1.1 - Microscopy

Light microscopes

• A microscope lets us see things too small to see with the naked eye, which is why microscopes are central to studying cells.
• The light microscope is the older, more common type. It uses light and a system of lenses to form a magnified image of a specimen.
• Light microscopes were first used in the 1590s.

Key features

• Magnification - Up to roughly 1,500 times - enough to see individual cells and larger sub-cellular structures inside them, such as the nucleus and chloroplasts.
• Living specimens - Can be used to view living specimens, so cells can be studied while they are still alive.

Electron microscopes

• An electron microscope forms an image using a beam of electrons instead of light.
• They were developed in the 1930s.
• They have both a much higher magnification and a much higher resolving power than a light microscope - so they can show far smaller structures in far greater detail.

Revealing the inside of the cell

• Electron microscopes let biologists see the internal structure of cells for the first time, including:
  • The inside of mitochondria and chloroplasts
  • Very small features such as plasmids and viruses, where a light microscope showed nothing
• Seeing this internal detail let scientists work out how each structure carries out its job - for example how the inside of a mitochondrion is suited to respiration and how the inside of a chloroplast is suited to photosynthesis.
• The development of electron microscopy therefore greatly increased our understanding of how cells are built and how they work.

Drawbacks

• Expensive and not portable.
• Need a complicated process to prepare the specimen.
• That preparation kills the specimen, so - unlike a light microscope - an electron microscope cannot be used to view living material.

Magnification and resolution

A microscope does two separate things to a specimen, and it's worth keeping them apart:

1. Magnification (magnifying power) - How many times bigger the microscope makes the specimen appear. A magnification of x100 means the image looks 100 times larger than the real object.
2. Resolution (resolving power) - How well a microscope can distinguish between two points that are very close together - seeing them as two separate points rather than as a single blur.

Why both matter

• A higher resolution produces a sharper, more detailed image, because close-together features stay separate instead of merging.
• Magnification and resolution are not the same thing. Magnifying an image more doesn't help if the resolution is too low - the enlarged image would just be a bigger blur.
• This is why the electron microscope reveals so much more than the light microscope: it has both a much higher magnification and a much higher resolving power, so the extra magnification actually shows extra detail.

Comparing light and electron microscopes

• The two types of microscope suit different jobs.
• A light microscope is cheap, portable, and can view living specimens - but its lower magnification and resolution mean it shows only cells and their larger internal structures.
• An electron microscope has far higher magnification and resolution, revealing fine sub-cellular detail - but it is expensive, not portable, and cannot be used on living material.

Light vs electron microscopes