aqa biology AS topic

Cell structure

Microscopy

• Microscopes are instruments that produce a magnified image of an object.
• A convex glass lens acts as magnifying glass, they are more effective if in pairs like a compound microscope.
• Long wavelengths of light means a light microscope can distinguish between two objects if they are 0.2μm or further. 
• This can be overcome by using beams of electrons rather than beams of light
• With shorter wavelengths, the beam of electrons in the electron microscope can distinguish between two objects only 0.1nm apart

Magnification 

• material under a microscope = object 
• appearance of material when viewed under microscope = image 
• Magnification = (size of image) / (size of real object)
• Size of real object = size of image/ magnification 
• Kilometer (km) =10³
• Metre (m) = 1
• Millimetre (mm) = 10-³
• Micrometre (μm) =10-⁶
• Nanometre (nm) =10^-9

Resolution 

• Resolution or resolving power of microscope is minimum distance apart that two objects can be in order for them to appear as separate items
• Resolving power depends on wavelength or form of radiation used
• In light microscope, it is about 0.2μm 
• Means that any objects which are 0.2μm or more apart will be seen separately but any objects closer than 0.2μm will appear as single item 
• Greater resolution means greater clarity, this image produced is clearer and more precise 
• increasing magnification increases size of an image but doesn’t always increase resolution 
• every microscope has limit of resolution 
• up to this point increasing the magnification will not do this.
• Object while appearing larger will be more blurred

Cell Fractionation

• Process where cells are broken up and the different organelles they contain are seperated. 
• Before it can take place it must be in
• Cold - to reduce enzyme activity that might break down the organelles
• Isotonic - to prevent organelles bursting or shrinking due to osmotic gain or loss of water 
• Buffered - so pH doesn't fluctuate as it could alter structure of the organelles or affect functioning of enzymes
• Homogenation - cells broken up by homogeniser (blender) to release organelles from cells. Fluid left is homogenate which is filtered to remove any complete cells and large pieces of debris
• Ultracentrifugation - process by which fragments in filtered homogenate are separate in machine called centrifuge = spins tubes of homogenate at very high speeds in order create a centrifugal force
• Filtrate is placed in a centrifuge and spun at slow speed
• Heaviest organelle sinks to form a pellet and fluid left is supernatant
• Supernatant is removed and transferred then spun again at a faster speed than before.
• The next heaviest organelles, mitochondria, are forced to the bottom of the tube 
• process is continued in this way so that at each increase in speed, next heaviest organelle is sedimented and separated out.
• Results:  Nuclei 1st at 1,000 revolutions per min. Then mitochondria at 3,500 revolutions per min. Then Lysosomes at 16,500 revolutions per min

The electron microscope

• Light microscopes have poor resolutions as result of relative;y long wavelength of light 
• …