Photosynthesis: Why Does It Occur? J. J. ~ a c ~ o n a l d ' Bishopbriggs High School, Scotland Photosynthesis is a complex multistep process that functions under nonstandard conditions. Respiration and photosynthesis share a common boundary a t the reversible limit, and photosynthesis is possible only beyond this boundaru. This discussiou is based on standard values. Combustion and Respiration of Glucose I n order to understand photosynthesis, we start with combustion and respiration of glucose. C6H1206+ 602 3 6C02 + 6H20 Standard function changes2 are
@= -2800 kJ mol-'
A G +~ TI%
-2900 kJ mol-'
ture of respiration: The energy changes that go with it depend on the way it is operated. Opposite of Respiration Now photosynthesis is usually described, blandly, a s the opposite of respiration. But thii is true only a t the reversible limit. Reversible Respiration
Work out is a maximum. Heat is in 100 kJ mol-'.
lu = -2900
kJ mol-' ( = AGO) q = +I00 kJ m K 1 ( = qgV)
Photosynthesis (= 9":)
Workinis2900 k J m o r l ( = m =AGO) Heat out is 100 kJ mor' ( = * = *gV) @ = AGe + q g
+I00 kJ mol-'
Combustion of glucose is3 VI?ry exothermic: 2800 k J of +2800 kJ mol-' +2900 kJ mol-' -100 kJ mol-' heat are evolved per mole of re:&ion, and no optional work is done. Respiration, however, harnesses some of Light t h e free-energy decrease a s work, so the heat loss per mole decreases. The higher the efficiency of the cell, the more work it does, and the less heat it e- flow during photosynthesis evolves. Some slow-moving, cold-bodied animals I e- flow during, rerdration I seem to operate on occasions a t efficiencies a s high a s 75%. At this effiGlucose Omen ciencv. the work done Der Oxygen watm and electrode electmde wbon dioxide and wafer mol-'. he heat evolved a t t h e same time is 2900 x 12H20 602 + 24H+ + 24e0.25 kJ = 725 k J minus the C6HI2O6 + 6H20 + 6C02 + 24H+ + 24e100 kJ absorbed d u e to me fonvard reaction occurs during respiration, the The fonuard reastion occurs during respiration, q g u p e n c e '2 = -625 k J reverse during photosynthesis. the reverse during photosynthesis. mol .- ~ - ~ If respiration could oper- I a t e a t - a n efficiency of Figure 1. Chemistry in a leaf. A respiration cell under standard condRions has an emf. of (about)1.5 V. Clock(2800/2900) x 100% = 97%, wise movement of electrons occurs during respiration;counterclockwisemovement occurs during photosynthethe process would break sis. Reversible operation requires the potentials of the electrochemical cell and photoelectric cell to be equal opposite. Respiration and photosynthesis are then exactly opposite in every way. Respiration does maxieven i n terms of heat and mum work infinitely slowly; photosynthesis has minimum work done on it infinitely slowly. transfer: The 100 k J of h e a t left over from G A Respiration Cell would equal that absorbed a s and the heat change would be zero. At efficiencies greater than 97%, therefore, A respiration cell consists of a glucose electrode and an respiration would become endothermic! At the reversible oxSgen~lectrodrI Fig. 1,. The e.rn'i: ol:rhe cell understandlimit, 2900 k J of work would be done, and 100 k J of heat ard conditions 1s calculated from AGz = -nFE - and is avwould be absorbed (infinitely slowly). This then is the naproximately 1.5 V This is the potential exerted by the cell when it is operated reversibly, that is, when the clockwise 1Former Principal Teacher of Chemistry. movement of electrons is equally opposed by a n external 'Filled-in symbols, such as AH represent rates. The usual unit for AHis kJ mol-' (of reaction).In the same wav. nand wreDresent rates e.m.f. The cell is then in metastable equilibrium. of energy lransfer ahere the transferoccurs a's heal and work Open A photoelectric cell supplies the opposing potential. symbols, sJcn as I H nowever, represenl Imte olfferencesberween When its potential is smaller than that of the electroml al ana fma states The Jsua un I for 3 H s KI O,OL e For a an0 w chemical cell, respiration occurs irreversibly and electrons the usual unit is also kilojoule, move clockwise. When its potential is greater, photosyn-
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Volume 72 Number 12 December 1995
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thesis occurs and electrons move counterclockwise. ~ o inT o s y N T ~ E s I s iork RESPIRATION When the opposed potentials are equal, the cell is on hold and operates reversith 100% with I009 ibly. The term reversible op'ficiency eration, unfortunately, implies t h a t something i s Work in per mole decreases; Work out per mole increases; ............ happening when nothing is! efficiency inmaser. efficiency increaser. Provided the incoming light continues to generate a he efficiency of photosynthesis is the minimum The efficiency of respiration is the optional work counterclockwise electron w k input, AGe if conditions am sfandard, divided done, -won,divided by the maximum possible at push equal and opposite to y the (much greater) wok-capable energy input, E. the reversible limit, urn.. ,which is -AGe if that generated by the cell, et E equal 20 000 U mol-l. mnditions are standard. This quotient b then neither respiration nor phofficiency = (AGelE) x 100% u p d as a percentage. tosynthesis will occur. FiesLet rm be -500 kJ mol-l. piration and photosyntheEfficiency = (uoog/-AGe) X 100% sis a r e now opposite processes in every sense, and this is the only time when they are so described. Photosynthesis therefore needs aninput of 2900W0f Figure 2 Respiration and photosynthesis share a common boundary. work per mole of glucose while,~simultaneously,100 k J of heat is expelled. Phocable 100 k J a t 100% efficiency to much larger values a t tosynthesis under standard conditions is ex other mi^!^ lower efficiencies. The larger values are not actually due to changes in the cell but rather to wasted incident light.' A Common Boundary The onlv " cenuinelv cell-derived art of this total heat loss For respiration to occur, it must be operated below the is that due to q%," that is, the 100 kJ, and this is constant reversible limit. Photosynthesis must therefore be operwhatever the efficiency. ated above the limit. I t is as if respiration and photosynthesis share a common boundary (Fig. 2). When we move Summary away from this boundary in order to make the processes go, the processes take on their own character and cease to Respiration and photosynthesis are exactly opposite only be opposites. The values of A@, AGO, and q% for each a t the reversible boundary between the two. This metasprocess are unchanged. But for respiration, a s the effltable equilibrium state is maintained as long as the inciciency decreases, G is transferred less a s work and more a s dent light promoting photosynthesis exerts a force equal heat, and w becomes less negative. Then q moves from and o~nositeto that exerted bv .resoiration. When the force positive to negative and becomes more negative. For phoinduced by light is less, respiration can occur and does so tosynthesis, a s efficiency decreases, more and more of the irreversibly. (The free-energy decrease is not all used as incident light is wasted while w (= AGO) and q (= q%) work; some is wasted a s heat.) When the force is greater, are unchanged. photosynthesis occurs. Let us assume that the production of 1 mol of glucose needs a n input of 20,000 W of work-capable energy (as 3~nfortunately, photosynthesis is usually described as endotherlight). The system stores 2900 k J of this as free energy and mic. This is a common misconceptiori. Although the value of A@, expels 100 k J a s heat together with (20,000 - 2900) k J = which is positive, tells us that the internal energy of the system in17,100 k J a s h e a t The system, 1mol of glucose and 6 mol creases by 2800 kJ mol", it does not specify how. As it happens, this of oxygen is 2800 k J better off, and this is its enthalpy increase is not due to the absorotion of heat but rather to the absoro(This is not a heat transfer.) The total heat gengain, tion of 2900 kJ of work-capablelight energy (which is stored as G) erated is 17,200 kJ. Efficiency is 14.5% (Fig. 2). minus the simultaneous heat loss of 100 kJ. Photosynthesis, therefore, under standard conditions, is 4As such, this waste heat belongs to the system that generates the light. exothermic a t all times. Heat loss varies from a n impracti-
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