Photosynthesis Also Known As… All the reasons you’ll ever need to chop down all the plants because we probably don’t really need them anyway right?

Photosynthesis: The Details  Photosynthesis is the result of two distinct processes – the light reactions and the Calvin cycle (dark reactions). 6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2  Remember…The light reactions provide the energy molecules to the Calvin cycle which will attach the carbons, oxygens and hydrogens together to make glucose.

The Light Reactions  The light reactions provide the necessary molecules needed to make glucose. In order to do this we must do the following: 1.Photoexcitation – Absorb energy of sunlight using chlorophyll molecules. 2.Electron transport – Move energized electrons through a series of reactions that will release energy that we can harness. These electrons eventually end up with NADP to make NADPH – an electron carrier (for our back pocket). 3.Chemiosmosis – Protons (H + ) are pumped across the membrane which builds a gradient that is relieved by ATP synthase which makes ATP.

1. Photoexcitation  Chlorophyll has double bonds that are made of electrons.  When a photon hits these double bonds, electrons are given the energy and go from a ground state to an excited state with more energy.  The electrons are released from the chlorophyll and are picked up by a primary electron acceptor. Chlorophyll is oxidized and the acceptor is reduced.

Photosystems  Photoexcitation takes place in the photosystems – there are two in the light reactions – P680 & P700. The numbers refer to the wavelength of light each responds to best.  P680 is also known as PS II.  P700 is known as PS I.  They are numbered as I and II in order of their discovery – not their occurrence.  A photosystem is made of a chlorophyll molecule, antenna chlorophylls, accessory pigments and a primary electron acceptor.

2. Electron Flow  There are two paths that electrons may take to get through the photosystems – non-cyclic and cyclic.  Non-cyclic electron flow occurs when there is ample ATP available and it gets the electrons to NADP + so it can form NADPH as soon as possible.  Cyclic electron flow occurs when ATP levels are low. The electrons follow a cyclic path that yields extra ATP molecules and then eventually gives the electrons to NADP +.

Non-cyclic Electron Flow 1.Photons hit P680 – electrons are excited and go to primary electron acceptor. 2.They are then passed on to PQ (plastoquinone) and then to the b 6 f cytochrome complex – a proton pump. Protons are pumped from the stroma into the thylakoid lumen. This will form a proton gradient that will couple with ATP Synthase to yield ATP. (a la cell respiration) 3.Electrons leave proton pump and go to PC (plastocyanin) and then are dumped onto P700. Here they receive another blast of energy from photons and once again sent to a primary electron acceptor. 4.The electron acceptor passes the electrons to ferredoxin (FD) and then the electrons are passed on to NADP reductase which will become reduced and form NADPH.

Cyclic Electron Flow  Everything about the cyclic flow is the same until you hit FD (ferredoxin).  NOW…Instead of going on to NADP, the electrons are re-inserted just before the proton pump (b 6 f complex) so they can do the whole “proton gradient makes a problem that is fixed with ATP synthase and we get ATP from it” thing.  This path can cycle several times before returning to the non-cyclic mode.  The next step, the Calvin cycle, requires more ATP than NADPH in order to make a glucose molecule.

3. Chemiosmosis  This step occurs during the electron movement associated with the light reactions.  You need both NADPH and ATP to run the Calvin cycle.  The ATP is made by chemiosmosis involving the proton pump (b 6 f), the proton gradient it produces and the enzyme ATP Synthase.  You have already seen this as part of the cellular respiration process inside the mitochondrion.

The Light Reactions

The Dark Side  You just made a bunch of ATP and NADPH from the light reactions.  You now use these electrons and energy to put a bunch of carbons together to make glucose – this is the job of the Calvin cycle.  The Calvin cycle can be broken down into three phases: 1.Carbon fixation – Putting carbons together. 2.Reduction reactions – Adding electrons & energy. 3.Regeneration of RuBP – Getting back to the start – it is a cycle after all.

Calvin Cycle: Step 1 Carbon Fixation  We will be approaching this in class as if we have six Calvin cycles all working together.  The source of carbon is carbon dioxide (CO 2 ).  A carbon atom from CO 2 is added to the 5-C ribulose biphosphate (RuBP) to form an unstable six-carbon molecule that quickly splits into two 3-C molecules of PGA or 3-phosphoglycerate.  The first molecule made in this process has three carbons – this is why this pathway of photosynthesis is known as the C 3 pathway.

Calvin Cycle: Step 2 Reduction Reactions  Each PGA is phosphorylated by ATP to form 1,3-biphosphoglycerate.  Next, a pair of electrons from NADPH reduces the 1,3-biphosphoglycerate to produce G3P - glyceraldehyde 3-phosphate.  What you have just done was add electrons to make bonds and a source of hydrogen for the glucose.

Calvin Cycle: Step 3 Regeneration of RuBP  You now have 6 molecules of G3P – five of them will be used to regenerate three molecules of RuBP. This process consumes three molecules of ATP. The RuBP can now be reused to fix more carbon from CO 2.  The G3P that is left over is now used to make carbohydrates.  This explanation is usually doubled to explain the formation of a glucose molecule that has six carbons.

All Together Now!

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