Biological cell

The cell is the basic unit of life.

Table of contents

1 Overview
2 Features of prokaryotic and eukaryotic cells

2.1 Prokaryotic cells
2.2 Eukaryotic cells

2.2.1 Diagram of a typical eukaryotic (animal) cell

3 History
4 Related topics

Overview

All known cells (except a few specialized cell types) have certain basic components in common:

Cytoplasm (the living substance which makes up most of the cell volume)
DNA, the genetic information that acts as a blueprint for the other components.
Proteins, the machinery of the cell.
A membrane, which envelopes the cell, separates its interior from the surroundings, strictly controls what moves in and out and maintains the electric potential of the cell,

They also share several abilities:

Reproduction by cell division.
Metabolism, including the taking in of raw material, using it to build cell components, or breaking it down for energy, and releasing byproducts.
Protein biosynthesis by transcription of DNA to RNA and then translation to protein.

These functions and abilities are expressed in the cell cycle: the "birth", growth, reproduction, and "death" of individual cells.

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.

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.)

Features of prokaryotic and eukaryotic cells

	Prokaryotes	Eukaryotes
typical organisms	bacteria	protists, fungi, plants, animals
typical size	~ 1-10 um	~ 10-100 um
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

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

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.

Diagram of a typical eukaryotic (animal) cell

Organelles:

History

1665 : Robert Hooke discovers cells in cork, then in living plant tissue using an early microscope.

...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).

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