Summary A Level Biology Cell Structure and Microscopes Notes
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Course
Unit 2 - Cells
Institution
AQA
This pack of notes contains detailed information about scanning, transmission and light microscopes, mitosis and the cell cycle, cancer and animal and plant organelles. These notes helped me achieve an A* in my term exams and an A in my A level exams.
What are microscopes? Instruments that produce a magnified image of an
object.
Microscope lenses work more effectively if the convex glass lenses are used in
pairs in a compound light microscope.
● Light microscopes - the relatively long wavelength of light rays means
that a light microscope can only distinguish between two objects if they
are 0.2μm, or further apart.
This limitation can be overcome by using beams of ELECTRONS rather than
beams of light.
● Electron microscopes - with their shorter wavelengths, a beam of
electrons in the electron microscope can distinguish between two
objects only 0.1nm apart.
Magnification
The material that is put under a microscope is referred to as the object. The
appearance of this material when viewed under the microscope is referred to
as the image.
,Magnification: the magnification of an object is how many times bigger the
image is when compared to the size of the real object.
Magnification = size of image/size of real object
Size of real object = size of image/magnification
When calculating magnification, make sure that the units of length are the
same for both the object and the image!
Resolution
Resolution/resolving power: the resolution or resolving power of a microscope
is the minimum distance apart that two objects can be in order for them to
appear as separate items.
● The resolving power depends on the wavelength or form of radiation
used.
● A light microscope has a resolution of about 0.2μm (any two objects that
are 0.2μm or more apart will be seen separately but if they are closer
than this, they will appear as a single item).
● Greater resolution means greater CLARITY.
● The image produced with a high resolution is clearer and more PRECISE.
● Increasing magnification increases the size of the image but doesn’t
always increase the resolution.
● Every microscope has a limit of resolution. Up to this point, increasing
the magnification will reveal more detail, but beyond this point
increasing the magnification will not do this.
● The object will still appear larger but more blurred instead!
, Cell fractionation
To study the structure and function of the various organelles that make up
cells, it is necessary to obtain large numbers of ISOLATED organelles.
Cell fractionation: the process where cells are broken up and the different
organelles they contain are separated out.
Process of cell fractionation:
1. Homogenation
2. Ultracentrifugation
BEFORE cell fractionation begins, the tissue is placed in a cold, buffered
solution which has the same water potential as the tissue. The solution is:
● Cold - to reduce enzyme activity that might break down the organelles.
● Buffered - so that the pH does not fluctuate. Any change in pH could
affect the functioning of enzymes and also alter the structure of the
organelles.
● Of the same water potential as the tissue - to prevent the organelles
from shrinking or bursting due to osmotic loss or gain of water.
Stages of cell fractionation:
1. HOMOGENATION
- The cells are broken up inside a homogeniser (blender)
, - The various organelles from the cell are released.
- The resulting fluid is called the homogenate.
- The homogenate is filtered to remove any complete cells and large
pieces of debris.
2. ULTRACENTRIFUGATION
Ultracentrifugation: the process by which the fragments in the filtered
homogenate are separated in a machine called a centrifuge.
The centrifuge spins tubes of homogenate at very high speeds in order to
create a centrifugal force.
For animal cells, the process goes like this:
○ The tube of filtrate is placed in the centrifuge and spun at a slow speed.
○ The heaviest organelles i.e. the nuclei are forced to the bottom of the
tube where they form a thin sediment or pellet.
○ The fluid at the top, supernatant, is then removed leaving only the
sediment of nuclei.
○ The supernatant is transferred to another tube and spun in the
centrifuge at a FASTER speed than before.
○ The next heaviest organelles i.e. the mitochondria are forced to the
bottom of the tube.
○ The process is continued in this way so that, at each increase in speed,
the next heaviest organelle is sedimented and separated out.
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