Quick questions on Methods of studying cells: microscopy, magnification and cell fractionation - AQA A-Level Biology

Optical (light) microscope. Uses light focused by glass lenses. Maximum magnification about times 1500; resolution about 200 nanometres (limited by the wavelength of light). Specimens can be living and in colour, and the equipment is cheap and portable. Cannot resolve small organelles such as ribosomes.

Cell fractionation separates organelles so they can be studied in bulk.

Uses light focused by glass lenses. Maximum magnification about times 1500; resolution about 200 nanometres (limited by the wavelength of light). Specimens can be living and in colour, and the equipment is cheap and portable.

A beam of electrons passes through a very thin specimen; denser regions absorb more electrons and appear darker. Resolution about 0.1 nanometres, so internal ultrastructure (cristae, ribosomes) is visible. But the specimen must be dead and in a vacuum, preparation is complex and can create artefacts, and the image is 2D and not in colour.

The tissue is placed in a solution that is:

The tissue is blended to break open the cells and release the organelles into the solution, then filtered to remove debris.

The filtered homogenate is spun, slowest first then increasing the speed in stages. The densest organelles sediment first as a pellet; the supernatant is decanted and spun again at a higher speed to pellet the next densest organelle. The order is:

Explain why an electron microscope has a higher resolution than a light microscope. [2 marks]

A cell image is 50 mm wide and the magnification is times 2500. Calculate the actual width in micrometres. [2 marks]

Explain why the solution used in cell fractionation must be isotonic and buffered. [2 marks]