Cell Structure Flashcards
What is an object?
What is magnification?
It is the material that is put under a microscope.
The magnification of an object is how many times bigger the image is when compared to the object.
What is the formula for finding magnification?
image/actual size
What is the resolution of a microscope?
It is the minimum distance apart that two objects can be in order for them to appear as separate items.
What is a tissue placed in before cell fractionation?
Explain why these conditions are used?
It is placed in a cold, buffered isotonic solution.
cold - to reduce enzyme activity that might break down the organelles.
isotonic (same water potential) - to prevent organelles bursting or shrinking as a result of osmotic gain or loss of water.
buffered - so that the pH does not fluctuate (any change in pH could alter the structure of the organelles or affect the functioning of enzymes.
What happens in homogenation?
Cells broken up by homogeniser - releases organelles from the cell - homogenate (resultant fluid) is filtered to remove any complete cells and large pieces of debris
What happens during ultracentrifugation? (for animal cells)
tube of filtrate placed in centrifuge and spun at low speed
heaviest organelles (nuclei), are forced to bottom to form the sediment
Fluid at top (supernatent) removed, leaving just the sediment of nuclei
supernatent is transferred to another tube and spun in the centrifuge at a faster speed than before
next heaviest organelle (mitochondria) are forced to bottom of tube
process continues , whereby at each increased speed, the next heaviest organelle is sedimented and separated
What are the two main advantages of electron microscopes?
electron beam has a very short wavelength and microscope can therefore resolve objects well - has a high resolving power
as electrons are negatively charged the beam can be focused using electromagnets
What can we say about the best modern electron microscopes in terms of specs? What are some requirements of electron microscopes?
can resolve objects that are just 0.1 nm apart - 2000 better than a light microscope
because electrons are absorbed/deflected by the molecules in the air, a near-vacuum must be created withing the chamber of an electron microscope for it to work effectively.
What are the two types of electron microscopes?
transmission electron microscope -TEM
scanning electron microscope - SEM
How does the TEM work?
consists of an electron gun that produces a beam of electrons that is focused onto the specimen by a condenser electromagnet.
How are observations established in the TEM?
The beam passes through a thin section of the specimen. Parts of the specimen absorb electrons and therefore appear dark. Other parts of the specimen allow the electron to pass through and so appear bright.
an image is produced on a screen and this can be photographed to give a photomicrograph.
Why cannot a TEM always achieve a 0.1 nm resolving power in practice?
There are difficulties preparing the specimen that limit the resolution that can be achieved
a higher energy electron beam is required and this may destroy the specimen
What are the main limitations of the TEM?
whole system must be in a vacuum so living specimens cannot be observed
a complex ‘staining’ process is required and even then the image is not in colour
the specimen must be extremely thin
image may contain artefacts
result is a flat 2-D image, and can be overcome by a slow, complicated process by taking a series of sections through a specimen to then build a 3-D image.
How does a SEM work? How is it similar to a TEM and how is it different?
SEM directs a beam of electrons on to the surface of the specimen form above, rather than penetrating it from below. Beam is then passed back and forth across a portion of the specimen in a regular pattern - electrons scattered by specimen and and this pattern depends on the contours of the specimen surface - a 3D image can be built up.
SEM has same limitations as TEM but specimens do not need to be extremely thin and electrons do not penetrate.
Basic SEM has a lower resolving power than a TEM, around 20 nm, but is still ten times better than a light microscope
What are the 9 most important organelles in eukaryotic cells?
Nucleus Mitochondrion Chloroplasts Endoplasmic reticulum Golgi apparatus Lysosomes Ribosomes Cell wall Vacuoles
What are the 5 parts of the nucleus and what is the function of the nucleus?
Nuclear envelope, nuclear pores, nucleoplasm, chromosomes, nucleolus
It is the control centre of the cell, produces mRNA and tRNA, manufactures ribosomes, retains genetic information.
