The role of ATP
ATP – adenosine triphosphate – provides an immediate source of energy for all cells in every living organism.
The diagram shows that one ATP molecule is made up of an organic base (adenosine) which is attached to a pentose sugar known as ‘ribose’. This sugar is then attached to three phosphate groups.
A lot of energy is stored in the ATP molecule and this energy is generated by the repulsion between the phosphate group and the negatively charged oxygen atoms which are bonded to each other. Most of the energy is stored in the bond of the last phosphate group and when energy is needed by cells, this bond is broken by a hydrolysis reaction catalysed by the ATPase enzyme. When one molecule of ATP is broken, adenosine diphosphate (ADP) and an inorganic phosphate group (Pi) are produced as well as about 30KJ of energy which is released (this value is measured under laboratory conditions but it is estimated that in cellular conditions, about 50KJ will be released). Although some of this energy is lost as heat, most of it is used by cells in their various biochemical processes. ATP is a more reliable source of energy as it is constantly being resynthesized; it is also a quicker process because only one bond needs to be broken to release energy which is unlike molecules such as glucose.
Methods of synthesizing ATP
Energy can become available inside cells to aid the synthesis of ATP by two methods:
Chemical potential energy can be transferred from glucose molecules to ATP molecules. This happens in a process known as glycolysis in the Krebs cycle stages of respiration.
Process of chemiosmosis (diffusion of ions across a selectively permeable membrane) – it happens during aerobic respiration in mitochondria and light-dependent stage of photosynthesis.
Uses of ATP
Muscle contraction – actin and myosin filaments in the muscle fibres depend on ATP to function especially myosin which uses energy released from hydrolysis of ATP to move along the actin filaments.
Active transport across membrane cells (the process of moving substances against a concentration gradient).
Synthesis of large molecules such as proteins or nucleic acids.
Used to produce glucose in the light-independent stage of photosynthesis.
The chemiosmotic theory of ATP production
The main site of ATP synthesis is in the mitochondrion. The main reactions of respiration in the Krebs cycle takes place in the matrix while the electron transfer chain reactions (light-dependent stage) and ATP production occurs on the inner membrane of the mitochondria. The inner membrane of the mitochondria is impermeable to protons so in order for the protons to go back into the matrix, they have to go through special pores in the stalked particles where the electron transfer chain is found. This movement of protons along their electrical concentration and PH gradients drives the synthesis of ATP.
References
Biological Science review volume 19, no 2, November 2006
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