|Lecture Notes: 27 August||
All of these characteristics come together in the fundamental unit of life, the cell. All cells share a few features (text Figure 1-3):
There are three domains of life recognized today (text Figure 1-4, figure below), with two fundamental types of cell organization: prokaryote (no nuclear envelope or membrane bound organelles) and eukaryote (membrane bound organelles and a nuclear envelope).
A typical idealized prokaryote cell is shown in text Figure 1-6. [overhead- E. coli cell]
Let's look at E. coli for a moment just to get an idea of its size, and also to get an idea of the sizes of various molecules. The image in your text and as shown above gives the major components.
E. coli cell x 100,000 [overhead] is an artists rendition of a typical E. coli cell, with the various components drawn to scale. E. coli cytosol [overhead] magnifies a square section of that cell to 1,000,000 times so that particles such as ribosomes, proteins and DNA are readily visible. This view leaves out all of the small molecules though, to simplify the visualization. Finally a corner of the square is magnified a further ten times and water and small metabolites are shown in a very thin slice of our bacterial cell.
To give some additional perspective, lets look at the relative sizes of these objects. [overhead-sizes of molecules, one million times magnification]
Cool facts about E. coli :70% water, 15% protein, 7% nucleic acids, 3% polysaccharides, 3%, lipids, 1% inorganic ions, & 0.2% metabolites.
Complex, generalized organisms such as E. coli exhibit an amazing level of redundancy in enzymes etc. For example, of the approximately 4,000 genes in E. Coli less than 300 have been found to be "essential," where essential means the organism cannot grow on rich medium if the gene is deleted. Many genes also appear to be "silent" under more restrictive conditions as well - that is the organism often has more than one pathway to accomplish a given metabolic activity. (Cornish-Bowden & Cárdenas, Nature 14 Nov 2002, p 129)
As mentioned earlier we will be focusing on eukaryotes in the rest of this course. Eukaryotes differ from prokaryotes in having a nucleus and cell organelles (their cells are physically compartmentalized). As a point of reference, an E. coli cell is about the size of a typical mammalian mitochondria.
Let's look at where different major metabolic pathways occur in a "typical" liver cell. (text Figure 1-7a) An image of a more generalized cell is shown below.
Not identified in the image above are:
Last modified 24 August 2008