The Light Reaction Created by Caitlynn Lied 2008

NADPH

NADP+

ATPNADPH

NADP+

PHOTOSYNTHESIS

ADP

H

O

H

ADPADP

H

OH

HO

H

ATP

ATP

H

HH

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.

HH

H

H

HH

H

Photosystem I

Photosystem II

Thylakoid Space

Stroma

The last chlorophyll molecule that the energy reaches is called the reaction center (also called P680 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

HH

H

H

HH

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

HH

HThe electron from the first PEA then replaces the one lost in Photosystem I

Photosystem II

H

HH

H

The O2 molecule leaves the plant, and the H+ moves off to a latter part of the cycle

Photosystem I

Stroma

Thylakoid Space

2H2O 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

HH

H

O

At this point, the reaction no longer needs Photosystem II

Photosystem II

Photosystem I

H

HHH

H

Stroma

Thylakoid Space

H

Photosystem I

Stroma

Thylakoid Space

H

OH

H

O

H

H

H

H

H

HH

H

NADPH

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

OH

H

O

H

Photosystem I

H

H

Stroma

Thylakoid Space

H

H

After that, ADP and a phosphate (also from the dark reaction) enters the ATP Synthase

The ATP Synthase pumps the H+ (originally from H2O) 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

ATPThen the ATP moves back to the dark reaction

And that’s the light reaction!