AS Level CIE Biology: 1.1 Microscopes

why are specimens viewed under a light microscope?

to allow some details of cellular material to be observed

how are pre-prepared permanent slides viewed

very thin layers of tissue are cut then stained and permanently mounted on a glass slide for repeated use

how do you prepare a slide using a liquid specimen?

add a few drops containing the liquid sample to clean a slide using a pipette then lower a coverslip over the specimen and gently press down to remove air bubbles

what do coverslips do?

protect the microscope lens from the liquids and help to prevent drying out

how do you prepare a microscope slide using a solid specimen?

use scissors/scalpel to cut a small sample of tissue and peel away/cut a very thin layer of cells from the tissue sample to ensure that theyre thin enough for light to pass through. place the sample onto a slide and a drop of water may be added. then apply iodine stain. gently lower coverslip over the specimen and press down to remove any air bubbles.

how do you prepare a slide using human cells?

brush teeth w toothbrush and paste to remove bacteria. take a sterile cotton swab and scrape the inside cheek for five-ten seconds. smear cotton swab on centre of microscope slide for two to three seconds. add drop of methylene blue solution, nucleus and mitochondria appear darker as stains negatively charged molecules in cell. place coverslip on top, lay down on one edge and gently lower other until flat to reduce bubble formation and absorb any excess solution with paper towel.

iodine stains…

starch blue-black and colors nuclei and plant cell walls pale yellow

crystal violet stains…

cell walls purple

methylene blue stains..

animal cell nuclei dark blue

congo red stains…

the background red as not taken up by cells to provide contrast

biological drawing rules:

title, magnification shown, can use scale bar, sharp pencil, plain white paper, clear single lines, no sketching/shading, large drawing, well defined structures, only visible structures should be drawn and should look like specimen, use proper proportions, clearly label with lines with no arrowheads, dont cross, drawn with ruler, are on one side and connected directly to part

magnification:

number of times that a real life specimen has been enlarged to give a larger view/image

Image/drawing size =

actual size of image x magnification

one meter

millimeters

millimeters to micrometers

times one thousand

micrometers to nanometers

times one thousand

what is an eyepiece graticule and stage micrometer used for?

measuring the size of an object when viewed under a microscope

units for eyepiece graticules

epu/eyepiece units.

one graticule division =

number of nanometers divided by number of graticule division

graticule divisions covered by object x magnification factor =

length of object in micrometers

total magnification=

eyepiece lens magnification x objective lens magnification

resolution:

microscope's ability to distinguish two separate points on an image as separate objects,

resolution limits…

magnification

resolution is limited by..

wavelength of light 400-700nm

max res. for light microscope

200nm

max res. for electron microscope

0.5nm

why do e ms have a much higher res than l ms?

electrons have a much smaller wavelength than visible light

light microscope size:

small and portable

light microscope preparation:

simple

light microscope maximum mag.

x2000

are specimens living or dead in light ms

can be both

electron microscope size:

large machines that are permanently installed in laboratories

which microscope needs a vacuum

electron, so electrons can travel through

electron microscope preparation

complex

max magnification

x500,000

can specimens be alive or dead in e ms

always dead

light ms primary use

looking at whole cells, small plant and animal organisms and tissues in organs

e ms primary use

scanning and transmission at organelles, viruses and DNA

how do e ms work??

fire a beam of e- at specimen

how do transmission e m work

fire electrons through a specimen

how do scanning em work

bounce e- off the surface of a specimen

what happens to electrons in both e ms

picked up by electromagnetic lens then showing image