- Basic concepts of cell theory
Cells: we begin as a single cell and develop into
an individual organism of trillions of cells: the basic structural unit
of life.
Most cells are invisible to the naked eye. (Cell theory applies to our
current earth environment, not the reducing, primitive earth environment:
spontaneous generation.)
- All organisms are composed of one
or more cells.
- The cell is the basic structural
and functional unit of life.
- All cells arise from pre-existing
cells.
(Shown by Pasteur in the mid 1850's.)
- Basic cell types
Two common types of cells/groups of organisms: prokaryotic
and eukaryotic.
- Prokarytic cells ("before
the nucleus" - primitive cell type)
figure 5.1: a prokaryotic cell
Prokaryotes are a magnitude smaller than eukaryotes. There are two main
groups of prokaryotes: bacteria and cyanobacteria. Both have cell membranes,
cell walls, and carry out the needed metabolic activities.
Usually prokaryotes are single-celled and don't have a true nucleus, but
do have DNA. Prokaryote do not contain many organelles, but do contain
ribosomes and a plasmid DNA. Ribosomes function in the sythesis of proteins.
The plasmid DNA is used by the bacterium in the production of enzymes which
defend them from viruses. We use plasmid DNA as a vector (carrier) in genetic
engineering (lecture 20). Examples: Escherichia coli
(E. coli): one strain of E. coli (the uncooked hamburger strain?) can kill
you; other strains are not toxic.
Prokaryotes reproduce by binary fission, (a process more primitive than
asexual reproduction in eukaryotes). As a prokaryote grows, it duplicates
its DNA, which then attaches to the cell membrane/ cell wall area; each
duplicate separates with one half of the original cell, which divides into
two cells, each containing a duplicate of the DNA. As each new cell grows,
this process is repeated, repeatedly.
Although we say prokaryotes have a single circular strand of DNA, it is
still a rather extensive amount of material; (it's circular in that it
doesn't have 'ends,' like a string: it's continuous).
Other examples of prokaryotes: they are small: (remember the clusters on
the point of a pencil). Our relationship to (need for) prokaryotes: cyanobacterium
(blue-green algae) inocorporate nitrogen into living tissues. Without their
doing so, we wouldn't be.
We use prokaryotes: in technology (e.g. headphones); the bacteria such
as that in Yellowstone's 'hot-pots,' produce enzymes that work at high
temperatures (too high for our enzymes to work). These enzymes are used
in genetic engineering. Prokaryotes eat (break down) just about anything:
used in soaps, oil spills. Enzymes from another bacterium, by increasing
the temperature at which water will freeze, help in making artificial snow.
Yet another is used to kill the larvae that cause river blindness. When
Ogden had a problem with the gypsy moth, it was solved by spraying a (biodegradable)
bacterium.
- Eukaryotic cells ("true nucleus"
- advanced cell type) plant cell animal
cell
Eukaryotes have a true nucleus. (Plant and animal
cells are eukaryotic.) Eukaryotic cells are complex, highly organized,
containing a variety of organelles. Some have a cell wall: plants (cell
wall primarily of cellulose) and fungi (cell wall primarily of chitin).
Animals: no cell wall. Outside boundary is a plasma membrane only.
Eukaryotes reproduce asexually (mitosis lecture 11)
and sexually (meiosis lecture 12). (e.g stramberry
plant: 'runners' develop into individual plants. Strawberries also reproduce
sexually: (flowers: male and female parts.) Asexual reproduction is very
uncommon in animals, but not absent: aphids, for example, reproduce can
reproduce asexually.
Usually eukaryotes are multicellular, but can be single-celled. Eukaryotic
organisms range in size from microns, such as the illions all over your
body (e.g. mites), to relatively huge: whales, elephants, etc..
- Common features of prokaryotic
and eukaryotic cells
- Cell coverings
All cells have cell coverings.
- Cell wall
All prokaryotes have cell walls. Plants and fungi
(eukaryotes) have cell walls as well.
- Plasma membrane Plasma
membrane
All living cells, prokaryotic or eukaryotic,
have plasma membrane. The plasma membrane is composed of a phospholipid
bilayer. Each layer has a fatty end (always on the inside because it is
non-polar) which is hydrophobic ("water-hating" fatty tails).
The phosphorous end is charged (negative) and so it interacts with water
("water-loving" hydrophilic). The bilayer, which is self-assembling,
contains proteins (various purposes: transport, etc.), cholesterol (prevent
the phospholipids from packing too tightly).
- Cytoplasm
The cytoplasm is everything in the cell but the cell
wall, cell membrane, and nucleus.
- Organelles: Ribosomes
Ribosomes are the common organelle in eukaryotes
and prokaryotes. Because every cell must produce proteins at some point,
ribosomes are required.
- Genetic information: DNA
Both cell types contain DNA.
- Metabolic similarities
Autotrophs and heterotrophs in both groups. Every
cell must have an energy source (be able to process ATP).
Review the summary table.
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