lecture 15.105 ATP synthesis, photosynthesis

Biological Sciences 105 Lecture 16, December 6, 2018 Copyright Steven M. Theg, 2018. All federal and state copyrights reserved for all original material presented in this course through any medium, including lecture or print. 1 Course evaluation forms. Please fill them out. Why all this talk about proton pumping? Mechanism of oxidative phosphorylation: We have seen a number of examples of substrate level phosphorylation wherein the energy stored in a molecule, PEP for example, is captured to make ATP. Not so with ATP synthesis in the mitochondria. Here the energy is captured in a proton gradient across the inner mitochondria membrane. H + in  H + out , generates a proton gradient. There are two parts to the gradient, a chemical part (ΔpH) and an electrical part (Δψ). Clearly the gradient contains some potential energy because if you allowed the H + to cross the membrane freely the gradient would collapse. So how much energy in the gradient? ΔG = RTln([C 2 ]/[C 1 ]) + zFΔψ Want to explore the meaning of this equation. It describes the energy required to make the gradient, and the energy available that can be derived from the gradient. It also gives us a jumping off point to understand something about electrical potentials and membranes. This applies, for instance, to nerve signal propagation. What happens when you have a gradient made with any set of ions, and you suddenly change the permeability of the barrier to one of the ions? Take K + .

Biological Sciences 105 Lecture 16, December 6, 2018 Copyright Steven M. Theg, 2018. All federal and state copyrights reserved for all original material presented in this course through any medium, including lecture or print. 2 K + moves from the left side to the right. But this sets up an electrical imbalance, negative to the left and positive to the right. Creates an electrical potential, Δψ. The chemical gradient makes K + want to move from left to right, but the electrical potential makes K + want to move from right to left. Eventually the driving force of the chemical gradient is exactly opposed by the Δψ, and at that moment all movement stops. What is the situation with no further movement in chemistry? Equilibrium. And the ΔG of a system in equilibrium? Zero = 0. Can use this to figure out the magnitude of the opposing Δψ. Δ? = 𝑅𝑇𝑙? [𝐶 2 ] [𝐶 1 ] + 𝑧?∆𝜓 = 0 Δ𝜓 = − 𝑅𝑇 𝑧𝐹 𝑙? [𝐶 2] [𝐶 1 ] This is called the equilibrium potential, or the Nernst potential, or the diffusion potential. This is the basis of establishment of a resting potential in cells. Especially important in nerve cells, where the potential can be quickly altered by opening and closing selective ion channels, allowing the potential change to be propagated down the length of the (long) cell (axon).

Biological Sciences 105 Lecture 16, December 6, 2018 Copyright Steven M. Theg, 2018. All federal and state copyrights reserved for all original material presented in this course through any medium, including lecture or print. 4 Experimental Proof of Mitchell’s Chemiosmotic Coupling hypothesis; No one believed at first. Mitchell’s Plot pH Jump Experiment (with thylakoids): 1. Acid bath, pH 5 2. Put into basic solution, pH 8, immediately. 3. ATP synthesis Now do this at different acidities of the 1 st stage bath. Threshold; think of water reservoir

Biological Sciences 105 Lecture 16, December 6, 2018 Copyright Steven M. Theg, 2018. All federal and state copyrights reserved for all original material presented in this course through any medium, including lecture or print. 5 Now repeat with different pH and salts in 1 st stage bath What does the ATPase look like? Sort of a lollipop in the membrane F0: Proton channel, multiple subunits, 1 present at ~ 10 – 12 copies F1: Reversible ATPase, 5 separate subunits, 9 in total = 3α3ß+γΔε. 3 equal catalytic groups. These can be separated into an ATPase part and a channel part. On the computer –

Biological Sciences 105 Lecture 16, December 6, 2018 Copyright Steven M. Theg, 2018. All federal and state copyrights reserved for all original material presented in this course through any medium, including lecture or print. 7 The energetically uphill reactions are those promoted by light absorption in Photosystem II (P680) and Photosystem I (P700). Differences between mitochondria and chloroplasts: Respiration is C 6 H 12 O 6 + 6 O 2  6 CO 2 + 6 H 2 O ΔG°’ = -2,480 kJ/mol Photosynthesis (green plants) is 6 CO 2 + 6 H 2 O + light  C 6 H 12 O 6 + 6 O 2 ΔG°’ = +2,480 kJ/mol The critical difference is light. Recall that light excites the special reaction center molecules P680 and P700, changing their redox potential from very (+) to very (-). As with mitochondria, there is a similar view of the ETC that talks about proteins and not redox. PC Fd

Biological Sciences 105 Lecture 16, December 6, 2018 Copyright Steven M. Theg, 2018. All federal and state copyrights reserved for all original material presented in this course through any medium, including lecture or print. 8 Note the proton pumping reactions. Green plants are obviously not the only photosynthetic organisms. How do they compare with others? Plants CO 2 + 2 H 2 O + light  CH 2 O + O 2 + H 2 O Other organisms use different things besides H 2 O, i.e., H 2 S, propanol, lactate) The general pattern for photosynthesis as CO 2 + 2 H 2 D + light  CH 2 O + 2 D + H 2 O with H 2 D as any H + or e - donor; D is the oxidized form Energy input: We will now calculate the efficiency of energy utilization. We will have in the recorded lecture information about the fixation of CO 2 into reduced compounds. But for now we will write –

Biological Sciences 105 Lecture 16, December 6, 2018 Copyright Steven M. Theg, 2018. All federal and state copyrights reserved for all original material presented in this course through any medium, including lecture or print. 10 In cyclic, some e - go back from Fd to the cytb 6 f complex, pumping H + as e - go back to PSI. Make ATP with those H + . No net gain of NADPH. So now with linear and cyclic e - flow: 2 H 2 O + 2 NADP + + 3 ADP + 3 Pi + 9 photons  O 2 + 2 NADPH + 2 H + + 3 ATP How efficient is photosynthesis? What is the representation hν? This is the energy in one photon. H = Plank’s Constant= 6.626 x 10 -34 Js. v (Greek letter nu) is the frequency of light = c/λ with c = 3 x 10 8 m/s. So for 700nm light (  ), v= (3 x 10^8 m/s)/(700 x 10^-9)= 4.29 x 10^14s-1. So the energetic content of a photon of 700nm light is hv = 6.626 x 10-34 Js x 4.29x10^14s-1 = 2.84 x 10 -19 J/photon A mole of the photons → x 6.02 x 10 23 photon/mol = 170 kJ/mol photons. (A mole of photons is called an Einstein) So 9 Einsteins of 700 nm light has 9 x 170kJ/mol = 1530 kJ Writing CO 2 +2H 2 0 +n(hv) → CH 2 O +H 2 0 +O 2  G°’= +473 kJ/mol

Biological Sciences 105 Lecture 16, December 6, 2018 Copyright Steven M. Theg, 2018. All federal and state copyrights reserved for all original material presented in this course through any medium, including lecture or print. 11 So photosynthetic efficiency is calculated as; 473 kJ/mol needed / 1530 kJ/mol input = 0.31 or 31% energy conversion