Cells 2 Flashcards
Optical microscope
uses light from a 2D image
Visible light
longer Wavelength so slower resolution 200nm
Lower magnification x 1500
SEM
uses electrons to form a 2d image
beams of electrons can surface knocking off electrons from a thin specimen which are gathered by a cathode ray tube to form an image
Higher resolution as electrons have a shorter wavelength
High magnification
TEM
uses electrons to form a 3d image
electromagnets focus beam of electrons onto specimen, transmitted more dense = more absorbed = darker appearance
electrons shorter wavelength so higher resolution 0.2nm
high mag - x 1500000
Disadvantages of optical microscope
2D image
Only used on thin specimens
low resolution ( can’t see internal structures or organelles e.g. ribosomes)
low mag
Disadvantages of SEM
Vacuum so can not see living organisms
lower resolution than TEM
Disadvantages of TEM
2d image
only used on thin specimens
vacuum so can not see LO
Advantages of TEM, SEM and Optical microscopes
Optical - can see living organisms
SEM
- 3D image
High resolution
High magnification
used on thick specimens
TEM
High Resolution and magnification
Magnification
how much bigger is the image is compared to the actual size
Resolution
The ability to distinguish between 2 points
Steps for measuring the size of an object with an eye piece graticule
Line up eyepiece graticule with stage micrometer
- Use stage micrometer to calculate the size of divisions on eyepiece graticule at a particular magnification
- Take the micrometer away and use the graticule to measure how many divisions
make up the object
- Calculate the size of the object by multiplying the number of divisions by the size of division
- Recalibrate eyepiece graticule at different magnifications
How to prepare a temporary mount
Use tweezers to place a thin section of specimen e.g. tissue on a water drop on a microscope slide
- Add a drop of a stain e.g. iodine in potassium iodide solution used to stain starch grains in plant cells
- Add a cover slip by carefully tilting and lowering it, trying not to get any air bubbles
Cell fractionation
Homogenise tissue using a blender
- Disrupts cell membrane / break open cell
- Release contents / organelles
2. Place in a cold, isotonic, buffered solution
- Cold reduces enzyme activity so organelles aren’t broken down
- Isotonic so water doesn’t move in/out of organelles by osmosis so they don’t burst / shrivel
- Buffered keeps pH constant so enzymes don’t denature
3. Filter homogenate
- Remove large, unwanted debris e.g. whole cells, connective tissue
4. Ultracentrifugation
a) Centrifuge homogenate in a tube at a low speed
b) Remove pellet of heaviest organelle and spin supernatant at a higher speed
c) Repeated at higher and higher speeds until organelles separated out, each time pellet is made of lighter organelles
d) Separated in order of mass/density: nuclei → chloroplasts → mitochondria → lysosomes → endoplasmic reticulum → ribosomes
How to distinguish between a cell artefact and organelle
repeatedly prepared specimen in different ways
If an object could only be seen with one preparation technique, but not another it was more likely to be an artefact than an organelle
Cell cycle
Eukaryotic cells that are able to divide
Interphase
Is for growth and repair of the cell
S phase
DNA replicates semi conservatively leading to 2 sister chromatids
G1 and G2
Number of organelles and volume of cytoplasm increases; protein synthesis; ATP content increased
Mitosis
Parent cell divides = two genetically identical daughter cells, containing identical/exact copies of DNA of the parent cell.
- Stages - ‘PMAT
Prophase
Chromosomes condense, becoming shorter and thicker = appear as two sister chromatids joined by a centromere
- Nuclear envelope breaks down and centrioles move to opposite poles forming spindle network
Metaphase
Chromsomes align at the equator
Spindle fibres attach to chromosomes by centromeres
Anaphase
Spindle fibres contract, pulling sister chromatids to opposite poles of the cell
- Centromere divides
Telophase
Chromosomes uncoil, becoming longer and thinner
- Nuclear envelope reforms = two nuclei
- Spindle fibres and centrioles break down
Cytokinesis
The division of the cytoplasm, usually occurs, producing two new cells
The importance of mitosis
Parent cell divides to produce 2 genetically identical daughter cells for…
- Growth of multicellular organisms by increasing cell number
- Repairing damaged tissues / replacing cells
- Asexual reproduction
Binary fission
Circular DNA and plasmids replicate Cytoplasm expands (cell gets bigger) as each DNA molecule moves to opposite poles of the cell
- Cytoplasm divides
- = 2 daughter cells, each with a single copy of DNA and a variable number of plasmids
