This is a merge-in-progress of material from cell (biology) and the User:Lexor/Temp/Cell (NCBI) article from NCBI's Science Primer.
The cell is the structural and functional unit of all living organisms. Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, (humans have an estimated 100,000 billion cells). The cell theory, first developed in the 19th century, 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.
The word cell comes from the Latin cella, a small room.
Each cell is a self-contained and self-maintaining entity: 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.
All cells share several abilities:
One classification of cells is based on whether they live alone or in groups. Organisms vary from single cells (called single-celled (or unicellular organisms) that function and survive more or less independently, through colonial forms with 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.
Before we can discuss the various components of a cell, it is important to know what organism the cell comes from. There are two general categories of cells based on their internal structure: prokaryotes and eukaryotes.
All cells whether prokaryotic or eukaryotic have 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. Inside the membrane is a salty cytoplasm (the substance which makes up most of the cell volume). All cells possess DNA, the hereditary material of genes and RNA, which contain the information necessary to express various proteins such as enzyme, the cell's primary machinery. Within the cell at any given time are various additional biomolecules. This article will briefly overview these primary components of the cell then continue to briefly describe their function.
Main article: Cell membrane
The outer lining of a eukaryotic cell is called the plasma membrane. A form of plasma membrane is also found in prokaryotes, but in this organism it 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 (fat-like molecules) and proteins. 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.
Main article: Cytoskeleton
The 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.
Main article: Cytoplasm
Inside the cell there is a large fluid-filled space called the cytoplasm, sometimes called the cytosol. In prokaryotes, this space is relatively free of compartments. In eukaryotes, the cytosol is the "soup" within which all of the cell's organelles reside. It is also the home of the cytoskeleton. The cytosol contains dissolved nutrients, helps break down waste products, and moves material around the cell through a process called cytoplasmic streaming. The nucleus often flows with the cytoplasm changing its shape as it moves. The cytoplasm also contains many salts and is an excellent conductor of electricity, creating the perfect environment for the mechanics of the cell. The function of the cytoplasm, and the organelles which reside in it, are critical for a cell's survival.
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. Human genetic material is made up of two distinct components: the nuclear genome and the mitochondrial genome. The nuclear genome is divided into 24 linear DNA molecules, each contained in a different chromosome. The mitochondrial genome is a circular DNA molecule separate from the nuclear DNA. Although the mitochondrial genome is very small, it codes for some very important proteins.
Main article: Organelle
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 organelles, that are adapted and/or specialized for carrying out one or more vital functions. Organelles are found only in eukaryotes and are always surrounded by a protective membrane.
Prokaryotes are distinguished from eukaryotes on the basis of nuclear organization, specifically their lack of a nuclear membrane. Prokaryotes also lack any of the intracellular organelles and structures that are characteristic of eukaryotic cells. Most of the functions of organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic plasma membrane. Prokaryotic cells have three architectural regions: appendages called flagella 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 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 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, performing dedicated functions, the aforementioned organelles.. Other differences include:
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) |
Typical animal cell | Typical plant cell | |
---|---|---|
Organelles |
|
|
Additional structures |
Main articles: Cell growth, Cell metabolism
Between successive cell divisions cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions; catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, where 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 Kreb's cycle, or citric acid cycle, occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions.
Main article: Cell division
Cell division involves a single cell (called a mother cell) dividing into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation ( vegetative reproduction) in unicellular organisms. 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 gametes 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.
Main article: Protein biosynthesis
Protein synthesis is the process in which the cell builds proteins. DNA transcription refers to the synthesis of a messager 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 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.
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.
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 amino acids 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.
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.
This article incorporates public domain material from Science Primer. NCBI. Archived from the original on 2009-12-08.
This is a merge-in-progress of material from cell (biology) and the User:Lexor/Temp/Cell (NCBI) article from NCBI's Science Primer.
The cell is the structural and functional unit of all living organisms. Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, (humans have an estimated 100,000 billion cells). The cell theory, first developed in the 19th century, 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.
The word cell comes from the Latin cella, a small room.
Each cell is a self-contained and self-maintaining entity: 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.
All cells share several abilities:
One classification of cells is based on whether they live alone or in groups. Organisms vary from single cells (called single-celled (or unicellular organisms) that function and survive more or less independently, through colonial forms with 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.
Before we can discuss the various components of a cell, it is important to know what organism the cell comes from. There are two general categories of cells based on their internal structure: prokaryotes and eukaryotes.
All cells whether prokaryotic or eukaryotic have 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. Inside the membrane is a salty cytoplasm (the substance which makes up most of the cell volume). All cells possess DNA, the hereditary material of genes and RNA, which contain the information necessary to express various proteins such as enzyme, the cell's primary machinery. Within the cell at any given time are various additional biomolecules. This article will briefly overview these primary components of the cell then continue to briefly describe their function.
Main article: Cell membrane
The outer lining of a eukaryotic cell is called the plasma membrane. A form of plasma membrane is also found in prokaryotes, but in this organism it 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 (fat-like molecules) and proteins. 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.
Main article: Cytoskeleton
The 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.
Main article: Cytoplasm
Inside the cell there is a large fluid-filled space called the cytoplasm, sometimes called the cytosol. In prokaryotes, this space is relatively free of compartments. In eukaryotes, the cytosol is the "soup" within which all of the cell's organelles reside. It is also the home of the cytoskeleton. The cytosol contains dissolved nutrients, helps break down waste products, and moves material around the cell through a process called cytoplasmic streaming. The nucleus often flows with the cytoplasm changing its shape as it moves. The cytoplasm also contains many salts and is an excellent conductor of electricity, creating the perfect environment for the mechanics of the cell. The function of the cytoplasm, and the organelles which reside in it, are critical for a cell's survival.
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. Human genetic material is made up of two distinct components: the nuclear genome and the mitochondrial genome. The nuclear genome is divided into 24 linear DNA molecules, each contained in a different chromosome. The mitochondrial genome is a circular DNA molecule separate from the nuclear DNA. Although the mitochondrial genome is very small, it codes for some very important proteins.
Main article: Organelle
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 organelles, that are adapted and/or specialized for carrying out one or more vital functions. Organelles are found only in eukaryotes and are always surrounded by a protective membrane.
Prokaryotes are distinguished from eukaryotes on the basis of nuclear organization, specifically their lack of a nuclear membrane. Prokaryotes also lack any of the intracellular organelles and structures that are characteristic of eukaryotic cells. Most of the functions of organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic plasma membrane. Prokaryotic cells have three architectural regions: appendages called flagella 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 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 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, performing dedicated functions, the aforementioned organelles.. Other differences include:
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) |
Typical animal cell | Typical plant cell | |
---|---|---|
Organelles |
|
|
Additional structures |
Main articles: Cell growth, Cell metabolism
Between successive cell divisions cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions; catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, where 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 Kreb's cycle, or citric acid cycle, occurs inside the mitochondria and is capable of generating enough ATP to run all the cell functions.
Main article: Cell division
Cell division involves a single cell (called a mother cell) dividing into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation ( vegetative reproduction) in unicellular organisms. 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 gametes 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.
Main article: Protein biosynthesis
Protein synthesis is the process in which the cell builds proteins. DNA transcription refers to the synthesis of a messager 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 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.
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.
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 amino acids 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.
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.
This article incorporates public domain material from Science Primer. NCBI. Archived from the original on 2009-12-08.