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IB Biology revision notes

Group 4 · Sciences · SL / HL

Theme A asks what all living things share and how they differ. The best place to start is the cell: the basic unit of life.

Course topics

A · Unity and diversity · Water, nucleic acids, cells, viruses, classification, evolution · 9 subtopics
B · Form and function · Biomolecules, membranes, gas exchange, transport, adaptation · 10 subtopics
C · Interaction and interdependence · Enzymes, respiration, photosynthesis, signalling, ecosystems · 9 subtopics
D · Continuity and change · DNA replication, cell division, inheritance, homeostasis, natural selection · 12 subtopics

Free lesson: A · Unity and diversity

Cell theory: All living things are made of cells, and cells come from existing cells
Prokaryotes: No nucleus, 70S ribosomes, DNA in a nucleoid (e.g. bacteria)
Eukaryotes: Nucleus and membrane-bound organelles, 80S ribosomes (e.g. animals, plants)

Feature	Prokaryote	Eukaryote
Nucleus	No (nucleoid)	Yes
Ribosomes	70S	80S (70S in mitochondria)
Membrane-bound organelles	No	Yes
DNA	Circular, naked, plus plasmids	Linear, with histones
Size	About 1–10 µm	About 10–100 µm

\[\text{magnification}=\frac{\text{image size}}{\text{actual size}}\]

Use the same units for both: 1 mm = 1000 µm.

Water is polar, so molecules form hydrogen bonds. This explains cohesion, adhesion, its high specific heat capacity and why it is a good solvent.

Worked example

A cell is 20 µm wide. In a drawing it is 40 mm wide. What is the magnification?

Put both in the same unit: 40 mm = 40,000 µm.
Magnification = image ÷ actual = 40,000 ÷ 20.
= ×2,000.
Write it with a × sign and no units.

Common mistakes

Mixing mm and µm in the magnification formula.
Saying prokaryotes have no ribosomes. They do: 70S.
Calling mitochondria the “powerhouse” without explaining aerobic respiration and ATP.
Confusing cell wall (plants, fungi, bacteria) with cell membrane (all cells).

Quick check

Which structure is found in prokaryotic cells?

70S ribosomes. Prokaryotes lack membrane-bound organelles but have 70S ribosomes.

An image is 5 mm long and the actual object is 10 µm. What is the magnification?

×500. 5 mm = 5,000 µm. 5,000 ÷ 10 = ×500.

Which organelle carries out aerobic respiration?

Mitochondrion. Mitochondria carry out the later stages of aerobic respiration and produce most of the cell's ATP.

Why does water have a high specific heat capacity?

Hydrogen bonds must be broken to raise its temperature. Energy goes into breaking hydrogen bonds before molecules speed up, so temperature rises slowly.

Practise this in the app. Interactive diagrams, quizzes, exam-style papers with mark schemes and a personal study planner. Open Biology in FortyFive →

FAQ

What topics are in IB Biology?

IB Biology (SL / HL) is organised into: A · Unity and diversity; B · Form and function; C · Interaction and interdependence; D · Continuity and change.

Are these IB Biology notes free?

Yes. The topic pages, key terms and the first lesson are free. The full lessons, interactive diagrams and exam-style practice papers with mark schemes are in the FortyFive app, which you can start for free.

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