How Cells Acquire Energy

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

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

Chloroplast Structure two outer membranes stroma inner membrane system (thylakoids connected by channels)

Photosynthesis Equation LIGHT ENERGY 12H2O + 6CO2 6O2 + C2H12O6 + 6H2O Water Carbon Dioxide Oxygen Glucose Water

Where Atoms End Up Reactants 12H2O 6CO2 Products 6O2 C6H12O6 6H2O

Two Stages of Photosynthesis sunlight water uptake carbon dioxide uptake ATP LIGHT-DEPENDENT REACTIONS ADP + Pi LIGHT-INDEPENDENT REACTIONS NADPH NADP+ P glucose oxygen release new water

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-violet light)

Pigments Color you see is the wavelengths not absorbed

Variety of Pigments Chlorophylls a and b Carotenoids Anthocyanins

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

Accessory Pigments Carotenoids, Phycobilins, Anthocyanins beta-carotene phycoerythrin (a phycobilin) percent of wavelengths absorbed wavelengths (nanometers)

Pigments in Photosynthesis Bacteria Pigments in plasma membranes Plants Pigments and proteins organized into photosystems that are embedded in thylakoid membrane system

Arrangement of Photosystems water-splitting complex thylakoid compartment H2O 2H + 1/2O2 P680 P700 acceptor acceptor pool of electron carriers PHOTOSYSTEM II stroma PHOTOSYSTEM I

Light-Dependent Reactions Pigments absorb light energy, give up e-, which enter electron transfer chains Water molecules split, ATP and NADH form, 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 This molecule (P700 or P680) is the reaction center of a photosystem

Pigments in a Photosystem reaction center

Electron Transfer Chain Adjacent to photosystem As electrons pass along chain, energy they release is used to produce ATP

Cyclic Electron Flow Electrons Electron flow drives ATP formation 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

Synthesis of ATP (chemiosmotic phosphorylation) second electron transfer chain photolysis e– e– ATP SYNTHASE first electron transfer chain NADP+ NADPH ATP PHOTOSYSTEM II PHOTOSYSTEM I ADP + Pi

Chemiosmotic Model of ATP Formation Electrons within the membrane of the chloroplast attract H+ protons The H+ protons are pumped inside the chloroplast membranes The Protons are allowed to pass out of the membrane through the CF1 particle that is rich in ADP + P plus phosphorylating enzymes.

Chemiosmotic Model for ATP Formation H+ is shunted across membrane by some components of the first electron transfer chain Gradients propel H+ through ATP synthases; ATP forms by phosphate-group transfer Photolysis in the thylakoid compartment splits water H2O e– acceptor ATP SYNTHASE ATP ADP + Pi PHOTOSYSTEM II

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

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

unstable intermediate 6 CO2 (from the air) Calvin- Benson Cycle CARBON FIXATION 6 6 RuBP unstable intermediate 12 PGA 6 ADP 12 ATP 6 ATP 12 NADPH 4 Pi 12 ADP 12 Pi 12 NADP+ 10 PGAL 12 PGAL 2 PGAL Pi P glucose

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 The plant is in trouble because it does not enough Carbon dioxide to undergo photosynthesis The plant still needs energy so it taps its own store of glucose

C4 Plants Carbon dioxide is fixed twice In mesophyll cells, carbon dioxide is fixed to form four-carbon oxaloacetate Oxaloacetate is stored as a crystal When times get bad (drought conditions), the plant can now convert the crystalline form of oxaloacetate back to Carbon dioxide and undergo photosynthesis.

Summary of Photosynthesis light 6O2 12H2O CALVIN-BENSON CYCLE C6H12O6 (phosphorylated glucose) NADPH NADP+ ATP ADP + Pi PGA PGAL RuBP P 6CO2 end product (e.g., sucrose, starch, cellulose) LIGHT-DEPENDENT REACTIONS 6H2O LIGHT-INDEPENDENT REACTIONS