Chapter 10 Photosynthesis

main idea: making glucose autotroph – self-feeder; -organism which makes its own food a) phototrophic – uses light b) chemotrophic – uses chemicals from its environment

Photosynthesis overview A. Photophase (Light reactions) – occur in grana -light dependent B. Synthetic phase (Dark reactions) – occur in stroma -light independent

Light reactions Sunlight – form of electromagnetic energy (radiation) that travels in waves Wavelength range is electromagnetic spectrum, part of which we see as colors of visible light (ROY G BIV) at 400 – 700 nm Particles of light are photons (or quanta) Amount of energy to wavelength is inversely related ex: short wavelength = greater energy of photon violet has 2x greater energy than red

Pigments – substances that absorb visible light; color seen is most reflected blue & red are absorbed by chlorophyll (which reflects green) Pigment’s absorption ability is measured quantitatively by spectrophotometer

Splitting of water 1 st thought that CO 2 was split 1930’s vanNiel studying photosynthesis in bacteria found these use H 2 S and removed H to make sugar, therefore, all phototrophs need an H compound & plants must split water, not CO 2

Van Niel’s experiment 1950’s experiment conducted using heavy oxygen 18 O CO 2 + H 2 O O 2 Respiration yields 686 kcal of free energy per mole (the exact amount required to reduce CO 2 to glucose)

Photooxidation of chlorophyll Pigments go from ground state to excited state when photons boost energy to higher levels (excited state is unstable) so energy is quickly passed or released as heat or light in some cases (fluorescence)

Photosystems primary electron acceptor – traps high- energy electrons that have absorbed photons; this energy stored powers ATP & NADPH synthesis accessory pigments clustered in antenna complex of several hundred molecules around a chlorophyll a at the reaction center

Photosystems Antenna complex, reaction center chlorophyll, & primary electron acceptor make up a photosystem (light harvesting unit built into the thylakoid membrane) 2 kinds : Photosystem I – contains p700 absorbs best at 700 nm (far red) Photosystem II – contains p680 absorbs best at 680 nm (red) *p700 & p680 are identical chlorophyll a molecules, but associated with different proteins

ATP formation ATP synthetases form ATP when thylakoid membranes pump protons from stroma to thylakoid compartment

Calvin cycle Cyclic metabolic pathway in stroma Uses ATP & NADPH from light reactions to reduce CO 2 to sugar CO 2 combines with RuBP (5-C sugar) to make a 3-C sugar (G3P) ultimately using ATP & electrons from NADPH

Calvin cycle steps Step 1: Carbon fixation -enzyme catalyzing RuBP is RuBP carboxylase (rubisco) -6-C intermediate splits into 2 3-PGA (each 3 CO 2 entering, 6 ATP used) Step 2: Reduction -NADPH 2 donates a pair of high energy electrons to form sugar (G3P) -2 G3P’s can be rapidly converted to one glucose molecule (18 ATP’s & 12 NADPH 2 )

Calvin cycle Step 3: Regeneration of CO 2 acceptor (RuBP) -5 molecules of G3P are rearranged into 3 molecules of RuBP

Summary For 1 G3P, 9 molecules of ATP & 6 molecules of NADPH Light reactions regenerate ATP & NADPH G3P from the Calvin cycle is the material used to synthesize other organic compounds such as glucose & other carbohydrates

Alternatives for carbon fixation photorespiration – -rubisco adds O 2 to the Calvin cycle instead of CO 2 -decreases photosynthetic output by siphoning organic materials from the Calvin cycle

Alternatives for carbon fixation C 4 plants: -preface Calvin cycle by fixing carbon into a 4-C compound (C 4 ) ex: sugar cane & corn -have structural adaptations (bundle sheath cells) -CO 2 is incorporated into organic compounds in mesophyll then they’re exported to bundle sheath cells (spatial separation) -PEP carboxylase adds CO 2 to PEP (phosphoenolpyruvate)

Alternative for carbon fixation CAM plants: (Crassulacean acid metabolism) -evolved in succulents as an adaptation to arid environments ex: cacti & pineapples -incorporate carbon into intermediate compounds during the night & when stomata close during the day, those compounds fuel the Calvin cycle – these steps occur at different times (temporal)