PowerLecture: Chapter 7 Where It Starts - Photosynthesis

Sunlight and Survival p.107

Photons Packets of light energy Each type of photon has fixed amount of energy

Visible Light Violet (380 nm) to red (750 nm) Longer wavelengths, lower energy Figure 7-2 Page 108

Visible Light Fig. 7-2, p.108 shortest wavelengths (most energetic) range of most radiation reaching Earth’s surface range of heat escaping from Earth’s surface longest wavelengths (lowest energy) gamma rays x rays ultraviolet radiation near-infrared radiation infrared radiation radio waves microwaves VISIBLE LIGHT 400 450 500 550 600 650 700 Wavelengths of light (nanometers) Fig. 7-2, p.108

Pigments Light-catching part of molecule often has alternating single and double bonds Electrons in these bonds move to higher energy levels by absorbing light

Variety of Pigments Chlorophylls a and b Carotenoids Anthocyanins Phycobilins

Wavelength absorption (%) Wavelength (nanometers) Chlorophylls Main pigments in most photoautotrophs Wavelength absorption (%) chlorophyll a chlorophyll b Wavelength (nanometers)

Accessory Pigments Carotenoids, Phycobilins, Anthocyanins beta-carotene phycoerythrin (a phycobilin) percent of wavelengths absorbed wavelengths (nanometers)

Pigments Fig. 7-3a, p.109

Pigments Fig. 7-3b, p.109

Pigments Fig. 7-3c, p.109

Pigments Fig. 7-3d, p.109

Pigments Fig. 7-3e, p.109

Pigments in Photosynthesis Bacteria Pigments in plasma membranes Plants Pigments and proteins organized into photosystems embedded in thylakoid membrane

T.E. Englemann’s Experiment Background Certain bacteria move toward areas of high oxygen concentration

T.E. Englemann’s Experiment

Photosynthesis Equation LIGHT ENERGY 12H2O + 6CO2 6O2 + C6H12O6 + 6H2O Water Carbon Dioxide Oxygen Glucose Water In-text figure Page 111

Photosynthesis Fig. 7-6a, p.111

Photosynthesis two outer membranes thylakoid compartment thylakoid membrane system inside stroma stroma Fig. 7-6b, p.111

light-dependant reactions light-independant reactions Photosynthesis SUNLIGHT H2O O2 CO2 NADPH, ATP light-dependant reactions light-independant reactions NADP+, ADP sugars CHLOROPLAST Fig. 7-6c, p.111

Light-Dependent Reactions Pigments absorb light energy, give up e-, which enter electron transfer chains Water molecules split, ATP and NADPH form, and oxygen is released Pigments that gave up e- ‘s get replacement e- ‘s

Light-Dependent Reactions photon Photosystem Light-Harvesting Complex Fig. 7-7, p.112

cross-section through a disk-shaped fold in the thylakoid membrane LIGHT- HARVESTING COMPLEX PHOTOSYSTEM II sunlight PHOTOSYSTEM I H+ NADPH e- e- e- e- e- e- NADP + + H+ e- H2O H+ H+ H+ H+ thylakoid compartment H+ H+ H+ H+ H+ H+ O2 H+ thylakoid membrane stroma ADP + Pi ATP cross-section through a disk-shaped fold in the thylakoid membrane H+ Fig. 7-8, p.113

Photosystem Function: Harvester Pigments Most pigments in photosystem are harvester pigments When excited by light energy, these pigments transfer energy to adjacent pigment molecules Each transfer involves energy loss

Photosystem Function: Reaction Center Energy is reduced to level that can be captured by molecule of chlorophyll a This molecule (P700 or P680) is the reaction center of a photosystem Reaction center accepts energy and donates electron to acceptor molecule

Electron Transfer Chain Adjacent to photosystem Acceptor molecule donates electrons from reaction center

Noncyclic Electron Flow Two-step pathway for light absorption and electron excitation Uses two photosystems: type I and type II Produces ATP and NADPH Involves photolysis (splitting of water)

Machinery of Noncyclic Electron Flow H2O second electron transfer chain photolysis e– e– ATP SYNTHASE first electron transfer chain NADP+ NADPH ATP PHOTOSYSTEM II PHOTOSYSTEM I ADP + Pi

Cyclic Electron Flow Happens when excess NADPH backs up the Photosystem II Electrons are donated by P700 in photosystem I to acceptor molecule flow through electron transfer chain and back to P700 No NADPH is formed

Chemiosmotic Model of ATP Formation Electrical and H+ concentration gradients are created between thylakoid compartment and stroma H+ flows down gradients into stroma through ATP synthase Flow of ions drives formation of ATP

Chemiosmotic Model for ATP Formation H+ is shunted across membrane by some components of the first electron transfer chain Gradients propel H+ through ATP synthases; ATP forms by phosphate-group transfer Photolysis in the thylakoid compartment splits water H2O e– acceptor ATP SYNTHASE ATP ADP + Pi PHOTOSYSTEM II

Light-Independent Reactions “Sugar Factory” part of photosynthesis Take place in the stroma Includes the Calvin-Benson cycle

THESE REACTIONS PROCEED IN THE CHLOROPLAST’S STROMA Calvin- Benson Cycle THESE REACTIONS PROCEED IN THE CHLOROPLAST’S STROMA Fig. 7-10a, p.115

Calvin-Benson Cycle Overall reactants Overall products Carbon dioxide ATP NADPH Overall products Glucose ADP NADP+ Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated

unstable intermediate 6 CO2 (from the air) Calvin- Benson Cycle CARBON FIXATION 6 6 RuBP unstable intermediate 12 PGA 6 ADP 12 ATP 6 ATP 12 NADPH 4 Pi 12 ADP 12 Pi 12 NADP+ 10 PGAL 12 PGAL 2 PGAL Pi P glucose

phosphorylated glucose Calvin- Benson Cycle 6CO2 ATP 6 RuBP 12 PGA 12 6 ADP 12 ADP + Calvin-Benson cycle 12 Pi ATP 12 NADPH 4 Pi 12 NADP+ 10 PGAL 12 PGAL 1 Pi 1 phosphorylated glucose Fig. 7-10b, p.115

The C3 Pathway In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA Because the first intermediate has three carbons, this is called the C3 pathway

Hot Dry days & C3 Plants On hot, dry days stomata close Inside leaf Oxygen levels rise Carbon dioxide levels drop Rubisco attaches RuBP to oxygen instead of carbon dioxide (this is photorespiration) Only one PGAL forms instead of two - slowing sugar formation

C3 Plants Fig. 7-11a1, p.116

C3 Plants upper epidermis palisade mesophyll spongy mesophyll lower stoma leaf vein air space Basswood leaf, cross-section. Fig. 7-11a2, p.116

Twelve turns of the cycle, not just six, to C3 Plants Stomata closed: CO2 can’t get in; O2 can’t get out Rubisco fixes oxygen, not carbon, in mesophyll cells in leaf RuBP 6 PGA + 6 glycolate Calvin-Benson Cycle 5 PGAL 6 PGAL CO2 + water 1 PGAL Twelve turns of the cycle, not just six, to make one 6-carbon sugar Fig. 7-11a3, p.117

C4 Plants Fig. 7-11b1, p.117

C4 Plants in hot dry weather Carbon dioxide is fixed twice In mesophyll cells, carbon dioxide is fixed to form four-carbon oxaloacetate Oxaloacetate is transferred to bundle-sheath cells Carbon dioxide is released and fixed again in Calvin-Benson cycle

C4 Plants upper epidermis mesophyll cell bundle- sheath cell lower Corn leaf, cross-section. Fig. 7-11b2, p.117

C4 Plants Stomata closed: CO2 can’t get in; O2 can’t get out Carbon fixed in the mesophyll cell, malate diffuses into adjacent bundle-sheath cell PEP oxaloacetate C4 Plants C4 cycle malate pyruvate CO2 In bundle-sheath cell, malate gets converted to pyruvate with release of CO2, which enters Calvin-Benson cycle RuBP 12 PGA…? Calvin-Benson Cycle 10 PGAL 12 PGAL 2 PGAL 1 sugar Fig. 7-11b3, p.117

CAM Plants Carbon is fixed twice (in same cells) Night Day Carbon dioxide is fixed to form organic acids Day Carbon dioxide is released and fixed in Calvin-Benson cycle

CAM Plants Fig. 7-11c1, p.117

CAM Plants stoma epidermis with thick cuticle mesophyll cell air space Fig. 7-11c2, p.117

Stomata stay closed during day, open for CO2 uptake at night only. CAM Plants Stomata stay closed during day, open for CO2 uptake at night only. C4 cycle operates at night when CO2 from aerobic respiration fixed C4 CYCLE CO2 that accumulated overnight used in C3 cycle during the day Calvin-Benson Cycle 1 sugar Fig. 7-11c3, p.117

Photosynthesis overview sunlight Light- Dependent Reactions 12H2O 6O2 ADP + Pi ATP NADPH NADP+ 6CO2 Calvin-Benson cycle 6 RuBP 12 PGAL Light- Independent Reactions 6H2O phosphorylated glucose end products (e.g., sucrose, starch, cellulose) Fig. 7-14, p.120

Carbon and Energy Sources glucose (stored energy, building blocks) sunlight energy Photosynthesis Aerobic Respiration 1. H2O is split by light energy. Its oxygen diffuses away; its electrons, hydrogen enter transfer chains with roles in ATP formation. Coenzymes pick up the electrons and hydrogen 1. Glucose is broken down completely to carbon dioxide and water. Coenzymes pick up the electrons, hydrogens. oxygen 2. The coenzymes give up the electrons and hydrogen atoms to oxygen-requiring transfer chains that have roles in forming many ATP molecules. 2. ATP energy drives the synthesis of glucose from hydrogen and electrons (delivered by coenzymes), plus carbon and oxygen (from carbon dioxide). carbon dioxide,water ATP available to drive nearly all cellular tasks Fig. 7-12, p.118

Fig. 7-16b, p.121