Cells (AQA AS Biology) PART 3 of 6 TOPICS

 

 

TOPICS: Structure of eukaryotic cells  Structure of prokaryotic cells and viruses  Methods of studying cells  All cells arise from other cells  Transport across cell membs  Cell recognition and the immune system

Methods of studying cells:

Optical microscopes

                Advantages:

• Cheaper than electron microscopes

Disadvantages:

• Magnification and resolution is not as electron microscopes

 

Transmission electron microscopes

                Advantages:

• Smaller wavelength of electron beam allows organelles smaller than mitochondria to be seen

 

Disadvantages:

• Specimen has to be dead

 

Scanning electron microscopes

                Advantages:

• Gives 3D structure of organelles

Disadvantages:

• It is very expensive

 

 

Cell fractionation has three processes:

1. Homogenisation is where the cell membrane is broken to release the organelles into an ice cold (preventing enzyme activity), isotonic (preventing shrivelling and bursting of organelles) buffer (to keep the pH constant to prevent the enzymes and organelles from denaturing).
2. Filtration is where the cell membrane is separated from the organelles as well as large debris using a gauze.
3. Ultracentrifugation is where the organelles are separated from each other. All the organelles are poured into a tube and centrifuged where it is spun at high speed at an angle. Heaviest organelles such as nuclei form a pellet at the bottom with all the lighter organelles at the top called the supernatant. The supernatant is drained off into another tube where it is spun at a higher speed at an angle to get the next heaviest organelles (mitochondria) and the process is repeated until all the organelles are separated.