Biology - cell structure

what are microscopes?

instruments that produce a magnified image of an object

how do lenses work more effectively?

if they are used in pairs in a compound light microscope

what the smallest distance between 2 objects that a light microscope can distinguish?

0.2 micrometres

what the smallest distance between 2 objects that an electron microscope can distinguish?

0.1 nanometres

what is the object of a microscope?

the material that is put under a microscope

what is the image of a microscope?

the appearance of the material when viewed under the microscope

what is magnification?

how many times bigger the image is when compared to the object

magnification =?

size of image / size of real object

what do you have to ensure when calculating magnification?

units are the same for the sizes measured

conversions table:

what is resolution/ resolving power of a microscope?

the minimum distance apart that 2 objects can be in order for them to appear separate

what does resolution depend on?

the wavelength/ form of radiation used

does increasing magnification increase resolution?

no - every microscope has a limit of resolution

what will happen at the limit of resolution?

increasing the magnification will just make the object appear more blurred

why do we want to obtain large numbers of isolated organelles?

to study structure and function

what is cell fractionation?

the process where cells are broken up and the different organelles they contain are separated out

what must happen before cell fractionation can begin?

the tissue must be placed in cold, buffered solution of the same water potential as the tissue

why must it be cold?

to reduce enzyme activity that might break down organelles

why must it be buffered?

so that the pH doesnt fluctuate. Any change in pH could alter the structure of the organelles or affect the functioning of enzymes

why must it be of the same water potential as the tissue?

to prevent organelles bursting or shrinking as a result of osmotic gain/ loss of water

what are the 2 stages to cell fractionation?

1. homogenation

2. ultracentrifugation

what is homogenation?

cells are broken up by a homogeniser. this releases the organelles from the cell. the resultant fluid (homogenate), is then filtered to remove any complete cells and large pieces of debris

what is ultracentrifugation?

the process by which the fragments in the filtered homogenate are separated in a machine called a centrifuge. this spices the tubes of homogenate at very high speed in order to create a centrifugal forces

what is the process for animal cells?

1. the tube of filtrate is placed in the centrifuge and spun at a slow speed

2. the heaviest organelles, the nuclei, are forced to the bottom of the tube, where they forms a thin sediment/ pellet

3. the fluid at the top of the tube (supernatant) is removed, leaving just the nuclei sediment

4. the supernatant is transferred to another tube and spun in the centrifuge at a faster speed than before

5. the next heaviest organelles, the mitochondria, are forced to the bottom of the tube

6. the process is continued in this way so that, at each increase in speed, the next heaviest organelle is sedimented and separated out

why is cell fractionation so good?

has allowed advances in biological knowledge by allowing a detailed study of the structure and function of organelles

what are the 2 main advantages of an electron microscope?

• electron beam has a very short wavelength and the microscope can therefore resolve objects well - high resolution

• as electrons are negatively charged the beam can be focused using electromagnets

what is the case because electrons are absorbed or deflected by molecules in air?

a near vacuum has to be created within the chamber of an electron microscope in order for it to work effectively

comparison of radiation pathways in light and electron microscopes?

what are the 2 types of electron microscope?

1. transmission electron microscope

2. scanning electron microscope

what is the TEM?

consists of an electron gun that produces a beam of electrons that is focused onto the specimen by a condensor electromagnet. the beam passes through a thin section of the specimen. parts of this specimen absorb electrons and therefore appear dark. other parts allow the electrons to pass through and so appear bright

what is the image produced on the screen that can be photographed called?

a photomicrograph

why is the resolving power of the TEM not always 0.1nm?

1. difficulties preparing the specimen limit the resolution that can be achieved

2. a higher energy electron beam is required and this may destroy the specimen

limitations of the TEM?

1. the system must be in a vacuum and therefore living specimens cant be observed

2. a complex staining process is required and even then the image isnt in colour

3. the specimen must be very thin

4. the image may contain artefacts

what are artefacts?

things that result from the way the specimen is prepared. Artefacts may appear on the finished micrograph but are not part of the natural specimen. it is therefore not always easy to be sure that what we see on a photomicrograph really exists in that form

why must the specimens be really thin?

to allow electrons to penetrate, resulting in a flat 2D image

how can you build the image up to being in 3D?

take a series of sections through the specimen and look at them through the photomicrographs produced

what is similar about the TEM and SEM?

same limitations, except the specimens dont need to be thin as electrons dont penetrate

what happens with the SEM?

the SEM directs a beam of electrons on to the surface of the specimen from above, rather than penetrating it from below. the beam is then passed back and forth across a portion of the specimen in a regular pattern. the electrons are scattered by the specimen and the pattern of this scattering depends on the contours of the specimen surface

how can you build up a 3D image?

by computer analysis of the pattern of scattered electrons and secondary electrons produced

whats the resolving power of a basic SEM?

20nm