How Cells Acquire Energy Chapter 6

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

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

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

Organelles of photosynthesis Chloroplasts

Photosynthesis Equation 12H 2 O + 6CO 2 6O 2 + C 2 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 Almost 1/3 is reflected back into space 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 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 Wavelength (nanometers) chlorophyll b chlorophyll a

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

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

Variety of Pigments Chlorophylls a and b Carotenoids Anthocyanins Phycobilins

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

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

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

Photosystems and Electron Transporters water-splitting complexthylakoid compartment H2OH2O2H + 1/2O 2 P680 acceptor P700 acceptor pool of electron transporters stromaPHOTOSYSTEM II PHOTOSYSTEM I

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 Light-Dependent Reactions

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

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

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

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

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

Machinery of Noncyclic Electron Flow photolysis H2OH2O NADP + NADPH e–e– ATP ATP SYNTHASE PHOTOSYSTEM IPHOTOSYSTEM II ADP + P i e–e–

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

Chemiosmotic Model of 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 transport system operates

Chemiosmotic Model of 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

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

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 CARBON FIXATION 6CO 2 (from the air) 66 RuBP PGA unstable intermediate 6 ADP 6 12 ATP NADPH PGAL glucose P PGAL 2 PiPi 12 ADP 12 P i 12NADP P i PGAL

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 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 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

Fissures in sea-floor where seawater mixes with molten rock Complex ecosystem is based on energy from these vents Bacteria are producers in this system Hydrothermal Vents

Light and Life at the Vents Vents release faint radiation at low end of visible spectrum These photons could be used to carry out photosynthesis Nisbet and Van Dover hypothesize that the first cells may have arisen at hydrothermal vent systems

Supporting Evidence Absorption spectra for ancient photosynthetic bacteria correspond to wavelengths measured at the vents Photosynthetic machinery contains iron, sulfur, manganese, and other minerals that are abundant at the vents

Summary of Photosynthesis light 6O 2 12H 2 O CALVIN- BENSON CYCLE C 6 H 12 O 6 (phosphorylated glucose) NADPHNADP + ATP ADP + P i PGA PGAL RuBP P 6CO 2 end product (e.g. sucrose, starch, cellulose) LIGHT-DEPENDENT REACTIONS