Biology 2.1.1

MAGNIFICATION: THE NUMBER OF TIMES LARGER AN OBJECT IS THAN THE ORIGINAL IMAGE

• can be adjusted on a compound microscope with the objective lenses
• MAGNIFICATION= SIZE OF IMAGE/ACTUAL SIZE

RESOLUTION: THE ABILITY TO DISTINGUISH BETWEEN TWO OBJECTS THAT ARE VERY CLOSE TOGETHER

• limited by the diffraction of light because diffraction causes the organelles to overlap in a specimen so the structures can no longer be seen as seperate entities and detail is lost.
• Resolution can be increased by using beams of electrons which have a wavelength thousands of times shorter than light. Electron beams are diffracted still but the shorter wavelength means the beams can be much closer before they overlap meaning objects which are much smaller can be seen seperately without diffraction blurring the image. 

MICROSCOPES:

TEM: Transmission electron microscope: a beam of electrons is transmitted through a specimen and focused to produce an image

SEM: a beam of electrons is sent across the surface specimen and reflected electrons are colected. 

Light Microsopes:

• cheaper
• small and portable
• simple sample prep
• natural colour or stained
• up to x2000 magnification
• 200nm resolving power
• specimens can be living or dead

Electron microsopes:

• expensive
• large and requires installation
• complex sample prep
• sample prep often distorts material
• vacuum required 
• B&W images produced 
• over x500,000 magnification
• TEM is 0.5nm and SEM is 3-10nm
• Specimens are dead

• Eyepiece graticule: used to measure the size of a sample under a microscope. It is a glass disc marked with a fine scale of 1 to 100, scale has no units and remains unchanged which ever objective lens is in place
• Every microscope and every lens has to be calibrated speerately using an eyepiece graticule and a slide micrometer
• Relative size divsions increase as magnification increases.The scale on the graticule at each magnification is callibrated using a stage micrometer.
• The scale on amicrometer slide is usually 100 divisions=1mm so 1 division= 10 micrometers.

Calibrating a x4 objective lens:

• put stage micrometer in plapce and the eyepiece graticule in the yepiece
• get the scale on micrometer slide in clear focus
• align the micrometer scale with the scale on the eyepiece and take a reading from the two scales.
• 1 graticule division= no. of micrometers/ no. of graticule divisions
• graticule divisions x magnification factor = measurement (micrometers)

Dry mount- solid specimens are viewed whole or sectioned, placed on centre of the slide with a cover slip over. E.g. hair. pollen.

Wet mount- specimens are suspended in liquid (e.g. water or immension oil) and a coverslip is placed on from an angle

Squash slides- a wet mount is prepared then lens tissue is used to gently press down the coverslip. Depending on the material, damage to coverslip is avoided by squashing samples between to slides

Smear slides- the edge of a slide is used to smear the sample, creating a thin, even coating on another slide, cover slip is placed over. E.g. blood.

Pre-preparing slides:

• FIXING- Preserving specimens using chemicals that leave them in as near natural state as possible. 
• SECTIONING-specimens are…