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ed for Keratin (red) and DNA (green).]] The cell is the structural and functional unit of all , are Unicellular , consisting of a single cell. Other organisms, such as Human s, are Multicellular , (humans have an estimated 100 trillion or 1014 cells; a typical cell size is 10 µm, a typical cell mass 1 nanogram). The Cell Theory , first developed in 1839 by Schleiden and Schwann , states that all Organism s are composed of one or more cells; all cells come from preexisting cells; all vital functions of an organism occur within cells, and cells contain the Hereditary Information necessary for regulating cell functions and for transmitting information to the next generation of cells. The word ''cell'' comes from the Latin ''cella'', a small room. The name was chosen by Robert Hooke when he compared the Cork cells he saw to the small rooms monks lived in. Some ( Lynn Margulis and Dorian Sagan, 1995) have argued that the cell is the smallest unit of Life . OVERVIEW Properties of cells Each cell is at least somewhat self-contained and self-maintaining: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Each cell stores its own set of instructions for carrying out each of these activities. s across.]] All cells share several abilities:
SUBCELLULAR COMPONENTS All cells, whether prokaryotic or eukaryotic, have a Membrane , which envelopes the cell, separates its interior from its environment, controls what moves in and out, and maintains the Electric Potential Of The Cell . Inside the membrane, a Salt y Cytoplasm takes up most of the cell volume. All cells possess DNA , the hereditary material of Gene s, and RNA , containing the information necessary to Build various Protein s such as Enzyme s, the cell's primary machinery. There are also other kinds of Biomolecule s in cells. This article will list these primary components of the cell, then briefly describe their function. Cell membrane: A cell's protective coat Main article: 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 ''cell membrane''. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a Double Layer Of Lipids ( Hydrophobic fat-like molecules) and Hydrophilic Phosphorous molecules. Hence the layer is called a Phospholipid Bilayer . Embedded within this membrane is a variety of other molecules that act as channels and pumps, moving different molecules into and out of the cell. Cytoskeleton: A cell's scaffold Main article: The cytoskeleton is an important, complex, and dynamic cell component made up of Microfilaments . 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 mobility. There is a great number of proteins associated with the cytoskeleton, each controlling a cell's structure by directing, bundling, and aligning filaments. Genetic material Two different kinds of genetic material exist: Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA). Most organisms use DNA for their long-term information storage, but some viruses ( Retrovirus es) have RNA as their genetic material. The biological information contained in an organism is Encoded in its DNA or RNA sequence. RNA is also used for information transport (e.g., MRNA ) and Enzymatic functions (e.g., Ribosomal RNA) in organisms that use DNA for the genetic code itself. Prokaryotic genetic material is organized in a simple circular DNA molecule (the bacterial Chromosome ) in the Nucleoid Region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called Chromosome s inside a discrete nucleus, usually with additional genetic material in some organelles like Mitochondria and Chloroplasts (see Endosymbiotic Theory ). A human cell has genetic material in the nucleus (the Nuclear Genome ) and in the mitochondria (the Mitochondrial Genome ). In humans the nuclear genome is divided into 46 linear DNA molecules called chromosomes. The mitochondrial genome is a circular DNA molecule separate from the nuclear DNA. Although the mitochondrial genome is very small, it codes for some important proteins. Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called Transfection . This can be transient, if the DNA is not inserted into the cell's Genome , or stable, if it is. Organelles Main article: 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 Organelle s, that are adapted and/or specialized for carrying out one or more vital functions. Membrane-bound organelles are found only 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'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 double 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 copied 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. ; Ribosomes (the protein production machine) : Ribosome s are found 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. ; Mitochondria and Chloroplasts (the power generators) : Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. As mitochondria contain their own genome that is separate and distinct from the nuclear genome of a cell, they play a critical role in generating energy in the eukaryotic cell, a process involving a number of complex Metabolic Pathway s. Chloroplasts are larger than mitochondria, and convert solar energy into a Chemical Energy ("food") via Photosynthesis . Like mitochondria, chloroplasts have their own genome. Chloroplasts are found only in photosynthetic eukaryotes, like plants and Algae . There is a number of plant organelles that are modified chloroplasts; they are broadly called Plastid s, and are often involved in storage. ; Endoplasmic reticulum and Golgi apparatus (macromolecule managers) : The synthesis, Detoxification and as a Calcium reservoir. The Golgi Apparatus , sometimes called a ''Golgi body'' or ''Golgi complex'' is the central delivery system for the cell and is a site for protein processing, packaging, and transport. Both organelles consist largely of heavily-folded membranes. ; Lysosomes and Peroxisomes (the cellular digestive system) : Lysosome s and Peroxisome s 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 Enzyme s, 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. 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. ; Centrioles : Centrioles help in the formation of mitotic appratus. Two centrioles are present in the animal cells. They are also found in some fungi and algae cells. ; Vacuoles : Vacuoles store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably '' Amoeba '' have contractile vacuoles, which are able to pump water out of the cell if there is too much water. ANATOMY OF CELLS Prokaryotic cells and Pili — proteins attached to the cell surface; a Cell Envelope consisting of a capsule, a Cell Wall , and a Plasma Membrane ; and a Cytoplasmic Region that contains the Cell Genome (DNA) and ribosomes and various sorts of inclusions. Other differences include:
Eukaryotic cells s: (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) Centriole s.]] There are two types of cells, eukaryotic and prokaryotic. Eukaryotic cells are usually found in multi-cellular organisms, while prokaryotic cells are usually on their own. Eukaryotic cells are about 10 times the size of a typical prokaryote and can be as much as 1000 times greater in volume. The major 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 , a membrane-delineated compartment that houses the eukaryotic cell's DNA. It is this nucleus that gives the eukaryote its name, which means "true nucleus." Other differences include:
CELL FUNCTIONS Cell growth and metabolism Main articles: Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual , in which the cell breaks down complex molecules to produce energy and reducing power, and Anabolism , wherein the cell uses energy and reducing power to construct complex molecules and perform other biological functions. Complex sugars consumed by the organism can be broken down into a less chemically-complex sugar molecule called Glucose . Once inside the cell, glucose is broken down to make adenosine triphosphate ( ATP ), a form of energy, via two different pathways. The first pathway, Glycolysis , requires no oxygen and is referred to as Anaerobic Metabolism . Each reaction is designed to produce some hydrogen ions that can then be used to make energy packets (ATP). In prokaryotes, glycolysis is the only method used for converting energy. The second pathway, called the Krebs cycle, or Citric Acid Cycle , occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions. of the cell (''light blue''), Gene s (DNA, ''dark blue'') are Transcribed into RNA . This RNA is then subject to post-transcriptional modification and control, resulting in a mature MRNA (''red'') that is then transported out of the nucleus and into the Cytoplasm (''peach''), where it undergoes Translation into a protein. mRNA is translated by Ribosome s (''purple'') that match the three-base Codon s of the mRNA to the three-base anti-codons of the appropriate TRNA . Newly-synthesized proteins (''black'') are often further modified, such as by binding to an effector molecule (''orange''), to become fully active.]] Creation of new cells Main article: Cell division involves a single cell (called a ''mother cell'') dividing into two daughter cells. This leads to growth in Multicellular Organism s (the growth of Tissue ) and to procreation ( Vegetative Reproduction ) in Unicellular Organism s. Prokaryotic cells divide by Binary Fission . Eukaryotic cells usually undergo a process of nuclear division, called Mitosis , followed by division of the cell, called Cytokinesis . A Diploid cell may also undergo Meiosis to produce haploid cells, usually four. Haploid cells serve as Gamete s in multicellular organisms, fusing to form new diploid cells. DNA Replication , or the process of duplicating a cell's genome, is required every time a cell divides. Replication, like all cellular activities, requires specialized proteins for carrying out the job. Protein synthesis Main article: Protein synthesis is the process in which the cell builds Protein s. DNA Transcription refers to the synthesis of a Messenger RNA (mRNA) molecule from a DNA template. This process is very similar to DNA replication. Once the mRNA has been generated, a new protein molecule is synthesized via the process of Translation . The cellular machinery responsible for synthesizing proteins is the Ribosome . The ribosome consists of structural RNA and about 80 different proteins. When the ribosome encounters an mRNA, the process of Translating an mRNA to a protein begins. The ribosome accepts a new Transfer RNA , or tRNA—the adaptor molecule that acts as a translator between mRNA and protein—bearing an Amino Acid , the building block of the protein. Another site binds the tRNA that becomes attached to the growing chain of amino acids, forming the a polypeptide chain that will eventually be processed to become a protein. DISEASES OF THE CELL Cancer Cancer , a class of disease, causes cells to multiply uncontrollably, invading other tissues either by direct growth into adjacent tissue or by implantation into distant sites by Metastasis . This uncontrollable multiplication is due to improper replication of the cell's DNA resulting in faulty set of instructions for cell function. Many mutation events may be required to transform a normal cell into a malignant cell. See the main article for details. ORIGINS OF CELLS Main article: The origin of cells has 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 life is viewed from the point of view of chains stable in a varying and sometimes aggressive environment, and may have been the main reason for which cells evolved. The latter is fundamental for the evolution of Biological Complexity . If freely-floating DNA molecules that code for Enzyme s are not enclosed into cells, the enzymes that benefit a given DNA molecule (for example, by producing nucleotides) will automatically benefit the neighbouring DNA molecules. This might be viewed as " Parasitism by default." Therefore the Selection 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 all the DNA molecule is 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 won't benefit from a positive mutation in a neighbouring molecule: this in turn 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 complete accuracy since the earliest molecules of life, probably up to the stage of cellular life, were most likely . However, the core of the reasoning is the same.) Biochemically, cell-like spheroids formed by Proteinoid s are observed by heating Amino Acid s with Phosphoric Acid as a catalyst. They bear much of the basic features provided by Cell Membrane s. Proteinoid-based protocells enclosing RNA molecules could (but not necessarily should) have been the first cellular life forms on Earth. Another theory holds that the turbulent shores of the ancient coastal waters may have served as a mammoth laboratory, aiding in the countless experiments necessary to bring about the first cell. Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendency to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through Ph.D. Origin of eukaryotic cells 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 Archaea n prokaryote cell – a theory termed the Endosymbiotic Theory . There is still considerable debate on if organelles like the for the origin of eukaryotic cells. HISTORY
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