Photosynthesis

• The process of converting energy from sunlight to energy in chemical bonds.

• 6CO2 + 6H2O + light C6H12O6 + 6O2

• Light absorbing pigments:– Chlorophyll a (P680 and P700)– Chlorophyll b– Carotenoids (red, orange, or yellow)

• Photosystems: PS I [P700 ] and PS II [P680 ]

Noncyclic Photophosphorylation

• Process of making ATP from ADP + Pi using energy derived from light

• 7 steps:1) Photosystem II2) Primary electron acceptor3) Electron transport chain4) Phosphorylation5) Photosystem I6) NADPH7) Photolysis

Noncyclic Photophosphorylation

• Summary– Takes light energy and electrons from H2O to

make the energy-rich molecules ATP and NADPH

– Light-dependent reactions, or light reactions

Cyclic Photophosphorylation

• Electrons from PS I are “recycled”

• Electrons from PS I join with the Electron Transport Chain and generate ATP, returning to PS I

Calvin-Benson Cycle(Dark Reactions)

• Fixes CO2

• Takes inorganic CO2 and incorporates it into an organic molecule that can be used in biological systems

• Goal: produce a single molecule of glucose (C6H12O6)

• Must repeat 6 times, using 6 CO2 molecules

Calvin-Benson Cycle(Dark Reactions)

• Carboxylation: 6 CO2 combine with Ru BP to produce 12 PGA– Enzyme: RuBP carboxylase, or Ribisco

• Catalysis the rxn CO2 and RuBP (ribulose biphosphate)

• Reduction: 12 ATP and 12 NADPH are used to convert 12 PGA to 12 PGAL

• Regeneration: 6 ATP are used to convert 10 PGAL to 6 RuBP

• Carbohydrate synthesis

Calvin-Benson Cycle(Dark Reactions)

• Summary– 6CO2 + 18 ATP + 12 NADPH

18 ADP + 18 Pi + 12 NADP+ + 1 glucose

Chloroplast

• Structure– Outer membrane– Inner membrane– Stroma

• Enzymes for the Dark Rxns

– Thylakoid• Contain the light-absorbing pigments and enzymes for the

light rxns

– Thylakoid compartment– Granum

Chemiosmotic Theory

• Describes the mechanism by which ADP is phosphorylated to ATP

• Steps:

1) H+ ions (protons) accumulate inside the thylakoids

2) A pH and electrical gradient (electrochemical) across the thylakoid membrane is created

3) ATP synthase generate ATP

Photorespiration

• Ribisco has the ability to fix O2, as well as carbon dioxide

• Problems:– Decrease efficiency of CO2 fixation– Products formed react with RuBP do not lead to

useful, energy-rich molecules• Peroxisomes breakdown these products

C4 Photosynthesis

• PEP (phosphoenolpyruvate)

• OAA (oxaloacetate)

• PEP carbooxylase

• Malate

• Bundle sheath cells

• Stomata are partially open during the day

• Plants found in hot, dry climates

• Sugar cane and crab grasses

CAM Photosynthesis

• PEP carboxylase

• OAA

• Malic acid

• Stomata are closed during the day and OPEN at night

• Vacuole

• Plants found in hot, dry climates with cool nights (deserts)