General Cell Information
>The cell is the smallest
unit of life.
>Your body has about
100 trillion cells.
>All cells have DNA
and cytoplasm.
>There are two basic
types of cells: prokaryotic and eukaryotic.
> Eukaryotic cells
differ from prokaryotic cells in that eukaryotic cells contain many membrane
bound
organelles.
Prokaryotic Cells
- have no nucleus
- have no membrane-bound organelles
- make up the kingdoms Monera (bacteria) and Archaea
[See more info on The Six Kingdoms]
- prokaryotes are more primitive than eukaryotes
- cells have a single circular chromosome
- have ribosomes surrounded by a cell membrane and a cell wall
- may have photosynthetic pigments (i.e. cyanobacteria) |
 |
- some prokaryotes have an external flagellum for locomotion or pili for
adhesion
- shapes: baccilli (rods), cocci (round), spirilla (helical)
- prokaryotes were the first forms of life on earth, evolving over 3.5
billion years ago |
Prokaryotic Structure
Internally,
prokaryotes have a simple internal structure, and no membrane-bound organelles.
Nucleoid
– DNA in the cell is generally found in this central region. Though it
isn't surrounded by a membrane, it is visibly separate from the rest of
the cell interior.
Ribosomes
– Ribosomes make the cytoplasm of prokaryotes look granular appearance
in electron micrographs. They are smaller than ribosomes in eukaryotic
cells, but they do the same job of translating the genetic message in messenger
RNA so as to produce proteins.
Storage
granules – Nutrients may be stored in the cytoplasm in the form of
lipids, glycogen , polyphosphate (and sometimes sulfur or nitrogen).
Endospore
- Some prokaryotes form spores that are very resistant to drought, high
temperatures and other hazards of nature. Once the hazard is removed, the
spore germinates to create a new prokaryotes.
Externally,
prokaryotes have some or all of the following structures:
Capsule
– Protective layer of polysaccharides (and sometimes proteins) around the
cell. It is often associated with pathogenic bacteria because it
serves as a barrier against phagocytosis by leukocytes (white blood cells).
 |
Outer
membrane – This lipid bilayer is found in Gram negative bacteria and
is the source of lipopolysaccharide (LPS) in these bacteria. LPS is toxic.
Cell
wall – The cell wall maintains the overall shape of a prokaryotic cell.
It is made up of peptidoglycan (polysaccharides and protein). The
three primary shapes in bacteria are coccus (round), bacillus (rods) and
spirillum (spiral or helical). Mycoplasma are bacteria that have no cell
wall and therefore have no definite shape.
Periplasmic
space – This is found only in prokaryotes with both an outer membrane
and plasma membrane (i.e. Gram negative bacteria). In the space between
the two membranes are enzymes and other proteins that help digest and move
nutrients into the cell.
Plasma
membrane – This is a lipid bilayer similar to the plasma membrane
of other cells. There are numerous proteins moving in or on this layer
that transport waste, ions, and nutrients across the membrane. |
Appendages:
prokaryotes may have the following appendages:
Pili
– These are hollow, hair-like extensions made of protein allow prokaryotes
to attach to other cells. One particular type, the sex pilus, allows
the transfer of DNA from one prokaryote to another. Pili are also called
fimbriae.
Flagella
- Flagella are long appendages which rotate by means of a "motor" located
just under the cytoplasmic membrane. They are used to move the cell around.
Prokaryotes may have one, a few, or many flagella in different places on
the cell.
Eukaryotic Cells
- have a nucleus
- have organelles (compartments
that enable a cell to function by making and releasing energy, helping
the cell to maintain homeostasis, and enabling a cell to reproduce)
- all other cells (other
than bacteria)
- found in the Protista,
Fungi, Plant, and Animal kingdoms
- eukaryotes date back
1.2-1.5 billion years ago
- have 1000 times more
DNA than prokaryotic cells |
 |
 |
Eukaryotic Structure :
- nucleus - controls the cell’s activities;
contains the DNA
- cell membrane - a tough, flexible
lipid bilayer made of phospholipids, protein, and other molecules.
- cytoplasm - jellylike substance
that fills the inside of a cell
- ribosomes - make proteins
- mitochondria - produce energy in
the cell
- endoplasmic reticulum - "tunnels"
in the cytoplasm that allow materials to move through the cell easier
- Golgi body - stores, processes,
and secretes materials |
Unlike animal cells,
plant cells have cell walls, vacuoles, & chloroplasts.
Plants only:
- cell wall - rigid surrounding
of plant cells
- chloroplasts - contain chlorophyll
in plants; this is where the plant’s food is produced
- vacuoles - large bodies in
plant cells that hold water, waste, etc. |
 |
Theories on the origin of eukaryotic organelles:
The two major theories are Endosymbiosis
and Autogenous models.
Endosymbiotis theory:
> This is the most popular scientific theory was first attributed to Lynn
Margulis, though related concepts by others (i.e. Mereschkowsky) have
been around for years.
> It is theorized that a larger anaerobic prokaryotic cell “swallowed”
a smaller aerobic one, and the aerobic prokaryote became an organelle ...
a mitochondrion ... of the larger cell. 
> This theory only applies to mitochondria and choloroplasts.
> Within the idea of endosybiosis, there are different theories as to the
exact type and sequence of evolution. Some of these have been proposed
by: Mereschkowsky (1905, 1910); Goksøyr (1967); Sagan
(1967); de Duve (1969); Stanier (1970); Raff and Mahler
(1972); Uzzel and Spolsky (1974); Bogorad (1975); Cavalier-Smith (1975);
John and Whatley (1975); Whatley et al. (1979); Doolittle (1980);
Van Valen and Maiorana (1980); Margulis (1981, 2000); Cavalier-Smith (1987);
Van Valen and Maiorana (1980); Zillig et al. (1989); Lake and Rivera
(1994); Gupta and Golding (1996); Moreira and Lopez-Garcia (1998); Horiike
et al. (2001).
> It is sometimes called SET (serial endosymbiosis theory), exogenous,
or xenogenous hypothesis.
> The elements of this theory are as follows:
1. Mitochondria of eukaryotes evolved from aerobic bacteria living within
their host cell.
2 The chloroplasts of eukaryotes evolved from endosymbiotic cyanobacteria
(autotrophic prokaryotes).
3. Eukaryotic cilia and flagella may have arisen from endosymbiotic spirochetes.
The basal bodies
from which eukaryotic cilia
and flagella develop would have been able to create the mitotic spindle
and thus made mitosis possible.
- Evidence
for the Endosymbiotis Theory:
1. Mitochondria and chloroplasts have their own DNA in circular molecules
like the prokaryotes.
2. These organelles have their own ribosomes that are smaller than those
in the cytoplasm. These ribosomes are the same size as those in prokaryotes.
3. The protein synthesis of these organelles is semi-independent of that
occurring in the cytoplasm and it is inhibited by the same antibiotic that
works on prokaryotes.
4. These organelles are found enclosed in membranes as though they were
captured in a vacuole of a larger cell.
5. Mitochondria and chloroplasts divide by fission, not mitosis.
Autogenous Theory:
1. Eukaryotes arose directly from a single prokaryote ancestor by compartmentalization
of functions brought about by infoldings of the prokaryote plasma membrane
2. This model is usually accepted for the endoplasmic reticulum, golgi,
and the nuclear membrane, and of organelles enclosed by a single membrane
(such as lysosomes).
3. According to the autogenous hypothesis, mitochondria and chloroplasts
have evolved within the protoeukaryote cell by compartmentalizing plasmids
or vesicles of DNA within a pinched off invagination of the cell membrane.
4. Key contributors have been Klein and Cronquist (1967) and Cavalier-Smith
(1975, 1978).
5. Organelles evolved gradually in steps.
Other theories:
- Thermoreduction model: eukaryotes came first, and prokaryotes
evolved (reductive evolution) from them; focuses more on gene and genome
organization
- Ox-tox model: the ancestor of mitochondria was an aerobic proteobacterium;
the host was an anaerobic, primitively amitochondriate eukaryote; addresses
the origin of mitochondria, not the complete eukaryote cell
- Panspermia (or Cosmozoan): cells came from somewhere else (outer
space) and seeded Earth
- Biblical (or Scientific Creationism): both types of cells came
from a Creator
ADDENDUM: The Six Kingdoms
1) Archaebacteria
- Archaebacteria are organized into three phyla of bactreria that are found
mainly in extreme habitats where little else can survive.
- All known Archaebacteria live without oxygen(anerobic) and obtain their
energy from inorganic molecules or from light.
2) Eubacteria
- Eubacteria (“true bacteria”) live in a wide variety of habitats and obtain
their energy needs in a variety of ways.
- The main phyla are organized by how they obtain energy:
1) heterotrophs: need to consume organic molecules for energy
2) autotrophs: make their own food
3) chemotrophs: obtain their energy from chemosynthetic breakdown of inorganic
(nonliving matter - no carbon) substances such as sulfur and nitorgen compounds.
3) Protists: single
and multicellular organisms that are plant-like, animal-like and fungi-like.
There are 12 phyla.
4) Fungi: many-celled
organisms that decompose dead matter in our environment. There are 4 phyla.
5) Plants: many-celled
organisms that are characterized by their tough cell walls and photosynthetic
abilities. There are 9 divisions.
6) Animals: a very
diverse group of 9 phyla and very large, numbering over one million identified
species. |
