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Cell (biology)
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Cell (biology)

In biology, 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.

Table of contents
1 Structure
2 Organisms
3 Types of cells: prokaryotic and eukaryotic
4 Functions
5 The origin of cells
6 History
7 Etymology
8 Related topics
9 External links

Structure

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 kingdomss: Animalia, Plantae and Fungi, are all eukaryotic.)

Comparison of features of prokaroytic and eukaryotic cells
  Prokaryotes Eukaryotes
typical organisms bacteria protists, fungi, plants, animals
typical size ~ 1-10 µm ~ 10-100 µm (sperm cells, apart from the tail, are smaller)
type of nucleus nucleoid region; no real nucleus real nucleus with double membrane
DNA circular (usually) linear molecules (chromosomes) with histone proteins
RNA-/protein-synthesis coupled in cytoplasm RNA-synthesis inside the nucleus
protein synthesis in cytoplasm
ribosomes 50S+30S 60S+40S
cytoplasmatic structure very few structures highly structured by intercellular membranes and a cytoskeleton
cell movement flagella made of flagellin flagella and cilia made of tubulin
mitochondria none one to several dozen (though some lack mitochondria)
chloroplasts none in algae and plants
organization usually single cells single cells, colonies, higher organisms with specialized cells
cell division Binary fission (simple division) Mitosis (core division)
Cytokinesis (cytoplasmatic division)

Prokaryotic cells

Eukaryotic cells

Eukaryotic cells are highly organized and composed of structurs known as
organelles that perform specific functions.

A typical animal cell

Organelles:
  1. Nucleolus
  2. Nucleus
  3. Ribosome
  4. Vesicle
  5. Rough endoplasmic reticulum (ER)
  6. Golgi apparatus
  7. Microtubule
  8. Smooth ER
  9. Mitochondria
  10. Vacuole
  11. Cytoplasm
  12. Lysosome
  13. Centrioles

Organelles (see diagram above)

Structures:

A typical plant cell

Organelles:

  1. Tonoplast
  2. Central vacuole
  3. Nucelus
  4. Rough endoplasmic reticulum
  5. Smooth endoplasmic reticulum
  6. Peroxisome
  7. Golgi apparatus
  8. Ribosomes
  9. Chloroplast
  10. Microfilaments
  11. Microtubules
  12. Mitochondrion

Structures:
  1. Plasma membrane
  2. Cell wall
  3. 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

...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.

Etymology

The word
cell comes from the Latin cella, a small room.

Related topics

External links