o In a light microscope, visible light is passed through the object under
the microscope and then through glass lenses.
o The lenses bend the light so that the image of the object is magnified.
o Important parameters in microscopy:
- Resolution (resolving power)
o Magnification is the ratio of an object’s image size to its real size.
o Resolution is a measure how clear the image is.
o Contrast emphasises differences in parts of the sample.
o Electron microscope focuses a beam of electrons through the object
or onto its surface.
o The scanning electron microscope is used for detailed study of the
surface of a specimen. The electron beam scans the surface of the
sample, which is coated with a thin film of gold.
o Transmission electron microscope is used to study the ultrastructure of
cells. It aims an electron beam through a very thin section of the
o Cell fractionation takes cells apart and separates the major organelles
and other subcellular structures from one another.
o It enables researchers to prepare specific cell components in bulk and
identify their functions.
Comparing eukaryotic and prokaryotic cells
o All cells have a selective barrier – plasma membrane
o The cytosol surrounds the membrane and it is where all the organelles
and other components are found.
o All cells contain chromosomes, which carry genes in the form of DNA.
o All cells have ribosomes, which makes proteins according to
instructions from the genes.
o The difference between eukaryotic and prokaryotic cells is the location
of their DNA.
o Eukaryotic cell’s DNA is found in the nucleus which is bounded by a
o Prokaryotic cell’s DNA is concentrated in a region that is not
membrane-enclosed – nucleoid.
o Eukaryotic cells are much larger than prokaryotic cells. o The plasma membrane acts as a selective barrier that allows sufficient
passage of oxygen, nutrients and wastes to the entire cell.
BACTERIA AND ARCHAEA
o Prokaryotic cells have a cell wall which maintains cell shape,
provides physical protection and prevents the cell from bursting in a
o Peptidoglycan is present in bacterial cell wall which is a network of
modified-sugar polymers cross-linked by short polypeptides.