Cell differentiation and the evolution of multicellularity

Cells (Unity and diversity) · Cell structure · note 9 of 9

Spec A2.2.13, A2.2.14

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Cell differentiation and the evolution of multicellularitySpec A2.2.13, A2.2.14

In short

Cell differentiation is the process by which cells become specialised to form tissues in multicellular organisms. Almost every cell has the same genes, but different patterns of gene expression, often triggered by changes in the environment, make cells develop differently. Multicellularity has evolved repeatedly and has the advantages of allowing larger body size and cell specialisation.

In a multicellular organism nearly all cells contain the same genome. Cell differentiation is the process by which cells develop into specialised types, forming tissues.

The basis of differentiation is different patterns of gene expression. In each cell type some genes are switched on (transcribed) and others are switched off. Changes in gene expression are often triggered by changes in the cell's environment, such as signal molecules from neighbouring cells or the cell's position in the embryo.

For example, only developing red blood cells express the genes for haemoglobin, and only certain cells in the pancreas express the insulin gene, although almost every body cell carries both genes.

Common mistake:

Differentiation does not remove genes. A specialised cell keeps the full genome; it expresses only some of its genes.

Evolution of multicellularity

Multicellularity has evolved repeatedly, separately in different lineages. Many fungi and eukaryotic algae, and all plants and animals, are multicellular.

Larger body size: an organism is not limited to the size of one cell, which is restricted by its surface area to volume ratio.
Cell specialisation: different cells carry out different functions (division of labour), so each function can be carried out more efficiently.

Quick check

State the three parts of cell theory.
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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?
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The length of specimen covered by one eyepiece unit changes with magnification.
What size of ribosome do prokaryotes have?
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70S.
Which substance makes up fungal cell walls?
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Chitin.
HL only Give two pieces of evidence for the endosymbiotic origin of mitochondria.
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They have 70S ribosomes and naked circular DNA (and can replicate by division).

Written and checked against the IB Biology HL specification · Updated October 2026

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.

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