cell fractionation

• cell fractionation


  the process of breaking up a sample of tissue then centrifuging at different speeds it also has 3 sections; homogenisation, filtration and ultracentrifugation
• Homogenisation


  homogenisation is the biological term of breaking up cells

  The sample of tissue must be placed in a cold isotonic buffer solution
• why does sample tissue have to be placed in an ice cold isotonic buffer solution ?

  ice cold - to reduce activity of enzymes(as it lowers the kinetic energy of particles)that break down organelles

  isotonic -must have the same water potential as cells being broken up to prevent water entering the cell via osmosis. which would cause them to expand and get damaged

  buffered -to prevent organelle proteins and enzymes from being denatured
• filtration


  homogenate (containing homogenised cells) is filtered through a gauze.to seperate any large cell debris or tissue debris that did not breakup.the organelles that are smaller than the debris pass through the gauze.this leaves a solution (known as the filtrate) that contains a mixture of organelles
• ultracentrifugation


  The filtrate is placed into a tube and the tube is placed in a centrifuge A centrifuge is a machine that separates organelles by spinning and fractionating parts of cell as when the homogeniser is spun due to centrifrugal force the heaviest organelle is pulled to the bottom ie. nuclei as this was the first and most densest organelle it was spun at a lower speed
• describe and explain why filtration is necessary to isolate ribosomes from a homogenised cell solution

  • filtration removes whole cells from a cell solution
  • due to small size ribosomes are able to pass through the filter meaning they can be isolated
• describe and explain why ultracentrifugation is necessary to isolate ribosomes from a homogenised cell solution


  • cell solution is spun at a low speed to remove nuclei
  • cell solution is spun at high speed until ribosomes collect in a pellet
• Describe how a sample of chloroplasts could be isolated from leaves

  1. Break open tissue and filter
  2. Place sample in ice-cold , isotonic , buffered solution
  3. Centrifuge and remove debris - nuclei
  4. Centrifuge at a higher speed and remove chloroplast