LE 10-3 Leaf cross section Vein Mesophyll Stomata CO2 O2 Mesophyll cell Chloroplast 5 µm Outer membrane Thylakoid Stroma Granum Thylakoid space Intermembrane space Inner membrane 1 µm

Chloroplasts are organelles that are the site of photosynthesis Leaves are the major locations of photosynthesis Their green color is from chlorophyll, the green pigment within chloroplasts Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast Through microscopic pores called stomata, CO2 enters the leaf and O2 exits

Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana Chloroplasts also contain stroma, a dense fluid

6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O Photosynthesis can be summarized as the following equation: 6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced

LE 10-5_3 H2O CO2 Light NADP+ ADP + CALVIN CYCLE LIGHT REACTIONS ATP NADPH Chloroplast [CH2O] (sugar) O2

The Two Stages of Photosynthesis: A Preview Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) The light reactions (in the thylakoids) split water, release O2, produce ATP, and form NADPH The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules

LE 10-7 Light Reflected light Chloroplast Absorbed Granum light Transmitted light

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

Chlorophyll a is the main photosynthetic pigment Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll

(INTERIOR OF THYLAKOID) LE 10-12 Thylakoid Photosystem STROMA Photon Light-harvesting complexes Reaction center Primary electron acceptor Thylakoid membrane e– Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)

A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes A photosystem consists of a reaction center surrounded by light-harvesting complexes The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center

A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

There are two types of photosystems in the thylakoid membrane Photosystem II functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm Photosystem I is best at absorbing a wavelength of 700 nm The two photosystems work together to use light energy to generate ATP and NADPH

Non Cyclic LE 10-13_5 ADP Electron Transport chain Primary acceptor H2O CO2 Light NADP+ ADP LIGHT REACTIONS CALVIN CYCLE ATP NADPH O2 Electron Transport chain [CH2O] (sugar) Primary acceptor Primary acceptor Electron transport chain Fd Pq e– e– H2O e– e– NADP+ Cytochrome complex 2 H+ NADP+ reductase + 2 H+ + 1/2 O2 NADPH Pc e– + H+ Energy of electrons e– P700 Light P680 Light ATP Photosystem I (PS I) Photosystem II (PS II)

NADPH Mill makes ATP Photosystem II Photosystem I LE 10-14 e– ATP e– Photon e– Photon Photosystem II Photosystem I

Noncyclic Electron Flow During the light reactions, there are two possible routes for electron flow: cyclic and noncyclic Noncyclic electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH

Primary acceptor Primary Fd acceptor Fd NADP+ Pq NADP+ reductase LE 10-15 Primary acceptor Primary acceptor Fd Fd NADP+ Pq NADP+ reductase Cytochrome complex NADPH Pc Photosystem I ATP Photosystem II

Cyclic Electron Flow Cyclic electron flow uses only photosystem I and produces only ATP Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle

LE 10-16 Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST Diffusion Intermembrane space Thylakoid space Electron transport chain Membrane Key ATP synthase Matrix Stroma Higher [H+] Lower [H+] ADP + P i ATP H+

A Comparison of Chemiosmosis in Chloroplasts and Mitochondria Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP The spatial organization of chemiosmosis differs in chloroplasts and mitochondria

LE 10-17 STROMA (Low H+ concentration) Cytochrome complex H2O CO2 Light NADP+ ADP LIGHT REACTIONS CALVIN CYCLE ATP NADPH STROMA (Low H+ concentration) O2 [CH2O] (sugar) Cytochrome complex Photosystem II Photosystem I Light NADP+ reductase Light 2 H+ Fd NADP+ + 2H+ NADPH + H+ Pq Pc H2O THYLAKOID SPACE (High H+ concentration) 1/2 O2 +2 H+ 2 H+ To Calvin cycle Thylakoid membrane ATP synthase STROMA (Low H+ concentration) ADP + ATP P i H+

Water is split by photosystem II on the side of the membrane facing the thylakoid space The diffusion of H+ from the thylakoid space back to the stroma powers ATP synthase ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place

The Calvin cycle has three phases: Carbon fixation (catalyzed by rubisco) Reduction Regeneration of the CO2 acceptor (RuBP)

LE 10-18_3 Input 3 (Entering one at a time) CO2 H2O CO2 Input Light 3 (Entering one at a time) NADP+ ADP CO2 LIGHT REACTIONS CALVIN CYCLE ATP Phase 1: Carbon fixation NADPH Rubisco O2 [CH2O] (sugar) 3 P P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 ATP 6 ADP 3 ADP CALVIN CYCLE 3 6 P P ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP+ 6 P i 5 P G3P 6 P Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction 1 P G3P (a sugar) Glucose and other organic compounds Output

The Calvin cycle uses ATP and NADPH to convert CO2 to sugar The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P) For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2

Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis The closing of stomata reduces access to CO2 and causes O2 to build up These conditions favor a seemingly wasteful process called photorespiration

Photorespiration: An Evolutionary Relic? In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound In photorespiration, rubisco adds O2 to the Calvin cycle instead of CO2 Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

C4 leaf anatomy and pathway Mesophyll cell Mesophyll cell CO2 Photosynthetic cells of C4 plant leaf PEP carboxylase Bundle- sheath cell The C4 pathway Oxaloacetate (4 C) PEP (3 C) Vein (vascular tissue) ADP Malate (4 C) ATP C4 leaf anatomy Pyruvate (3 C) Bundle- sheath cell Stoma CO2 CALVIN CYCLE Sugar Vascular tissue

C4 Plants C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

CAM Plants CAM plants open their stomata at night, incorporating CO2 into organic acids Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle

LE 10-20 Sugarcane Pineapple C4 CAM CO2 CO2 Mesophyll cell CO2 incorporated into four-carbon organic acids (carbon fixation) Night Organic acid Organic acid Bundle- sheath cell CO2 CO2 Day Organic acids release CO2 to Calvin cycle CALVIN CYCLE CALVIN CYCLE Sugar Sugar Spatial separation of steps Temporal separation of steps

The Importance of Photosynthesis: A Review The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells In addition to food production, photosynthesis produces the oxygen in our atmosphere