Methods of studying cells Flashcards – AQA A-Level Biology - (7402)

How does an optical microscope produce a magnified image?

It uses visible light and glass lenses to magnify a specimen.

How does light interact with the specimen in a transmission light microscope?

Light passes through a thin specimen.

What provides contrast between structures in a transmission light microscope?

Differences in light absorption or staining.

What types of specimens can optical microscopy be used to observe?

Living cells and prepared specimens.

What limits optical microscope resolution, and what is its typical maximum resolution?

The wavelength of visible light limits resolution to about 200 nm.

What does the resolution limit of optical microscopy mean for observing cell organelles?

Many organelles are too small to be distinguished clearly.

What are two potential drawbacks of staining cells for optical microscopy?

It may require cells to be killed and can alter the specimen.

How does a transmission electron microscope (TEM) produce an image?

A beam of electrons is transmitted through a very thin specimen.

What focuses the electrons in a TEM?

Electromagnetic lenses.

How is contrast produced in a TEM image?

Different parts of the specimen transmit or scatter electrons to different extents.

How can contrast in a TEM image be increased?

By heavy-metal staining.

What does a TEM image show about a specimen's internal structure?

A two-dimensional section, rather than the whole three-dimensional specimen.

Why can a TEM resolve much finer detail than an optical microscope?

Electrons have a short wavelength, allowing much greater resolution.

Why can't living cells be observed with a TEM?

The specimen must be thin and prepared for use in a vacuum.

How can specimen preparation affect the reliability of a TEM image?

It can alter structures and introduce artefacts.

How does a scanning electron microscope (SEM) form an image?

It scans a focused electron beam across the specimen’s surface and detects signals produced by beam–surface interactions.

What does an SEM image reveal about a specimen?

It shows surface detail with a three-dimensional appearance, rather than primarily showing internal structures.

How does SEM resolution generally compare with that of optical and transmission electron microscopes?

SEM generally has greater resolution than an optical microscope, but lower resolution than a transmission electron microscope.

Why can living cells not be observed using SEM?

Specimens are prepared for examination in a vacuum.

What effect can SEM preparation have on a specimen?

It may alter the specimen.

What is the purpose of cell fractionation?

To separate cell components so they can be studied.

How are cells initially disrupted in cell fractionation?

By homogenisation in a cold, buffered, isotonic solution.

Why is the homogenisation solution kept cold?

To slow enzyme activity.

Why is the homogenisation solution buffered?

To help maintain pH.

Why is an isotonic solution used for homogenisation?

To reduce osmotic damage to organelles.

What does filtering the homogenate remove?

Large debris and unbroken cells.

How is the filtrate processed through successive centrifugation stages?

It is centrifuged at increasing speeds, transferring the supernatant for further centrifugation after each stage.

What is the difference between the pellet and supernatant after centrifugation?

The pellet contains sedimented components; the supernatant is the remaining liquid.

How do component size and density affect the speed needed for sedimentation?

Larger, denser components generally sediment at lower speeds; smaller components require faster spinning.

How pure are the fractions produced by cell fractionation?

They contain different cell components, but separation need not be perfectly pure.