Biology Model Answer's

Describe and explain how you would use cell fractionation and ultracentrifugation to obtain a

sample of nuclei from muscle tissue.

• Homogenise the tissue to break open cells, to release organeles.

• Then filter to remove tissue and debris

• Cold solution to prevent enzyme activity

• isotonic solution is used to make the water potential equal and prevent organelles bursting.

• The buffered solution is used to stop enzymes (proteins) from denaturing.

• Then the centrifuge spins at a lower speed so that nuclei moves to the bottom.

Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells.

• TEM use electrons and optical use light.

• TEM Allows for a greater resolution and therefore can be used to see smaller organeelles can be observered

• However with TEM = only view dead specimens but Optical can view live specimens also.

• Moreover TEM doesnt have colour but optical does

• TEM requires thinner specimens

• TEM requires a more complex preparation

• TEM focuses using magnets but optical use glass lenses.

Describe the principles and the limitations of using a transmission electron microscope to investigate cell structure.

• Electrons pass through thin specimen;

  • denser parts absorbing more electrons and thus appear darker.

• Electrons have short wavelengths so give a high resolution.

• But…

  • It cant look at living specimen

  • specimen must be very thin

  • image is not in 3d

  • and artefacts are present

b

b