THE SHORT STORY OF PHOTOSYNTHESIS:

Photosynthesis converts light energy to chemical energy.

(glucose + fructose = sucrose)

Light energy is used to make ATP and NADPH, which are later used to make sugars from carbon dioxide. ATP is a cell’s energy molecule.

ATP and NADPH, along with oxygen gas are the products of the light reactions of photosynthesis.

• Plant cells can’t efficiently store ATP and NADPH, or transport them efficiently to other tissues that require energy.

• Plants can’t use ATP or NADPH directly as carbon skeletons for cell growth.

So what do plants do with ATP and NADPH?

ATP & NADPH are used to convert CO2 into stable, easily transported sugars.

Photosynthesis occurs in two contrasting series of reactions:

Light dependent reactions & The Calvin cycle

I -Light dependent reactions

1) Chlorophyll in the photosystems (located in the grana) absorbs light causing electrons to gain energy and move.

2) The energy rich compounds ATP and NADPH are formed to carry energy & protons to the Calvin cycle (In the stroma).

3) Water is split apart in a process to provide electrons (& protons) to the light reactions. Oxygen is also released.

water

electrons

protons

II –The Calvin Cycle or Light Independent reactions (a.k.a. “Dark Reactions”)

Dr. Calvin

1) It happens in the stroma. This is the solution of chemicals that surrounds the thylakoid membranes

2) ATP and NADPH from the light reactions are used

• to fix inorganic carbon (CO2) into organic carbon, and

• produce high energy sugars such as glucose.

3) The plant uses the sugars for energy (cell respiration- breaks down the sugars to make more ATP for cellular activities) and for building more complex compounds such as starches, cellulose, etc.