Ch 10: Photosynthesis
Photosynthesis
 Anabolic Pathway (endergonic)
 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
o Solar energy to chemical energy
 Autotrophs – Plants, Bacteria, Protists
 CO2 is reduced because it gains electrons (from hydrogen)
 H2O is oxidized because its electrons (hydrogen) are taken
Solar Radiation
 Light is electromagnetic radiation
 Travels in rhythmic waves
o Wavelength is distance b/w crests of waves
o Shorter wavelength = Higher energy
 Photon – discrete unit of light that behaves as a particle
Pigments
 Compounds that absorb light energy
 Chlorophyll a – main pigment in plants
 Accessory Pigments:
o Chlorophyll b
o Carotenoids (~600) – harness light energy & protect from photodamage
 Carotenes
 Xanthophylls
Chloroplast Structure
 Two membranes w/ space in b/w
 Stroma (NOT stoma) – fluid w/ DNA, ribosomes, enzymes
 Thylakoids – membranous ‘discs’ (third membrane)
o Photosystems & ATP Synthase
 Granum – stacks of thylakoids
Photosynthesis Overview
 Light Dependent Rxns (thylakoid membrane)
o Absorb visible light
o Split water & take electrons (oxidized)
o Create ATP, NADPH, & O2
 Light Independent Rxns (stroma)
o Carbon fixation – inorganic CO2 made into carbohydrates
o Uses ATP & NADPH
 Light Dependent Reactions
o Photosystems II & I found in thylakoid membrane
o Protein complex w/ a reaction center & 250-400 pigments
o PS I was found before PS II, but is second in order of action
o Photosystem II
 Photon energy absorbed by pigments & passed until it reaches reaction center
 Special dimer of Chlorophyll a (P680)
 Dimer’s electrons excited & passed (now P680+) to primary electron acceptor
 Pheophytin is reduced
 P680+ very strong oxidizing agent
 Water is split by enzyme, electrons passed to P680 (reduced), O2 released
o Linear Electron Flow
 Pheophytin passes electrons down an ETC & energy used to pump H+ into thylakoid lumen
 H+ moves across ATP Synthase into stroma (photophosphorylation)
 PS I goes thru similar events as PS II
 P700 excited, electrons passed to primary electron acceptor
 End w/ NADP+ forming NADPH
 P700+ gains electron from ETC from PS II
o
Cyclic Photophosphorylation
 Uses PS I only
 Primary electron acceptor can also feed electrons back into ETC in b/w PS II & PS I (instead
of NADP+)
 Builds proton gradient
 Creates ATP only
 No Oxygen or NADPH produced
 Light Independent Reactions
o In stroma, ATP & NADPH are used to create sugars out of CO2
o Calvin Cycle – complex cycle of reactions by which sugars created
 Glyceraldehyde-3-phosphate (G3P) is produced
 Can be linked to form bigger carbs
o Three Phases
 Carbon Fixation
 CO2 (C atom) put on ribulose bisphosphate (RuBP; 5C)
o Catalyzed by Rubisco
o 3 CO2 fixed to 3 RuBP to complete one turn of cycle (18 total C)
 6C product splits into two 3C molecules
o Six 3C molecules made in total
 Reduction
 6 ATP & 6 NADPH used to reduce 3C molecules & create G3P
 One ‘leaves’ cycle as net gain
 Regeneration of CO2 acceptor
 3 ATP used to turn leftover G3P back into RuBP to restart cycle
o Net energy input – 9 ATP & 6 NADPH
Water Problems
 If dry, plants will close stomata
 Conserves H2O, but reduces CO2, & O2 builds up
 Rubisco can bind O2 or CO2
o Photorespiration – rubisco adds O2 to RuBP
 CO2 is final breakdown product
 No ATP or sugar made
 C4 Plants
o Minimize photorespiration by making CO2 into oxaloacetate (4C) in mesophyll
o Catalyzed by PEP carboxylase
 High CO2 affinity, so even works at low concentrations
o 4 C compounds moved to bundle sheath cells, where CO2 released to Calvin cycle
 CAM Plants
o Crassulacean acid metabolism
o Open stomata at night
 CO2 incorporated into organic acids
o Stomata close in day & CO2 released to Calvin Cycle