Cell (biology)

The cell is the structural and functional unit of all living organisms, sometimes called the "building blocks of life." 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 = 1014 cells).

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 salty cytoplasm takes up most of the cell volume. All cells possess DNA, the hereditary material of genes and RNA, which contain the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. This article will list these primary components of the cell then briefly describe their function.

Related Topics:
Membrane - Electric potential of the cell - Salt - Cytoplasm - DNA - Gene - RNA - Build - Protein - Enzyme - Biomolecule

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Cell membrane - a cell's protective coat

Main article: Cell membrane

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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 (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.

Related Topics:
Double layer of lipids - Protein

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Cytoskeleton - a cell's scaffold

Main article: Cytoskeleton

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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.

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Cytoplasm - a cell's inner space

Main article: Cytoplasm

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Inside the cell there is a large fluid-filled space called the cytoplasm, which includes both to the mixture of ions and fluids in solution within the cell, and to the organelles contained in it. The term cytosol refers only to the fluid itself.

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In prokaryotes, the cytoplasm is relatively free of compartments. In eukaryotes, it normally contains a large number of organelles. 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, which is important in certain cell functions like cell signaling.

Related Topics:
Cytoplasmic streaming - Cell signaling

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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 (retroviruses) 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 RNA for the genetic code itself.

Related Topics:
Deoxyribonucleic acid - Ribonucleic acid - Retrovirus - Encoded - MRNA - Enzymatic - Ribosomal

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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 chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory).

Related Topics:
Chromosome - Nucleoid region - Endosymbiotic theory

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A human cell, i.e. has genetic material in the nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome). 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.

Related Topics:
Nuclear genome - Mitochondrial genome

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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.

Related Topics:
Transfection - Genome

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Organelles

Main article: Organelle

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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. Membrane-bound organelles are only found in eukaryotes.

Related Topics:
Organs - Organelle

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• 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: Ribosomes 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 mentioned earlier, mitochondria contain their own genome that is separate and distinct from the nuclear genome of a cell. Mitochondria play a critical role in generating energy in the eukaryotic cell, and this process involves a number of complex metabolic pathways. 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 are a number of plant organelles that are modified chloroplasts; they are broadly called plastids and are often involved in storage.
• Endoplasmic reticulum and Golgi apparatus - macromolecule managers:: The endoplasmic reticulum (ER) is the 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, which has ribosomes on its surface, and the smooth ER, which lacks them. Translation of the mRNA for those proteins that will either stay in the ER or be exported from the cell occurs at the ribosomes attached to the rough ER. The smooth ER is important in lipid 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: 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. 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 - They 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.