David W. Deamer Department of Zoology University of California Davis, 95616
II
ATP Ivn+hesis: The Current Controversy
Adenosine triphosphate (ATP) is the primary source of readily available energy for numrrous 1)iological pvocesses. Activities as dircrgent as muscular cffort and creat,ivethought require tbeir quota of A'I'P hydrolysis, as do also such relat,ively simple processes as protein synthesis and maintenance of osmotic equilibrium in red blood cells. The complex involvement of ATP in some major biological energy pathways is outlined in Figure 1. To summarize briefly,
Figure 1. Biologic01 energy tronrfer through ATP. The major pothwoys of biologicol energy tronsfer ore rhown in this greatly simplified diagram. Light energy is trapped b y chloroplartr in the photoreduction and photophorphorylotion processes in which nicotinomide adenine dinvcieotide phorphote (NADPI is reduced to NADPH and adenine diphorphote IADPI and inorganic phosphate (Pi1 combine to form adenosine triphosphote IATPI. The mechonirm, by which there reactions occur ore unknown. NADPH and ATP then enter ond drive the Calvin cycle in which C 0 2 and Hz0 ore fixed in the form of various reduced carbon compounds. in mitochondrio, the energy of reduced carbon compound. Is utilized to drive oxidative phoxphorylotion. The ATP which is synthesized in mitochondri~is the primary energy source for many important biologicd functions, including muscle contradim, nerve impuke tranrmitrion, and biosynthetic reactions wch as protein 3ynthe~ir.
light energy from the sun is trapped by chloroplasts, mhich are chlorophyll-containing organelles of green plants. Some of the energy is utilized to synthesize ATP from adenosine diphosphate (ADP) and inorganic phosphate (P,) and some to produce reducing power in the form of reduced nicotinamide adenine dinucleotide phosphate (NADPH). The resulting ATP and NADPH then drive the carbon fixation cycle (Calvin cycle) in mhich water and carbon dioxide react to form reduced carbon compounds such as carbohydrates. The reduced carbon compounds may then serve as an energy source in the more familiar oxidative metabolism of animals. In nearly all animal cells, mitochondria oxidize reduced carbon compounds and use this energy to phosphorylate ADP. This is called oxidative phosphorylation, as compared to the photophosphorylation which takes place in plants. The resulting ATP may then energize numerous biochemical reactions, inclnding protein synthesis, contractile processes such as 198
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lourno1 of Chemical Education
t,hose which occur in muscle and cilia, and ion transport, mhich is essential to the functioning of nerve cells and kidney tubules. A molecular model of ATP is shown in Figure 2. The
Figure 2. Adenosine triphorphate (Alp). The drawing. illustrate ATP ar o .pace-filling molecule (below1 and with covolent bonds only [above). An extended configuration is rhown for ciority.
molecule is composed of adenine, ribose, and phosphate moieties. When the terminal phosphate is hydrolyzed, approximately 7,000 cal per mole are released. For this reason, the terminal anhydride bond is called a high-energy bond, and for convenience, the bond is represented by-(read squiggle). The energy of ATP hydrolysis drives simple biochemical reactions by energetically down-hill group transfer reactions. However, the mechanisms by mhich ATP is utilized in the important processes of muscle contraction and ion t,ransport are not at all clear. Electron Transport and the Chemical Intermediate Hypothesis of Phosphorylation
From what has been described above, it is obvious that one of the fundamental problems in biology today is to understand the means by which ATP is generated in mitochondria and chloroplasts. Several factors in-
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