Photosynthesis

1.         General

            a.         6CO₂ + 6H₂O ➝ C₆H₁₂O₆ + 6O₂ - note that the reaction is the reverse of cellular respiration

            b.         light energy is captured by low energy CO₂ and converted into high energy C₆H₁₂O₆

            c.         it was long believed that CO₂ was split up so that C was incorporated into glucose and O₂ was released. Actually, C goes into glucose and O into glucose and water (as a product); water (as a reactant) is split to yield the oxygen product

            d.         redox - in cellular respiration electrons move from high energy glucose to O₂ making low energy water; in photosynthesis, low energy water is split and electrons move to CO₂ making high energy glucose; solar energy provides the “power” needed for the reduction

2.         Leaf structure

            a.         grana

            b.         thylakoid

            c.         stroma

3.         Two Phases of Photosynthesis

            a.         Light reactions - occur in thylakoid membrane

            b.         absorbing energy - Why do plants look green if they absorb red and blue light?

                        i.         pigment molecules are arranged in photosystems of a few hundred. These arrangements are called an antenna complex

                        ii.        a photon of light strikes a pigment molecule and an electron absorbs the energy, becoming excited. The energy of this electron is passed from one molecule to another until it reaches the reaction center chlorophyll. There, the primary electron acceptor captures the energy in a redox reaction before it is lost.

            c.         Electron Flow

                        i.         photosystem II absorbs light energy and the energy is transferred to the primary electron acceptor

                        ii.        electrons from photosystem II are replaced by splitting water

                        iii.       electrons are passed along an electron transport chain to photosystem I and the energy is used to make ATP (note: as in cellular respiration, the ETC does not actually make any ATP)

                        iv.       electrons in pigment molecules of photosystem I are excited again by absorbing light energy and are used to reduce NADP+ to NADPH

                        v.         electrons are replaced by those coming down the electron transport chain fro photosystem II.

            d.         Carbon Fixation (Calvin Cycle) - occurs in the stroma - the purpose is to reduce low energy CO₂ into high energy glucose. The energy needed comes from the ATP produced by the light reactions while the electrons needed come from the NADPH produced by the light reactions. That is, the light reactions function to provide some of the materials needed by the Calvin cycle.

                        i.         Carbon Fixation

                                    (1)       a carbon from CO₂ is attached to a 5C compound to form a 6C compound

                                    (2)       the 6C compound breaks in half

                        ii.        Reduction

                                    (1)       each 3C compound gets another phosphate and is reduced by NADPH

                        iii.       Regenerate starting compound

                                    (1)       some 3C product is used to make glucose and other compounds

                                    (2)       some is used to regenerate the starting material