2.1.1 Cell Structure Flashcards
Outline how a student could prepare a temporary mount of tissue for a light microscope.
- Obtain thin section of tissue e.g. using ultratome or by maceration.
- Place plant tissue in a drop of water.
- Stain tissue on a slide to make structures visible.
- Add coverslip using mounted needle at 45° to avoid trapping air bubbles.
Describe how light microscopes work.
- Lenses focus rays of light and magnify the view of a thin slice of specimen.
- Different structures absorb different amounts and wavelengths of light.
- Reflected light is transmitted to the observer via the objective lens and eyepiece.
Describe how a transmission electron microscope (TEM) works.
- Pass a high energy beam of electrons through a thin slice of specimen.
- More dense structures appear darker since they absorb more electrons.
- Focus image onto fluorescent screen or photographic plate using magnetic lenses.
Describe how a scanning electron microscope (SEM) works.
- Focus a beam of electrons onto a specimen’s surface using electromagnetic lenses.
- Reflected electrons hit a collecting device and are amplified to produce an image on a photographic plate.
Describe how a laser scanning confocal microscope works.
- Focus a laser beam onto a small area on a sample’s surface using objective lenses.
- Fluorophores in the sample emit photons.
- Photomultiplier tube amplifies the signal onto a detector. An image is produced pixel by pixel in the correct order.
How should the field of view in microscopy be recorded?
Draw a diagram with a sharp
pencil. Do not use sketchy lines
or shading. Include a scale bar. Annotate visible structures.
State an equation to calculate the actual size of a structure from microscopy.
actual size = image size / magnification
Define magnification and resolution.
Magnification: factor by which the image is larger than the actual specimen.
Resolution: smallest separation distance at which 2 separate structures can be distinguished from one another.
Why do samples need to be stained for light microscopes?
Coloured dye binds to the structures. Facilitates absorption of wavelengths of light to produce image. Differential staining: contrast between heavily & lightly stained areas distinguishes structures.
State the magnification and resolution of a compound optical microscope.
magnification: x 2000
resolution: 200 nm
State the magnification and resolution of a TEM.
magnification: x 500 000
resolution: 0.5 nm
State the magnification and resolution of an SEM.
magnification: x 500 000
resolution: 3 - 10 nm
Explain how to use an eyepiece graticule and stage micrometer to measure the size of a structure.
- Place micrometer on stage to calibrate eyepiece graticule.
- Line up scales on graticule and micrometer. Count how many graticule divisions are in 100μm on the micrometer.
- Length of 1 eyepiece division = 100μm / number of divisions.
- Use calibrated values to calculate actual length of structures.
Describe the structure of the nucleus.
● Surrounded by a nuclear envelope, a
semipermeable double membrane.
● Nuclear pores allow substances to enter/exit.
● Dense nucleolus made of RNA & proteins assembles ribosomes.
Describe the function of the nucleus.
● Contains DNA coiled around chromatin into chromosomes.
● Controls cellular processes: gene
expression determines specialisation & site of mRNA transcription, mitosis,
semiconservative replication.