Light Dependent Reactions

Photosystem

A collection of pigment molecules (chlorophyll) that serve as the light collecting unit. The photosystems are embedded in the thylakoid membrane

Purpose of light-dependent reactions

Produce ATP and NADPH that are needed for the light-independent reactions (Calvin cycle)

Where do light dependent reactions occur

In the thylakoid membrane which is found in the chloroplast

Photosystem II

A Photosystem is a collection of molecules in which 100’s of pigment molecules are embedded

Step 1

chlorophyll molecules absorb energy from the sun which increases the energy level of the electrons in chlorophyll molecules. The molecules become excited and are passed to the electron into the electron transport chain. The electrons move down the chain from Photosystem II to Photosystem I. Energy is released and used to transport hydrogen ions from the storms into the thylakoid space.

Step 2

The electrons that were lost must now be replaced. Enzymes in the thylakoid membrane break apart water molecules into 2 electrons, 2 H+ ions, and 2 oxygen atoms. These electrons replace the high-energy electrons that chlorophyll lost to the electron transport chain.

Step 3

The hydrogen ions from the water are released inside the thylakoid. The oxygen is considered a waste product and is released back into the atmosphere.

Step 4

PS1 receives electrons from PS2

Step 5

Chlorophyll molecules in Photosystem 1 absorb energy from the sun and use it to re-energize the electrons.

Step 6

Electrons are passed down a second electron transport chain to the electron acceptor called NADP+. NADP+ joins with hydrogen ions and two electrons. This is referring to the are found outside the thylakoid called the stroma.

Stroma

A dense liquid that surrounds the thylakoids

Step 7

Hydrogen ions flow from an area of high concentration inside the thylakoid to an area of low concentration in the stroma. The hydrogen flows through a protein enzyme called ATP synthase. As the hydrogen flows through ATP synthase, the protein rotates like a turbine being turned by water.

Step 8

The ATP snynthase binds a phosphate to ADP as the protein rotates forming ATP.

Step 9

Hydrogen ions are actively pumped back inside the thylakoid space to keep the concentration of hydrogen very high inside the thylakoid. NADPH and ATP are formed to use in the Calvin cycle