Cell Structure

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CELL FRACTIONATION:

  • The process where cells are broken up and the different organelles they contain are separated out.
  • Before cell fractionation can begin, the tissue is placed in a cold, buffered solution of the same water potential as the tissue.
  • The solution is:
    • Cold - to reduce enzyme activity that might break down the organelles.
    • Is of the same water potential as the tissue - to prevent organelles bursting or shrinking as a result of osmotic gain or loss of water.
    • Buffered - so that the pH does not fluctuate. Any change in pH could alter the structure of the organelles or affect the functioning of enzymes.
  • There are 2 stages to cell fractionation:
    • Homogenisation:
      • Cells are broken up by a homogeniser (blender).
      • This releases the organelles from the cell.
      • The resultant fluid, known as homogenate, is then filtered to remove any complete cells and large pieces of debris.
    • Ultracentrifugation:
      • The process by which the fragments in the filtered homogenate are separated in a centrifuge.
      • This spins tubs of homogenate at very high speed in order to create a centrifugal force.
      • For animal cells, the process is:
        • The tube of filtrate is placed in the centrifuge and spun at a slow speed.
        • The heaviest organelles, the nuclei, are forced to the bottom of the tube, where they form a thin sediment or pellet.
        • The fluid at the top of the tube (supernatant) is removed, leaving just the sediment of nuclei.
        • The supernatant is transferred to another tube and spun in the centrifuge at a faster speed than before.
        • The next heaviest organelles, the mitochondria, are forced to the bottom of the tube.
        • The process is continued in this way so that, at each increase in speed, the next heaviest organelle is sedimented and separated out.

EUKARYOTIC CELL STRUCTURE:

  • THE NUCLEUS:
    • Contains the organism's hereditary material and controls the cell's activities.
    • Usually spherical and between 10 and 20 micrometres in diameter, the nucleus has a number of parts:
      • The nuclear envelope:
        • A double membrane that surrounds the nucleus.
        • Its outer membrane is continuous with the endoplasmic reticulum of the cell and often has ribosomes on its surface.
        • It controls the entry and exit of materials in and out of the nucleus and contains the reactions taking place within it.
      • Nuclear pores:
        • Allow the passage of large molecules, such as mRNA, out of the nucleus.
        • There are typically around 3000 pores in each nucleus, each 40-100nm in diameter.
      • Nucleoplasm:
        • The granular, jelly-like material that makes up the bulk of the nucleus.
      • Chromosomes:
        • Consist of protein-bound, linear DNA.
      • The nucleolus:
        • A small spherial region within the nucleoplasm.
        • It manufactures rRNA and assembles the ribosomes.
        • There may be more than 1 nucleolus in a nucleus.
    • Functions:
      • Act as the control centre of the cell through the production of mRNA and tRNA and hence protein synthesis.
      • Retain the genetic material of the cell in the form of DNA and chromosomes.
      • Manufacture rRNA and ribosomes.
  • THE MITOCHONDRION:
    • Usually rod-shaped and 1-10 micrometres in length.
    • Made up…

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