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Eine Zelle ist in der Regel definiert als eine Einheit oder Kompartimente, umschlossen von einer Grenze oder Mauer, die in der Regel Teil einer größeren Struktur. In biologischer Hinsicht Zellen sind die strukturellen und funktionellen Einheiten von allen lebenden Organismen. Lebenden Zellen zeigen überwältigende Beweise für Intelligent Design aufgrund ihrer vielen minimal komplexen molekularen Maschinen. Zellen sind so unglaublich kompliziert, dass wir erst am Anfang zu verstehen, ihre internen Abläufe und in der Tat viele Funktionen in der Zelle noch insgesamt Geheimnis.
Some organisms, such as [[bacteria]], are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, or have many cells—an estimated 100,000,000,000,000 cells! Each cell is an amazing world unto itself: it can take in [[nutrients]], convert these nutrients into [[energy]], carry out specialized functions, and reproduce as necessary. Even more amazing is that each cell stores its own set of instructions for carrying out each of these activities.[http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html]
In the early twentieth century, the cell was viewed as essentially a blob of protoplasm. It was simply an unobservable collection of gelatin molecules, the inner workings of which were not yet understood. On the other hand, 21st century technology reveals that although the tiniest bacterial cells are incredibly small, weighing less than 10-12 grams, the entire cell is incredibly integrated and each part works as part of a team.
An analogy sometimes used, is the comparison of a cell to a city. For instance, the workers can be compared to the [[protein]], the powerplant to the [[mitochondria]], the roads to the actin fibres and [[microtubules]], the trucks to the Kinosin and Dynein, the factories to the [[ribosomes]], the library to the nucleic acid, the recycling centre to the [[Lysosome]], the police to the chaperones, and the post office to the [[golgi apparatus]].
As technology increases, science continuously opens black boxes within already opened ones, and as more and more of these are being exposed, the phenomenal complexity of the whole system pushes evolutionary theories to breaking point.
== Cell Types ==
[[Image:Cell_types.png|thumb|350px|right|Comparison of a typical animal cell ([[eukaryote]]) and bacterial cell ([[prokaryote]]).]]
Cells are identified as one of two broad categories; [[prokaryote]] or [[eukaryote]], which have several characteristics in common. All multicellular organisms have eukaryotic cells that take on vastly different roles and form specialized [[tissues]]. All prokaryotes are single-celled organisms (bacteria).
=== Eukaryotes ===
: ''Main Article: [[Eukaryote]]''
Eukaryotes include [[fungi]], [[animals]], and [[plants]] as well as some unicellular organisms ([[protists]]). Eukaryotic cells are about 10 times the size of a prokaryote and can be as much as 1000 times greater in volume. The major and extremely significant difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is the presence of a [[cell nucleus|nucleus]], a membrane-delineated compartment that houses the eukaryotic cell’s DNA. It is this nucleus that gives the eukaryote—literally, true nucleus—its name.
Eukaryotic organisms also have other specialized structures, called organelles, which are small structures within cells that perform dedicated functions. As the name implies, you can think of organelles as small organs. There are a dozen different types of [[organelles]] commonly found in eukaryotic cells.[http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html]
=== Prokaryotes ===
: ''Main Article: [[Prokaryote]]''
Bacteria are [[prokaryotes]], which differ from [[eukaryotes]] in that their [[DNA]] is not organized inside a [[cell nucleus|nucleus]]. Prokaryotes also have only one [[chromosome]], which is circular instead of linear. Although [[prokaryote|Prokaryotic cells]] are sometimes called "simple cells", they perform most of the same metabolic processes as [[eukaryote|eukaryotic cells]]. Many of these reaction are simply not sequestered inside organelles. For example, both prokaryotes and eukaryotes perform [[photosynthesis]] and [[cellular respiration]], but only eukaryotes have [[chloroplasts]] and [[mitochondria]] respectively.
Prokaryotes are unicellular organisms that do not develop or differentiate into multicellular forms. Some bacteria grow in filaments, or masses of cells, but each cell in the colony is identical and capable of independent existence. The cells may be adjacent to one another because they did not separate after cell division or because they remained enclosed in a common sheath or slime secreted by the cells. Typically though, there is no continuity or communication between the cells. Prokaryotes are capable of inhabiting almost every place on the earth, from the deep ocean, to the edges of hot springs, to just about every surface of our bodies.[http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html]
===Cell Types===
Each individual begins life as a simple cell, sown together by the bonding of the male [[sperm]] with the female [[egg]]. This single cell contains the digital code necessary for the formation of other cells to perform the many functions of the body. These include our taste buds, fat cells, skin cells, blood cells and many more. All in all, the human body consists of some thirty trillion cells!
Three basic categories of cells make up the mammalian body: [[germ cells]], [[somatic cells]], and [[stem cells]]. Each of the approximately 100,000,000,000,000 cells in an adult human has its own copy, or copies, of the genome, with the only exception being certain cell types that lack nuclei in their fully differentiated state, such as red blood cells. The majority of these cells are diploid, or have two copies of each chromosome. These cells are called somatic cells. This category of cells includes most of the cells that make up our body, such as skin and muscle cells. Germ line cells are any line of cells that give rise to gametes—eggs and sperm—and are continuous through the generations. Stem cells, on the other hand, have the ability to divide for indefinite periods and to give rise to specialized cells.[http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html]
== Cellular Structures ==
[[Image:Plant_cell_structure.png|right|350px]]
All cells are surrounded by a [[membrane]] that is composed of a double layer (bilayer) of phospholipids. Inside this plasma membrane is the cytoplasm that consists of a fluid called the protoplasm, and contains one or more [[organelles]] depending on the cell type. The protoplasm is composed of about 65% water and contains about a billion dissolved molecules per cell. The water provides a favorable environment for the thousands of biochemical reactions that take place inside the cell.
* [[Cytoplasm]] - '''Cytoplasm''' is the jellylike mass that composes the most part of the [[cell]], and is sometimes called the cytosol. It surrounds the [[cell nucleus|nucleus]] and contains the organelles. It contains dissolved nutrients, helps break down waste products, and moves material around the cell through a process called [[cytoplasmic streaming]].[http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html]
* [[Cytoskeleton]] - The cytoskeleton is an important, complex, and dynamic cell component. It acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell; and moves parts of the cell in processes of growth and motility. There are a great number of proteins associated with the cytoskeleton, each controlling a cell’s structure by directing, bundling, and aligning filaments.[http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html]
* [[Plasma membrane]] - The outer lining of a eukaryotic cell is called the plasma membrane. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of proteins and lipids, fat-like molecules. Embedded within this membrane are a variety of other molecules that act as channels and pumps, moving different molecules into and out of the cell. A form of plasma membrane is also found in prokaryotes, but in this organism it is usually referred to as the cell membrane.[http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html]
* [[Nucleic acid]] - Two different kinds of genetic material exist: [[deoxyribonucleic acid]] (DNA) and [[ribonucleic acid]] (RNA). Most organisms are made of DNA, but a few [[viruses]] have RNA as their genetic material. The biological information contained in an organism is encoded in its DNA or RNA sequence. Prokaryotic genetic material is organized in a simple circular structure that rests in the cytoplasm. Eukaryotic genetic material is more complex and is divided into discrete units called genes.[http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html]
==Organelles==
[[Image:Biological_cell.png|thumb|350px|Diagram of a typical animal cell ([[eukaryote]]). Organelles:(1) [[nucleolus]] (2) [[cell nucleus|nucleus]] (3) [[ribosome]] (4) [[vesicle]] (5) [[endoplasmic reticulum|rough endoplasmic reticulum]] (ER) (6) [[golgi apparatus]] (7) cytoskeleton (8) smooth ER (9) [[mitochondria]] (10) [[vacuole]] (11) [[cytoplasm]] (12) [[lysosome]] (13) [[centrioles]]]]
: ''Main Article: [[Organelle]]''
* [[cell nucleus|nucleus]] - holds genomic DNA in eukaryotes. The nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes and is the place where almost all DNA replication and RNA synthesis occur. The nucleus is spheroid in shape and separated from the cytoplasm by a membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or synthesized, into a special RNA, called mRNA. This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. In prokaryotes, DNA processing takes place in the cytoplasm.[http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html]
* [[Mitochondria]] - performs cellular respiration in eukaryotes. Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Mitochondria contain their own DNA that is separate and distinct from the nuclear genome of a cell. Mitochondria have two functionally distinct membrane systems separated by a space: the outer membrane, which surrounds the whole organelle; and the inner membrane, which is thrown into folds or shelves that project inward. These inward folds are called cristae. The number and shape of cristae in mitochondria differ, depending on the tissue and organism in which they are found, and serve to increase the surface area of the membrane.
* [[Chloroplast]] - convert light energy from the sun into ATP through a process called photosynthesis. Like mitochondria, they are surrounded by a double membrane with an intermembrane space. They also have their own DNA and are involved in energy metabolism; and both also have reticulations, or many foldings, filling their inner spaces.[http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html]
* [[Ribosome]] - translates [[mRNA]] into [[polypeptides]] in both prokaryotes and eukaryotes. The ribosome is a large complex composed of many molecules, including RNAs and proteins, and is responsible for processing the genetic instructions carried by an [[mRNA]]. The process of converting an mRNA's genetic code into the exact sequence of amino acids that make up a protein is called [[translation]]. [[Protein]] synthesis is extremely important to all cells, and therefore a large number of ribosomes—sometimes hundreds or even thousands—can be found throughout a cell. Ribosomes float freely in the [[cytoplasm]] or sometimes bind to another organelle called the [[endoplasmic reticulum]]. Ribosomes are composed of one large and one small subunit, each having a different function during protein synthesis.[http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html]
* [[Endoplasmic reticulum]] - a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that will float freely in the cytoplasm. The ER has two forms: the rough ER and the smooth ER. The rough ER is labeled as such because it has ribosomes adhering to its outer surface, whereas the smooth ER does not. Translation of the mRNA for those proteins that will either stay in the ER or be exported (moved out of the cell) occurs at the ribosomes attached to the rough ER. The smooth ER serves as the recipient for those proteins synthesized in the rough ER.[http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html]
* [[Golgi apparatus]] - processes and packages various macromolecules into vesicles. [[Proteins]] to be exported are passed to the Golgi apparatus, sometimes called a Golgi body or Golgi complex, for further processing, packaging, and transport to a variety of other cellular locations.[http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html]
* [[Lysosome]] - performs cellular digestion. 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 rich in digestive enzymes, naturally occurring proteins that speed up biochemical processes. For example, lysosomes can contain more than three dozen enzymes for degrading proteins, nucleic acids, and certain sugars called polysaccharides. All of these enzymes work best at a low pH, reducing the risk that these enzymes will digest their own cell should they somehow escape from the lysosome. Here we can see the importance behind compartmentalization of the eukaryotic cell. The cell could not house such destructive enzymes if they were not contained in a membrane-bound system.
* [[Vacuole]] - storage cavity.
* [[Cilia]]
* [[Flagellum]]
== Gallery ==
<gallery>
Image:OnionX100.jpg|Onion cells - 100X magnification
Image:Cholera bacteria SEM.jpg|Scanning electron micrograph (SEM) of Cholera [[bacteria]]
Image:Red White Blood cells.jpg|SEM of [[Red blood cell]], [[white blood cell]], and platelet
Image:Salmonella.jpg|SEM of Salmonella typhimurium bacteria invading cultured [[human]] cells
Image:EMpylori.jpg|Bacteria with [[flagellum]]
Image:TreponemaPallidum.jpg|Treponema pallidum bacteria - a [[spirochaete]]
</gallery>
== References ==
* [http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html What is a Cell?] by the National Center for Biotechnology Information.
== External Links ==
===Creationist links===
* [http://creationresearch.org/crsq/abstracts/sum35_4.html The Putative Evolution of the Animal Eukaryote Cell Ultrastructure] by Jerry Bergman. CRSQ 35(4):221-231. March 1999.
===Secular links===
* [http://www.jcb.org/ Journal of Cell Biology]
== See Also ==
* [[Biologie]]
[[Category:Biologie]]
[[Category:Zellbiologie]]
[[en:Cell biology]]
[[fr:Biologie de la cellule]]
[[zh:细胞生物学]]
