Cell Structure

• What is microscopy?

  Microscopes are instruments that produce a magnified image of an object. A convex lens can act as a magnifying glass, but act more effectively used in pairs in a light microscope. Also scientists have discovered how to magnify an image using electrons.
• What is the wavelength of light?

  The wave length of light is relatively long meaning can only distinguish objects 0.2$\mu$m apart. This is a limitation.
• What is the wavelength of electrons?

  Wavelength of electrons - beams of electrons have a shorter wavelength so can distinguish objects 0.1nm apart. This overcomes the limitations of light microscopy.
• What is the equation for magnification?

  magnification = image size/real object size
• What is unit conversion table for measures of length?

  Kilometre(Km): 10$^3$m

  Metre(m): 1m

  Millimetre(mm): 10$^{-3}$m

  Micrometre($\mu$m): 10$^{-6}$m

  Nanometre(nm): 10$^{-9}$m
• What is resolving power?

  Resolving power - the minimum distance apart that 2 objects can be in order for them to appear separate. It depends on the wavelength of type of radiation used, higher resolving power means the image is more clear and precise.
• What will increasing magnification do to the resolution?

  Increasing magnification does not always increase resolution, all microscopes have a limit to their resolution and it depends on the type of radiation they use. When magnification is increased beyond its limit the image will appear larger but more blurred.
• What is cell fractionation?

  A process that allows scientists to study the structure of organelles by breaking them up and separating out the organelles.
• What must be done to the tissue before cell fractionation?

  It must be placed in a cold, buffered solution that is the same water potential as the tissue.
• Why does the solution the sample is placed in have to be cold?

  Cold - to reduce enzyme activity that might break down organelles you are wanting to study.
• Why does the solution have to be buffered?

  Buffered - so the pH of the solution does not fluctuate, any change could alter the shape of organelles or affect the functioning of enzymes.
• Why does the solution have to have the same water potential as the tissue?

  Same water potential - prevent organelles bursting or shrinking as a result of osmotic gain(bursts-lysis) or loss of water(shrinks - crenation).
• What are the 2 stages of Cell fractionation?

  Homogenation and Ultracentrifugation.
• Describe the process of homogenation.

  Cells are broken up by a homogeniser, releasing organelles from the cell. The resultant fluid is called a homogenate, it is filtered to remove any complete cells and larger debris.
• What are other methods of breaking up cells in homogenation?

  Grinding, mincing, chopping, pressure changes, osmotic shock, freeze-thawing, ultra-sound.
• Describe ultracentrifugation.

  The process by which the fragments in filtered homogenate are separated in a centrifuge. It spins test tubes at a very high speed to create centrifugal force.
• What is the order of organelle fractionation?

  Nuclei$\rightarrow$ Chloroplasts $\rightarrow$ Mitochondria $\rightarrow$ Lysosomes $\rightarrow$ Endoplasmic Reticulum $\rightarrow$ Ribosomes
• What is the process of ultracentrifugation?

  1.A tube of filtered homogenate placed in centrifuge spun at low speed.

  2. The heaviest organelles forced to the bottom, forcing a sediment pellet.

  3. The fluid at the top of the tube(supernatant) is removed, leaving sediment.

  4. The supernatant spun at higher speed in another tube.

  5. The next heaviest organelle form thin sediment at bottom of tube.

  6. Process is repeated, at each increase in speed, next heaviest organelles is sedimented and separated until all organelles are separated.
• How does an optical microscope work?

  Visible light passes through the specimen, into a magnifying lens of the microscope, then into the objective viewing lens. The eye the forms an image in the brain.
• What are the advantages of using an optical microscope?

  The specimen can be alive, easy use, cheaper, images produced are coloured.
• What are the disadvantages of using an optical microscope?

  Magnification is lower and there is a limit to the resolution, so cannot view internal structures of the cell.
• What is the definition of magnification?

  The ability to make small objects seem larger so you can see things you would not be able to see, previously.
• What are the 2 types of electron microscopes?

  Scanning electron microscope (SEM) and Transmission electron microscope (TEM)
• How does the Transmission electron microscope work?

  Uses magnets to focus an electron beam to pass through a very thin sample, the denser regions absorb electrons and appear darker on the electron micrograph. In less dense regions electrons can easily pass through appearing lighter. It must take place in a vacuum. The resolution is 0.1nm and the magnification is x500,000
• What are the micrographs produced by TEM's like?

  They are 2D black and white images, the colour can be put into the image by computer programming afterwards, detail of organelle structures are visible, the sample is dead due to the vacuum.
• How does the Scanning electron microscope work?

  Magnets focus a beam of electrons to bounce off/scan the surface of the sample. The electrons interacts with the molecules on the surface and light is emitted. Takes place in a vacuum but has lower magnification and resolution then TEM. Magnification = x100,000 and Resolution = 20nm
• What do the micrographs produced by SEM's look like?

  They are 3D and black and white however they are still dead as takes place in a vacuum. Allows you to see the shape of the molecule/cell as a whole.
• Why do electron microscopes need to take place inside a vacuum?

  Therefore, the particles in the air do not deflect the electrons out of the beam of alignment, this would affect the image produced.
• What are the advantages of using Electron microscopes?

  Advantages: greater resolution, greater magnification, detailed images of content of cell and 3D structure, can view the cell as a whole, digital images can be enlarged and manipulated.
• What are the disadvantages of using an electron microscope?

  Disadvantages: expensive, training required to use, cannot view living specimen, vacuum is required, images are black and white, complex staining process, sample must be extremely thin.
• What is the definition of a graticule?

  A glass disc placed in the eyepiece of a microscope. A scale is etched on the graticule, that is 10mm long and divided into 100 sub-divisions.
• What is a stage micrometer?

  A slide that aids in calibrating an eyepiece graticule for a specific objective lens. The scale on this is usually 2mm long and its smallest sub-divisions are 0.01mm(10$\mu m$)
• How do you calibrate an eyepiece graticule?

  1.Line up the graticule and the stage micrometer.

  2. Discover how many graticule units go into a certain number of micrometer units.

  3. Divide down to get how many graticule units is equivalent to 1 micrometer units.

  4. 1 micrometer unit is equal to 10$\mu m$, so to discover how much 1 graticule unit is do 10 $\div$ how many graticule units goes into 1 micrometer unit.
• What is the ultrastructure of a cell?

  A cell is adapted to carry out a particular function depending on its function it will have different organelles that suit its function.
• What is the shape of the nucleus and what parts does it consist of?

  It is spherical and usually 10-20$\mu m$ in diameter. It consist of the parts: nuclear envelope, nuclear pores, nucleoplasm, chromosomes and the nucleolus.
• What is the structure and function of the nuclear envelope?

  Structure: a double membrane surrounding the nucleus, the outer membrane is continuous with the endoplasmic reticulum.

  Function: it controls the entry and exit of materials to the nucleus and contains the reactions that take place within the nucleus.
• What are nuclear pores?

  Holes/channels in the nuclear envelope that are around 40-100nm in diameter. There are around 3000 pores in each nucleus. They allow the passage of large molecules, such as messenger RNA, out of the nucleus.
• What is the nucleoplasm?

  It is the granular, jelly-like material that makes up the bulk of the nucleus.
• What are chromosomes?

  Chromosomes are thread-like structures made of linear DNA and proteins that carry genetic information in the form of genes.
• What is the nucleolus and what is its function?

  The nucleolus is a small spherical region within the nucleoplasm, there may be more then one in the nucleus. The function of the nucleolus is to manufacture ribosomal RNA and assembly of ribosomes.