A note on the order of my lectures:
So far we have discussed the cell itself and divided its several functions amongst organelles that carry them out. We have also discussed the properties of membranes and how diffusion operates across them as a passive event. As a consequence diffusion can be opposed, but requires energy input. Lastly, we covered energy and how it may be converted into various forms or used to do work. Within the cell this work is often guided by enzymes.
Where we are going:
In the next section we will address how energy is captured by living things from the environment and converted into a form that may be stored. In the chapter after that, we will consider how this captured energy can be brought out of storage and converted into a useful form for enzymes to use in getting specific jobs done.
As stated above, the purpose of photosynthesis is to convert energy from the environment (solar energy) into a new chemical form (glucose) that can be stored for later use by cells. The process of photosynthesis is completed, in eukaryotic cells, entirely within organelles called chloroplasts. These are organelles that are theoretically descended from prokaryotic cells that engaged in symbiotic relationships with larger cells but are now inseparable parts of the larger cells. As such, we recognize that there are other cells that can carry out photosynthesis, but we will restrict our discussion to that carried out in plant cells.
The basic reaction occurs in two phases, the light reactions and the dark reactions. Despite their names, both occur at the same time, typically when it is light.
The light reactions are when photons from the sun transmit energy into pigment molecules in the chloroplast. From there, electrons carry the energy from one molecule to the next in an electron transport chain that functions to pump protons (H+) across the membrane. In this way an electrochemical-, or proton-, gradient is established. This gradient is a form of potential energy that can be released when protons diffuse back across the membrane passively, through ATP synthase proteins that form channels through the membrane. When H+ ions pass through this channel energy is captured to synthesize ATP through a process called chemiosmosis. This is very analogous to the way that dams capture the energy of water passing through. The high energy electron is finally passed off to form NADPH, a high energy electron shuttle. Because the reaction cannot repeat until the electron is replaced in the photosystem, one is taken from H2O, which splits to form O2 and more H+ ions. The end result of the light reactions is the formation of ATP and NADPH (and O2 as a waste product) from solar energy and H2O.
This summary does not include details reactions starting from Photosystems I and II specifically. Nor does it include the cyclic reaction.
The dark reactions will be covered in our next class a little more extensively, but basically, their function is to use the ATP and NADPH produced in the light reactions as power to synthesize glucose from CO2.