Studying cells

Question 1

What is magnification, and how do you calculate it?

Answer 1

The number of times greater an image is than the size of the real object.

Question 2

What does resolution mean?

Answer 2

The minimum distance apart that two objects can be distinguished as separate objects.

Question 3

What are the principles of optical microscopes?

Answer 3

1. Light is focused using glass lenses. Light passes through the specimen, and different structures absorb different amounts and wavelengths of light.

Question 4

What are the principles of transmission electron microscopes?

Answer 4

1. Electrons are focused on the sample using electron magnets.
2. Electrons pass through the specimen, and denser parts absorb more electrons, so they appear darker.

Question 5

What are the principles of scanning electron microscopes?

Answer 5

1. Electrons are focused on the sample using electromagnets.
2. Electrons defect off the specimen surface.

Question 6

Compare the limitations between optical microscopes, TEMs and SEMs.

Answer 6

1. Optical microscopes generate a 2D image of a cross-section, TEMs generate a 2D image of a cross-section, and SEMs generate a 3D image of a surface.
2. Optical microscopes have a lower resolution compared to electron microscopes, as light has a longer wavelength compared to electron beams. TEMs have higher resolution compared to SEMs
3. You can’t see internal structures using optical microscopes and SEMs, but you can see them using TEMs.
4. Optical microscopes have a lower magnification compared to electron microscopes. TEMs have higher magnification compared to SEMs
5. In optical microscopes, the sample has to be thin. In TEM, the sample has to be very thin, and in SEMs, it does not need to be thin.
6. You can view live specimens with the optical microscopes and not the electron microscopes, as the TEMs and SEMs require a vacuum.
7. The optical microscopes have a simple preparation, and the electron microscopes have a complex preparation; this means artefacts are often present.
8. Optical microscopes can produce coloured images, and electron microscopes do not show colour.

Question 7

Describe and explain the principles of cell fractionation.

Answer 7

1. Homogenise the tissue to break open the cells and release the organelles.
2. Place in an ice-cold, buffered, isotonic solution: Ice cold to slow down enzyme activity, so the organelles are not damaged. Isotonic so water doesn’t move in or out of organisms via osmosis, so they don’t burst. Buffered to maintain the pH and ensure the enzymes don’t denature.
3. Filter the homogenate to remove any unbroken cells or cell debris.

Question 8

Describe and explain the principles of ultracentrifugation.

Answer 8

1. Centrifuge the homogenate at low speeds and time: To remove the pellet of the most dense organelle.
2. Respin the supernatant at increasing speeds and times to remove pellets of less dense organelles.