what is the role of the hydrogen ion gradient in light reactions?

Question

cant find the answer in my text book

Answer 1

Overall, both photosynthesis and respiration are about the same thing: converting energy in the form of "electron power" into an easier to use format: ATP. The difference is that in respiration, we start with already energetic electrons (found int C-C and C-H bonds in sugar) whereas in photosynthesis, we 'zap' electrons with a photon of light and ENERGIZE them.

 

To your question: how do we convert 'electron energy' into the energy stored in ATP (where it is in the form of POTENTIAL energy of having lots of negative charges held together)? It's a complicated, two-step process.

 

FIRST,  we convert 'electron power' to 'hydrogen power'. This occurs because as the electrons are passed in the Electron Transport Chain, at several points their movement is used to drag/force protons (H+) from one side of a membrane to the other. Continued passage of electrons in this way creates an IMBALANCE across the membrane--many more protons accumulate on the (lumen) side than the (stroma) side. This imbalance is a different form of potential energy--a GRADIENT of protons, high concentration on one side of the membrane, low concentration on the other.

 

We now convert the PROTON energy into ATP-stored energy by way of a totally cool machine called ATP Synthase. It 'allows' the protons to return to the low concentration side, but they must turn the rotor in the synthase as they go through. The synthase, in turn, is 'holding' ADP and phosphate (Pi), and in turning, these components are physically driven together in such a way that a covalent bond forms--creating ATP!

 

Short form: electron energy is converted to proton gradient by events occurring during travel in electron transport chain

proton gradient energy is converted to ATP stored energy by action of protons driving movement of ATP synthase

 

Grab your textbooks and look closely at whatever figure shows the electron transport chain and pay attention to electron movement, 'pumping' of protons, and ATP synthase at the end.