Photosynthesis How Plants Make Food from Sunlight and Low Energy Molecules

Photoautotrophs

Photoautotrophs Photoautotrophs Carbon source is carbon dioxide Carbon source is carbon dioxide Energy source is sunlight Energy source is sunlight Heterotrophs Heterotrophs Get carbon and energy by eating autotrophs or one another Get carbon and energy by eating autotrophs or one another Carbon and Energy Sources

Photoautotrophs Capture sunlight energy and use it to carry out photosynthesis Capture sunlight energy and use it to carry out photosynthesis Plants Plants Some bacteria Some bacteria Many protistans Many protistans

T.E. Englemann’s Experiment Background Certain bacterial cells will move toward places where oxygen concentration is high Certain bacterial cells will move toward places where oxygen concentration is high Photosynthesis produces oxygen Photosynthesis produces oxygen

T.E. Englemann’s Experiment Hypothesis Movement of bacteria can be used to determine optimal light wavelengths for photosynthesis 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 Algal strand placed on microscope slide and illuminated by light of varying wavelengths Oxygen-requiring bacteria placed on same slide Oxygen-requiring bacteria placed on same slide

T.E. Englemann’s Experiment

Results Bacteria congregated where red and violet wavelengths illuminated alga Conclusion Conclusion Bacteria moved to where algal cells released more oxygen--areas illuminated by the most effective light for photosynthesis

Linked Processes Photosynthesis Energy-storing pathway Energy-storing pathway Releases oxygen Releases oxygen Requires carbon dioxide Requires carbon dioxide Aerobic Respiration Energy-releasing pathway Requires oxygen Releases carbon dioxide

Focusing in on the location of photosynthesis in a plant

Location and structure of chlorophyll molecules in plants

Photosynthesis Equation 12H 2 O + 6CO 2 6O 2 + C 6 H 12 O 6 + 6H 2 O watercarbon dioxide oxygenglucose water LIGHT ENERGY

Two Stages of Photosynthesis sunlightwater uptakecarbon dioxide uptake ATP ADP + P i NADPH NADP + glucose P oxygen release LIGHT INDEPENDENT- REACTIONS LIGHT DEPENDENT- REACTIONS new water

Continual input of solar energy into Earth’s atmosphere Continual input of solar energy into Earth’s atmosphere Almost 1/3 is reflected back into space Almost 1/3 is reflected back into space Of the energy that reaches Earth’s surface, about 1% is intercepted by photoautotrophs Of the energy that reaches Earth’s surface, about 1% is intercepted by photoautotrophs Sunlight Energy

Electromagnetic Spectrum Shortest Gamma rays wavelength X-rays UV radiation Visible light Infrared radiation Microwaves LongestRadio waves wavelength

Visible Light Wavelengths humans perceive as different colors Wavelengths humans perceive as different colors Violet (380 nm) to red (750 nm) Violet (380 nm) to red (750 nm) Longer wavelengths, lower energy Longer wavelengths, lower energy

Photons Packets of light energy Packets of light energy Each type of photon has fixed amount of energy Each type of photon has fixed amount of energy Photons having most energy travel as shortest wavelength (blue-green light) Photons having most energy travel as shortest wavelength (blue-green light)

Pigments Light-absorbing molecules Light-absorbing molecules Absorb some wavelengths and transmit others Absorb some wavelengths and transmit others Color you see are the wavelengths NOT absorbed Color you see are the wavelengths NOT absorbed Wavelength (nanometers) chlorophyll b chlorophyll a

Pigments in Photosynthesis Bacteria Bacteria Pigments in plasma membranes Pigments in plasma membranes Plants Plants Pigments embedded in thylakoid membrane system Pigments embedded in thylakoid membrane system Pigments and proteins organized into photosystems Pigments and proteins organized into photosystems Photosystems located next to electron transport systems Photosystems located next to electron transport systems

Pigments in a Photosystem reaction center (a specialized chlorophyll a molecule)

Pigments absorb light energy, give up e - which enter electron transport systems 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 Water molecules are split, ATP and NADH are formed, and oxygen is released Pigments that gave up electrons get replacements Pigments that gave up electrons get replacements Light-Dependent Reactions

Photosystem Function: Harvester Pigments Most pigments in photosystem are harvester pigments Most pigments in photosystem are harvester pigments When excited by light energy, these pigments transfer energy to adjacent pigment molecules When excited by light energy, these pigments transfer energy to adjacent pigment molecules Each transfer involves energy loss Each transfer involves energy loss

Photosystem Function: Reaction Center Energy is reduced to level that can be captured by molecule of chlorophyll a 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 This molecule (P700 or P680) is the reaction center of a photosystem Reaction center accepts energy and donates electron to acceptor molecule Reaction center accepts energy and donates electron to acceptor molecule

Cyclic Electron Flow electron acceptor electron transport system e–e– e–e– e–e– e–e– ATP

Electron Transport System Adjacent to photosystem Adjacent to photosystem Acceptor molecule donates electrons from reaction center 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 As electrons flow through system, energy they release is used to produce ATP and, in some cases, NADPH

Cyclic Electron Flow Electrons Electrons are donated by P700 in photosystem I to acceptor molecule are donated by P700 in photosystem I to acceptor molecule flow through electron transport system and back to P700 flow through electron transport system and back to P700 Electron flow drives ATP formation Electron flow drives ATP formation No NADPH is formed No NADPH is formed

Energy Changes Potential to transfer energy (voids) H2OH2O 1/2 O 2 + 2H + (PHOTOSYSTEM II) (PHOTOSYSTEM I) e–e– e–e– e–e– e–e– second transport system NADPH first transport system

Noncyclic Electron Flow Two-step pathway for light absorption and electron excitation Two-step pathway for light absorption and electron excitation Uses two photosystems: type I and type II Uses two photosystems: type I and type II Produces ATP and NADPH Produces ATP and NADPH Involves photolysis - splitting of water 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)

Synthesis part of photosynthesis Synthesis part of photosynthesis Can proceed in the dark Can proceed in the dark Take place in the stroma Take place in the stroma Calvin-Benson cycle Calvin-Benson cycle Light-Independent Reactions

Calvin-Benson Cycle Overall reactants Overall reactants Carbon dioxide Carbon dioxide ATP ATP NADPH NADPH Overall products Glucose ADP NADP + Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated

Melvin Calvin

The Calvin cycle (Layer 1)

The Calvin cycle (Layer 2)

The Calvin cycle (Layer 3)

Using the Products of Photosynthesis Phosphorylated glucose is the building block for: Phosphorylated glucose is the building block for: sucrose sucrose The most easily transported plant carbohydrate The most easily transported plant carbohydrate starch starch The most common storage form The most common storage form

In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA 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 Because the first intermediate has three carbons, the pathway is called the C3 pathway The C3 Pathway

Photorespiration in C3 Plants On hot, dry days stomata close On hot, dry days stomata close Inside leaf Inside leaf Oxygen levels rise Oxygen levels rise Carbon dioxide levels drop Carbon dioxide levels drop Rubisco attaches RuBP to oxygen instead of carbon dioxide Rubisco attaches RuBP to oxygen instead of carbon dioxide Only one PGAL forms instead of two Only one PGAL forms instead of two

C4 Plants Carbon dioxide is fixed twice Carbon dioxide is fixed twice In mesophyll cells, carbon dioxide is fixed to form four-carbon oxaloacetate In mesophyll cells, carbon dioxide is fixed to form four-carbon oxaloacetate Oxaloacetate is transferred to bundle- sheath cells Oxaloacetate is transferred to bundle- sheath cells Carbon dioxide is released and fixed again in Calvin-Benson cycle Carbon dioxide is released and fixed again in Calvin-Benson cycle

Figure C 4 leaf anatomy and the C 4 pathway

CAM Plants Carbon is fixed twice (in same cells) Carbon is fixed twice (in same cells) Night Night Carbon dioxide is fixed to form organic acids Carbon dioxide is fixed to form organic acids Day Day Carbon dioxide is released and fixed in Calvin-Benson cycle Carbon dioxide is released and fixed in Calvin-Benson cycle

Figure C 4 and CAM photosynthesis compared

Figure A review of photosynthesis