To give some perspective, lets look at the relative sizes
of these objects. [overhead-sizes of molecules, one million times
magnification]
There are two main cell types: prokaryote and eukaryote.
A typical idealized prokaryote cell is shown in Figure 1-6 on
p 8. [overhead- E. coli cell] A typical idealized eukaryote
(animal) cell is shown in Figure 1-8 on p 10 of Voet. [overheads-
Animal cell, Plant cell]
Compartmentation in Eukaryotes
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. [overhead-Animal
cell]
Cytosol: Glycolysis and most of gluconeogenesis; Pentose
Phosphate shunt; Fatty acid biosynthesis.
Mitochondria: Kreb's Citric Acid Cycle; Electron transport
system and Oxidative Phosphorylation; Fatty acid oxidation; Amino
acid catabolism; Interconversion of carbon skeletons.
Endoplasmic Reticulum: Lipid Synthesis; Steroid synthesis;
Phase one detoxification reactions; Biosynthesis and modification
of membrane and export proteins.
Golgi Complex: Further modification of membrane and
export proteins.
Nucleus: DNA replication, synthesis and processing
of messenger RNA's.
Nucleolus: localized region of the nucleus in which
ribosomal RNA's are synthesized and processed.
Plasma Membrane: Active and passive transport systems;
receptors and signal processing systems (synthesis of various
second messengers etc.).
Ribosomes: protein synthesis (associated with ER for
membrane and export proteins).
Glycogen Granules: enzymes of glycogen synthesis and
breakdown. including branching and debranching.
Chloroplasts (plants only): Light capturing processes
and electron transport & oxidative phosphorylation for photosynthesis;
Calvin cycle (dark reactions of photosynthesis).
Glyoxisomes (plants only): location of glyoxalate
cycle.
Let's look at a "typical" plant cell for
a moment. All of the organelles we saw in animals are here as
well, but with a few additions: [overhead-Plant cell]
Chloroplasts: Light capturing processes and electron
transport & oxidative phosphorylation for photosynthesis;
Calvin cycle (dark reactions of photosynthesis).
Glyoxisomes: location of glyoxalate cycle.
Cell wall: made up of cellulose glued together with
lignin (a plastic like polymer) - maintains cell integrity against
high osmotic pressure, gives cell rigidity.
Vacuole: storage of dilute aqueous solutions, provides
fluid for osmotic pressure.
A prokaryote cell: size and composition
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. Figure 1.25 [overhead- E. coli cell x 100,000]
is an artists rendition of a typical E. coli cell, with
the various components drawn to scale. Figure 1.26 [overhead-
E. coli cytosol] magnifies a square section of that cell
another ten 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.
Pathway Diagrams
