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Cells in culture, stained for keratin
The cell is the fundamental structural and functional unit of all living organisms. The cell theory, first developed in the 1800s, states that all organisms are composed of one or more cells; all cells come from preexisting cells; all
vital functions of an organism occur within cells and that cells contain the hereditary information necessary for regulating cell
functions and for transmitting information to the next generation of cells.
Structure
Surface of an embryonic mouse cell with scale indicated
Organisms
Organisms vary from single cells (called single-celled
organisms) that function and survive more or less independently, through colonial forms with multiple
similar cells living together, to multicellular forms in which cells are specialized and do not generally
survive once separated. There are 220 types of cells and tissues that make up the multicellular human body.
Types of cells: prokaryotic and eukaryotic
Two basic types of cells are described: prokaryotic and eukaryotic. Prokaryotic cells are structurally simple. They are found only in
single-celled and colonial
organisms. In the three-domain system of Scientific classification, prokaryotic cells are placed
in the domains Archaea and Eubacteria. Eukaryotic cells have organelles with their
own cell membranes. Single-celled eukaryotic organisms are very diverse, but many colonial and multicellular forms also exist.
(The multicellular kingdoms: Animalia, Plantae and Fungi, are all
eukaryotic.)
Prokaryotic cells
- The cytoplasm of prokaryotes (the liquid which makes up most of the cell volume) is diffuse and granular due to
ribosomes (protein factories) floating in the cell.
- The plasma membrane (a phospholipid bilayer) separates the interior of the cell from its environment and serves as a
filter and communications beacon.
- Most prokaryotes have a cell wall (some exceptions are
Mycoplasma (a bacterium) and Thermoplasma (an archaeon)). It consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents
the cell from "exploding" from osmotic pressure against a hypotonic
environment.
- A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium Borrelia burgdorferi,
which causes Lyme disease). Even without a real nucleus, the DNA
is somehow condensed in a nucleoid. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Plasmids can carry additional functions, such as
antibiotic resistance.
- Some prokaryotes have flagella which enable them to move actively
instead of passively drifting.
Eukaryotic cells
Eukaryotic cells are highly organized and composed of structurs known as organelles that perform specific functions.
- The cytoplasm of eukaryotes does not appear as granular as that of prokaryotes, since an important part of the ribosomes are
bound to the endoplasmic reticulum.
- The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not
be present.
- The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are highly condensed (e.g. folded around histones). All chromosomal DNA is stored in the
cell nucleus, separated from the cytoplasm by a membrane. Some
eukaryotic organelles can contain some DNA.
- Eukaryotes can become mobile using cilia or flagella. The flagella are more complex than those of
prokaryotes.
A typical animal cell
Organelles (see diagram above)
Structures:
A typical plant cell
Organelles:
- Tonoplast
- Central vacuole
- Nucelus
- Rough endoplasmic reticulum
- Smooth endoplasmic reticulum
- Peroxisome
- Golgi apparatus
- Ribosomes
- Chloroplast
- Microfilaments
- Microtubules
- Mitochondrion
Structures:
- Plasma membrane
- Cell wall
- Plasmodesma
Human body cells
The body contains trillions of cells.
Functions
All cells share several abilities:
Many cell functions are carried out by enzymes.
Energy use
The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules. This energy
is derived from metabolic pathways.
Moving of proteins
A typical mammalian cell contains up to 10,000 different proteins.
The origin of cells
The origin of cells has much to do with the origin of life , and was
one of the most important steps in evolution of life as we know it. The birth of
the cell marked the passage from prebiotic chemistry to biological life.
Origin of the first cell
If we see life forms from the point of view of replicators, that is DNA molecules in the actual life, cells satisfy two fundamental conditions : protection from the
outside environment and confinement of biochemical activity. The former condition is needed to maintain the fragile DNA chains stable in a varying and sometimes aggressive environment, and probably was the main reason
for which cells evolved. The latter is fundamental for the evolution of biological complexity.
If we have,let's imagine, freely-floating DNA molecules that code for enzymes that are
not enclosed into cells, the enzymes that advantage a given DNA molecule (for example,by producing nucleotides) will
automatically advantage also the neighbouring DNA molecules. You can see it as "parasitism by default". Therefore the evolutive pressure on DNA molecules will be much lower,since there is
not a definitive advantage for the "lucky" DNA molecule that produces the better enzyme over the others: all molecules in a given
neighbourhood are almost equally advantaged. If we have the DNA molecule enclosed in a cell, then the enzymes coded from the
molecule will be kept close to the DNA molecule itself. The DNA molecule will directly enjoy the benefits of the enzymes it
codes, and not of others. This means other DNA molecules can't benefit of a positive mutation in a neighbouring molecule :
this means that positive mutations give immediate and selective advantage to the replicator bearing it, and not on others. This
is thought to have been the one of the main driving force of evolution of life as we know it. (Note. This is more a metaphor
given for simplicity than a possible truth, since probably the earliest molecules of life, probably up to the stage of cellular
life, were RNA molecules , acting both as replicators and enzymes : see RNA world hypothesis . But the core of the reasoning is the
same.)
Biochemically, cell-like spheroids formed by proteinoids are observed by
heating aminoacids with phosphoric acid as a catalyst. They bear much of the basic features provided by cell membranes. Proteinoid-based protocells enclosing RNA molecules could (but
not necessarily should) have been the first cellular life forms on Earth.
Origin of the eukaryotic cell
The eukaryotic cell seems to have evolved from a symbiotic community of
prokaryotic cells. It is almost certain that DNA-bearing organelles like the mitochondria and the chloroplasts are what remains of
ancient symbiotic oxygen-breathing bacteria and cyanobacteria, respectively, where the rest of the cell seems to be derived from an ancestral archaean prokaryote cell. There is still considerable debate on if organelles like the
hydrogenosome predated the
origin of mitochondria, or viceversa : see the hydrogen hypothesis for the origin of eukaryotic cells.
History
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- ...I could exceedingly plainly perceive it to be all perforated and porous, much like a Honeycomb...these pores or cells , were not very deep, but consisted of a great many little boxes... –
Hooke describing his observations on a thin slice of cork.
- 1839 : Theodor Schwann and Matthias Jakob Schleiden elucidate the principal that
plants and animals are made of cells, concluding that cells are a common unit of structure and development, thus founding the
Cell Theory.
- The belief that life forms are able to occur spontaneously (generatio spontanea) is contradicted by Louis Pasteur (1822-1895).
- Rudolph Virchow states that cells always emerge from cell divisions (omnis cellula ex cellula).
- 1931: Ernst Ruska builds first transmission electron microscope (TEM) at
the University of Berlin. By 1935 he has built an EM with
twice the resolution of a light microscope, revealing previously unresolvable organelles.
- 1953: Watson and Crick made their first announcement on the double-helix structure for DNA on February 28.
Etymology
The word cell comes from the Latin
cella, a small room.
Related topics
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