Why does the rate of photosynthesis increase, peak and then decrease as temperature increases?

Why does the resistance of a metal wire increase with temperature?

• I know the A-level explanation (for the yanks it's basically for 16-18 year olds), which is that the metal ions vibrate more; hence there are, supposedly, more collisions between the ions and the electrons, blah blah blah... But the thing i don't understand, is that, overall, there is just the same amount of space for the same amount of electrons to flow in the same amount of time whether hotter or cooler. I mean the opposite can be argued, by saying that; as the temperature is increased, the rate of collisions decreases because the ions are giving way for the electrons to go through, hence there is a decrease in the rate of collisions and the resistance therefore decreases. I know this is wrong because of experimental evidence, but the explanation for this can be argued just as much as the one above. Is there a more plausible explanation as to why the increase in temperature increases the resistance of a metal wire? Thanks in advance for any replies!

• Answer:

  "as the temperature is increased, the rate of collisions decreases" What is the justification for this in light of the kinetic theory of matter? Rate of collisions/vibrations increase with increasing temperature. That is the first thing we learn in Class VIII/IX Physics...