Everything Life Sciences Grade 10

FACT

Sections for TEM

have to be so thin

that they have to be

prepared using a

special piece of

equipment called

an ultramicrotome.

FACT

Transmission

electron

microscopes can

magnify an image

50 million times.

Figure 3.4:

Transmission elec-

tron microscope

in use.

used a simple microscope with only one lens to observe blood cells. He was the first scientist
to describe cells and bacteria through observation under microscope. By combining two or
more lenses, the magnification of the microscopes was improved, thus allowing scientists to
view smaller structures.

Thedissecting microscopeis an optical microscope used to view images in three dimensions
at low resolution. It is useful for low-level magnification of live tissue. The development of
thelight microscope, (Figure 3.5) which uses visible light to magnify images allowed for up
to 1000X magnification of objects through which scientists were able to view individual cells
and internal cell structures such as the cell wall, membrane, mitochondria and chloroplasts.
However, although the light microscope allowed for 1000X magnification, in order to see
even smaller structures such as the internal structure of organelles, microscopes of greater
resolving power (with up to 10 000X magnification) were required.

With the development ofelectron microscopesthe microscopic detail of organelles such
as mitochondria and chloroplasts became easier to observe. TheTransmission Electron Mi-
croscope(TEM) was developed first, followed by theScanning Electron Microscope(SEM).
TEM is used to view extremely thin sections of material. Beams of electrons pass through
the material and are focused by electromagnetic lenses. In SEM the electrons are bounced
off the surface of the material and thus produce a detailed image of the external surface of
the material. They produce a 3D image by picking up secondary electrons knocked off the
surface with an electron collector. The image is then amplified and viewed on a screen.
Examples of each of the image types produced by these microscopes are given in Figures 3.1
to 3.3.

The world through a microscope

Figure 3.1:SEM: A natural com-

munity of bacteria growing on a

single grain of sand.

Figure 3.2: SEM: These pollen

grains show the characteristic

depth of field of SEM micro-

graphs.

Figure 3.3: TEM: Image of

chloroplast, showing thylakoid

discs within a eukaryotic cell.

The apparatus most commonly used in lab microscopy exercises is a simple light micro-
scope. Figure 3.1 shows an annotated diagram of a light microscope with a description of
the function of each part. The main parts are described in the table that follows and the
function of each part is explained.

Chapter 3. The basic units of life 63