IB DP Biology: Cell structure and microscopy (DP Biology)

This study guide covers section A2.2 (Cell structure) of the IB Biology guide, first assessment 2025, for DP Biology. Statements A2.2.1 to A2.2.11 are for both SL and HL. Statements A2.2.12 to A2.2.14 are additional higher level, so they are HL only and are marked that way below.

Use it with the revision notes and the practice set. See also the IB DP Biology hub and the printable checklist. Not sure where to start? Take a free 10-minute diagnostic. For the whole theme, see Theme A: Unity and Diversity.

What this unit covers

Statement	Topic	Level
A2.2.1	Cell theory and deduction	SL and HL
A2.2.2	Microscopy skills, graticule, magnification, scale bars	SL and HL
A2.2.3	Developments in microscopy	SL and HL
A2.2.4	Structures common to all cells	SL and HL
A2.2.5	Prokaryote cell structure	SL and HL
A2.2.6	Eukaryote cell structure	SL and HL
A2.2.7	Processes of life in unicellular organisms	SL and HL
A2.2.8	Animal, fungal and plant cells	SL and HL
A2.2.9	Atypical cell structure	SL and HL
A2.2.10	Cells and structures in micrographs	SL and HL
A2.2.11	Drawing and annotation	SL and HL
A2.2.12	Endosymbiosis	HL only
A2.2.13	Cell differentiation	HL only
A2.2.14	Evolution of multicellularity	HL only

A2.2.1 Cells as the basic unit of life

Cell theory has three parts. All living organisms are made of one or more cells. The cell is the smallest unit that carries out the functions of life. New cells come only from existing cells.

The nature of science point is deduction: a theory generates a prediction about a specific case.

Worked example. A team finds an unfamiliar organism in a hot spring. Cell theory predicts that it is made of at least one cell, and A2.2.4 adds that it will have DNA, watery cytoplasm and a plasma membrane.

A2.2.2 Microscopy skills

The guide expects practical experience of making temporary mounts, staining, focusing, measuring with a graticule, calculating size and magnification, drawing scale bars and photographing the view. Measuring with an instrument is quantitative observation.

Focusing. Start on the lowest power with the coarse focus, then sharpen with the fine focus. After changing to a higher objective, use the fine focus alone.

Staining. Most cells are almost transparent, so a stain such as iodine solution or methylene blue adds contrast.

Calibrating an eyepiece graticule

An eyepiece graticule is a ruler inside the eyepiece with arbitrary divisions. Its value changes with each objective lens, so you calibrate it against a stage micrometer, which has divisions of known length.

Worked example. Under one objective, 40 graticule divisions line up with 25 stage micrometer divisions of 10 µm each.

• Length of 25 stage divisions = 25 × 10 = 250 µm
• One graticule division = 250 ÷ 40 = 6.25 µm
• An onion epidermal cell that spans 14 divisions is 14 × 6.25 = 87.5 µm long.

Under a higher-power objective the value of a division is smaller, so you recalibrate for every objective.

Magnification, image size and actual size

Use the triangle:

magnification = image size ÷ actual size

Put image size and actual size in the same unit before dividing. 1 mm = 1000 µm and 1 µm = 1000 nm.

Worked example (electron micrograph). A printed micrograph carries a scale bar 25 mm long labelled 1 µm.

• Magnification = 25 mm ÷ 1 µm = 25 000 µm ÷ 1 µm = ×25 000
• A mitochondrion measures 46 mm on the print. Actual length = 46 mm ÷ 25 000 = 0.00184 mm = 1.84 µm.

Worked example (light micrograph). A photograph taken at ×150 shows a cell 48 mm long. Actual length = 48 ÷ 150 = 0.32 mm = 320 µm.

Making a scale bar

A scale bar is a line on the image that stands for a stated real length. If a magnification is known, bar length on the page = real length × magnification.

Worked example. You want a 2 µm bar on an image at ×12 000. Bar length = 2 µm × 12 000 = 24 000 µm = 24 mm. Draw a 24 mm line labelled “2 µm”. A scale bar stays correct if the image is resized.

A2.2.3 Developments in microscopy

Electron microscopy. Electrons have a much shorter wavelength than visible light, so electron microscopes give far higher resolution (the ability to tell two close points apart). They show ultrastructure such as ribosomes and membranes. Transmission electron microscopes image thin slices. Scanning electron microscopes show surfaces in three dimensions. The costs: specimens are dead, in a vacuum and stained with heavy metals, so images are greyscale and may contain artefacts.

Freeze fracture. A frozen specimen is cracked. Membranes tend to split between their two lipid layers, exposing embedded proteins. A replica of the surface is examined by electron microscopy.

Cryogenic electron microscopy (cryo-EM). Samples are frozen so quickly that water forms a glass-like state, not ice crystals. No stain or chemical fixing is needed, so molecules keep their natural shape. Many images are combined to work out molecular structure.

Fluorescent stains and immunofluorescence. A fluorescent stain absorbs light of one colour and emits another, so the stained structure glows against a dark background. In immunofluorescence a fluorescently labelled antibody binds one specific protein, showing where it is in the cell.

A2.2.4 Structures common to all cells

All cells have:

• DNA as genetic material. It stores the instructions for making proteins and is copied before division.
• Cytoplasm that is mostly water. Water is a solvent, so metabolic reactions take place in solution.
• A plasma membrane made of lipids. Phospholipids form a barrier that lets the cell keep an internal environment different from outside.

A2.2.5 Prokaryote cell structure

The guide’s model is a Gram-positive bacterium such as Bacillus or Staphylococcus.

Component	Detail	Function
Cell wall	Thick wall outside the membrane	Supports the cell and resists bursting
Plasma membrane	Lipid-based	Controls exchange
Cytoplasm	Watery, contains ribosomes, no membrane-bound organelles	Site of metabolism
Naked DNA in a loop	Not wrapped around histones, not enclosed by a membrane; lies in the nucleoid region	Genetic material
70S ribosomes	Smaller than eukaryotic ribosomes	Protein synthesis

Prokaryote structure varies, and you do not need details of those variations.

A2.2.6 Eukaryote cell structure

Eukaryote cytoplasm is compartmentalized: membranes create regions with different conditions and enzymes, so incompatible reactions run at once.

Structure	Features	Function
Nucleus	Double membrane with pores; chromosomes of DNA bound to histones	Stores DNA, controls gene expression
80S ribosomes	Free in cytoplasm or on rough ER	Protein synthesis
Mitochondrion	Double membrane, inner membrane folded	Aerobic respiration, making ATP
Rough ER	Sacs with ribosomes	Makes proteins for secretion
Smooth ER	Tubes, no ribosomes	Makes lipids
Golgi apparatus	Stack of flattened sacs	Modifies, sorts and packages proteins into vesicles
Vesicles and vacuoles	Single membrane	Transport, storage
Lysosome	Vesicle of digestive enzymes	Breaks down unwanted material
Microtubules and microfilaments	Protein fibres of the cytoskeleton	Shape, support, movement of organelles and the cell

A2.2.7 Processes of life in unicellular organisms

A single cell must do everything. The eight functions to link to structures are reproduction, growth, metabolism, nutrition, excretion, movement, homeostasis and response to stimuli.

Function	Paramecium (heterotroph)	Chlamydomonas (photosynthetic)
Nutrition	Engulfs bacteria into food vacuoles	Photosynthesis in its chloroplast
Movement	Beating cilia	Two flagella
Homeostasis	Contractile vacuole removes excess water	Contractile vacuoles do the same
Response to stimuli	Swims away from harmful chemicals	Eyespot helps it swim towards light
Excretion	Waste leaves across the membrane	Waste leaves across the membrane
Reproduction	Fission (asexual)	Asexual, and can be sexual

Metabolism and growth occur in both: chemical reactions run in the cytoplasm, and the cell grows before it divides.

A2.2.8 Animal, fungal and plant cells

Feature	Animal	Fungus	Plant
Cell wall	None	Present, contains chitin	Present, contains cellulose
Vacuole	Small, temporary	Often present, varying in size	Large central sap vacuole for storage and turgor
Chloroplasts and other plastids	None	None	Chloroplasts; other plastids store starch or pigments
Centrioles	Present	Generally absent	Absent in flowering plants
Cilia and flagella	Often present	Rare	Absent in flowering plants

A2.2.9 Atypical cell structure

Number of nuclei is one way to show atypical structure.

• Aseptate fungal hyphae have no cross walls, so one continuous cytoplasm holds many nuclei.
• Skeletal muscle fibres form when many cells fuse, so one fibre has many nuclei.
• Mature mammalian red blood cells have no nucleus, leaving more room for haemoglobin, but they cannot make new proteins.
• Phloem sieve tube elements have no nucleus when mature. This leaves an open channel for sap.

A2.2.10 Identifying cells and structures in micrographs

Prokaryote, plant or animal?

• Very small (a few µm), no nucleus, pale nucleoid region: prokaryote.
• Cell wall, large sap vacuole or chloroplasts: plant.
• Nucleus but no wall or chloroplasts: animal.

Structures in electron micrographs.

Structure	How it looks
Nucleus	Large, double membrane with gaps (pores), dark clumps of chromatin
Mitochondrion	Oval or rod shape, double membrane, folded inner membrane
Chloroplast	Large, double membrane, stacks of flattened sacs inside
Sap vacuole	Large, pale, single membrane, often pushes other organelles aside
Golgi apparatus	Stack of curved, flattened sacs with small vesicles budding off
Rough ER	Flattened sacs with dark dots (ribosomes) on the surface
Smooth ER	Network of tubes with no dots
Ribosomes	Tiny dark dots, free or attached
Chromosomes	Dark rods, visible in dividing cells
Cell wall	Thick layer outside the thin plasma membrane
Plasma membrane	Thin double line of two dark layers around the cytoplasm
Secretory vesicles	Small round membrane-bound sacs near the Golgi apparatus or the cell surface
Microvilli	Finger-like projections of the surface that increase surface area
Nucleoid region	Pale area without a membrane, containing the DNA

A2.2.11 Drawing and annotating electron micrographs

You may be asked to draw the nucleus, mitochondria, chloroplasts, sap vacuole, Golgi apparatus, rough and smooth ER and chromosomes, and also the cell wall, plasma membrane, secretory vesicles and microvilli. Use a sharp pencil, clear continuous lines, no shading and ruled label lines that do not cross. Include a scale bar or magnification. Annotation means adding the function to the label, not just the name.

Example annotations for a mitochondrion.

• Outer membrane: boundary of the organelle.
• Inner membrane folded into cristae: large surface area for the enzymes that make ATP.
• Matrix: fluid containing enzymes for the early stages of aerobic respiration.
• Secretory vesicle: carries proteins to the plasma membrane for release.
• Microvilli: increase surface area for absorption.

HL only: A2.2.12 Origin of eukaryotic cells by endosymbiosis

The theory says that all eukaryotes descend from a common unicellular ancestor that already had a nucleus and reproduced sexually. Mitochondria then arose when this ancestor took in an aerobic prokaryote that was not digested but lived on inside it. In some lineages, chloroplasts arose later in a similar way from a photosynthetic prokaryote.

Evidence: mitochondria and chloroplasts have 70S ribosomes, naked circular DNA, and can replicate inside the cell. The nature of science point is that a theory is strong when it explains many observations and supports predictions.

HL only: A2.2.13 Cell differentiation

Cells in an organism share the same genes, yet a muscle cell and a nerve cell differ greatly. Differentiation, the process of becoming specialized, depends on different patterns of gene expression: some genes are active in a cell type and others are not. These patterns are often triggered by changes in the cell’s environment, such as chemical signals from neighbouring cells.

HL only: A2.2.14 Evolution of multicellularity

Multicellularity evolved repeatedly. Many fungi, eukaryotic algae and all plants and animals are multicellular. The advantages are larger body size and cell specialization: different cells can concentrate on different jobs rather than every cell doing everything.

Common errors

• Writing that prokaryotes have “no DNA” or “no ribosomes”. They have both, but the DNA is naked and the ribosomes are 70S.
• Saying plant cell walls contain chitin. That is fungi.
• Saying that differentiating cells lose genes. They switch genes on or off.

Official syllabus

This resource is aligned to the IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization, section A2.2 (Cell structure).