Plant Pigments Ch 10 – Photosynthesis

Autotrophs Photoautotrophs, chemoautotrophs

Phoosynthesis as a Redox Process Respiration is an exergonic redox process. Energy is released from the oxidation of sugar. The electrons associated with the sugar’s hydrogens, lose potential energy as the carriers transport them to oxygen.

Photosynthesis is an endergonic redox process What molecule is being reduced? Carbon dioxide What is the source of energy? Light The light boosts the energy of the electrons as they are moved from water to sugar Water is split, electrons are transferred from the water to carbon dioxide reducing it into sugar.

Location Summary ½ million chloroplasts/mm2 of leaf surface Mesophyll layer O2 exits and CO2 enters leaf through stomata Stomata in close proximity to chloroplasts – WHY? Typical mesophyll cell has 30-40 chloroplasts

Summary Equation 6CO2 + 6H2O ---> C6H12O6 + 6O2 What is the equation for photosynthesis? 6CO2 + 6H2O ---> C6H12O6 + 6O2 How do the light reactions capture solar energy? Photosynthetic pigments There are many different pigments that are associated with the light reactions of photosynthesis

Scientific Research How did Van Neil’s research support the hypothesis of H2O (rather than CO2) as O2 source? How did Engelmann’s experiment reveal relationship between O2 production and pigment colors?

Photosynthesis: 2 parts LIGHT REACTIONS (location?) (PHOTOPHOSPHORYLATION) - conversion of light energy to chemical energy CALVIN CYCLE (location?) (DARK REACTIONS) – transforms atmospheric CO2 to organic molecule; uses energy from light rxn to reduce to sugar.

LIGHT REACTION location? purpose? react/prod? DARK REACTION

Chlorophyll A What happens during light reaction? Light energy is converted into chemical energy. How is this done? Ground state to excited state (e- elevate to an orbital w/ more potential energy) What molecule is capable of passing the light energy to an electron? Only chl a

Chlorophyll & other pigments release photon of light (fluorescence) without an e- acceptor What is the acceptor in the light reaction? NADP+

If the chl a is the only pigment that actually passes the light energy to electrons, then how is it that there is photosynthesis at 500 nm of light?

Thylakoids - 3 major pigments Chlorophyll a – dominant pigment. Red & blue absorption Chlorophyll b and carotenoids Slightly different absorption Funnel energy to chloro a (photosystem) PHOTOPROTECTION (carotenoids)

Why do the leaves Turn Color in the Fall? What causes the leaves to be green to begin with? Chlorophyll a and b The colors that we see are a reflection of the wavelengths of light that the pigments cannot absorb. Why do leaves turn colors in the fall? Why do the leaves fall off of the trees, and die soon after the color change?

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The transfer of energy to the electrons and from the electrons to the primary electron acceptor is the first step of the light reactions. There are two photosystems - I and II. How does the chl a differ in each? It differs only in the different proteins with which it associates.

Noncyclic Flow

Cyclic Flow

Cyclic Electron Flow

Cyclic Photophosphorylation Electrons are used to pump protons into the thylakoid space. As the ions diffuse out through ATP synthase the energy lost is used to generate ATP What is produced during cyclic electron flow? ATP

Benefits of Cyclic Pathway Noncyclic e- flow produces ATP and NADPH in roughly equal quantities Calvin cycle consumes more ATP than NADPH Cyclic electron flow allows chloroplast to generate extra ATP to satisfy the Calvin cycle

Where does photosystem I get its electrons? From photosystem II Where does photosystem II get its electrons? From water What is the product of cyclic photophosphorylation? ATP What is the product of non-cylcic ? ATP and NADPH

Produces ATP and NADPH on the stroma side of the thylakoid The proton gradient (pH gradient), across thylakoid membrane is substantial When illuminated, the pH in thylakoid space drops to about 5 and the pH in stroma increases to about 8, a thousandfold difference in H+ concentration Produces ATP and NADPH on the stroma side of the thylakoid