Where It Starts – Photosynthesis Chapter 6

Sunlight as an Energy Source Photosynthesis runs on a fraction of the electromagnetic spectrum, or the full range of energy radiating from the sun

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

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

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

Light-catching part of molecule often has alternating single and double bonds These bonds contain electrons that are capable of being moved to higher energy levels by absorbing light Pigment Structure

Variety of Pigments Chlorophylls a and b Carotenoids Xanthophylls Phycobilins Anthocyanins

Chlorophylls Main pigments in most photoautotrophs Wavelength absorption (%) Wavelength (nanometers) chlorophyll b chlorophyll a

Carotenoids Found in all photoautotrophs Absorb blue-violet and blue-green that chlorophylls miss Reflect red, yellow, orange wavelengths Two types –Carotenes - pure hydrocarbons –Xanthophylls - contain oxygen

Xanthophylls Yellow, brown, purple, or blue accessory pigments

Phycobilins & Anthocyanins Red to purple pigments Phycobilins –Found in red algae and cyanobacteria Anthocyanins –Give many flowers their colors

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

T.E. Englemann’s Experiment

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

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

Photosynthesis Equation

Organelles of photosynthesis Chloroplasts

Inside the Chloroplast Two outer membranes enclose a semifluid interior, the stroma Thylakoid membrane inside the stroma

Inside the Chloroplast Photosystems are embedded in thylakoids, containing 200 to 300 pigments and other molecules that trap sun’s energy Two types of photosystems: I and II

Photoautotrophs –Carbon source is carbon dioxide –Energy source is sunlight Heterotrophs –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 –Plants –Some bacteria –Many protistans

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

Two Stages of Photosynthesis

Excitation of Electrons Excitation occurs only when the quantity of energy in an incoming photon matches the amount of energy necessary to boost the electrons of that specific pigment Amount of energy needed varies among pigment molecules

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 transfer chains

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

Electron Transfer Chains Adjacent to photosystem Acceptor molecule donates electrons from reaction center As electrons flow through chain, 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 transfer chain and back to P700 Electron flow drives ATP formation No NADPH is formed

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

ATP and NADPH Formation

ATP Formation When water is split during photolysis, hydrogen ions are released into thylakoid compartment More hydrogen ions are pumped into the thylakoid compartment when the electron transfer chain operates

ATP Formation Electrical and H + concentration gradient exists between thylakoid compartment and stroma H + flows down gradients into stroma through ATP synthesis Flow of ions drives formation of ATP

Energy Transfers

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

Calvin-Benson Cycle

Building Glucose PGA accepts –phosphate from ATP –hydrogen and electrons from NADPH PGAL (phosphoglyceraldehyde) forms When 12 PGAL have formed –10 are used to regenerate RuBP –2 combine to form phosphorylated glucose

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

In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA The C3 Pathway 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

C4 Plants

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

Summary of Photosynthesis