* How are membranes in the chloroplast similar to the plasma membrane? How are they different? * W4 W4

* To identify the reactants and products of the light dependent reaction * To determine the mechanism involved in transferring energy from light to ATP and NADPH

* Visible light is radiation (which is the transmission of energy through space as a wave or particle). * It is a small section of what we call the electromagnetic spectrum. * Electromagnetic radiation travels in packets called photons.

* Pigments can absorb photons that have the right energy level to excite the atom or molecule. * Photons that are not at the right energy are reflected. * reflected colours are those our eyes perceive on an object.

* As seen yesterday, integral to the thylakoid membrane are photosystems, which are protein-based complexes composed of dozens of photosynthetic pigments. * When a photon of the right energy hits a pigment, it transfers the energy to the reaction centre.

* 1. A photon hits Photosystem II and releases an electron from the reaction centre (called P680 because of the light it absorbs). * 2. This causes a chain of redox reactions whereby stronger and stronger *oxidizing agents take the electron from the molecule before it in the chain. * 3. Successive transfer of the electron allows for hydrogen to be pumped across the thylakoid membrane into the lumen by the cytochrome complex. * 4. The original electrons lost from P680 are replenished by splitting a water molecule. O 2 is formed through this reaction!

* Successive oxidations by increasingly electronegative molecules results in an electron transport chain. * As potential is decreasing along the chain, this energy can be harnessed to do work, such as pumping ions.

* 4. the electrons that were passed down the chain end up in the reaction centre of Photosystem I (P700), and another photon is needed to re-energize it so it can be released. * 5. The chain ends in the reduction of a molecule of NADP + into NADPH. At this point, there is a large gradient of H + across the membrane, as well as some high energy NADPH electron carriers.

* ATP Synthase uses the electrochemical gradient (from H + ) that has been built up in the thylakoid lumen to catalyze the addition of P i to ADP to create ATP. The Proton Motive Force (or PMF) is what drives this reaction. * XDY XDY * When it results from energy from light, such as in this case, we call ATP synthesis photophosphorylation.

* Create a list of all the reactants and products from these reactions (exclude enzymes, proteins, cofactors, and other components of the membrane, as they are not used up).

* A secondary pathway has been adapted for periods of low NADP + in the chloroplast. We call it cyclic electron flow (as opposed to non cyclic electron flow just described). * In this pathway, only Photosystem I is used. * Photons excite the electrons in P700, which reduce the cytochrome complex which can pump Hydrogen ions. * As a hydrogen ion gradient is still being built up, ATP synthase can catalyze ATP synthesis. * No NADP + is needed in this mechanism, but as a result, no NADPH is formed.

* The end result in both cases is ATP. Though normally, the chloroplast is also able to reduce NADP + into NADPH as well.

* 2TY 2TY

* 1. Look up the molecules involved in electron transport in the thylakoid membrane and be amazed at how many there are (you don’t need to know any specific names aside from maybe the cytochrome complex). * 2. Where does O 2 come from in Photosynthesis? * 3. Do one (or more) of the following: * A) Find a good video that shows all the steps in sequence, without going into too much detail. Share it with your classmates on facebook, or by sending it to me. * B)Create a rhyme or mnemonic that helps to remember the steps in the LDR. * C)Draw a diagram (without copying another!) that shows the steps in the LDR.