The simplest known photosynthetic system is bacteriorhodopsin (BR) , a 26KD memb

Question

The simplest known photosynthetic system is bacteriorhodopsin (BR) , a 26KD membrane protein found in halophilic bacteria. BR is a simple light-activated proton pump which translocates one H+ per photon absorbed. The protons are then used to synthesize ATP with a typical ATP synthase enzyme. The peak absorption of BR is in the green light range (565 nm photons).

a. If 3 H+/ATP is required for synthesis what is the % efficiency of this photosynthetic system? Assume the DG’ for ATP hydrolysis in -12 kcal/mol (-50. kJ/mol) under the real cellular conditions at 37oC.

b. Look up a value for the efficiency of a typical green plant (PSI and PSII) photosynthetic system. What is it (there are any different estimates out there—pick one)? How does it compare? What are some of the disadvantages of the simple BR system as compared to the extremely complex green plant system?

Explanation / Answer

(a) To calculate energy of photon (E) at wavelength (lambda) 565nm, we have the relationship E = h.c/(lambda)......eqn.1 where h is Planck's constant (h = 6.626 x 10-34 J.s), and c is velocity of light (c= 3x 108 m/s), and lambda is wavelength which is in our case is 565nm or 565 x 10-9 m. Putting all the values in eqn 1 and calculating for E, we get

E = 3.6 x 10-19 J. We now multiply this value with Avagadro's No. N = 6.022×1023 to get energy of photon/mol.

Therefore, energy of photon per mol = 3.6 x 10-19 x 6.022×1023 = 2.16 x 105 J/mol or 216 kJ/mol.

Now since three photons need to be absorbed for one ATP synthesis. Therefore, total energy input = 648 kJ/mol.

Now energy from ATP = 50kJ/mol.

Hence the % efficiency of this photosynthetic system is 100 - 100*(648 - 50)/648 = 7.7%

(b) Well we know that report value pf photosysnthesis for plants range anywhere from 0.1 % to 3%. So let's pick 2% as the photosynthetic efficiency of plants. This means that photosynthetic system BR is much more efficient than green plant (PSI and PSII) photosynthetic system.

In complex phototrophic systems like that of plants chlorophylls or bacteriochlorophylls are used rather than bacteriorhodopsin.Chlorophylls in complex phototrophic systems are aided by other pigments known as "antennas" in capturing light energy whereas these pigments are not present in bacteriorhodopsin-based systems.

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