mechanism errors, such as hydrolysis wording or enzyme graph wording, into flashcards.

FAQ

What is the hardest part of IB Biology molecular biology?

The hardest part is usually linking molecular structure to biological function. You need to connect polarity to water properties, protein shape to enzyme action, base sequence to protein synthesis, and energy transfer to respiration and photosynthesis.

What is the difference between condensation and hydrolysis?

Condensation joins molecules and releases water. Hydrolysis breaks bonds using water. A common exam mistake is reversing water's role, so practise writing both reactions with the bond formed or broken.

Why does enzyme activity fall after the optimum temperature?

After the optimum temperature, heat disrupts bonds that maintain the enzyme's tertiary structure. The active site changes shape, substrates bind less effectively, and the reaction rate falls.

What is the difference between transcription and translation?

Transcription makes mRNA from a DNA template. Translation uses mRNA codons at a ribosome to build a polypeptide. A simple exam chain is DNA -> mRNA -> amino acid sequence -> polypeptide.

Are respiration and photosynthesis exact opposites?

No. Their summary equations are related, but the pathways are not simply the same process in reverse. Treat them as linked energy-transfer processes with different locations, stages, and enzyme-controlled steps.