Studying Cells
We have already studied this: use your notes from
the introductory topic to complete this section of
your homework booklet.
Homework reminder:
Make sure your homework booklet is up to date
when you hand it in on Thursday:
- All of the first section (3.2.1.1 and 3.2.1.2)
including exam questions.
You should detach this section (make sure your
name is on it!) to hand in.
Task 1 – Labelling cell diagrams
Let’s check the work from last lesson.
Studying Cells
• We can study whole cells or organelles using
microscopes.
• Optical (light) microscope
• Electron microscope
▫ Transmission
▫ Scanning
You need to be able to describe how these work and
their limitations.
Light Microscope
Resolution: 200nm
• Any images closer together than 200nm will be seen as 1
object
• Due to magnitude of the wavelength of light
• 2 objects can only be seen if light can pass between them
• Human eye resolution = 100μm
Eyepiece lens: X10
Objective lenses: x4, x10,
x40
x100 (oil immersion)
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Higher magnifications
and resolutions than
optical microscopes
Uses a beam of
electrons rather than
light and
electromagnets rather
than glass lenses
Scanning EM used to
view surface details
Transmission EM used
to view internal
structures
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Read the information on the sheets.
Summarise the key points into a table of
similarities and differences between optical
and electron microscopes.
Include:
◦ How they work
◦ How specimens are prepared
◦ Their limitations
Cell Fractionation and
Ultracentrifugation
Cells consist of a number of organelles.
We can examine their structure and function if we
can isolate them.
Cell fractionation = breaking up a cell to release its
organelles.
Ultracentrifugation = a method for the separation
of the organelles by density.
Extracting organelles from cells
14 of 37
© Boardworks Ltd 2008
Extracting organelles from cells
15 of 37
© Boardworks Ltd 2008
Steps of Cell Fractionation & Ultra Centrifugation
Cell Fractionation
1. Tissue to be studied is cut into small pieces and
placed into an ICE COLD, ISOTONIC BUFFER
solution.
Why?
ICE COLD to stop enzyme activity.
ISOTONIC (same concentration/water potential
as cytoplasm) to prevent osmosis which would
damage organelles).
BUFFER to keep pH constant so proteins are not
denatured.
Steps of Cell Fractionation & Ultra Centrifugation
Cell Fractionation
2. Ground into smaller pieces using a
HOMOGENISER to release the organelles
from the cell.
3. The HOMOGENATE is filtered to remove
any complete cells and large debris i.e. cell
walls/membranes.
Steps of Cell Fractionation & Ultra Centrifugation
Ultracentrifugation
1. Homogenate is placed in a test tube and
centrifuged.
The faster the speed at which the tube is
spun, the greater the force generated.
2. Larger, more dense fragments collect at the
bottom of the tube to form a PELLET.
Smaller ones remain near the top suspended
in a liquid called the SUPERNATENT.
Steps of Cell Fractionation & Ultra Centrifugation
Ultracentrifugation
3. Pellet is removed and the supernatant
remaining is re-spun at a faster speed (more
force).
4. Process is repeated at higher and higher
speeds.
Important info!
Organelle sizes are
similar in all cells so
we can predict when
they will form a
pellet.
Since the whole
process involves
centrifuging at
different speeds it
is called
DIFFERENTIAL
CENTRIFUGATION.
Cut and stick!
• Cut out the cards and arrange them in the
correct order.
• Write down the reasons that we use a cold,
isotonic buffer solution (if you can’t
remember, look it up in a textbook!).
Homework reminder again:
Make sure your homework booklet is up to date
when you hand it in on Thursday:
- All of the first section (3.2.1.1 and 3.2.1.2)
including exam questions.
- You should detach these pages to hand in.
- You also need to start filling in the section
on the work we’ve covered today, ready to
hand in on Monday next week.