Cell structure — IB Diploma Biology SL
Cell theory, microscopy skills and techniques, prokaryote and eukaryote cells, atypical cells, and micrographs.
Revision notes
7 short notes, in the order of the specification. Each one in short:
Cells are the basic structural unit of all living organisms: every organism consists of one or more cells. Typical cells share three structures: DNA as the genetic material, a cytoplasm composed mainly of water, and a plasma membrane composed of lipids that encloses the cytoplasm. Each is needed for the cell to function and reproduce.
Microscopy skills
PracticalMicroscopy skills include making temporary mounts, staining, focusing with the coarse and fine adjustments, measuring with a calibrated eyepiece graticule and taking photographs. Magnification is image size divided by actual size, so actual size equals image size divided by magnification. Always convert both measurements to the same unit, such as micrometres, before dividing.
Electron microscopes use a beam of electrons, which has a much shorter wavelength than light, so they have far higher resolution and reveal cell ultrastructure. Freeze fracture splits frozen membranes to show the proteins inside them, cryogenic electron microscopy reveals the 3D structure of proteins, and fluorescent stains and immunofluorescence show where specific molecules are inside cells.
Prokaryote cells have a cell wall, plasma membrane, cytoplasm, naked DNA in a loop and 70S ribosomes, but no nucleus; Gram-positive eubacteria such as Bacillus and Staphylococcus are the type studied. Eukaryote cells have a compartmentalised cytoplasm with 80S ribosomes, a nucleus with a double membrane and pores, membrane-bound organelles and a cytoskeleton.
A unicellular organism must carry out all the functions of life within one cell: homeostasis, metabolism, nutrition, movement, excretion, growth, response to stimuli and reproduction. In Paramecium, for example, contractile vacuoles expel excess water for homeostasis, cilia provide movement, food vacuoles digest food, and the cell reproduces by dividing in two.
Plant, fungal and animal cells differ in their cell walls, vacuoles, plastids, centrioles, cilia and flagella. Plants have cellulose walls, large permanent vacuoles and chloroplasts; fungi have chitin walls and no plastids; animals have no wall but have centrioles. Atypical cells include multinucleate aseptate fungal hyphae and skeletal muscle fibres, and red blood cells and sieve tubes without nuclei.
In light and electron micrographs, cells are identified as prokaryote, plant or animal from features such as the nucleus, cell wall, vacuole and chloroplasts. In electron micrographs you should recognise structures including the mitochondrion, Golgi apparatus and rough and smooth endoplasmic reticulum. Drawings use clear pencil lines and ruled labels, and annotations state each structure's function.
Exam questions
6 exam-style questions (18 marks), each with its mark scheme.
Answer the questionsFlashcards
27 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of cells (unity and diversity) on one page, so you can see where this subtopic fits.
Open the mind mapPrint it
Free PDFs to print or save.
Quick check questions
State the three parts of cell theory.
Show answer
All organisms consist of one or more cells; cells are the basic structural unit of life; cells only arise from pre-existing cells.
Why must an eyepiece graticule be calibrated for each objective lens?
Show answer
The length of specimen covered by one eyepiece unit changes with magnification.
What size of ribosome do prokaryotes have?
Show answer
70S.
Which substance makes up fungal cell walls?
Show answer
Chitin.
Frequently asked questions
What is the difference between prokaryotic and eukaryotic cells?
Prokaryotic cells have no nucleus: their DNA is a naked loop in the cytoplasm, and they have 70S ribosomes and no membrane-bound organelles. Eukaryotic cells have a nucleus with a double membrane and pores, chromosomes of DNA bound to histones, 80S ribosomes, membrane-bound organelles and a cytoskeleton.
How do you calculate magnification in biology?
Magnification equals the size of the image divided by the actual size of the specimen. Measure the image with a ruler, convert both values to the same unit, usually micrometres (1 mm = 1000 µm), then divide. Rearranged, actual size equals image size divided by magnification, which is how cell sizes are found from micrographs.
How do you calculate magnification using a scale bar?
Measure the length of the scale bar on the image with a ruler, convert it to the same unit as the value written on the bar, then divide the measured length by that value. For example, a 20 mm bar labelled 0.5 µm gives 20 000 µm divided by 0.5 µm, a magnification of ×40 000.
Why do electron microscopes have higher resolution than light microscopes?
Electron microscopes have higher resolution because a beam of electrons has a much shorter wavelength than light. They can distinguish points about 1–2 nm apart, compared with about 200 nm for a light microscope, so they reveal ultrastructure such as ribosomes, membranes and the internal structure of mitochondria and chloroplasts.
Written and checked against the IB Biology SL specification · Updated October 2026