Cellular Ultrastructure
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Eukaryotic Cells Prokaryotic Cells Cell Fractionation

 

All living things are made of cells, and cells are the smallest units that can be alive. Life on Earth is classified into five kingdoms, and they each have their own characteristic kind of cell. However the biggest division is between the cells of the prokaryote kingdom (the bacteria) and those of the other four kingdoms (animals, plants, fungi and protoctista), which are all eukaryotic cells. Prokaryotic cells are smaller and simpler than eukaryotic cells, and do not have a nucleus.

    Prokaryote =   "before carrier bag"  i.e. without a nucleus
Eukaryote =    "good carrier bag"    i.e. with a nucleus

 We'll examine these two kinds of cell in detail, based on structures seen in electron micrographs (photos taken with an electron microscope). These show the individual organelles inside a cell.  

Eukaryotic Cells  [back to top]

Prokaryotic Cells  [back to top]

  Summary of the Differences Between Prokaryotic and Eukaryotic Cells  

Prokaryotic Cells

Eukaryotic cells  
small cells (< 5 mm)

larger cells (> 10 mm)

always unicellular often multicellular
no nucleus or any membrane-bound organelles, such as mitochondria always have nucleus and other membrane-bound organelles
DNA is circular, without proteins DNA is linear and associated with proteins to form chromatin
ribosomes are small (70S) ribosomes are large (80S)
no cytoskeleton always has a cytoskeleton
motility by rigid rotating flagellum made of flagellin motility by flexible waving undulipodium, made of tubulin
cell division is by binary fission cell division is by mitosis or meiosis
reproduction is always asexual reproduction is asexual or sexual
huge variety of metabolic pathways common metabolic pathways


 
Endosymbiosis

Prokaryotic cells are far older and more diverse than eukaryotic cells. Prokaryotic cells have probably been around for 3.5 billion years, while eukaryotic cells arose only about 1 billion years ago. It is thought that eukaryotic cell organelles like nuclei, mitochondria and chloroplasts are derived from prokaryotic cells that became incorporated inside larger prokaryotic cells. This idea is called endosymbiosis, and is supported by these observations:

 

Cell Fractionation  [back to top]

This means separating different parts and organelles of a cell, so that they can be studied in detail. All the processes of cell metabolism (such as respiration or photosynthesis) have been studied in this way. The most common method of fractionating cells is to use differential centrifugation:

1.       Cut tissue (e.g. liver, heart, leaf, etc) in ice-cold isotonic buffer.  Cold to stop enzyme reactions, isotonic to stop osmosis, and buffer to stop pH changes.

2.       Grind tissue in a blender to break open cells.

3.       Filter.  This removes insoluble tissue (e.g. fat, connective tissue, plant cell walls, etc).  This filtrate is not called a cell-free extract, and is capable of carrying out most of the normal cell reactions.

 4.       Centrifuge filtrate at low speed

            (1 000 x g for 10 min)

 

 5.       Centrifuge supernatant at medium speed

            (10 000 x g for 30 min)

 6.       Centrifuge supernatant at high speed

            (100 000 x g for 1 hour)

 7.       Centrifuge supernatant at very high speed

            (300 000 x g for 3 hours)

  8.       Supernatant is now organelle-free cytoplasm

A more sophisticated separation can be performed by density gradient centrifugation. In this, the cell-free extract is centrifuged in a dense solution (such as sucrose or caesium chloride). The fractions don't pellet, but instead separate out into layers with the densest fractions near the bottom of the tube. The desired layer can then be pipetted off. This is the technique used in the Meselson-Stahl experiment (module 2) and it is also used to separate the two types of ribosomes. The terms 70S and 80S refer to their positions in a density gradient

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Last updated 23/05/2004