Photosynthesis reactions |

The light-dependent reaction is the first step of photosynthesis
This takes place in the Thylakoid membranes of chloroplasts and involves the green pigment chlorophyll
Chlorophyll is found in structures called photosystems that are embedded in the thylakoid membrane
Between the photosystems is a system of protein channels called electron carriers that make up the electron transport chain
In the light-dependent reaction, ATP, reduced NADP, and O2 are made by phosphorylation

Light energy strikes photosystem 2 (PSII) and is absorbed
The light energy excites electrons in the chlorophyll in PSII
These electrons, now at a higher energy level, move down the electron transport chain to photosystem 1 (PSI)
As they move down the electron transport chain, they lose energy that is used to actively transport H+ ions into the thylakoid against their concentration gradient
The H+ ions flow down their concentration gradient through the ATP synthase enzyme channel which causes ADP and a phosphate group (Pi) to be combined to form ATP
The H+ ions bond to NADP to form reduced NADP (NADPH)

Light strikes PSI
The electrons are moved in a loop from PSI to the electron carriers and back again
This process only forms small amounts of ATP

The light-independent reaction, also known as the Calvin Cycle, is the second stage of photosynthesis
It takes place in the stroma of the chloroplasts
It forms a molecule called triose phosphate (TP) that can be converted into glucose and other substances

An enzyme called rubisco (ribulose bisphosphate carboxylase) is used to combine CO2 with a molecule called ribulose bisphosphate (RuBP) to form 2 molecules of glycerate phosphate (GP)
2xGP is converted to three molecules of TP using the energy from the breakdown of 2ATP to ADP and 2NADPH to NADP
Some TP is converted to glucose. The rest is used to regenerate RuBP using the energy from the breakdown of 1 ATP to ADP

The ATP and NADPH used in the Calvin Cycle come from the light-dependent reaction