Chapter 10: PHOTOSYNTHESIS 1. Overview of Photosynthesis 2. Light Absorption 3. The Light Reactions 4. The Calvin Cycle
1. Overview of Photosynthesis Chapter Reading pp. 185-190, 206-207
What is Photosynthesis? The process of converting light energy (kinetic) into energy stored in the covalent bonds of glucose molecules (potential). 6 CO 2 + 6 H 2 O Light energy C 6 H 12 O 6 + 6 O 2 Carbon dioxide Water PHOTOSYNTHESIS Glucose Oxygen gas carried out by photoautotrophs plants, phytoplankton, cyanobacteria (any photosynthetic organism) the basis of almost all ecosystems all food energy ultimately comes from the sun source of all atmospheric oxygen (O 2 )
Photosynthetic Organisms (a) Plants (c) Unicellular protist 10 µm (e) Purple sulfur bacteria 1.5 µm (b) Multicellular alga (d) Cyanobacteria 40 µm
Photosynthesis occurs in Chloroplasts Leaf cross section Vein Mesophyll Stomata CO 2 O 2 Chloroplast Mesophyll cell 5 µm
Chloroplast Chloroplast Structure Stroma Granum Thylakoid Thylakoid space Inner membrane Outer membrane Intermembrane space 1 µm
The Fate of Atoms Involved in Photosynthesis Reactants: 6 CO 2 12 H 2 O Products: C 6 H 12 O 6 6 H 2 O 6 O 2 Revealed by experiments involving radioactive isotopes in key molecules: 14 C in CO 2 and 18 O in H 2 O and CO 2
Two Stages of Photosynthesis H 2 O CO 2 Light Light Reactions NADP + ADP + P i ATP NADPH Calvin Cycle Chloroplast O 2 [CH 2 O] (sugar)
2. Light Absorption Chapter Reading pp. 190-193
The Electromagnetic Spectrum 10 5 nm 10 3 nm 1 nm 10 3 nm 10 6 nm 1 m (10 9 nm) 10 3 m Gamma rays X-rays UV Infrared Microwaves Radio waves Visible light 380 450 500 550 600 650 700 750 nm Shorter wavelength Higher energy Longer wavelength Lower energy
Chlorophyll absorbs non-green light Chloroplast Light Reflected light Green light passes on through or is reflected, causing the leaves to appear green Absorbed light Transmitted light Granum only wavelengths with exact amount of energy to excite an e - to a higher orbital are absorbed
Spectrophotometry TECHNIQUE White light 1 Refracting prism Chlorophyll Photoelectric solution tube 2 3 4 Galvanometer Spectrophotometers measure the amount of light passing through a sample: Slit moves to pass light of selected wavelength Green light The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. measures % absorbance OR Blue light The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light. measures % transmittance
Light-Absorbing Pigments Chlorophyll a & b, and carotenoids RESULTS Chlorophyll a Chlorophyll b Carotenoids CH 3 in chlorophyll a CHOin chlorophyll b Porphyrin ring: light-absorbing head of molecule; note magnesium atom at center (a) Absorption spectra 400 500 600 700 Wavelength of light (nm) (b) Action spectrum Aerobic bacteria Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown (c) Engelmann s experiment 400 500 Filament of alga 600 700
Energy of electron Electrons absorb Photons electrons excited to higher energy orbitals e Excited state Heat Photon Chlorophyll molecule Photon (fluorescence) Ground state (a) Excitation of isolated chlorophyll molecule (b) Fluorescence
3. The Light Reactions Chapter Reading pp. 194-199
Thylakoid membrane Photosystems Photon Light-harvesting complexes Photosystem Reaction-center complex STROMA Primary electron acceptor Each photosystem in the thylakoid membrane consists of: e an array of lightabsorbing pigments Transfer of energy Special pair of chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) a reaction center containing 2 molecules of chlorophyll a and a primary e - acceptor
The Light Reactions Produces ATP (chemical energy) & NADPH (reducing power). 2 H + + 1 / 2 O 2 3 H 2 O e e Primary acceptor e 2 P680 Pq 4 Cytochrome complex 5 Pc Primary acceptor e P700 Fd e e 7 Light 8 NADP + reductase NADP + + H + NADPH 1 Light 6 ATP Photosystem II (PS II) Pigment molecules Photosystem I (PS I)
H 2 O ½ O 2 + 2 H + + 2 *e - 1 PS I PS II 2 e - transport chain (ETC) pumps H + into thylakoid
PS II 4 2 e- to NADPH PS I 3 ATP Synthase uses H + flow to make ATP
4 Stages of the Light Reactions 1) H 2 O split to O, 2 H + & 2 high energy e - (*e - ) in PS II sunlight H 2 O O 2 + H + + *e - 2) Energy released by a series of *e - transfers is used to generate H + gradient H + accumulates inside the thylakoid membrane 3) H + gradient used to make ATP (chemiosmosis) 4) *e - re-energized in PS I, passed on to NADP + *e - ends up in NADPH (an electron carrier)
Electron Energy Levels ATP e e e e e e NADPH Mill makes ATP e Photosystem II Photosystem I
ATP in Respiration vs Photosynthesis Mitochondrion Chloroplast MITOCHONDRION STRUCTURE Intermembrane space Inner membrane Electron transport chain H + Diffusion CHLOROPLAST STRUCTURE Thylakoid space Thylakoid membrane Key Higher [H + ] Lower [H + ] Matrix ATP synthase ADP + P i H + ATP Stroma
Summary of the Light Reactions STROMA (low H + concentration) Light Photosystem II 4 H + Cytochrome complex Light Photosystem I Fd NADP + reductase 3 NADP + + H + Pq NADPH THYLAKOID SPACE (high H + concentration) H 2 O e e 1 1 / 2 O 2 +2 H + 2 4 H + Pc To Calvin Cycle STROMA (low H + concentration) Thylakoid membrane ATP synthase ADP + P i H + ATP
4. The Calvin Cycle Chapter Reading pp. 199-204
Overview of the Calvin Cycle A series of reactions called the Calvin cycle that synthesize glucose from CO 2 and H 2 O: ATP, NADPH CO 2 + H 2 O C 6 H 12 O 6 (glucose) uses energy stored in ATP and NADPH produced by the light reactions can occur in dark (doesn t require light directly) also occurs during daylight! takes place in the stroma of chloroplasts outside the thylakoids
The Calvin Input 3 (Entering one at a time) CO 2 Cycle Rubisco Phase 1: Carbon fixation 3 P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate 3-Phosphoglycerate (RuBP) P 6 6 ADP ATP 3 ATP 3 ADP Calvin Cycle 6 P P 1,3-Bisphosphoglycerate Phase 3: Regeneration of the CO 2 acceptor (RuBP) 5 P G3P 6 P Glyceraldehyde-3-phosphate (G3P) 6 6 NADP + 6 P i NADPH Phase 2: Reduction Output 1 P G3P (a sugar) Glucose and other organic compounds
C 4 Pathway helps retain H 2 O C 4 leaf anatomy The C 4 pathway Photosynthetic cells of C 4 plant leaf Mesophyll cell Bundlesheath cell Vein (vascular tissue) Mesophyll cell CO 2 PEP carboxylase Oxaloacetate (4C) Malate (4C) PEP (3C) ADP ATP Stoma Mechanism to store carbon from CO 2 in soluble form, allowing stomata closure during day to conserve water Bundlesheath cell CO 2 Pyruvate (3C) Sugar Vascular tissue Calvin Cycle
C 4 compared to CAM Crassulacean Acid Metabolism Sugarcane Mesophyll cell Bundlesheath cell C 4 Organic acid Calvin Cycle Sugar CO 2 1 Pineapple CO 2 incorporated into four-carbon organic acids (carbon fixation) Organic acids release CO 2 to Calvin cycle CAM Organic acid CO 2 CO 2 2 Calvin Cycle Sugar CO 2 Night Day a slightly different way to fix CO2 at night for use during the day without opening stomata (a) Spatial separation of steps (b) Temporal separation of steps
Summary of Photosynthesis H 2 O CO 2 Light Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain NADP + ADP + P i ATP NADPH RuBP 3-Phosphoglycerate Calvin Cycle G3P Starch (storage) Chloroplast O 2 Sucrose (export)
Key Terms for Chapter 10 photoautotroph chloroplast, thylakoid, stroma chlorophyll, carotenoids ATP, NADPH, photosystem, reaction center electron transport chain (ETC) ATP synthase Light reactions, Calvin cycle C 4 and CAM carbon fixation Relevant Chapter Questions 1-8, 10, 12