IB Biology Cell Biology: Cells, Membranes, and Division

Cell biology feels like a chapter of definitions until a question asks you to explain why cells stay small, why membranes control movement, or why mitochondria support endosymbiotic theory. The marks are not in naming the topic; they are in connecting structure, function, and evidence.

This guide rebuilds Chapter 1 into a source-backed revision route for IB Biology students.

Quick Answer

For IB Biology cell biology revision, focus on these high-yield links:

• cell theory states that living things are made of cells, cells are the smallest unit of life, and cells come from pre-existing cells
• exceptions such as striated muscle, aseptate fungal hyphae, and giant algae show the model has edge cases
• high surface area to volume ratio improves exchange efficiency
• prokaryotes lack a nucleus; eukaryotes are compartmentalised by membrane-bound organelles
• membrane structure explains diffusion, osmosis, facilitated diffusion, active transport, endocytosis, and exocytosis
• stem cells self-renew and differentiate, but therapy questions need benefits and risks
• mitosis supports growth, repair, asexual reproduction, and embryonic development

Core Concept That Gets Marks

Cell biology answers usually need one of three moves:

Cell Theory and Its Exceptions

Cell theory has three core statements:

IB Biology often tests whether you can handle exceptions. Striated muscle fibres are long and multinucleate. Aseptate fungal hyphae have continuous cytoplasm and many nuclei. Giant algae such as Acetabularia are single cells but can be large and complex.

The point is not that cell theory fails. The point is that biological models can be useful while still having edge cases.

Weak Answer vs Mark-Worthy Answer

Surface Area to Volume Ratio

Surface area to volume ratio explains why cells remain small and why exchange surfaces are folded or specialised. As a cell gets larger, volume increases faster than surface area. Demand rises faster than exchange capacity.

Useful exam sentence:

A higher surface area to volume ratio increases exchange efficiency because substances cross the membrane over shorter diffusion distances.

Worked Example 1: Why Cells Stay Small

Question: Explain why very large cells can have difficulty exchanging materials efficiently.

Markscheme-style answer: As cell size increases, volume increases faster than surface area. This lowers the surface area to volume ratio, so there is less membrane surface available per unit volume and longer diffusion pathways. Exchange may become too slow to meet the cell's metabolic needs.

Why this scores: It gives the mathematical relationship, the membrane consequence, and the biological consequence.

Functions of Life and Emergent Properties

Unicellular organisms can perform all functions of life independently: metabolism, response, homeostasis, growth, excretion, reproduction, and nutrition. In exam answers, link the function to an example rather than only listing the terms.

Emergent properties arise when smaller parts interact to produce a new property at a higher level of organisation:

atoms -> molecules -> cells -> tissues -> organs -> organ systems -> organism

Stem Cells and Differentiation

Stem cells can self-renew and differentiate. Most body cells contain the same genome, but different genes are active in different specialised cells.

Therapy questions need balance. Stem cells can replace damaged cells, but answers should also consider source, compatibility, tumour risk, and ethical concerns.

Prokaryotes and Eukaryotes

Avoid writing that prokaryotes have no organelles at all. They have ribosomes, but not membrane-bound organelles.

Organelles and Ultrastructure

Resolution is the ability to distinguish two close points. Magnification is how much larger an image appears than the real object:

magnification = image size / actual size

Membrane Structure and Transport

The membrane is based on a phospholipid bilayer. Phospholipids are amphipathic: hydrophilic heads face the aqueous environment, and hydrophobic tails face inward.

Worked Example 2: Facilitated Diffusion vs Active Transport

Question: A student says that any transport using a membrane protein must require ATP. Explain why this is incorrect.

Markscheme-style answer: Facilitated diffusion uses channel or carrier proteins but moves substances down a concentration gradient, so it does not require ATP. Active transport uses protein pumps or co-transporters to move substances against a gradient and requires energy directly or indirectly.

Why this scores: It separates protein use from energy use and mentions the direction of the gradient.

Origin of Cells and Endosymbiosis

Biogenesis states that cells arise from pre-existing cells. Abiogenesis is the idea that early life came from non-living chemical processes before cells existed.

Endosymbiotic theory explains how mitochondria and chloroplasts may have originated from prokaryotes engulfed by early eukaryotic cells. Evidence includes their own DNA, 70S ribosomes, double membranes, bacterial size, antibiotic susceptibility, and replication from pre-existing organelles. To practise this evidence path, use A2.1 origins of cells questions.

Mitosis and the Cell Cycle

Mitosis is used for growth, asexual reproduction in some eukaryotes, tissue repair, and embryonic development.

Cyclins and cyclin-dependent kinases help control cell cycle progression. If regulation fails, uncontrolled division can produce tumours.

Question-Type Breakdown for IB Biology Cell Biology

Exam-Ready Mini Checklist

Before moving on, check:

• Did I state cell theory accurately?
• Did I know at least three exceptions and why they matter?
• Did I link SA:V to membrane area and diffusion path length?
• Did I say prokaryotes lack a nucleus, not DNA?
• Did I separate facilitated diffusion from active transport?
• Did I explain endosymbiotic evidence, not just the theory?
• Did I describe what happens during each mitosis stage?

Use A2.2 cell structure questions for targeted practice, then broaden into the IB Biology question bank. Turn repeated transport and mitosis errors into flashcards.

FAQ

What is the most important part of IB Biology cell biology?

The most important part is connecting structures to functions. IB questions often reward precise links: membrane structure explains transport, surface area to volume ratio explains exchange rate, organelles explain compartmentalisation, and mitosis explains growth and repair.

Why is surface area to volume ratio tested so often?

Surface area to volume ratio links cell size to exchange efficiency. It explains why cells are small, why cells divide, and why exchange surfaces such as villi and alveoli are folded or highly branched.

What is the difference between facilitated diffusion and active transport?

Facilitated diffusion uses membrane proteins but moves substances down a concentration gradient without ATP. Active transport moves substances against a gradient and requires energy directly or indirectly.

What evidence supports endosymbiotic theory?

Mitochondria and chloroplasts have their own DNA, 70S ribosomes, double membranes, bacterial-like size, and replicate from pre-existing organelles. These features support the idea that they originated from engulfed prokaryotes.

How should I revise mitosis for IB Biology?

Revise mitosis as a sequence of physical events, not just PMAT labels. For each stage, know what happens to chromosomes, spindle fibres, nuclear membranes, and the cytoplasm.