Lecture 44: Photosynthesis and Redox NO YES

In eukaryotes, photosynthesis occurs in chloroplasts

The major steps of photosynthesis Photon absorption excites an electron – Described in lectures 28 and 29 Electron flow allows H + to be pumped into a region of high concentration/positive charge – Focus of this lecture H + flow out of the high concentration/positive charge region drives ATP synthesis – Focus of Monday’s lecture ATP is used to power carbon fixation, generating e.g. sugars – Maybe next year?

Monday Today The major steps of photosynthesis

The photosynthesis electron transport chain

Why do electrons flow this way?

Outline for today’s lecture Photosynthesis – Overview and some cellular context – The photosynthesis electron transport chain Reduction and oxidation – Terminology – Role of electric potential – Predicting net direction of electron flow Coupling of electron flow to H + “pumping” – H + production from water splitting – The “Q cycle” mechanism

First, some key terminology: reduction and oxidation Reduction is the gain of an electron. Oxidation is the loss of an electron. The nomenclature reflects the tendency of oxygen, a highly electronegative atom, to partially or fully steal e - from other molecules. O2O2 e-e-

First, some key terminology: reduction and oxidation Reduction is the gain of an electron. Oxidation is the loss of an electron. Whenever one molecule is oxidized, another is reduced (unless the e - is truly liberated). The oxidized and reduced forms of a molecule constitute a redox pair. reduced form oxidized form

First, some key terminology: reduction and oxidation Reduction is the gain of an electron. Oxidation is the loss of an electron. When a molecule in water is reduced, it often picks up a H +, too: Adding/removing a net hydrogen (H + & e - ) is called hydrogenation/dehydrogenation

First, some key terminology: reduction and oxidation Fun fact: hydrogenation of fatty acids is used to make margarine

First, some key terminology: reduction and oxidation To understand the net direction of electron flow in the photosynthesis electron transport chain, we would like to know: What is the change in energy associated with a balanced redox reaction?, e.g.

Electrons can decrease their electric potential energy U e by moving to molecules where they experience a more positive electric potential  Electron flow and electric potential

Electrons experience electric potentials inside molecules Thorium nucleus (90 protons) Oxygen nucleus (8 protons) 97 other electrons?!

Electrons can minimize their electric potential energy U e by moving to molecules where they experience a more positive electric potential  We can measure relative values of  experimentally.

Electron flow between molecules, driven by electric potential differences, can be measured Metal #1 Metal #2

Electrons flow to molecules with more positive internal electric potentials

Magnesium Graphite

Measuring “reduction potential” of biological redox pairs The previous examples used the same solution in both cells but different electrode types. Now suppose we use identical electrodes, but different solutions. Electrons will flow toward the solution containing the “greedier” redox pair. We can use this electron flow to measure the electrical potential of a solution relative to a standard: we call this the “reduction potential”.

What will happen if A oxidized has a higher affinity for electrons than H + ? ? The electric potential on the left is more positive (reduction potential is positive)

What will happen if A oxidized has a lower affinity for electrons than H + ? ? The electric potential on the left is less positive (reduction potential is negative)

Reduction potential E o

Reduction potentials along the photosynthesis electron transport chain

Relationship between reduction potential and energy differences E 0 ’ = mV E 0 ’ = mV What is the change in electric potential energy  U e on transferring two electrons from ferredoxin (Fd 2- ) to NADP + at standard state?

Question 2: How is H + “pumping” driven by electron flow?

Proton “pumping” mechanism The Expectation The Reality

H + production from water splitting

Photosynthesis and aerobic respiration

Plastoquinone (Q) as a “proton pump” Photosystem II (initial site of excitation) Cytochrome b 6 -f complex

Plastoquinone (Q) as a “proton pump” Photosystem II (initial site of excitation) Cytochrome b 6 -f complex

Plastoquinone (Q) as a “proton pump” Photosystem II (initial site of excitation) Cytochrome b 6 -f complex

Plastoquinone (Q) as a “proton pump” Cytochrome b 6 -f complex

Plastoquinone (Q) as a “proton pump” Cytochrome b 6 -f complex

Plastoquinone (Q) as a “proton pump” Cytochrome b 6 -f complex

Plastoquinone (Q) as a “proton pump” Cytochrome b 6 -f complex

Plastoquinone (Q) as a “proton pump” Cytochrome b 6 -f complex This clever recycling of electrons in a loop is called the “Q cycle”

Electron transport chains in photosynthesis and respiration both use Q cycles

What we hope you learned today How a H + gradient is generated during photosynthesis – Why electrons move down an electron transport chain from carrier to carrier – How electron movement is coupled to proton “pumping” in the Q cycle How to describe redox reactions How to build a battery