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Chapter 10: Photosynthesis

Light energy capture

Leaves & Pigments

Photosystems

System II

System I

The Z Scheme

Calvin Cycle

Carbon Fixation

Regulation

Energy Storage

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Light energy, in the form of photons, can be absorbed

by electrons, moving them to an excited state

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Leaf cross section

Mesophyll cell

Chloroplast

Granum

Energy from sunlight is absorbed by pigments in the

thylakoid membranes inside chloroplasts

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Pigments are molecules that absorb only certain

wavelengths of light, the wavelengths not absorbed are

transmitted or reflected which we perceive as color

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Photosynthetic organisms often contain several

pigments to increase their action spectrum

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Pigments have a long hydrophobic tail to anchor them

in the membrane and a head to absorb light energy

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The pigments Chlorophylls a and b and Carotenoids are

the main photosynthetic pigments in plants

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RESONANCE-ENERGY TRANSFER

Photon

Chlorophyll and -Carotene molecules

in antenna complex

Reactioncenter

Energy in these excited electrons has 3 different

potential fates

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Higher

Photon

Chlorophyll moleculeLower

Energy

ofelectron

and/orFluorescence

Heat

REDOX

Reaction center

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Overview of Photosynthesis

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1. The light-dependent reaction(the photo stage)- in thethylakoid membranes

2. The light-independent reaction(Calvin cycle; the synthesis stage)- in the stroma of chloroplasts

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Energy reaches the reaction center of Photosystem II,

chlorophyll is oxidized & a high-energy electron is

donated to the electron acceptor pheophytin

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Pheophytin passes the high-energy electrons to an ETC

containing plastoquinone and a cytochrome complex

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The ETC pumps H+into the thylakoid lumen triggering

chemiosmosis and ATP synthesis in the chloroplast

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Energy reaches the reaction center of Photosystem I,

chlorophyll is oxidized & 2 high energy electrons are

passed through a series of redox reactions to Ferredoxin

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Anoxygenic Photosynthesis in

Purple & Purple Sulfur bacteria

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Cyanobacteria, algae, and plants pass electrons through

both photosystems in a Z-scheme generating both ATP

(energy) and NADPH (reducing power)

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Electrons are transferred between PS II and PS I by

Plastocyanin (PC), are passed to NADP+, and are

replenished by stripping them from H2

O

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In Cyclic Electron Transport, PS I passes re-excited

electrons back to PS II to generate more ATP

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The energy generated in the light-dependent reactions

(ATP and NADPH) fuels the Calvin Cycle in the stroma

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Step 1 of the Calvin Cycle fixes carbon by combining 3

molecules of Ribulose Bisphosphate (RuBP) and 3 CO2

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Step 2 uses energy to reduce the 3-carbon molecules

made in step 1 to glyceraldehyde-3-phosophate (G3P)

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Step 3 uses 5 out of 6 G3P molecules (and more ATP) to

regenerate the 3 RuBP that went into step 1

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Rubisco (enzyme which combines RuBP and CO2) may

react RuBP with O2in a reaction called photorespiration

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Leaves can regulate the rate of the Calvin Cycle by

controlling CO2passage through gates called stomata

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C4 Plants regulate the Calvin Cycle through spatial

regulation (physical separation of Rubisco and air)

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Crassulacean Acid Metabolism (CAM) Plants also use

the C4 cycle but as part of temporal regulation

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Photosynthetic protein synthesis is regulated by light,

temperature, CO2and sugar concentrations

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Sugar (glucose & fructose) synthesized from G3P is

stored as sucrose (small & mobile) or starch (large)

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