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Cell Biology

Adenosine Triphosphate (ATP)

ATP is found in all living organisms from bacteria to humans. Viruses cannot generate or store energy in the form of ATP. This is one of the reasons that viruses are considered to be non-living entities because they have to depend on the host cell for provision of energy and for the ribosomes to manufacture virus proteins.

ATP is the primary energy currency of the cell. It is a molecule composed of some fancy molecules not readily available out in the natural environment. Adenine is a nucleobase, one of four nucleic acids in DNA and RNA. The adenine is attached to the 5-carbon sugar ribose, which is also found in RNA. Adenine and ribose combine to form adenosine to which a tail of three phosphate groups (PO4) is attached to make ATP. Energy is released when one of the bonds with a phosphate group is broken, leaving ADP (adenosine diphosphate) which is immediately recycled back to ATP in the mitochondrion.

The amount of energy released when the bond to the last phosphate group on the ATP tail is broken, is very close to what is needed to drive a typical biological reaction. Release of the third phosphate group is exothermic and it is coupled with a reaction in the cell that is endothermic. ATP is an energy coupling agent, not a fuel. All the life processes depend upon ATP.

Professor Jerry Bergman explains it this way:

“Generally, ATP is connected to another reaction – a process called coupling which means the two reactions occur at the same time and at the same place, usually utilizing the same enzyme complex. Release of phosphate from ATP is exothermic (a reaction that gives off heat) and the reaction it is connected to is endothermic (requires energy input in order to occur). The terminal phosphate group is then transferred by hydrolysis to another compound, a process called phosphorylation, producing ADP, phosphate (Pi) and energy.” [Bergman, J. 1999. ATP: The Perfect Energy Currency for the Cell. Creation Research Society Quarterly, 36 (1): 2-9.]

The enzyme ATP synthase attaches PO4 to ADP. Each revolution of the synthase machine requires a flow of protons ( H+). At maximum speed it turns at 200 revolutions per second producing 600 ATPs per second.

In prokaryotes, the ATP synthase is located in the plasma membrane around the cell. This rotary machine is less complex than in eukaryotes but is nevertheless still fancy, composed of a number of precisely shaped proteins. Its rotary base is powered by the flow of protons. In eukaryotes the synthase rotating machine is located in the chloroplasts and the mitochondria. These machines sit in plasma membranes inside these organelles which obviously also had be present for the synthase machine to work.

So, the bottom line is that formation of the ATP molecule had to be perfect to work, and the machinery to produce it also had to be perfect. And there had to be anchoring plasma membranes to provide a platform for the synthase machinery. Without ATP, no cell can perform functions necessary for life. On this basis alone, we can see that the living cell is irreducibly complex. It could not gradually develop an energy storage and release system through spontaneous processes.

Related Terms

  • Mitochondria
  • Bacteria