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Types of cellsOne way to classify cells is whether they live
alone or in groups. Organisms vary fromsingle cells (called
single-celled orunicellularorganisms) that function and survivemore
or less independently, through colonialforms with cells living
together, tomulticellularforms in which cells are specialized. 220
types of cells and tissues make
up the multicellularhuman body.
Cells can also be classified into two categories based on their
internal structure.
Prokaryoticcells are structurally simple. They are found only in
single-celled
and colonial organisms. In the three-domain system ofscientific
classification,prokaryotic cells are placed in the domainsArchaea
and Eubacteria.
Eukaryoticcells have organelles with their own membranes.
Single-celled
eukaryotic organisms such as amoebae and some fungi are very
diverse, butmany colonial and multicellular forms such as plants,
animals, and brown algaealso exist.
The cells of eukaryotes and prokaryotes. - This figure
illustrates a typical human cell(eukaryote) and a typical bacterium
(prokaryote). The drawing on the lefthighlights theinternal
structures of eukaryotic cells, including the nucleus, the
nucleolus,mitochondria, and ribosomes. The drawing on the
rightdemonstrates how bacterialDNA is housed in a structure called
the nucleoid, as well as other structures normallyfound in a
prokaryotic cell, including the cell membrane, the cell wall, the
capsule,ribosomes, and a flagellum.
Subcellular componentsAll cells whether prokaryotic or
eukaryotic have a membrane, which envelopes the cell,
separates its interior from its environment, controls what moves
in and out, andmaintains the electric potential of the cell. Inside
the membrane, a saltycytoplasm takesup most of the cell volume. All
cells possess DNA, the hereditary material ofgenes andRNA, which
contain the information necessary to build various proteins such
asenzymes, the cell's primary machinery. There are also other kinds
ofbiomolecules incells.
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Cell membrane - a cell's protective coat
The cytoplasm of a eukaryotic cell is surrounded by a plasma
membrane. A form of
plasma membrane is also found in prokaryotes, but is usually
referred to as the cellmembrane. This membrane serves to separate
and protect a cell from its surroundingenvironment and is made
mostly from a double layer of lipids (fat-like molecules)
andproteins. Embedded within this membrane are a variety of other
molecules that act aschannels and pumps, moving different molecules
into and out of the cell.
Cytoskeleton - a cell's scaffold
Schematic of typical animal cell,showing subcellular
components.Organelles: (1) nucleolus (2)
nucleus (3) ribosome (4) vesicle,(5)rough endoplasmic reticulum
(ER),(6) Golgi apparatus, (7)Cytoskeleton, (8) smooth ER,
(9)mitochondria, (10) vacuole, (11)cytoplasm, (12) lysosome,
(13)centrioles
Schematic of typical plant
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The cytoskeleton is an important, complex, and dynamic cell
component. It acts toorganize and maintain the cell's shape;
anchors organelles in place; helps duringendocytosis, the uptake of
external materials by a cell; and moves parts of the cell
inprocesses of growth and motility. There are a great number of
proteins associated withthe cytoskeleton, each controlling a cell's
structure by directing, bundling, and aligning
filaments.
Cytoplasm - a cell's inner space
Inside the cell there is a large fluid-filled space called the
cytoplasm, which includesboth to the mixture of ions and fluids in
solution within the cell, and to the organellescontained in it. The
term cytosolrefers only to the fluid itself.
In prokaryotes, the cytoplasm is relatively free of
compartments. In eukaryotes, itnormally contains a large number of
organelles. The cytosol contains dissolvednutrients, helps break
down waste products, and moves material around the cell
through a process called cytoplasmic streaming. The nucleus
often flows with thecytoplasm, changing its shape as it moves. The
cytoplasm also contains many salts andis an excellent conductor of
electricity, which is important in certain cell functions likecell
signaling.
Genetic material
Two different kinds of genetic material exist: deoxyribonucleic
acid (DNA) andribonucleic acid (RNA). Most organisms use DNA for
their long term informationstorage, but some viruses (retroviruses)
have RNA as their genetic material. Thebiological information
contained in an organism is encoded in its DNA or RNA
sequence. RNA is also used for information transport (e.g. mRNA)
and enzymaticfunctions (e.g. ribosomal RNA) in organisms that use
RNA for the genetic code itself.
Prokaryotic genetic material is organized in a simple circular
DNA molecule (thebacterial chromosome) in the nucleoid region of
the cytoplasm. Eukaryotic geneticmaterial is divided into
different, linear molecules called chromosomes inside a
discretenucleus, usually with additional genetic material in some
organelles like mitochondriaand chloroplasts (see endosymbiotic
theory).
A human cell, i.e. has genetic material in the nucleus (the
nuclear genome) and in themitochondria (the mitochondrial genome).
The nuclear genome is divided into 46 linear
DNA molecules called chromosomes. The mitochondrial genome is a
circular DNAmolecule separate from the nuclear DNA. Although the
mitochondrial genome is verysmall, it codes for some important
proteins.
Foreign genetic material (most commonly DNA) can also be
artificially introduced intothe cell by a process called
transfection. This can be transient, if the DNA is not insertedinto
the cell's genome, or stable, if it is.
http://en.wikipedia.org/wiki/Endocytosishttp://en.wikipedia.org/wiki/Cytoplasmic_streaminghttp://en.wikipedia.org/w/index.php?title=Cell_signaling&action=edithttp://en.wikipedia.org/wiki/DNAhttp://en.wikipedia.org/wiki/RNAhttp://en.wikipedia.org/wiki/Retrovirushttp://en.wikipedia.org/wiki/Genetic_codehttp://en.wikipedia.org/wiki/MRNAhttp://en.wikipedia.org/wiki/Enzymehttp://en.wikipedia.org/wiki/Ribosomehttp://en.wikipedia.org/wiki/Chromosomehttp://en.wikipedia.org/wiki/Nucleoid_regionhttp://en.wikipedia.org/wiki/Chromosomehttp://en.wikipedia.org/wiki/Endosymbiotic_theoryhttp://en.wikipedia.org/wiki/Genomehttp://en.wikipedia.org/wiki/Mitochondrial_genomehttp://en.wikipedia.org/wiki/Transfectionhttp://en.wikipedia.org/wiki/Genomehttp://en.wikipedia.org/wiki/Endocytosishttp://en.wikipedia.org/wiki/Cytoplasmic_streaminghttp://en.wikipedia.org/w/index.php?title=Cell_signaling&action=edithttp://en.wikipedia.org/wiki/DNAhttp://en.wikipedia.org/wiki/RNAhttp://en.wikipedia.org/wiki/Retrovirushttp://en.wikipedia.org/wiki/Genetic_codehttp://en.wikipedia.org/wiki/MRNAhttp://en.wikipedia.org/wiki/Enzymehttp://en.wikipedia.org/wiki/Ribosomehttp://en.wikipedia.org/wiki/Chromosomehttp://en.wikipedia.org/wiki/Nucleoid_regionhttp://en.wikipedia.org/wiki/Chromosomehttp://en.wikipedia.org/wiki/Endosymbiotic_theoryhttp://en.wikipedia.org/wiki/Genomehttp://en.wikipedia.org/wiki/Mitochondrial_genomehttp://en.wikipedia.org/wiki/Transfectionhttp://en.wikipedia.org/wiki/Genome
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Organelles
The human body contains many different organs, such as the
heart, lung, and kidney,with each organ performing a different
function. Cells also have a set of "little organs",called
organelles, that are adapted and/or specialized for carrying out
one or more vital
functions. Membrane-bound organelles are only found in
eukaryotes. Cell nucleus - a cell's information center: The cell
nucleus is the most
conspicuous organelle found in a eukaryotic cell. It houses the
cell'schromosomes and is the place where almost all DNA replication
and RNAsynthesis occur. The nucleus is spheroid in shape and
separated from thecytoplasm by a double membrane called the nuclear
envelope. The nuclearenvelope isolates and protects a cell's DNA
from various molecules that couldaccidentally damage its structure
or interfere with its processing. Duringprocessing, DNA is
transcribed, or copied into a special RNA, called mRNA. ThismRNA is
then transported out of the nucleus, where it is translated into a
specificprotein molecule. In prokaryotes, DNA processing takes
place in the cytoplasm.
Ribosomes - the protein production machine: Ribosomes are found
in both
prokaryotes and eukaryotes. The ribosome is a large complex
composed ofmany molecules, including RNAs and proteins, and is
responsible for processingthe genetic instructions carried by an
mRNA. The process of converting anmRNA's genetic code into the
exact sequence of amino acids that make up aprotein is called
translation. Protein synthesis is extremely important to all
cells,and therefore a large number of ribosomessometimes hundreds
or eventhousandscan be found throughout a cell.
Mitochondria and chloroplasts - the power generators:
Mitochondria are self-
replicating organelles that occur in various numbers, shapes,
and sizes in thecytoplasm of all eukaryotic cells. As mentioned
earlier, mitochondria contain theirown genome that is separate and
distinct from the nuclear genome of a cell.Mitochondria play a
critical role in generating energy in the eukaryotic cell, andthis
process involves a number of complex metabolic pathways.
Chloroplastsarelarger than mitochondria, and convert solar energy
into a chemical energy("food") via photosynthesis. Like
mitochondria, chloroplasts have their owngenome. Chloroplasts are
found only in photosynthetic eukaryotes like plants andalgae. There
are a number of plant organelles that are modified
chloroplasts;they are broadly called plastids and are often
involved in storage.
Endoplasmic reticulum and Golgi apparatus - macromolecule
managers::The endoplasmic reticulum (ER) is the transport network
for molecules targetedfor certain modifications and specific
destinations, as compared to molecules thatwill float freely in the
cytoplasm. The ER has two forms: the rough ER, which hasribosomes
on its surface, and the smooth ER, which lacks them. Translation
ofthe mRNA for those proteins that will either stay in the ER or be
exportedfromthe cell occurs at the ribosomes attached to the rough
ER. The smooth ER isimportant in lipid synthesis, detoxification
and as a calcium reservoir. The Golgi
http://en.wikipedia.org/wiki/Cell_nucleushttp://en.wikipedia.org/wiki/Nuclear_envelopehttp://en.wikipedia.org/wiki/Transcribedhttp://en.wikipedia.org/wiki/Ribosomehttp://en.wikipedia.org/wiki/Ribosomehttp://en.wikipedia.org/wiki/Translation_(genetics)http://en.wikipedia.org/wiki/Translation_(genetics)http://en.wikipedia.org/wiki/Mitochondrionhttp://en.wikipedia.org/wiki/Metabolic_pathwayhttp://en.wikipedia.org/wiki/Chloroplastshttp://en.wikipedia.org/wiki/Chloroplastshttp://en.wikipedia.org/wiki/Chemical_energyhttp://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/wiki/Algaehttp://en.wikipedia.org/wiki/Plastidhttp://en.wikipedia.org/wiki/Endoplasmic_reticulumhttp://en.wikipedia.org/wiki/Lipidhttp://en.wikipedia.org/wiki/Detoxificationhttp://en.wikipedia.org/wiki/Calciumhttp://en.wikipedia.org/wiki/Golgi_apparatushttp://en.wikipedia.org/wiki/Cell_nucleushttp://en.wikipedia.org/wiki/Nuclear_envelopehttp://en.wikipedia.org/wiki/Transcribedhttp://en.wikipedia.org/wiki/Ribosomehttp://en.wikipedia.org/wiki/Ribosomehttp://en.wikipedia.org/wiki/Translation_(genetics)http://en.wikipedia.org/wiki/Mitochondrionhttp://en.wikipedia.org/wiki/Metabolic_pathwayhttp://en.wikipedia.org/wiki/Chloroplastshttp://en.wikipedia.org/wiki/Chemical_energyhttp://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/wiki/Algaehttp://en.wikipedia.org/wiki/Plastidhttp://en.wikipedia.org/wiki/Endoplasmic_reticulumhttp://en.wikipedia.org/wiki/Lipidhttp://en.wikipedia.org/wiki/Detoxificationhttp://en.wikipedia.org/wiki/Calciumhttp://en.wikipedia.org/wiki/Golgi_apparatus
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apparatus, sometimes called a Golgi bodyorGolgi complexis the
centraldelivery system for the cell and is a site for protein
processing, packaging, andtransport. Both organelles consist
largely of heavily folded membranes.
Lysosomes and peroxisomes - the cellular digestive system :
Lysosomes
and peroxisomes are often referred to as the garbage disposal
system of a cell.Both organelles are somewhat spherical, bound by a
single membrane, and richin digestive enzymes, naturally occurring
proteins that speed up biochemicalprocesses. For example, lysosomes
can contain more than three dozen enzymesfor degrading proteins,
nucleic acids, and certain sugars called polysaccharides.Here we
can see the importance behind compartmentalization of the
eukaryoticcell. The cell could not house such destructive enzymes
if they were notcontained in a membrane-bound system.
Centrioles - They help in the formation of mitotic appratus. Two
centrioles arepresent in the animal cells. They are also found in
some fungi and algae cells.
Table 1: Comparison of features of prokaroytic and eukaryotic
cells
Prokaryotes Eukaryotes
Typical organisms bacteria, archaea protists, fungi, plants,
animals
Typical size ~ 1-10 m ~ 10-100 m (sperm cells, apart fromthe
tail, are smaller)
Type ofnucleus nucleoid region; no realnucleus
real nucleus with double membrane
DNA circular (usually) linear molecules (chromosomes)
withhistoneproteins
RNA-/protein-synthesis
coupled in cytoplasm RNA-synthesis inside the nucleusprotein
synthesis in cytoplasm
Ribosomes 50S+30S 60S+40S
Cytoplasmaticstructure
very few structures highly structured by endomembranesand a
cytoskeleton
Cell movementflagella
made offlagellin flagella and cilia made oftubulin
Mitochondrianone one to several dozen (though some lack
mitochondria)
Chloroplasts none in algaeand plants
Organization Organization usually single cells single
cells,colonies, higher multicellular organismswith specialized
cells
Cell division Binary fission (simpledivision)
MitosisMeiosis
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