In complex multicellular organisms, eukaryotic cells become specialised…

The idea

Most somatic cells in a multicellular organism contain identical genomes. Specialisation occurs because transcription factors activate only specific genes, altering the cell's shape and organelle proportions to suit its function. For example, a pancreatic acinar cell contains extensive rough endoplasmic reticulum and Golgi apparatus to synthesise and secrete digestive enzymes. A neutrophil possesses numerous lysosomes with hydrolytic enzymes to digest engulfed pathogens. A sperm cell features a mid-piece packed with mitochondria to supply ATP for its flagellum. Some highly specialised cells even lose their DNA entirely; mature mammalian red blood cells lack a nucleus to maximise space for haemoglobin.

What earns the marks

• Describe how differential gene expression leads to specialisation by altering cell shape and organelle proportions without changing the genome of somatic cells.
• Identify the specific organelle that is abundant or modified in a named specialised cell, such as extensive rough endoplasmic reticulum in pancreatic cells.
• Link the abundance of a specific organelle directly to the cell's function, for example, mitochondria releasing ATP for active transport or movement.
• Explain structural losses as functional adaptations, such as the absence of a nucleus in a mature red blood cell to accommodate more haemoglobin.

Common mistakes and how to put them right

• Stating that all cells in a multicellular organism contain identical DNA.

  Put it right by noting that mature mammalian red blood cells lack a nucleus and DNA, while gametes are haploid.

• Writing that sperm cells or muscle cells 'produce energy'.

  Put it right by specifying that their abundant mitochondria release ATP during aerobic respiration.

• Claiming a red blood cell 'has no organelles' when asked about a specific missing structure.

  Put it right: Explicitly naming the absent structure, such as the nucleus.

• Confusing differentiation with mutation.

  Put it right by explaining that specialisation usually involves changes in gene expression via transcription factors, not changes to the DNA base sequence.

Exam tips

• When describing cellular adaptations, explicitly link the structural feature to the process it performs and its ultimate function for the cell.
• If presented with an unfamiliar cell in a table or micrograph, identify which organelles are unusually abundant to deduce the cell's primary function.