3.2.1.3 Electron microscopes

Optical (light) microscopes

• They use light to form an image

• They have a max resolution of 0.2 micrometers

• This means you cannot use it to view organelles smaller than 0.2 micrometers

• That includes ribosomes, endoplasmic reticulum and lysosomes

• The max useful magnification is about x1500

Electron microscopes

• They use beams of electrons to form an image

• Higher resolution that optical microscopes, so more detailed images

• Max resolution of about 0.0002 micrometers

• Max useful magnification is about x1500000

• Produce black and white images

What are the 2 types of electron microscopes?

Transmission electron microscope (TEM) and scanning electron microscope (SEM)

TEM

• It uses electromagnets to focus a beam of electrons, which is then transmitted through the specimen

• Denser parts of the specimen absorb more electrons, which makes them look darker on the image you end up with

• It gives high resolution images, so you can see the internal structure of organelles like chloroplasts

• You have to view the specimen in a vacuum, so it is not good for looking at living organisms

• Vacuum ensures electrons are not absorbed by air molecules

• It can also be used on thin specimen

• 2D images produced

TEM method

• Very thin slices are made, and then stained with heavy metals

• Electrons do not pass through the heavily stained parts because they are absorbed

• Those that pass through are focused by electromagnets onto a fluorescent screen or photographic

• Allows us to see inside cells and organelles

SEM

• It scans a beam of electrons across the specimen

• This knocks off electrons from the specimen, which are gathered in a cathode ray tube to form an image

• The images you end up with show the surface of the specimen and they can be 3D

• It is good because it can be used on thick specimen, but it gives lower resolution images than TEM

What is a graticule?

A more suitable ruler for measuring cells

Stage graticule

It shows true lengths

Eyepiece graticule

• It has regular divisions and they need to be calibrated for each magnification

• It remains constant no matter what magnification the cells are viewed at

How to measure cell size using an eyepiece graticule

• Measure using eyepiece graticule

• Calibrate eyepiece graticule against ruler

• Take a number of measurements to calculate a mean