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Photosynthesis
1. Describe the interaction between chlorophyll and photolysis
2. Recognize the reactants, products, and function for carbon dioxide fixation

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In a nutshell, photosynthesis is the process by which plants (and a few other organisms) convert light energy, carbon dioxide, and water into carbohydrates and oxygen.

Photosynthesis basics
     - Photosynthesis captures the energy in light and uses it (along with carbon dioxide & water) to produce carbohydrates (sugar, starch, cellulose).
     - Though oxygen is given off as a by-product in normal photosynthesis, some organisms (e.g. heliobacteria) can produce carbohydrates but not oxygen. These are called anoxygenic photosynthesizers.
     - Oxygenic photosynthesizers use chlorophyll a, while the anoxygenic photosynthesizers use bacteriochlorophyll instead.
     - This process primarily occurs in plants, but also in some algae and bacteria.
     - The process of photosynthesis takes place in the chloroplasts, specifically using chlorophyll.
     - Photosynthesis in plants takes place primarily in the leaves.
     - The chemical reaction involved in photosynthesis is: 6CO₂ + 6H₂O (+ light energy) => C₆H₁₂O₆ + 6O₂.
     - Photosynthesis and cellular respiration are "opposites" ... cellular respiration takes glucose and produces energy, while photosynthesis takes (light) energy and produces glucose.

What occurs in photosynthesis?
     - Photons of light are captured and hit the chlorophyll (which raises electrons to highter energy levels).
     - A series of oxidation-reduction reactions occur.
     - The energy from the light is used to make ATP and NADPH.
     - Carbohydrates are then created, to be used as food in the cellular respiration process (to produce energy).
     - Photosynthesis is a 2-part process that involves light and dark reactions.

     Light reaction:
         - This step (which MUST take place in the light) converts light energy to chemical energy.
         - Inside the chloroplasts are thylakoids. The stacks of thylakoids are called grana.
         - The light reaction can be divided into Photosystem I and Photosystem II.
                 - Photosystem II - splits water (photolysis) and obtains electrons to drive proton translocation;
                                                  oxygen is given off as a by-product;
                                                  [H2O + light + ADP + P => O2 + ATP + e-]
                            - Chlorophyll (and several other pigments such as beta-carotene) are organized in clusters in the thylakoid membrane and absorb the light.
                            - Each of these differently-colored pigments can absorb a slightly different color of light and pass its energy to the central chlorphyll molecule to do photosynthesis.
                            - The main types of chlorophyll are chlorophyll a and chlorophyll b.
                            - Light strikes chlorophyll in such a way as to excite electrons to a higher energy state.
                            - The absorbed energy and water go to an electron acceptor.
                            - Photolysis uses the light energy to break apart water molecules to obtain electrons that will be used to replace the electrons lost by the photosystems to the electron transport systems.
                            - Oxygen is released at this point. The electron acceptor then goes into Photosystem I.
                 - Photosystem I - energizes the electrons for the reduction of NADPH;
                                                [e- + NADP+ + H => NADPH]
                           - As the electron acceptor goes from II to I, ADPs take on energy and become ATPs.
                           - The energy in the ATPs is converted into NADP+.
                           - The NADP+ loses a hydrogen and turns into NADPH. The stored energy in the ATPs and NADPH then goes into the dark reaction portion of photosynthesis.

     Dark reaction:
         - The dark reaction takes place in the stroma (a cytoplasm-like fluid) within the chloroplast.
         - This reaction converts carbon dioxide and five-carbon molecules into carbohydrates (sugar, starch, cellulose) in the Calvin cycle.

         - This part of photosynthesis does not need light to occur. But the products of the light reaction (ATP, NADPH) are utilized (to make carbon-to-carbon bonds).
         - Carbon dioxide is captured by the chemical ribulose biphosphate (RuBP).
         - It takes 6 molecules of carbon dioxide to make 1 molecule of glucose.
         - Carbon fixation (building organic molecules by adding carbon onto a chain) occurs in the Calvin cycle.
    The Calvin (or Calvin-Benson) Cycle:
         - Step 1: three C5 molecules combine with 3 carbon dioxide molecules to make six 3-carbon molecules
         - Step 2: 6 ATPs and 6 NADPHs provide the energy for the combining of the 3-carbon molecules into 6-carbon molecules (glucose)
         - Step 3: the five 3-carbon molecules are recycled to be used in Step 1 again
         - The chemical equation for the Calvin cycle is: C5 + CO2 + ATP + NADPH ---> C6H12O6

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