Recap- Photosynthesis, the process that uses light energy to make carbon compound glucose and oxygen. To put in simpler terms the reaction is as follows:
Carbon dioxide + water à Glucose + Oxygen
All free energy must take a certain route to enter organisms; this energy arises from the solar energy that is trapped by photosynthesis.
In photosystem II this is where ATP synthesis takes place. In photosystem I, this is where NADP is reduced. This is known as non-cyclic photophosphorylation, it will become clear as to why it is called this later on.
A second photon hits photosystem I, and again, electron excitement takes place. The electron moves through the electron transport chain, but this time rather than ATP being synthesised, the electron joins with hydrogen and NADP, to form reduced NADP (NADPH.) Remember, NADP is a coenzyme (a molecule used to aid a reaction.) When NADP is in a reduced form, NADPH, this means the coenzyme has gained a proton (and electron.) The reduced NADP is then used for a later reaction in photosynthesis. The reason why this reaction is called non-cyclic photophosphorylation is firstly because the process is not cyclic (unlike a reaction which will next be discussed.) Secondly, ‘photo’ means light, photophosphorylation means light is used to phosphorylate a molecule (ADP into ATP.)
Cyclic photophosphorylation is used to make ATP, this process produces no NADPH and no oxygen. In this reaction, the electron in Photosystem I is excited by a photon but the electron takes an alternative route.
Remember, both non-cyclic and cyclic photophosphorylation are light dependent reactions and will therefore cannot take place at night. But photosynthesis does still occur at night, at a lower rate…
Photosynthesis does not stop there, we have the light independent reaction, also called the Calvin cycle. As the name suggests, it does not need light to take place. This is where carbon dioxide comes into use. Carbon dioxide is taken up through the stomata on the surface of leaves, the carbon dioxide then diffuses to the chloroplasts. As the reaction is cyclic, the end product of the previous cycle is used in the next reaction, this is ribulose bisphosphate, RuBP for short (a 5-carbon compound.) The enzyme ribulose bisphosphate carboxylase (RuBisCO) fixes carbon dioxide onto RuBP to form a 6-carbon compound. The 6 carbon compound is unstable and immediately splits into 2 molecules. The intermediates are converted into triose phosphate molecules. From this, one carbon is taken to be used for carbon synthesis (for example making glucose.) The other 5 carbons are used to regenerate RuBP. The cycle requires 3 ATP and 2 reduced NADP molecules from the light dependent reaction in order to take place.
Therefore as one cycle produces 1 carbon for carbon skeletons, to make 1 molecule of glucose, the cycle has to go around 6 times. This means 12 NADPH molecules and 18 ATP molecules required from the light dependent reactions. And here we have photosynthesis finished! Well, a simpler version of photosynthesis…