How does photosynthesis work?

I wonder how photosynthesis works. I read the Wikipedia page, but I'm still confused.

My main gap of understanding is (I assume):

I know a photon changes the velocity of an electron. But I do not (fully) understand how a photon creates chemical reactions. Does the atom lose its electron due to this velocity increase and thereby attracting the electrons from the other atoms?

Is that all? Does this imply there is always an electron lost in photosynthesis or is that electron absorbed elsewhere (in another atom)?

3 Answers

A better way of thinking about it, without getting too bogged down in details, is that the plant absorbs the energy of photons and uses it to drive chemical processes - ultimately the synthesis of sugar out of carbon dioxide and water - that are energetically "uphill", and would not happen spontaneously without the energy from the photons. The energy from the photons is used by the plant to move electrons from one place to another, but no electrons are actually generated or consumed in the process ... photosynthesis is a cycle, so you always end up back where you started, with some molecules of $\ce{CO2}$ and $\ce{H2O}$ having been consumed (along with a lot of light energy), and some extra molecules of sugar and $\ce{O2}$ having been produced.

The places where the photonic energy is used to mobilize the electrons are protein complexes called photosystem I and photosystem II, which are concentrated in the chloroplasts of green plants. The details of how this happen are still not completely understood, but a LOT of progress has been made fairly recently using fancy equipment like laser spectrometers that can measure physicals processes that occur on timescales shorter than a nanosecond. This has allowed researchers to map the progress of the electron as it moves from one part of the protein complex to another, following absorption of the photonic energy.

Does that help at all?

• A little tiny bit. Thing is that I was more or less aware of what you wrote. Nice text anyway. Thanks. – mick Jan 10 '15 at 22:06

No, photons of visible light don't ionize molecules during photosynthesis - electrons are transferred via redox reactions in electron transport chain - it seems to me that fragment of the article was maybe a little misleading.

In some steps, a red light photon has enough energy (~0.5 e.v.) to raise an electron to a higher, metastable orbit (activating it), but might not actually remove the electron from the molecule (see http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic+Electron+Transport+and+ATP+Synthesis (very clear), http://hyperphysics.phy-astr.gsu.edu/hbase/biology/psetran.html and http://hyperphysics.phy-astr.gsu.edu/hbase/biology/psetran.html).