2.1.1 Observing Cells ( A level Biology Edexcel B)

What is Cell Theory?

Cell theory is a unifying concept that states that cells are a fundamental unit of structure, function and organisation in all living organisms.It also posits that all living things are composed of one or more cells, and all cells arise from pre-existing cells.

What does unifying concept mean?

Applies to all living organisms

What does fundamental unit mean?

the basic unit of living organisms

What does structure mean?

organisms are made from cells

What does magnifcation mean?

How many times larger an image is compared with the actual object.

what does Resolution / resolving power mean?

The ability to distinguish between two objects that are close together.

What is the formula for image size?

Image size = Actual size x Magnification

how much is a cm in mm?

1 cm is 10 mm

how much is a mm in micromemtres

one mm is 1000 micrometres

how much is a micrometre in nanometres

one micrometres is 1000 nanometres

how much is a mm in nanometres

one mm is 1 million nanometres

What are the 4 main points of a light microscope?

• Uses visible light

• Light passes through the specimen

• The objective lens magnifies the specimen

• The eyepiece lens magnifies the image further

What is the formula for Total Magnification?

Total Magnification = Objective lens x Eyepiece lens

What are the Advantages of a light microscope?

• Can observe living specimen

• Relatively Cheap

• Relatively light and portable - allows you to relatively observe cells / tissue in a natural state

• Relatively Straightforward to use

What are the disadvantages of a light microscope?

• Limited powers of resolution and magnification

• Preservation and staining tissue can produce artefacts in tissues being observed

What does an electron microscope use?

an electron microscope uses beams of electrons instead of visible light

How does the use of electrons in an electron microscope allow for more detail?

• Electrons have a much shorter wavelength than visible light.

• Shorter wavelength → greater resolving power.

What do you need to use an electron microscope ?

A vacuum - living specimens cant be observed

Why is extensive preparation on an electron microscope bad?

Because it can cause artefacts

Why do electron microscopes have a greater resolution than light microscopes?

Electron microscopes have greater resolution because electrons have a much shorter wavelength than visible light.

What is a major disadvantage of an electron microscope ?

An electron microscope is large and must be kept at room temperature and pressure and needs to maintain an internal vacuum - difficult to meet these conditions

How does the function of electrons differ in a TEM and an SEM?

• In a TEM , electrons pass through the specimen

• In a SEM , electrons scan the surface

What sort of image does a TEM and SEM produce?

• A TEM produces a 2D image

• A SEM produces a 3D image

What do TEM and SEM mainly show?

• A TEM mainly show internal structures

• A SEM mainly shows surface structures

What is the Resolution and magnification of a TEM compared to a SEM

• A TEM has a higher resolution and magnification than a SEM

What is Staining used for when observing cells?

Staining increases contrast between structures, making them easier to see and identify.

Why is staining used?

Biological specimens can have very little natural contrast, so staining makes particular structures more visible.

What are the 4 main stains used in observing cells?

• Haematoxylin

• Methylene blue

• Acetocarmine

• Iodine

What does Haematoxylin highlight?

stains the nuclei of both plant and animal cells , purple, blow , brown

What does Methylene blue highlight?

stains the nucleus of animal cells blue

What does Acetocarmine highlight?

stains the dividing chromosomes in dividing nuclei of both plant and animal cells

What does iodine highlight?

stains starch-containing material in plant cells blue black

Why are heavy metal ions like lead and uranium used in electron microscopy?

Heavy-metal ions can increase contrast in electron microscopy because they scatter electrons

What is meant by false colour?

False colours can be added to an electron microscope image after it has been produced to make different structures easier to distinguish.

What is an Artefact?

An artefact is a feature in a microscope image that was introduced during preparation or examination of the specimen and was not present in the original specimen.