Adenosine Triphosphate (ATP) in cells
All cells use ATP (see figure 1) as the primary energy-carrying primary molecule to which energy from the breakdown of food molecules – carbohydrates, fats & proteins – is transferred, then transferring this energy to cell functions (Vander et al, 1998, p.67). Discovered in 1929, Adenosine 5′-triphosphate (ATP) is primarily known as a multifunctional nucleotide that does not to store energy, but rather transfers it to the intracellular processes that require it (Vander et al. 1998 p.67). The molecular structure of ATP consists of a purine base (adenine), attached to pentose (ribose) and three phosphate groups. Synthesis of ATP occurs through glycolysis (in cytosol), cellular respiration (in mitochondria) and photosynthesis (in chloroplasts)(Kimball, J 2006). As ATP is extremely rich in chemical energy, it is harnessed for the chemical and mechanical work in association with the growth, maintenance and reproduction of living organisms (May, P 1997). To use an analogy, the ATP molecule can be described as a highly efficient chemical battery – working to distribute energy to the energy-requiring processes throughout the body.
Figure 1: Farabee 2007 Figure 1.1: chemistryland.com 2007
THE STRUCTURE, PHYSICAL & CHEMICAL PROPERTIES OF ATP
The chemical structure of ATP is based upon three components (see Figure 1); ribose: a five Carbon sugar, adenine the base and the string of phosphate groups. The centre component which is a sugar molecule is pentose (ribose) – consisting of carbon, oxygen and hydrogen atoms. Attached to the 1’ carbon atom of ribose is the second group called adenine (the base). Adenine is a purine base because it is made up of carbon, nitrogen and hydrogen atoms, and therefore has double (fused) rings of nitrogen and carbon atoms (Vander et al. 1998, p.31). As Adenine is a nitrogen-containing compound (also known as a nitrogenous base) (Farabee 2007). The third component attached to the 5’ carbon atom of ribose is a string of three phosphate groups, which are the key to the activity of ATP (Science Encyclopaedia, 2007).
HOW ATP RELEASES ENERGY
ATP is a type of organic molecule referred to as a nucleotide or nucleoside. Nucleotides are chemical compounds are basically made up of 3 components; a pentose sugar, one or more phosphate groups and a heterocyclic (adenosine) base. The part of the nucleotide molecule that doesn’t include the phosphate group in called a nucleotide. This is why ATP (has 3 phosphate groups) is called a nucleotide, or nucleoside triphosphate.
Figure 2: Chemical Structure of ATP, www.sparknotes.com
As nucleotides can have different numbers of phosphate groups associated with the molecule, the specific name of the nucleotide reflects its number of phosphate groups. As pictured (Figure 2), ATP contains three phosphate groups; the highest energy form of the three pictured. During metabolic reactions, these phosphate groups can be cut loose, releasing stored energy for the utilisation of a biological process; so that ATP yields either Adenosine diphosphate (ADP) – two phosphate groups, or adenosine monophosphate (AMP) – one phosphate group, (sparknotes.com). These 3 compounds; AMP, ADP and ATP are part of a group called nucleotides because they are the monomers of nucleic acids (Vander et al. 1998, p.67).
ATP molecules have covalent bonds, where two atoms share an electron (or electrons), in an attempt to satisfy their unfilled outermost valence shell in order to make them a more stable molecule. These covalent bonds are weak with generally low bond energy (Kimball, J 2006), yet release a great deal of energy when broken and are generally controlled by specific catalysts, called enzymes (Bettelhaim et al. 1991, p.70).
ATP + H20 ïƒ ADP + Pi + H+ + 7kcal/mol
Cellular respiration harvest chemical energy from food and stores it as energy in these covalent bonds (between phosphate groups) that are broken for exergonic (energy producing) reactions within the cell. During the process called hydrolysis (see Figure 3), these bonds are broken as energy is released, yielding the compounds ADP and AMP (Kimball, J. 2006). ATP is extremely rich in chemical energy, with the greatest amount of energy (approximately 7 kcal/mole) stored in the bond between the second and third phosphate groups, known as the pyrophosphate bond (Farabee 2007). As this high energy bond (occasionally depicted by wavy lines ~) is found between the phosphate groups, more energy is thus extracted when both phosphate groups are removed (Kimball, J. 2006)
Figure 3: Hydrolysis of ATP, Zymes.com 2007
The net molecular change of energy in the decomposition of ATP into ADP and an inorganic phosphate, is -12 k Cal/mole in vivo (inside of a living cell), and -7.3 kCal/mole in vitro, or in laboratory conditions (Opentopia, 2007 & Bettelheim et al, 1991, p. 602). This huge release in energy makes the decomposition of ATP extremely exergonic and very useful as a means for chemically storing energy (Bettelheim et al, 1991, p.602). The free energy can be a further used for locomotion (muscle contraction), chemosynthesis and in the active transport of ions and molecules across cell membranes (Kimball, J 2006)
ATP is usually not synthesised from scratch, rather it is simply ‘recharged’ (Zymes.com 2007). In metabolism ADP regains the phosphorus group to produce ATP and water (see Figure 4).
ADP + Pi + 7kcal/mol ïƒ ATP + H20
The process of re-phosphorylation uses the chemical energy obtained from the oxidation and breakdown of food into basic components such as sugars and lipids, photochemical reactions (sunlight) together with the presence of coenzyme Q10 (Darling, D 2007).
Figure 4: Re-phosphorylation of ATP, Zymes.com 2007
Glycolysis is the specific pathway by which the body gets energy from monosaccharides, the most important of which is glucose (Bettelheim et al. 1991, pp.606-8). In the first steps of glucose metabolism, energy is consumed, rather than released (see figure 4).
?? then takes place, where carbohydrates (chains of monosaccharides or simple sugars) react with water molecules – weakening the bonds – causing them to split in two parts creating simple forms such as glucose and fructose. The metabolised fats yield fatty acids and glycerol. The essential components then go through a process conducted in the inner membrane of a mitochondrion called glycolysis (Farabee, MJ 2007) Co-enzymes enter electron transport chain or respiratory chain. The electron transport system that is embedded in the membrane transfers protons from the inner compartment to the outer. When the protons flow back to the inner compartment the energy of their movement is used to add phosphate to ADP, forming ATP. This is where the majority of ATP production takes place. The other is through citric acid cycle/oxidative phosphorylation, and beta-oxidation (Opentopia 2007)
Figure 6: Ophardt C E Elmhurst College Virtual Chembook
This process is also able to switch directions when the input of additional energy (plus a phosphate group) “recharges” ADP into ATP.
If all phosphate groups are removed, a nucleotide becomes a nucleoside such as adenosine. From “Cellular nucleotides and nucleosides”. http://www.web-books.com/MoBio/Free/Ch3A4.htm” s
FUNCTION & LOCALISATION IN BODY
Human cells require energy of approximately 200- 300 moles of ATP daily (Opentopia, 2007), where ATP in the human body is equivalent to 0.1 mole in total quantity (Bettelheim 1991). Through its generation and degeneration (ATP molecules are recycled around 2000-3000 times/day), the amount of ATP and ADP remains fairly constant. This equates to 1 kilogram of ATP that is created, processed and recycled in the body every single hour (Opentopia, 2007).
TABLE OF PROPERTIES: Adenosine Triphosphate (ATP)
5-(6-aminopurin-9-yl)-3,4-dihydroxy-oxolan-2-yl methoxy-hydroxy-phosphoryl oxy-hydroxy-phosphory oxyphosphonic acid
Chemical formula C10 H16 N5 O13 P3
Molecular mass 507.181 g mol-1 (C 23.68%, H 3.18%, N 13.81%, O 41.01%, P 18.32%)
Melting point 1000°C
Density 67 g/cm3
CAS number 56-65-5
Figure 5, Merck 2001 & Opentopia
The overall function of ATP in the human body is to release high amounts of energy by breaking the phosphate-phosphate bonds through hydrolysis. The energy that is released is thus able to carry out cellular functions such as the active transport of molecules across cell membranes, the synthesis of macromolecules /proteins, as well as the support of daily activities such as locomotion (Opentopia, 2007). ATP is located and mainly produced inside the mitochondrion of cells.
ATP is intrinsic to the continual energy-transfer supply that all cells require in order to function. Generated through the processes of photosynthesis and cellular respiration, the molecule ATP serves as an immediate source of energy for the mechanical workings such as protein synthesis, cellular movements and locomotion (muscle contraction) in cells.
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