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Chapter 7: PHOTOSYNTHESIS

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Chapter 7: PHOTOSYNTHESIS Chapter 7: PHOTOSYNTHESIS 1. Overview of Photosynthesis 2. The “Light” Reactions 3. The “Dark” Reactions 1. Overview of Photosynthesis What is Photosynthesis? The process of converting light energy (kinetic) into energy stored in the covalent bonds of glucose molecules (potential). Light energy 6 CO2 + 6 H2O C6H12O6 + 6 O2 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 (O2) Photosynthesis vs Respiration essentially the reverse of each other Photosynthesis occurs in Chloroplasts Leaf Cross Section Mesophyll Cell Leaf Chloroplast CO2 O2 Chloroplast The Chloroplast outer membrane inner membrane stroma thylakoid granum Photosynthesis consists of 2 sets of Reactions The light-dependent or “Light” Reactions: H2O CO2 • convert sunlight Chloroplast energy into Light chemical energy NADP+ (stored in ATP & ADP +P NADPH) CALVIN LIGHT CYCLE REACTIONS (in stroma) “Dark” Reactions (in thylakoids) ATP E (Calvin cycle): lec tro ns NADPH • use chemical energy from light reactions O Sugar to make glucose 2. Light-dependent (“Light”) Reactions Light Reactions occur in Thylakoids A variety of light-absorbing pigments & electron transport proteins are embedded within the thylakoid membrane The Pigments absorb “Visible” Light Chlorophyll a & b: • the major pigments (absorb red, blue…, reflect green) Carotenoids (e.g., β-carotene) • accessory pigments (absorb green, blue, reflect red, yellow) Absorption Range for each Pigment blue red Chlorophyll absorbs “non-green” light energy Light Reflected light • green light passes on through or is reflected, causing the leaves to appear green Absorbed Chloroplast light Transmitted light + - H2O ½ O2 + 2 H + 2 *e 1 PS I PS II 2 e- transport chain (ETC) pumps H+ into thylakoid 2 e- to PS II 4 NADPH PS I 3 ATP Synthase uses H+ flow to make ATP Light Energy absorbed by Pigments Fuels 4 General Steps of the “Light Reactions”: + - - 1) H2O split to O, 2 H & 2 high energy e (*e ) in PS II sunlight + - H2O O2 + 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) Analogy of e– Light reactions ATP e– e– NADPH e– e– e– Mill n o t makes o h ATP P e– n o t o h P Photosystem II Photosystem I Chloroplast Summary of the “Light” Reactions Stroma (low H+ concentration) H+ Light Light H+ ADP + P ATP H+ NADP+ + H+ NADPH H+ Thylakoid membrane H+ H+ H2O 1 + H + + O + 2 H+ H+ + H H 2 H H+ 2 H+ Photosystem II Electron Photosystem I ATP synthase transport chain H+ Thylakoid space (high H+ concentration) 3. Light-independent (“Dark”) Reactions The “Dark” Reactions A series of reactions called the Calvin cycle that synthesize glucose from CO2 and H2O: ATP, NADPH CO2 + H2O C6H12O6 (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 “Dark” Reactions Involves an anabolic pathway known as the Calvin cycle: Calvin cycle • endergonic reactions of this pathway are fueled by ATP & NADPH from the “light” reactions Don’t • resulting sugars can be memorize used as a source of this!! energy or to build other organic molecules Summary of Photosynthesis stroma Key Terms for Chapter 7 • photoautotroph • chloroplast, thylakoid, stroma • chlorophyll, carotenoids • ATP, NADPH • electron transport chain (ETC) • ATP synthase • Light reactions, dark reactions, Calvin cycle Relevant Review Questions: 1-6, 8-10, 12.
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