
Photosynthesis How Plants Make Food from Sunlight and Low Energy Molecules
Carbon and Energy Sources • Photoautotrophs • Carbon source is carbon dioxide • Energy source is sunlight • Heterotrophs • Get carbon and energy by eating autotrophs or one another
Photoautotrophs • Capture sunlight energy and use it to carry out photosynthesis • Plants • Some bacteria • Many protistans
T.E. Englemann’s Experiment Background • Certain bacterial cells will move toward places where oxygen concentration is high • Photosynthesis produces oxygen
T.E. Englemann’s Experiment Hypothesis • Movement of bacteria can be used to determine optimal light wavelengths for photosynthesis
T.E. Englemann’s Experiment Method • Algal strand placed on microscope slide and illuminated by light of varying wavelengths • Oxygen-requiring bacteria placed on same slide
T.E. Englemann’s Experiment Results Bacteria congregated where red and violet wavelengths illuminated alga Conclusion Bacteria moved to where algal cells released more oxygen--areas illuminated by the most effective light for photosynthesis
Photosynthesis Energy-storing pathway Releases oxygen Requires carbon dioxide Aerobic Respiration Energy-releasing pathway Requires oxygen Releases carbon dioxide Linked Processes
Photosynthesis Equation LIGHT ENERGY 6O2 + C6H12O6 + 6H2O 12H2O + 6CO2 oxygen glucose water carbon dioxide water
Two Stages of Photosynthesis sunlight water uptake carbon dioxide uptake ATP ADP + Pi LIGHT DEPENDENT-REACTIONS LIGHT INDEPENDENT-REACTIONS NADPH NADP+ glucose P oxygen release new water
Sunlight Energy • Continual input of solar energy into Earth’s atmosphere • Almost 1/3 is reflected back into space • Of the energy that reaches Earth’s surface, about 1% is intercepted by photoautotrophs
Electromagnetic Spectrum Shortest Gamma rays wavelength X-rays UV radiation Visible light Infrared radiation Microwaves Longest Radio waves wavelength
Visible Light • Wavelengths humans perceive as different colors • Violet (380 nm) to red (750 nm) • Longer wavelengths, lower energy
Photons • Packets of light energy • Each type of photon has fixed amount of energy • Photons having most energy travel as shortest wavelength (blue-green light)
Pigments • Light-absorbing molecules • Absorb some wavelengths and transmit others • Color you see are the wavelengths NOT absorbed chlorophyll a chlorophyll b Wavelength (nanometers)
Pigments in Photosynthesis • Bacteria • Pigments in plasma membranes • Plants • Pigments embedded in thylakoid membrane system • Pigments and proteins organized into photosystems • Photosystems located next to electron transport systems
Pigments in a Photosystem reaction center (a specialized chlorophyll a molecule)
Light-Dependent Reactions • Pigments absorb light energy, give up e- which enter electron transport systems • Water molecules are split, ATP and NADH are formed, and oxygen is released • Pigments that gave up electrons get replacements
Photosystem Function: Harvester Pigments • Most pigments in photosystem are harvester pigments • When excited by light energy, these pigments transfer energy to adjacent pigment molecules • Each transfer involves energy loss
Photosystem Function: Reaction Center • Energy is reduced to level that can be captured by molecule of chlorophyll a • This molecule (P700 or P680) is the reaction center of a photosystem • Reaction center accepts energy and donates electron to acceptor molecule
Cyclic Electron Flow e– electron acceptor electron transport system e– e– ATP e–
Electron Transport System • Adjacent to photosystem • Acceptor molecule donates electrons from reaction center • As electrons flow through system, energy they release is used to produce ATP and, in some cases, NADPH
Cyclic Electron Flow • Electrons • are donated by P700 in photosystem I to acceptor molecule • flow through electron transport system and back to P700 • Electron flow drives ATP formation • No NADPH is formed
Energy Changes second transport system e– NADPH e– first transport system e– Potential to transfer energy (voids) e– (PHOTOSYSTEM I) (PHOTOSYSTEM II) 1/2 O2 + 2H+ H2O
Noncyclic Electron Flow • Two-step pathway for light absorption and electron excitation • Uses two photosystems: type I and type II • Produces ATP and NADPH • Involves photolysis - splitting of water
Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (Layer 1)
Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (Layer 2)
Light-Independent Reactions • Synthesis part of photosynthesis • Can proceed in the dark • Take place in the stroma • Calvin-Benson cycle
Overall reactants Carbon dioxide ATP NADPH Overall products Glucose ADP NADP+ Calvin-Benson Cycle Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated
Using the Products of Photosynthesis • Phosphorylated glucose is the building block for: • sucrose • The most easily transported plant carbohydrate • starch • The most common storage form
The C3 Pathway • In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA • Because the first intermediate has three carbons, the pathway is called the C3 pathway
Photorespiration in C3 Plants • On hot, dry days stomata close • Inside leaf • Oxygen levels rise • Carbon dioxide levels drop • Rubisco attaches RuBP to oxygen instead of carbon dioxide • Only one PGAL forms instead of two
C4 Plants • Carbon dioxide is fixed twice • In mesophyll cells, carbon dioxide is fixed to form four-carbon oxaloacetate • Oxaloacetate is transferred to bundle-sheath cells • Carbon dioxide is released and fixed again in Calvin-Benson cycle
CAM Plants • Carbon is fixed twice (in same cells) • Night • Carbon dioxide is fixed to form organic acids • Day • Carbon dioxide is released and fixed in Calvin-Benson cycle