AQA AS Biology Cell Structure Notes

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Magnification = size of image / size of real object

Cell Fractionation

Cell fractionation- has two stages:

1. Homogenation- sample placed in cold, buffered, isotonic solution (cold to halt enzyme controlled reactions, buffered to prevent pH changes, isotonic to prevent osmosis). Then homogenised in a blender, breaking down cells and releasing contents. Then filtered to remove any whole cells or large debris.

2. Ultracentrifugation- tube of filtrate place in centrifuge and spun at a low speed. Heaviest organelles (nuclei) forced to bottom of tube and form a pellet. Supernatant removed, and process repeated at higher speeds (next to pellet would be mitochondria).

Electron Microscope

Main advantages are that electron beam has a very short wavelength so the microscope has a high resolving power, and the beam can be focused using electromagnets as the electrons are negatively charged. Disadvantages include the fact that the sample must be dead as the whole thing takes place in a vacuum to avoid deflection of the electrons, specimen must be very thin to allow electrons to pass through, image may contain artefacts, and a complex staining process is required (TEM).

There are two main types of electron microscope- TEM ans SEM.

- TEM (transmission electron microscope) produces a black and white image