Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosynthesis

LE 10-3 Leaf cross section Vein Mesophyll Stomata CO 2 O2O2 Mesophyll cell Chloroplast 5 µm Outer membrane Intermembrane space Inner membrane Thylakoid space Thylakoid GranumStroma 1 µm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosynthesis as a Redox Process A simplified equation of photosynthesis that indicates only the net consumption of water is as follows: Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced 6 CO H 2 O + Light energy  C 6 H 12 O O 2

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Two Stages of Photosynthesis: A Preview Photosynthesis consists of the: – light reactions (the photo part) and – Calvin cycle (the sugar synthesis part) The light reactions split water, release O 2, produce ATP, and form NADPH The Calvin cycle forms sugar from CO 2, using ATP and NADPH made in the light reactions.

H2OH2O LIGHT REACTIONS Chloroplast Light ATP NADPH O2O2 1- The light reactions (in the thylakoids) split water, release O 2, produce ATP, and form NADPH H 2 O → 2H + + 2e - + ½ O 2

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings H2OH2O LIGHT REACTIONS Chloroplast Light ATP NADPH O2O2 NADP + CO 2 ADP P + i CALVIN CYCLE [CH 2 O] (sugar) 2- The Calvin cycle (in the stroma) forms sugar from CO 2, using ATP and NADPH made in the light reactions.

Visible light Gamma rays X-rays UV Infrared Micro- waves Radio waves 10 –5 nm 10 –3 nm 1 nm 10 3 nm10 6 nm 1 m (10 9 nm) 10 3 m nm Longer wavelength Lower energy Shorter wavelength Higher energy The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation Visible light consists of colors we can see, including wavelengths that drive photosynthesis

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Comparing the Wavelengths of the Different Colors of the Visible Light

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosynthetic Pigments: The Light Receptors Pigments are substances that absorb visible light Different pigments absorb different wavelengths Wavelengths that are not absorbed are reflected or transmitted Leaves appear green because chlorophyll reflects and transmits green light

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The main photosynthetic pigment is called – Chlorophyll a. The accessory pigments are: – chlorophyll b – carotenoids absorb excessive light that would damage chlorophyll. Types of carotenoids: – Carotenes (are pure hydrocarbons, made solely from C & H. For example:  -carotene in carrots) – Xanthophylls (are oxygenated hydrocarbons, made of C, H & O. For example lycopene, the red pigment in tomatoes) Photosynthetic Pigments

LE Thylakoid Photon Light-harvesting complexes Photosystem Reaction center STROMA Primary electron acceptor e–e– Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Thylakoid membrane How a Photosystem Harvests Light Energy

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Light Reactions During the light reactions, there are two possible routes for electron flow: 1- the linear (noncyclic) electron flow. 2- the cyclic electron flow.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Linear Electron Flow The linear (noncyclic) electron flow: – Is the primary pathway of the light reactions. – Occurs in the thylakoid membranes. – Involves both photosystems (PSI & PSII). – Splits water (H 2 O → 2H + + 2e - + ½ O 2 ) – Uses light energy to produces ATP and NADPH

LE 10-13_1 Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 The Linear Electron Flow

LE 10-13_2 Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 e–e– e–e– + 2 H + H2OH2O O2O2 1/21/2 The Linear Electron Flow

LE 10-13_3 Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 e–e– e–e– + 2 H + H2OH2O O2O2 1/21/2 Pq Cytochrome complex Electron transport chain Pc ATP The Linear Electron Flow

LE 10-13_4 Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 e–e– e–e– + 2 H + H2OH2O O2O2 1/21/2 Pq Cytochrome complex Electron transport chain Pc ATP P700 e–e– Primary acceptor Photosystem I (PS I) Light The Linear Electron Flow

LE 10-13_5 Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 e–e– e–e– + 2 H + H2OH2O O2O2 1/21/2 Pq Cytochrome complex Electron transport chain Pc ATP P700 e–e– Primary acceptor Photosystem I (PS I) e–e– e–e– Electron Transport chain NADP + reductase Fd NADP + NADPH + H H + Light The Linear Electron Flow

ATP Photosystem II e–e– e–e– e–e– e–e– Mill makes ATP e–e– e–e– e–e– Photon Photosystem I Photon NADPH A mechanical analogy for the linear electron flow of the light reactions

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Cyclic Electron Flow The cyclic electron flow: Occurs in the thylakoid membranes. Uses only photosystem I Excited electrons leave P700 and return to it. Does not produce NADPH or O 2 Produces only ATP. Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle

LE Photosystem I Photosystem II ATP Pc Fd Cytochrome complex Pq Primary acceptor Fd NADP + reductase NADP + NADPH Primary acceptor The Cyclic Electron Flow P700

LE STROMA (Low H + concentration) Light Photosystem II Cytochrome complex 2 H + Light Photosystem I NADP + reductase Fd Pc Pq H2OH2O O2O2 +2 H + 1/21/2 2 H + NADP + + 2H + + H + NADPH To Calvin cycle THYLAKOID SPACE (High H + concentration) STROMA (Low H + concentration) Thylakoid membrane ATP synthase ATP ADP + P H+H+ i [CH 2 O] (sugar) O2O2 NADPH ATP ADP NADP + CO 2 H2OH2O LIGHT REACTIONS CALVIN CYCLE Light

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Calvin cycle has three phases: – Carbon fixation phase catalyzed by the enzyme: ribulose bishosphate carboxylase oxygenase (rubisco) – Reduction: Making the sugar G3P. – Regeneration of the CO 2 acceptor 1,5-Ribulose Bisphosphate (RuBP) The Phases of the Calvin Cycle 231/calvin.html

LE Light CO 2 H2OH2O Light reactionsCalvin cycle NADP + RuBP G3P ATP Photosystem II Electron transport chain Photosystem I O2O2 Chloroplast NADPH ADP +P i 3-Phosphoglycerate Starch (storage) Amino acids Fatty acids Sucrose (export)