Expert: Florence M Rollwagen - 1/16/2009

Question
How do the light dependent reactions of photosynthesis relate to the calvin cycle?

Answer
Hi M A:  Thanks for your question.

Photosynthesis is the process of synthesizing sugars using the energy of light to power the reaction.  It has several interconnected pathways.  To understand this process (and respiration), you need to remember that the breaking of chemical bonds yields energy (captured in ATP) and the formation of chemical bonds uses energy (provided by ATP).

The light reactions of photosynthesis include the excitation of chlorophyll electrons by light which splits the water molecule into hydrogen and oxygen.  Sunlight provides the energy to release the energy in the H2O bonds.  This energy is captured by ATP.  A second light reaction captures the excited electron and drains its energy by passing it down the electron transport chain, producing NADPH.  Each time the electron energy is stepped down, a proton is pumped across the thylakoid membrane (a structure in the chloroplast).  These protons (hydrogen nuclei) then form a gradient across the membrane.  When these protons fall back down their gradient, ATP is formed.

Now that we have gathered energy and made ATP, it's time to spend that ATP to form chemical bonds.  To do this, the plant captures carbon from the environment in the form of CO2.  The carbon is bonded together to form glucose (a six carbon sugar).  This is the dark reaction, or Calvin Cycle.

Here are two websites that will give more detail:
http://en.wikipedia.org/wiki/Photosynthesis
http://www.wanafunzi.com/Sequence%201/Unit%206%20Cell%20Energy/Photosynthesis%20...

and a movie:
http://www.wwnorton.com/college/biology/discoverbio3/core/content/ch8/animations...

Hope this helps!  Write back if you have more questions.

FM Rollwagen, PhD