2.1.3 - Methods of studying cells Flashcards
- Define magnification
Magnification is how much bigger the image is than the specimen
- Define resolution
Resolution is the minimum distance apart that the two objects can be in order for them to appear as separate items.
- Describe the procedure to prepare a slide
● Add a drop of water to the slide
● Remove a thin section of tissue and place it onto the slide (flat as possible)
● Add 1 drop of iodine dissolved in potassium iodide to stain the sample (This is only correct if it is plant tissue)
● Lower a coverslip on top using a mounting needle
- Explain why it was important that the sections of tissue were thin
● A thin section allows more light through;
● allows a single layer of cells to be viewed.
- Explain why you should push down hard on the cover slip but should not push the cover slip sideways.
● Push hard to squash the tissue to create a single layer of cells.
Do not push sideways as this will cause the cells to roll together.
- A plant cell was observed with an optical microscope. Describe how the length of the cell could be estimated.
● Use a stage micrometer to help us calibrate the size of the eyepiece graticule.
● Measure the length of the plant cell with an eyepiece graticule.
Calibrating a stage micrometer
1) Find out the length of the stage micrometer? e.g. 10mm long = 10000μm
2) Find out how many divisions there are? e.g. 100 divisions
3) Work out how big each division is in μm? e.g. 10000/100 = 100μm = 1 division
4) Place the stage micrometer on the stage
5) Line up the divisions on the eyepiece graticule with those of the micrometer
6) Let’s say 100 units on the eyepiece graticule fit 30 units of the stage micrometer
7) Now work out the length of one eyepiece graticule unit in μm
1) Place your slide on the stage
2) Measure the length of the cell using eyepiece graticule units
3) Multiply the number of eyepiece graticule units by the length of one eyepiece graticule unit in μm (part 7 – see above)
Describe the properties of scanning electron microscope
Check notes
Describe the properties of transmission electron microscope
Check notes
Describe the properties of light microscope
Check notes
- Why are electron microscopes used to view cells?
● They have a HIGH resolution
● because electrons have a shorter wavelength than light.
● This allows you to view internal structures/organelles of a cell.
(Remember using this – Light microscope = Low resolution = Longer wavelength)
- Name two structures in a eukaryotic cell that cannot be identified using an optical microscope.
● Mitochondrion / ribosome / endoplasmic reticulum / lysosome / cell-surface membrane
- Describe the principles and the limitations of using a transmission electron microscope to investigate cell structure.
Principles:
1. Electrons pass through / enter (thin) specimen;
2. Denser parts absorb more electrons;
3. (So) denser parts appear darker;
4. Electrons have short wavelength so give high resolution;
Limitations:
5. Cannot look at living material / Must be in a vacuum;
6. Specimen must be (very) thin;
7. Artefacts present;
8. Complex staining method / complex / long preparation time;
9. Image not in 3D / only 2D images produced.
- What is the advantage of TEM (transmission) compared to SEM (scanning) is
● Higher resolution
● Allows internal structures within cells to be seen
- What is the advantages of SEM (scanning) compared to TEM is
● Thin sections do not need to be prepared
● shows surface of specimen
● can have 3-D images
- Write an equation to calculate magnification and draw a formula triangle
Magnification = Image length / Actual Length
- How many micrometers in a millimeter?
1mm = 1000μm
An epithelial cell has been magnified 1500 times. The image length is 34mm. Calculate the actual length of the epithelial cell in micrometers.
22.6 micrometres
- If a scale bar is provided this represents the size of what? How do we use it?
The scale bar represents the size of the image.
1) Measure the scale bar, using a ruler, in mm
2) Convert to micrometers - Use value as image size
3) The value written underneath the scale bar is actual size
4) Use these both to work out magnification
5) NOTE: Some questions will then ask you to work out the actual size of a specimen. Use the magnification you have just calculated and the image size (from the image provided of the specimen) to do this.
- What is meant by ‘cell fractionation’ and why would scientists want to do it?
● Separating out the contents of a cell into the different ‘fractions’ (i.e. different parts).
● This usually means separating out the different organelles by DENSITY.
● This is useful for scientists because it allows them to study individual organelles
- What is homogenization? What is the point of it?
● Using a blender (or a homogenizer) to break open the cell membrane to release all of the organelles inside
- Why is the solution filtered?
● To remove any whole cells or large cell debris before carrying out centrifugation
- The cells are kept in a specific type of solution: (you must memorise the whole sentence!)
A. Why is it cold? To reduce enzyme activity TO prevent digestion of organelles
B. Why is it isotonic? To prevent osmosis, SO that the ORGANELLES do not shrivel or burst/ lysis
C. Why is it buffered? So that the pH is kept constant, SO that proteins/ enzymes are not denatured
- What does ultracentrifugation mean? Describe what happens:
● Ultracentrifugation is used to separate very small things – such as different lengths of DNA/RNA
● The sample is put into a tube
● It is spun at a low speed (low Revolutions Per Minute = RPM)
● The most dense organelle forms a sediment/pellet at the bottom of the test tube
● The other organelles remain suspended in the supernatant
● The supernatant is removed and put into a clean test tube and spun at higher speed
● The second most dense organelle forms a sediment at the bottom etc.
