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Explain the basis of ATP generation in mitochondria and chloroplasts. How does this differ from the Substrate level Phosphorylation found in glycolysis?

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Introduction

Explain the basis of ATP generation in mitochondria and chloroplasts. How does this differ from the Substrate level Phosphorylation found in glycolysis? Introduction: ATP is the universal currency of free energy in biological systems (Stryer (1995)). Living organisms use it as a free-energy donor to supply free energy for three major purposes: the performance of mechanical work in muscular contraction and other cellular movements, the active transport of molecules and ions, and the synthesis of macromolecules and other biomolecules from simple precursors. However, ATP must be synthesised using free energy obtained from the environment. Phototrophs obtain this energy by trapping light energy; chemotrophs obtain it by the oxidation of foodstuffs produced by the phototrophs. ATP is not a long-term storage form of free energy - rather it is an immediate donor of free energy. Most ATP is consumed within a minute of synthesis. Consequently the turnover of ATP is very high - a resting human consumes about 40kg in 24 hours (Stryer (1995)). The mechanism of ATP Synthesis: In 1941, Lipmann and Kalckar elucidated the central role of ATP in energy exchanges in biological systems. The molecule itself is a nucleotide consisting of an adenine, a ribose and a triphosphate unit. The latter is the key feature in its role as an energy carrier. The two phosphoanhydride bonds (P-O-P) in the triphosphate unit release a large amount of free energy when they are hydrolysed; this is why ATP is an energy rich molecule (the amount of free energy released is, of course, smaller than for some other bonds - which is why ATP is used: because it allows the controlled release of packets of free energy). About ?7.3kcal/mol is released by the hydrolysis of ATP to ADP and orthophosphate (Pi) or when ATP is hydrolysed to AMP and pyrophosphate (PPi). However, under typical cellular conditions the actual ?G of these reactions is approximately -12 kcal/mol, because of the effect of the ionic strength of the medium and concentrations of magnesium and calcium ions. ...read more.

Middle

Light is captured by chlorophyll molecules, which are associated with multiprotein complexes, called photosystems I and II. Each photosystem consists of an antenna complex and a photochemical centre. The function of the antenna complex is to funnel light energy to the photochemical reaction centre by the process of resonance energy transfer. The photochemical centre in photosystem II (PSII) uses this energy boost an electron to a high enough level to reduce an electron acceptor (plastoquinone) and oxidise an electron donor (water). The oxidation of water to provide free electrons is an unfavourable reaction; consequently it is coupled with the capture of light energy (an energetically favourable reaction) to result in the splitting of water (called photolysis). 2H2O + 4hV [light] ? O2 + 4H+ + 4e- [used to reduce plastoquinone] This reaction occurs in the space enclosed by the thylakoid membrane (the thylakoid lumen), which is impermeable to protons. It therefore results in the build-up of protons in the lumen, producing a concentration gradient across the thylakoid membrane. The membrane is permeable to magnesium and chloride ions so these can move freely across the membrane to stabilise the charge: therefore it is not possible to build-up a charge difference. Because of this the concentration gradient produced is not large enough to generate all the ATP required by reductive biosynthesis. The gradient is increased by a second series of reactions, similar to those found in mitochondria, which pump protons from the stroma across the thylakoid membrane into the thylakoid lumen. Increasing the proton gradient increases the proton motive force, resulting in the production of more ATP. As in mitochondria these reactions involve an electron transport chain which consists of a number of complexes containing redox couples imbedded in the thylakoid membrane. As the complex is reduced and then oxidised by the passing of electrons down the transport chain it undergoes conformational changes (between the oxidised and reduced forms) ...read more.

Conclusion

required for glycolysis. The alternative path of regeneration is by aerobic respiration, which is much slower because oxygen must be transported from the lungs and proton gradients produced in the mitochondria to drive the synthesis of ATP and regenerate NAD+. Cellular reserves of ATP, creatine phosphate and anaerobic respiration of muscle glycogen only power a 100m sprint. However, for a 1000m run part of the ATP consumed must come from oxidative phosphorylation, because anaerobic respiration (substrate level phosphorylation) can not produce enough ATP. Therefore of necessity the championship velocity for the 1000m event is 7.6 m/s compared with 10.1 m/s for the 100m sprint. A marathon runner must also mobilise fat reserves in order to produce enough ATP to complete the race. The fatty acids must be transported from the adipose tissues to the muscle cells and need to be degraded, hence mobilisation of fat reserves is even slower and the pace of the race is reduced. ATP can be synthesised from a variety of different compounds, using a variety of different mechanisms (substrate level phosphorylation, oxidative phosphorylation and photophosphorylation). Each has particular advantages. Plants are rooted in the soil and cannot move to obtain food like chemotrophs (if they did, then all complex molecules would be rapidly used up), consequently they must synthesise ATP from a ubiquitous substance that is continually renewed: sunlight. Animals can move and obtain food that, when processed, synthesises ATP. But because animals move they must be able to respond much faster than plants can. Therefore they must have mechanisms by which they can produce a sudden burst of ATP, in order to respond to their environment. As a result, several different compounds are used as sources of ATP and these can be mobilised at different rates, by using different mechanisms: substrate level phosphorylation or oxidative phosphorylation. The latter is more efficient, producing the most ATP, but it is also slower. Animals must control ATP is synthesis if they are to respond efficiently and successfully to external stimuli. ...read more.

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