Photosynthesis: can you live with it?

I know some of you have mixed feelings about photosynthesis. You like the oxygen, but you are worried that a question about an oscillating electric field initiating photosynthesis might be bad for your health.

To the right is a poll regarding photosynthesis. What it asks about is how you would feel about a supplemental extra-credit question; so you wouldn't have to do it at all if you don't want to, and you could get extra credit if you did do it.  It would focus on the physics part of photosynthesis --the very beginning, in which an electric field helps an electron get into an excited state 

Here are my thoughts on the initial part of the photosynthesis process. The part where a “photon” is absorbed and an electron makes a quantum leap from its ground state to an excited state. I won't use the word photon, because: 1) we haven't defined that, and 2) it is actually not helpful for describing transitions or calculating their rates. I will describe the initiation of photosynthesis in terms of electric fields pushing on electric charges. These are things we have been studying this quarter.

At the Mg2+ site, before any light arrives, the electrons are sitting quietly in their ground state. This is a very stable state--a neon-like closed shell (2s2, 2p6). The electrons are quiet and happy. They see no reason to change.

An electric field comes from the sun. It propagates across millions of miles of empty space using the magic of induction to transform its energy into a magnetic form and then back again trillions and trillions of times. (This is the nature of light. It is composed of oscillating electric and magnetic fields.)

When the electric field, which has the wave-like form which we discussed in class, passes through the chlorophyll it exerts a force on the electrons in the Mg2+. One electron in particular will begin to oscillate due to the influence of the electric field wave. It moves up and down in the same way that a floating buoy in the ocean moves up and down when ocean waves pass by it.  The force on the electron is given by qE(t), where q is the charge of the electron and E(t) is the electric field of the light wave.

In this oscillating phase, the electron is in a superposition state, in which it is partially in an excited state and partially still in the ground state.  After a little while, the ligands began to notice this oscillating, partially excited electron. They grab for the excited electron. Their chance* of success is not 100%, but if they are successful they will spirit the excited electron away and use its energy to initiate a complex chemical and biological process (6H2O + 6CO2 --> …..). 

From a physics point of view, the key step is the transfer of energy from the electric field to the electron. This occurs via the qE interaction which gets the oscillatory movement of the electron started.  The force on a charge due to an electric field, qE, was one of the first things we learned about this quarter.

* This is the essentially a quantum measurement process and so it has a probabilistic aspect to it. You do not need to worry too much about that unless it is something that particularly interests you.