Cell Structure Flashcards
2.1 Microscopy
- m to cm (x100)
- cm to mm (x10)
- mm to μm (x1000)
- μm to nm (x100)
Light Microscope
- cannot distinguish objects closer than ½ wavelength of light
- max resolution = 200 nm
- max magnification = 1,500 X
- observes eukaryotic/whole cells (nuclei, mitochondria & chloroplasts)
- CANNOT observe ribosomes, lysosomes & endoplasmic ret.
Electron Microscopes
- beam of electrons used have a smaller wavelength than light
- max resolution = 0.2 nm
- max magnification = 1,500,000 X
- CAN observe ribosomes, lysosomes & endoplasmic ret.
- CANNOT observe live specimens as water must be removed
- requires very thin specimen & careful staining
- stains can create artefacts
- cannot produce coloured images
Transmission Electron Microscope (2D)
- max mag. = 500,000 X
- electromagnets are used to focus electron beams which are then transmitted through specimen
- denser specimen absorbs more electrons
- dense areas of cell appear darker (contrast)
- allows internal structure to be seen
Scanning Electron Microscope (3D)
- max mag. = 100,000 X
- scans electron beams across specimen which bounce of surface creating an image
- can be used on thick specimen
- specimen must be coated with a metal
- allows external structure to be seen
- lower resolution than TEMs
Laser Scanning Confocal Microscope
- cells are stained with fluorescent dyes
- cells are scanned with laser beams that reflect off the dye
- multiple depths of organism is scanned to produce a 3D image
- specific proteins & structure within cell can be observed
- higher res. than light microscopes
- slow process
- possibility of photodamage
Magnification & Resolution
- image/actual
- total mag = eyepiece mag x objective lens mag
- light is diffracted as it passes through specimen
- light rays overlap as points get closer which can “blur” images
Slide Preparation
Dry Mount e.g. hair
- solid specimen cut into very thin slices (or whole) aka ‘sectioning’
- specimen placed in centre of slide
- cover slip placed over specimen
Wet Mount e.g. cheek cell
- used for specimen suspended in water or immersion oil
- cover slip is placed at an angle to avoid air bubbles
Slide Preparation 2
Squash Slide e.g. root tip
- wet mount prepared first
- a lens tissue is used to press down cover slip to prevent finger residue
Smear Slide e.g. blood
- slide used to smear specimen to create a thin layer
- cover slip is placed over the thinnest section of specimen at a 45° angle
Staining
- ‘Crystal Violet’ & ‘methylene blue’ are positively charged dyes that stain negatively charged material in cytoplasm
- ‘Nigrosin’ & ‘Congo red’ are negatively charged dyes that are repelled by cytosol
- ^ the dye remains outside cell so it stands out from background
Differential Staining
Gran stain technique:
- separates bacteria into two groups (gram-positive & gram-negative)
- counterstains are used
Acid-fast technique:
- separates species of mycobacterium from other bacteria
- chloroplasts don’t need stains as they show up green (natural colour)
- dyes used in electron microscopes cause tissues to show up black or different shades of grey
Calibration
- an eyepiece graticule has no fixed units
- a stage micrometer is a stage with a 1mm line on it
- ^ used to find out how many µm each graticule unit represents
- each objective lens needs to be calibrated separately
- micrometer & eyepiece grat. must be aligned
2.4 Eukaryotic Cells
- DNA is contained in a membrane-bound nucleus and is a linear molecule
- DNA is coiled around histone proteins
- contains membrane-bound organelles
- has larger ribosomes than prokaryotes
Cell Wall
- freely permeable to most substances (including water molecules)
- offers structural support to cell
- support is provided by cellulose
- microfibril mesh produces strength
- helps resist hydrostatic pressure; making cell wall rigid to maintain structure
- cell wall in fungi is composed of chitin
Plasma Membrane
- controls exchange of materials between the internal and external cell environment
- partially permeable due to phospholipid bilayer
