Section 2:Cell Structure Flashcards
Define magnification
Measure of enlargment
Define resolution
A measure of how clear an object is OR the minimum distance apart two objects can be in order for them to apear as seperate items
How is total magnification calculated?
magnification of eyepiece lens x magnification of objective lens
How is the image size calculated?
total magnifcation x actual size
How do you convert mm into µm?
Multiply by 1000
How do light microscopes work?
A condenser lens on the microscope is used to focus a beam of light coming from the object
Three points
Give advantages of the light microscope
- Cheaper than electron microscopes
- Able to see the image in its true colour
- Specimens can be dead or living
Two points
Give disadvantages of light microscope
- Unable to see smaller organisms e.g. rhibosomes
- Resolution is limited due to the wavelength of light wave
Three points
How does an electron microscope work?
- Beam of electrons are passed through the specimen
- This is focused by electromagnets onto a fluorescent screen to produce to produce an image which can be photographed
- Parts of the specimen which absorbed electrons appear dark
Four points
Give disadvantages of electron microscopes
- The whole system must be in a vaccum~living specimens can’t be observed
- Specimen must be extremely thin (TEM only)
- Artefacts may appear
- A complex staining process is required, and even then the image is not in colour
What are the different types of electron microscopes?
Transmission electron microscope(TEM) and scanning electron microscope(SEM)
What is different in the methods for SEM and TEM?
TEM~beam of electrons is passed through the specimen and certain parts absorb the electrons
SEM~beam of electrons is passed over the surface of the specimen and collects the electrons scattered over the surface
What is different about the images produced by a TEM and a SEM?
SEM produces a 3D image, whereas TEM can only produce a 2D image
What is different about the resolving power of the SEM and TEM?
SEM has a lower resolving power than the TEM
What are the two stages in cell fractionation?
- Homogenation~breaking up the cells
- Ultracentrifugation~seperating the organelles out
Three points
Before cell fractionation the tissue is placed in a solution. Describe the features of the solution with explanations
- Cold~reduce enzyme activity that may break down organelles
- Isotonic(same WP as the tissue)~to prevent organelles bursting or shrinking due to osmotic gain or loss of water
- Buffered~keep pH constant to prevent structure of organelles altering or enzymes function changing
Two points
How can cells be homogenised?
- Using a pestle and mortar
- Using an electric blender
What is the fluid mixture left after homogenisation?
The homogenate
Why is the homogenate filtered?
To remove parts of the cell which have not been broken up properly
Four points
Describe how ultracentrifugation is carried out
- The filtrate is placed in the centrifuge
- At a low speed for a set period of time
- A pellet is formed at the bottom of the test tube containing the largest organelle
- Repeat this at higher speeds until the desired organelle forms a pellet
Define supernatant
The fluid left at the top of the test tube after being placed in a centrifuge
Why is it possible to sepearte cell organelles using centrifugation?
Organelles differ in density~denser organelles seperate off first
What is the difference between eukaryotic and prokaryotic cells?
Eukaryotic cells have a true nucleus unlike prokaryotic cells
What is the main function of the cell surface membrane and describe its structure?
Controls entry and exit of substances from the cell
* Phospholipid bilayer
* Carrier or channel proteins, may be appear on the inner or outer surfaces
