An atomic force microscope can magnify a living cell, whereas electron microscopes and scanning tunneling microscopes cannot. What requirement of scanning tunneling microscopes precludes the imaging of living specimens?

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Understand the principle of how a scanning tunneling microscope (STM) works: it relies on the quantum tunneling of electrons between a conductive tip and the specimen surface when they are extremely close.

Recognize that for STM to function, the specimen must be electrically conductive or at least semiconductive to allow the tunneling current to flow.

Consider the nature of living cells, which are generally non-conductive or poorly conductive due to their composition (mostly water and organic molecules).

Realize that because living cells do not provide the necessary conductive surface, the STM cannot generate the tunneling current required to form an image.

Conclude that the requirement of electrical conductivity in the specimen is what precludes the imaging of living specimens by scanning tunneling microscopes.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Principle of Scanning Tunneling Microscopy (STM)

STM operates by measuring the tunneling current between a conductive tip and the specimen's surface, requiring the sample to be electrically conductive or semiconductive. This electrical requirement limits STM's ability to image non-conductive or living biological samples directly.

Sample Requirements for Electron and Scanning Tunneling Microscopes

Electron microscopes require samples to be fixed, dehydrated, and often coated with conductive materials, making them unsuitable for live imaging. Similarly, STM needs a conductive surface and vacuum or controlled environments, which living cells cannot tolerate.

Advantages of Atomic Force Microscopy (AFM) for Live Cell Imaging

AFM uses a mechanical probe to scan the surface topology without needing electrical conductivity or vacuum conditions. This allows AFM to image living cells in their natural, hydrated state, preserving cell viability during observation.

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