How Cells Acquire Energy Chapter 7

Photoautotrophs Capture sunlight energy and use it to carry out photosynthesis Plants Some bacteria Many protistans

Figure 10.2 Photoautotrophs (a) Plants Figure 10.2 Photoautotrophs (c) Unicellular protist 10 µm (e) Purple sulfur bacteria 1.5 µm (b) Multicellular alga (d) Cyanobacteria 40 µm

Linked Processes Photosynthesis Aerobic Respiration Energy-storing pathway Releases oxygen Requires carbon dioxide Aerobic Respiration Energy-releasing pathway Requires oxygen Releases carbon dioxide

Figure 10.3 Zooming in on the location of photosynthesis in a plant Leaf cross section Vein Mesophyll Stomata CO2 O2 Chloroplast Mesophyll cell Outer membrane Figure 10.3 Zooming in on the location of photosynthesis in a plant Thylakoid Intermembrane space 5 µm Stroma Granum Thylakoid space Inner membrane 1 µm

Chloroplast Structure two outer membranes stroma inner membrane system (thylakoids connected by channels) Figure 7.3d, Page 116

Photosynthesis Equation LIGHT ENERGY 12H2O + 6CO2 6O2 + C2H12O6 + 6H2O Water Carbon Dioxide Oxygen Glucose Water In-text figure Page 115

Where Atoms End Up Reactants 12H2O 6CO2 Products 6O2 C6H12O6 6H2O In-text figure Page 116

Two Stages of Photosynthesis sunlight water uptake carbon dioxide uptake ATP LIGHT-DEPENDENT REACTIONS ADP + Pi LIGHT-INDEPENDENT REACTIONS NADPH NADP+ P glucose oxygen release new water In-text figure Page 117

Electromagnetic Spectrum Shortest Gamma rays wavelength X-rays UV radiation Visible light Infrared radiation Microwaves Longest Radio waves wavelength

Visible Light Wavelengths humans perceive as different colors Violet (380 nm) to red (750 nm) Longer wavelengths, lower energy Figure 7.5a Page 118

Photons Packets of light energy Each type of photon has fixed amount of energy Photons having most energy travel as shortest wavelength (blue-violet light)

Pigments Color you see is the wavelengths not absorbed Light-catching part of molecule often has alternating single and double bonds These bonds contain electrons that are capable of being moved to higher energy levels by absorbing light

Light Reflected light Chloroplast Absorbed Granum light Transmitted Fig. 10-7 Light Reflected light Chloroplast Figure 10.7 Why leaves are green: interaction of light with chloroplasts Absorbed light Granum Transmitted light

Variety of Pigments Chlorophylls a and b Carotenoids Xanthophils

Wavelength absorption (%) Wavelength (nanometers) Chlorophylls Main pigments in most photoautotrophs Wavelength absorption (%) chlorophyll a chlorophyll b Wavelength (nanometers) Figure 7.6a Page 119 Figure 7.7 Page 120

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

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

Arrangement of Photosystems water-splitting complex thylakoid compartment H2O 2H + 1/2O2 P680 P700 acceptor acceptor pool of electron carriers PHOTOSYSTEM II stroma PHOTOSYSTEM I Figure 7.10 Page 121

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

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

(INTERIOR OF THYLAKOID) Fig. 10-12 Photosystem STROMA Photon Primary electron acceptor Light-harvesting complexes Reaction-center complex e– Thylakoid membrane Figure 10.12 How a photosystem harvests light Pigment molecules Transfer of energy Special pair of chlorophyll a molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)

Electron Transfer Chain Adjacent to photosystem Acceptor molecule donates electrons from reaction center As electrons pass along chain, energy they release is used to produce ATP

Cyclic Electron Flow Electrons Electron flow drives ATP formation are donated by P700 in photosystem I to acceptor molecule flow through electron transfer chain and back to P700 Electron flow drives ATP formation No NADPH is formed

Primary acceptor Primary Fd acceptor Fd NADP+ Pq + H+ NADP+ reductase Fig. 10-15 Primary acceptor Primary acceptor Fd Fd NADP+ + H+ Pq NADP+ reductase Cytochrome complex NADPH Pc Figure 10.15 Cyclic electron flow Photosystem I Photosystem II ATP

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 Figure 7.13a Page 123

Electron transport chain Fig. 10-13-5 Electron transport chain Primary acceptor Primary acceptor 4 7 Electron transport chain Fd Pq e– 2 e– 8 e– H2O e– NADP+ + H+ Cytochrome complex 2 H+ NADP+ reductase + 3 1/2 O2 NADPH Pc e– e– P700 5 P680 Light 1 Light 6 6 ATP Figure 10.13 How linear electron flow during the light reactions generates ATP and NADPH Pigment molecules Photosystem I (PS I) Photosystem II (PS II)

Potential to transfer energy (volts) Energy Changes second transfer chain e– NADPH first e– transfer chain Potential to transfer energy (volts) e– e– (Photosystem I) (Photosystem II) H2O 1/2O2 + 2H+ Figure 7.13b Page 123

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 synthesis 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 Figure 7.15 Page 124

Fig. 10-17 STROMA (low H+ concentration) Cytochrome complex Photosystem II Photosystem I 4 H+ Light NADP+ reductase Light Fd 3 NADP+ + H+ Pq NADPH e– Pc e– 2 H2O 1 1/2 O2 THYLAKOID SPACE (high H+ concentration) +2 H+ 4 H+ To Calvin Cycle Figure 10.17 The light reactions and chemiosmosis: the organization of the thylakoid membrane Thylakoid membrane ATP synthase STROMA (low H+ concentration) ADP + ATP P i H+

Light-Independent Reactions Synthesis part of photosynthesis Can proceed in the dark Take place in the stroma Calvin-Benson cycle

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 Figure 7.16 Page 125 P glucose

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

Photorespiration in 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 Only one PGAL forms instead of two

C3 and C4 plant structure compared

C4 Plants 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

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

i CO2 Light NADP+ ADP Calvin Cycle Light Reactions ATP NADPH Fig. 10-5-4 H2O CO2 Light NADP+ ADP + P i Calvin Cycle Light Reactions ATP Figure 10.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle NADPH Chloroplast [CH2O] (sugar) O2

Satellite Images Show Photosynthesis Atlantic Ocean  Photosynthetic activity in spring Figure 7.20 Page 128