What is the difference between cyclic and non-cyclic photosynthesis?

These are the two types of photophosphorylation and they occur in the light-dependent stage of photosynthesis. We will start with non-cyclic photophosphorylation as this is the step we expect to see when the plant is sufficiently supplied with water and sunlight. Non-cyclic photophosphorylation involves both photosystems. Water will first be broken down into 4H+ ions and 4e- (electrons) and oxygen. This will happen in the thylakoid space - remember that the thylakoids are the ones that stack up to make the granum? The electrons produced will pass down the electron transport that stems from photosystem II first and then they will reach photosystem I. They then pass down another electron transport chain. At the end, the electrons are reduced to reduce NADP to NADPH. As the electrons pass down these chains of transport they release energy. This energy is used to pump H+ ions into the thylakoid space. The hydrogen ions then pass through the ATP synthase enzyme to form ATP from ADP. So in non-cyclic photophosphorylation, you make oxygen, from splitting the water molecule, you make ATP using the H+ ions and you make NADPH. In cyclic photophosphorylation, you only use photosystem I. There is no splitting of water - the electrons only come from the light harvesting complex. When the electrons reach the end of the chain, they return back to the start of the chain instead of helping to make NADPH as there is no NADP to accept them. They electrons passing down the chain means H+ ions are still pumped into the thylakoid membrane so we still make ATP but this is the only product of cyclic photophosphorylation.