AS Biology - Cell Structure - Organelles

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Protein Production

Proteins are made at the ribosomes.

Ribosomes on the rough endoplasmic reticulum make proteins that are excreted or attached to the cell membrane.

Free ribosomes in the cytoplasm make proteins that stay in the cytoplasm.

New proteins at the rough endoplasmic reticulum are folded and processed (e.g. sugar chains are added).

They're transported from the endoplasmic reticulum to the Golgi apparatus in vesicles.

At the Golgi apparatus proteins may undergo more processing (e.g. sugar chains trimmed or added).

Proteins enter more vesicles to be transported around the cell.

Example: Extracellular enzymes (like digestive enzymes) move to the cell surface and are excreted

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Functions of Cytoskeleton

Organelles in cells are surrounded by the cytoplasm

The cytoplasm is more than just a solution of chemicals - its got a network of protein threads running through it --------> the CYTOSKELETON.

Eukaryotic cells: 

Protein threads are arranged as microfilaments and microtubules.

Microfilaments = small solid strands 
Microtubules = tiny protein cylinders

4 main functions:
(1) Microtubules and microfilaments help support the cell's organelles, keeping them in position.
(2) Help strengthen and maintain its shape.
(3) Responsible for transport of materials within the cell. E.g. movement of chromosomes when they separate during cell division depends on the contraction of microtubules in the spindle.
(4) Proteins of the cytoskeleton can also cause the cell to move. E.g. movement of cilia and flagella is caused by the cytoskeleton protein filaments that run through them. In the case of single cells that have a flagellum, the cytoskeleton propels the whole cell. 

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Cell Fractionation

There are 3 types of fractionation:


Homogenisation = breaking up the cells

  • Can be done in several different ways e.g. vibrating/grinding the cells up in a blender. This breaks up the cell surface membrane and releases the organelles into the solution.

Filtration = getting rid of the big bits

  • The homogenised cell solution is filtered through a gauze to separate large cell debris/tissue debris, like connective tissue, from organelles.
  • Organelles are much smaller than the debris so they pass through easily.
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Cell Fractionation Continued...

Ultracentrifugation = separating the organelles

  • Cell filaments are poured into a test tube. The tube is put into a centrifuge (a machine that separates materials by spinning), and is spun at low speed.
  • The heaviest organelles, like nuclei, go to the bottom and form a thick sedament - the pellet.
  • Other organelles stay suspended in the fluid above - the supernatant.
  • The supernatant is drained off and poured into another tube and spun in the centrifuge at higher speed.
  • The heaviest organelles, this time mitochondria, form the pellet at the bottom.
  • The supernatant containing the rest of the organelles is drained off and spun in the centrifuge at an even higher speed.
  • This process is repeated at higher speeds until all organelles are separated out. Each time the pellet consists of lighter organelles.

Mass of organelles (heaviest------>lightest)

Nuclei--->Mitochondria--->Lysosomes--->Endoplasmic Reticulum--->Ribosomes

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