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).

Cell functions

Cell growth and metabolism

Main articles: Cell growth, Cell metabolism

Related Topics:
Cell growth - Cell metabolism

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Between successive cell divisions cells grow through the functioning of cellular metabolism.

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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.

Related Topics:
Cell - Catabolism - Anabolism

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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.

Related Topics:
Glucose - ATP

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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.

Related Topics:
Glycolysis - Anaerobic metabolism

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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.

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Making new cells

Main article: Cell division

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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.

Related Topics:
Multicellular organism - Tissue - Vegetative reproduction - Unicellular organism

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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.

Related Topics:
Prokaryotic - Binary fission - Eukaryotic - Mitosis - Cytokinesis - Diploid - Meiosis - Haploid - Gamete

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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.

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Protein synthesis

Main article: Protein biosynthesis

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Protein synthesis is the process in which the cell builds proteins.

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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.

Related Topics:
Transcription - Messenger RNA - Translation

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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.

Related Topics:
Ribosome - Translating - Transfer RNA - Amino acid

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