chapter 11

  • photosynthesis:

    • 6CO2 + 12H2O + Light energy → C6H12O6 + 6O2 + 6H2O

  • in photosynthesis, water is oxidized and carbon dioxide is reduced

    • water loses its electrons when it’s split in the light reactions

  • photosynthesis occurs in two main steps:

    • light reactions in the thylakoid membrane

    • dark reactions in the stroma (calvin cycle)

  • photosynthesis is endergonic, it requires the energy given to it by sunlight

  • electromagnetic spectrum:

    • is the entire range of electromagnetic energy

    • from highest to lowest energy: gamma rays, xray, uv, light, infrared, microwaves, radio waves

    • a spectrophotometer measures a pigment’s ability to absorb various wavelengths

    • pigments in the chloroplast: chlorophyll a, chlorophyll b (accessory pigment), carotenoids

    • whatever color a pigment doesn’t absorb, it reflects & appears as

      • this is why green light isn’t the most effective for photosynthesis

  • light reactions

    • split h2o for e- and h+, release o2, reduce NADP+ to NADPH, generate ATP

    • the ATP and NADPH produced here is to be used in the calvin cycle

    • electrons + light = excited

    • photosystems: PSII (P680) & PSI (P700)

      • pigment complex → absorbs solar energy and passes it to the

      • reaction center (P680,P700) (association of proteins that holds chlorophyll a molecules) → where the electrons in the center get energized that they escape from the reaction center and move to the

      • primary electron acceptor → accepts excited electrons and is reduced as a result

    • P680 is a component of PSII (the chlorophyll a molecules within PSII that absorb light most efficiently)

  • in the light reactions, there are cyclic and noncyclic electron flows

    • noncyclic (linear) electron flow → the primary pathway, involves both photosystems and produces both ATP and NADPH

      • a photon hits a pigment in PSII, exciting P680

      • an excited electron in P680 is transferred to the primary electron acceptor, getting oxidized into P680+

      • water splits into 2e-, 2H+, and oxygen

        • here, the electrons from water go to P680+, reducing it back into P680

        • the hydrogen ions are released into the thylakoid space

        • the oxygen atom combines with another oxygen atom from a different water molecule and forms O2 to be released

      • electrons are passed through the ETC in a series of redox reactions from the electron acceptor of PSII to PSI

        • the ETC includes the electron carrier plastoquinone (Pq), a cytochrome complex, and a protein called plastocyanin (Pc)

        • energy released by the electron transfer here is used to pump more H+ into the thylakoid space, creating a proton gradient across the thylakoid membrane

      • potential energy stored in the gradient drives ATP production by chemiosmosis

      • onto PSI, light energy excites P700 which loses an electron to the primary electron acceptor of PSI, oxidizing it into P700+

        • P700+ accepts an electron passed down from PSII via the electron transport chain to become P700

        • this is how PSI keeps renewing electrons, from PSII

      • electrons are passed from the primary electron acceptor of PSI down a second electron transport chain to the protein ferredoxin (Fd)

        • NO ATP IS PRODUCED BY THIS ETC

      • the enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+

        • two electrons are needed to reduce NADP+ to NADPH

        • also removes an H+ from the stroma

        • the electrons of NADPH are at a higher energy level than in H2O, so they’re more available for the reactions of the calvin cycle

    • cyclic electron flow → secondary pathway, where excited electrons cycle back from Fd to the cytochrome complex instead of being transferred to NADP+, this produces extra ATP

      • this pathway only uses PSI

      • electrons are passed to a P700 chlorophyll in the PSI reaction center via plastocyanin (Pc)

      • ATP is produced, which is why we have more ATP than NADPH as a result of the whole light reactions

  • the calvin cycle

    • CO2 fixation

      • CO2 is attached to the carbon dioxide acceptor RuBP (five-carbon sugar) by the enzyme RuBisCO

      • this forms a 6-carbon molecule, which immediately splits into two 3-carbon molecules (3-phosphoglycerate, or 3-PGA)

    • CO2 reduction

      • each molecule of 3-PGA is altered through phosphorylation by 6 ATP and reduction by 6 NADPH to produce one G3P (3-carbon still)

        • 3-PGA → BPG → G3P

      • for every 3CO2, 6G3P are formed. from these, only one counts (the rest are used in the next step to regenerate RuBP)

    • RuBP regeneration

      • the 5 molecules of G3P not used / not counted as net carbohydrate gain are rearranged and undergo reactions to yield 3 RuBP

      • 3ATP used here

    • so, since each turn of the calvin cycle generates 1 G3P, we need two turns of the cycle to produce one molecule of glucose, since two G3P molecules make one glucose

    • the light reactions regenerate the 9ATP and 6NADPH used here