Cross section of a typical leaf: Photons of light stream into the leaf. Much of it is reflected away (the green part of the visible light spectrum). But some light energy is captured by the leaf in very special “light catchers” called pigments. This begins an amazing process that ends in the production of a powerful, energy rich molecule of sugar.

Diagram of the processes in the chloroplast:

The light-dependent reactions and light-independent reactions occur together in this very important and abundant organelle.

The light-dependent reaction uses water taken in by the plant. When the water (H2O) is split, the H+ ions are used to reduce NADP+ and ADP to more highly energized NADPH and ATP. These two new types of molecules can then be used to drive the light-independent reaction (named the Calvin cycle, from one of its discoverers, Melvin Calvin). What’s left over from the water molecule is oxygen (O2), which is released.

The Calvin cycle now uses carbon dioxide (CO2) from the surrounding air, with the energy in the NADPH and ATP molecules, to produce sugars. The sugar glucose (C6H12O6) can now be used to fuel glycolysis in the cell’s cytosol.

Light photons strike pigments (chlorophyll a, b, and others) found in the thylakoid membranes inside the chloroplast (see A). This excites an electron, which is transferred to an electron acceptor (see B). This occurs in a process called “Photosystem II.” This more energetic state allows the Electron Transport Chain (an exergonic reaction, or energy-releasing process) that reforms the energy molecule ATP.

A second photosystem (Photosystem I) boosts the electrons’ energy level again to another electron acceptor. From here, another ETC can utilize the enzyme NADP+ Reductase, which will reform the power molecule NADPH (see C, D).

Both the NADPH and ATP are now ready to be used to drive the light-independent reactions of the Calvin cycle.

2007 Created by Wally Blankenship, RHHS, Rock Hill, SC

Shown in greater detail above, the Calvin cycle doesn't need light to power its process. That’s why it’s called the light-independent cycle. Once inaccurately called the “Dark reaction,” this cycle occurs almost constantly, day and night. While photosynthesis occurs inside the thylakoid membranes, this cycle occurs outside the thylakoid stacks, in the chloroplast’s fluid, called stroma.

The plant takes in CO2 from the atmosphere. This will be the source of carbon that’s needed for a sugar molecule. As the CO2 diffuses through the chloroplast, it is met by Rubisco, an enzyme that assembles the carbons in to a 6-carbon chain. This is the Carbon fixation phase. This acid is now converted from an acid to an aldehyde by significant power from NADPH and ATP being oxidized. This Reduction phase successfully reduces the 6-carbon molecule to a higher potential energy molecule of glucose. The final phase of the Calvin cycle is a Regeneration of CO2

acceptor, RuBP. This molecule is then readied as Rubisco, awaiting more CO2 to enter.