The Light Reaction Created by Caitlynn Lied 2008 NADPH NADP+ ATP NADPH NADP+ ADP H O H H O H H O H ATP

H H H H Photosystem I Photosystem II Sunlight filled with photons shines onto the plant The photons excite the electrons in the chlorophyll molecules Stroma Thylakoid Space The electrons move to the outer energy levels As the electron moves from an excited state (high energy) to ground state (low energy), the difference in energy is passed to a neighboring chlorophyll molecule. H H H

H H H H Photosystem I Photosystem II Thylakoid Space Stroma The last chlorophyll molecule that the energy reaches is called the reaction center (also called P 680 chlorophyll) The molecule becomes so energized that it throws off its electron, which is accepted by the Primary Electron Acceptor (PEA) The energy continues to move to every chlorophyll molecule in the photosystem H H H

H H H H Photosystem I Photosystem II Thylakoid Space Stroma While energy was being passed in Photosystem II, the same was happening in Photosystem I The PEA moves along a pathway known as an Electron Transport Chain to Photosystem I. Passing along a H –pump. NOTE: The reaction center in Photosystem II is called P680, but its called P700 in Photosystem I- the difference is in what type of wavelength the molecule absorbs H H H The electron from the first PEA then replaces the one lost in Photosystem I

Photosystem II H H H H The O 2 molecule leaves the plant, and the H+ moves off to a latter part of the cycle Photosystem I Stroma Thylakoid Space 2H 2 O molecules split to produce 4 hydrogen ions, 1 oxygen gas molecule, and 4 electrons. These electrons replaces the ones lost in the reaction center in Photosystem II As the PEA traveled along the E.T.C, energy from the electron was used to actively transport a hydrogen ion from the stroma into the thylakoid space HH H H H H O H O

At this point, the reaction no longer needs Photosystem II H O H H O H Photosystem II Photosystem I H H HH H Stroma Thylakoid Space H H H H H H

H Photosystem I Stroma Thylakoid Space H O H H O H H H H H H H H NADPHNADP+ Now, NADP+ enters the NADP Reductase from the dark reaction The electron from the PEA and a Hydrogen Ion from the stroma combine with the NADP+ to form one NADPH Then the NADPH leaves to go back to the dark reaction H

H O H H O H Photosystem I H H Stroma Thylakoid Space H H ADP After that, ADP and a phosphate (also from the dark reaction) enters the ATP Synthase The ATP Synthase pumps the H+ (originally from H 2 O) to the stroma through passive transport As H+ passes through the membrane, enough energy is supplied to combine an ADP + P to form one ATP molecule. H H H H ATP Then the ATP moves back to the dark reaction

Photosystem IPhotosystem II H O H NADPHNADP+ ADP ATP