This has enabled biologists to see and understand many more sub-cellular…

The idea

Before electron microscopy, cell biology relied on light microscopes, which resolved the nucleus, cytoplasm, cell membrane and sometimes chloroplasts, but little else. Electron microscopy revealed organelles such as mitochondria with their cristae, ribosomes, the endoplasmic reticulum, Golgi apparatus and lysosomes, and showed the double membrane of the nucleus and the internal membranes of chloroplasts. Seeing these structures allowed biologists to propose functions: ribosomes as sites of protein synthesis, mitochondria as sites of aerobic respiration and the Golgi apparatus in modifying and packaging proteins. For example, the observation that pancreatic cells are packed with rough endoplasmic reticulum supported the idea that they secrete proteins. Structure therefore informs function, and improved microscopy has driven advances in cell biology, medicine and biotechnology.

What earns the marks

• Electron microscopy revealed organelles that light microscopy could not resolve, including ribosomes, endoplasmic reticulum, Golgi apparatus and lysosomes.
• Observing a structure allows biologists to infer its function, for example ribosomes as sites of protein synthesis.
• Correlating organelle abundance with cell role, such as many mitochondria in muscle cells, supports structure-function reasoning.
• Advances in microscopy have contributed to fields such as medicine, genetics and biotechnology.
• Understanding sub-cellular structure underpins later topics including transport, enzymes and cell division.

Common mistakes and how to put them right

• Listing organelles without linking each to the function it performs.

  Put it right: Pair every structure with its role, such as ribosomes with protein synthesis.

• Implying that light microscopes show no organelles at all.

  Put it right: They do show the nucleus, cytoplasm, cell membrane and often chloroplasts, but not finer structures.

• Treating the history of microscopy as a list of dates rather than explaining how better resolution led to new biological understanding.

Exam tips

• Use the structure-function link explicitly: name the organelle, then state what its structure allows it to do.
• Choose examples that match the cell type, such as many mitochondria in a sperm cell or muscle cell.
• If asked about evidence, describe what was observed and the conclusion drawn, keeping observation and inference distinct.