A biologist would use an electron microscope to study sub-cellular structures, most commonly a transmission electron microscope (TEM).

Quick Scoop: The Core Idea

To see sub-cellular structures (like mitochondria, ribosomes, or membranes), you need extremely high resolution—far beyond what normal light microscopes can provide.

  • Light microscopes resolve down to about 200–300 nanometers, which is enough for whole cells and some organelles, but not fine internal details.
  • Electron microscopes use beams of electrons instead of light and can resolve structures at the nanometer scale, making them ideal for detailed views of internal cell architecture.

So, in the context of a typical biology exam or textbook question:

Answer: Transmission electron microscope (TEM) (or simply “electron microscope”).

Why Not Just a Light Microscope?

Modern light microscopy (including confocal and advanced fluorescence methods) is excellent for live-cell imaging and many subcellular structures, because its resolution is well matched to many organelles and dynamic processes. However:

  • Very small structures below the diffraction limit (around 200 nm) cannot be clearly resolved with standard light microscopy.
  • Electron microscopy breaks this limit and allows visualization of much finer details, such as membrane layers, nuclear pores, and detailed organelle ultrastructure.

A Quick Story-Like Picture

Imagine a biologist studying the intricate folds inside a mitochondrion. With a high-end fluorescence microscope, they can see where the mitochondria are and how they move in a living cell. But when they want a detailed “blueprint- style” image of those internal folds and the surrounding membranes, they prepare ultra-thin sections of the cell and look at them under a TEM, revealing crisp, black‑and‑white nanometer-scale detail.

TL;DR: For sub-cellular structures in the classic sense of fine internal detail, the expected answer is transmission electron microscope (TEM) (or simply electron microscope).