Phosphate Group
A phosphate group is an ion that contains one phosphorous atom bonded to four oxygen atoms. It is typically derived from a phosphoric acid and is anionic, having a negative three charge. When a phosphate group is attached to an organic compound it is bonded to that molecule by an oxygen molecule. Organic molecules containing phosphate groups are found throughout the body and perform a variety of functions.
One area that phosphate groups are found extensively is in nucleic acids, including DNA, and RNA. These molecules consist of nucleotides which contain a five carbon sugar, an organic base and a phosphate group. Nucleotides are formed in two steps; first, the sugar and the organic base react, then this unit reacts with phosphoric acid. Nucleic acids are formed by the polymerization of nucleotide monomers. Each of these monomers are connected through their phosphate groups. In this way, the phosphate group acts to stabilize the structure of the backbone of the nucleic acid.
Another important function of phosphate groups in living organisms is providing a source of energy for cells to do work. Cells do three main types of work, mechanical, chemical and transport. Mechanical work includes such things as controlling the flow of cytoplasm within the cell, moving chromosomes during reproduction, or contracting during muscle movements. Chemical work involves the metabolic reactions that would not occur spontaneously. Examples are protein synthesis and glycogenesis. Transport work involves the pumping of substances in and out of a cell. In most cases, a phosphate-containing compound called adenosine triphosphate (ATP) provides the power for all types of cellular work.
ATP is an organic molecule that is similar to the nucleotides found in nucleic acids. It has a nitrogen containing base and a five carbon sugar. However, unlike a nucleotide, it contains three phosphate groups instead of one. These phosphates are connected to each other making this part of the ATP molecule unstable. When it reacts with water, a phosphate is released along with a certain amount of energy. This is the basic reaction used by cells to power chemical reactions.
An example of how ATP works is found in protein synthesis. Proteins are composed of smaller units called amino acids which do not react spontaneously. During protein synthesis, enzymes in the cell cause a phosphate group to be transferred from ATP to an amino acid. This process, known as phosphorylation, energizes the amino acid and allows it to react with other amino acids to form a protein. A similar phosphorylation reaction occurs in nearly all other cases where a cell performs work.
While phosphate groups can energize reactions, cells also use them to store energy. This is done by converting energy from catabolic reactions such as those found during cellular respiration and photosynthesis to ATP. In this reaction, adenosine diphosphate (ADP) is reacted with a phosphate group to form ATP and water.
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