A2.2Theme A — Unity and diversity

Cell structure — IB Biology SL Revision Notes & Topical Questions

10 min read 13 parts Lesson 8 of 11 2 quizzes

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Lesson summary

A2.2 Cell structure — IB BIOLOGY SL notes: Describe cell theory, microscopy and the structure of prokaryotic and eukaryotic cells, including atypical cells.Cell theory — all living organisms are made of one or more cellsCells are the basic functional unit of living organisms.New …

Shared features
Deductive reasoning
Inductive reasoning
Data types
Liquid mount
Solid mount

Section 1 of 13Cell theory

1Cell theory

Cell theory all living organisms are made of one or more cells
Cells are the basic functional unit of living organisms.
New cells are produced from pre-existing cells.

2History

Hooke named cells in the 1660s from cork, and Schleiden and Schwann proposed cell theory in 1837

3Shared features

Shared features every cell has a membrane, genetic material and enzyme-catalysed reactions

4Deductive reasoning

Deductive reasoning moves from a general theory to a specific prediction that is tested
Cell theory predicts that a newly discovered organism will contain one or more cells.

5Inductive reasoning

Inductive reasoning many specific observations lead to a general theory

6Data types

Data types measurements with instruments are quantitative observations and colour or the presence of structures are qualitative

Section 2 of 13Using a light microscope

Using a light microscope

Parts

Parts eyepiece lens, objective lenses, stage with clips, light source, condenser, and coarse and fine focus

Liquid mount

Liquid mount add drops to the slide with a pipette and lower the coverslip to avoid bubbles

Solid mount

Solid mount cut a thin layer with a scalpel or forceps, stain if needed and lower the coverslip
Wear gloves and take care with sharp tools.

Place the slide on the stage and fix it with the clips.

Start on the low-power objective and use the coarse focus until the specimen is in focus.

Move to a higher objective only after focusing, and never move the stage down while changing objective.

At high power use only the fine focus.

Blurry image
- return to low power, check the specimen is thin enough and check for contamination

Section 3 of 13Magnification and measurement

Magnification

Magnification image size / actual size

Convert all measurements to the same units, and magnification has no units.
1 mm = 1000 µm and 1 µm = 1000 nm.

Scale bar

Scale bar measure the bar with a ruler, convert to the units written on it and divide by the real length

Adding a scale bar

Make the bar about 20% of the specimen length.
Write the real length it represents underneath.

Eyepiece graticule

Eyepiece graticule a scale in the eyepiece with no fixed units
Calibrate it for each objective lens with a stage micrometer.

Place a stage micrometer on the stage and line up the two scales.

Calculate the value of one graticule division.

Replace the micrometer with the specimen.

Count the divisions across the cell.

Multiply the divisions by the value of one division to find the actual size.

Section 4 of 13Light and electron microscopes

Plant cell structures in a micrograph-style view

Plant cells viewed with a transmission electron microscope

Plant cells viewed with a light microscope

Feature	Light	TEM	SEM
Radiation	Light	Electrons	Electrons
Resolution	About 200 nm	About 0.2 nm	About 5 nm
Image	Colour, 2D	Black and white, 2D section	Black and white, 3D surface
Living specimens	Yes	No	No
Useful magnification	Up to about ×1500	Over ×500 000	High, but below TEM
Preparation	Simple and quick	Thin sections and vacuum, artefacts possible	Coated whole specimen and vacuum
Portability	Small and portable	Large and fixed	Large and fixed

Magnification

Magnification how many times larger the image is than the object

Resolution

Resolution the ability to see two close objects as separate

Light cannot resolve points closer than half its wavelength, which is about 200 nm.
The shorter wavelength of electrons gives far higher resolution.

Light microscope

Light microscope shows eukaryotic cells, nuclei and possibly mitochondria and chloroplasts
It cannot show ribosomes, the endoplasmic reticulum or lysosomes.

TEM

TEM electrons pass through a thin section and denser parts absorb more, so they appear darker

SEM

SEM electrons bounce off the surface, giving a 3D image of the surface

Artefacts

Artefacts features from specimen preparation that look like real structures

Section 5 of 13Developments in microscopy

1Fluorescent stains

Fluorescent stains dyes bind to specific structures and glow under UV

2Immunofluorescence

Immunofluorescence antibodies carrying fluorescent dyes bind to chosen proteins, such as virus proteins

3Freeze fracture

Freeze fracture frozen cells are split along membranes in a vacuum to give a planar view of membrane interiors
It revealed proteins spanning the bilayer.

4Cryo-electron microscopy

Cryo-electron microscopy solutions of proteins are flash-frozen and imaged
Software builds a 3D model from images of individual molecules.
It has improved knowledge of virus structure, membranes and protein synthesis.

Section 6 of 13Features of all cells and prokaryotes

Features of all cells and prokaryotes

Plasma membrane

Plasma membrane a phospholipid bilayer that controls what enters and leaves
Its proteins act in cell recognition, communication and transport.

Cytoplasm

Cytoplasm watery cytosol with ions, small molecules and macromolecules, where many reactions occur

DNA

DNA genetic material, which lets new cells form from old ones and controls enzyme production

Prokaryote

Prokaryote a cell without a nucleus or membrane-bound organelles, 0.1-5 µm across

The two domains are Bacteria, such as E. coli, and Archaea, often found in extreme environments.
DNA is a naked circular molecule in the nucleoid, with small plasmids that may carry antibiotic resistance.
Ribosomes are 70S.
Most have a peptidoglycan wall that gives shape and stops bursting.

Gram positive

Gram positive a thick wall holds crystal violet and stains purple, as in Staphylococcus

Extras

Extras a slime capsule resists drying and immune attack, flagella move the cell and pili attach to surfaces and transfer DNA

Section 7 of 13Eukaryotic organelles

Feature	Prokaryote	Eukaryote
Nucleus	Absent, DNA in the nucleoid	Present with a double membrane
DNA	Circular and naked, with plasmids	Linear, wrapped around histones
Ribosomes	70S	80S in the cytoplasm
Organelles	None membrane-bound	Mitochondria, ER, Golgi and more
Cell wall	Peptidoglycan	Cellulose (plants), chitin (fungi), none (animals)
Typical size	1-5 µm	10-100 µm

Eukaryotic organelles

Compartmentalisation

Compartmentalisation gives higher enzyme and substrate concentrations, isolates harmful enzymes and allows separate optimum pH

Nucleus

Nucleus a double membrane with pores for mRNA and ribosome subunits out and enzymes in
Chromatin is linear DNA wrapped around histone proteins.
The nucleolus makes ribosomes.

Rough ER

Rough ER flattened sacs covered with 80S ribosomes that make proteins for secretion

Golgi apparatus

Golgi apparatus modifies proteins and lipids and packages them into vesicles
Cargo includes secreted hormones such as insulin, lysosome enzymes and membrane proteins.

Mitochondrion

Mitochondrion double membrane with folded cristae, containing the enzymes of aerobic respiration, circular DNA and ribosomes

Lysosome

Lysosome a vesicle with hydrolytic enzymes that digests worn-out organelles, and is used in immunity and apoptosis

Microtubules

Microtubules tubulin cylinders about 25 nm wide forming the cytoskeleton for support and movement

Section 8 of 13Plant, animal and fungal cells

Feature	Plant	Animal	Fungus
Cell wall	Cellulose	None	Chitin, glucans, glycoproteins
Vacuole	Large and permanent	Small and temporary	Small and non-permanent
Chloroplasts	In photosynthetic cells	None	None
Centrioles	Absent	Present	Absent
Cilia or flagella	None	Often present	None in true fungi
Carbohydrate store	Starch	Glycogen	Glycogen

Chloroplast

Chloroplast larger than a mitochondrion with a double membrane
Thylakoids stack into grana joined by lamellae.
The light reactions occur in the thylakoids and the Calvin cycle in the stroma.

Vacuole

Vacuole bounded by the tonoplast and stores sap

Cell wall

Cell wall extracellular and freely permeable, giving plant cells a fixed shape
Plasmodesmata link neighbouring plant cells through the walls.

Animal cilia

Animal cilia move an egg along the oviduct or sweep fluids in the respiratory tract

Fungal shape

Fungal shape fungal cells have walls but may still change shape

Section 9 of 13Functions of life and atypical cells

Functions of life and atypical cells

Seven functions

Seven functions metabolism, reproduction, homeostasis, growth, response, excretion and nutrition

Unicellular organisms

Unicellular organisms carry out every function in one cell
Paramecium uses cilia to move and a contractile vacuole to remove water.
Chlamydomonas has a chloroplast, an eyespot and flagella.

Red blood cells

Red blood cells have no nucleus, giving more room for haemoglobin, and a biconcave shape for a large surface area

Aseptate fungal hyphae

Aseptate fungal hyphae no cross-walls, so many nuclei share one cytoplasm

Skeletal muscle

Skeletal muscle long multinucleate fibres formed by the fusion of cells, up to 300 mm long

Phloem sieve tubes

Phloem sieve tubes no nucleus, mitochondria or ribosomes, so they depend on companion cells

Challenge to cell theory

Challenge to cell theory these atypical cells question the idea that cells work independently

Section 10 of 13Interpreting micrographs and drawing cells

Interpreting micrographs and drawing cells

Look for a nucleus to decide prokaryote or eukaryote.

Look for a cell wall or large vacuole to decide plant or animal.

Identify the abundant organelles and relate them to function.

Recognising organelles

Recognising organelles mitochondria are oval with cristae, and chloroplasts are larger with grana
Nuclei have a membrane and dark nucleolus, and vacuoles look pale.

Examples

Examples many lysosomes and mitochondria suggest a phagocytic cell such as a macrophage
Microvilli and many mitochondria suggest an absorptive cell lining the small intestine.

Drawing conventions

Title, magnification recorded and a scale bar if used.
A sharp HB pencil with clear single lines and no shading.
A drawing that fills the space with correct proportions.
Label lines drawn with a ruler, kept to one side, not crossing and without arrowheads.

Plan diagrams

Plan diagrams draw tissue outlines only and never individual cells

Section 11 of 13More from the specification

Electron micrograph
- A photograph taken through an electron microscope.

Section 12 of 13Key terms

Term	Meaning
Cell theory	All organisms are made of cells, cells are the basic unit of life and cells arise from existing cells.
Resolution	The smallest distance at which two points can still be seen as separate.
Eyepiece graticule	A scale in the eyepiece that is calibrated with a stage micrometer to measure specimens.
Prokaryote	A cell without a nucleus or membrane-bound organelles.
Eukaryote	A cell with a nucleus and membrane-bound organelles.
Nucleoid	The region of a prokaryotic cell holding its naked circular DNA.
Compartmentalisation	The division of a cell into membrane-bound regions so that processes stay apart.
Atypical cell	A cell, such as a multinucleate one or one without a nucleus, that does not fit the standard pattern.

Section 13 of 13Exam technique

Confusing magnification with resolution — magnification is the enlargement and resolution is the detail that can be separated

Mixing units in magnification calculations — convert image and actual size to the same unit first

Saying electron microscopes can view living cells — the vacuum means specimens must be dead

Saying all eukaryotic cells have a nucleus — mature red blood cells and sieve tube elements do not

Last updated: 26 October 2026

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A ⇔ A = I / M ⇔ I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.