PHOTOSYNTHESIS Chapter 10

PHOTOSYNTHESIS Overview: The Process That Feeds the Biosphere Photosynthesis Is the process that converts light (sun) energy into chemical energy

ENERGY CONVERSION Light energy ECOSYSTEM CO 2 + H 2 O Photosynthesis in chloroplasts Cellular respiration in mitochondria Organic molecules + O 2 ATP powers most cellular work Heat

SUMMARY EQUATION FOR PHOTOSYNTHESIS 6 CO H 2 O + Light energy C 6 H 12 O O H 2 O Photosynthesis is a redox process Water is oxidized, carbon dioxide is reduced It is an anabolic pathway with endergonic reactions It requires energy

SUMMARY EQUATION FOR PHOTOSYNTHESIS Energy  6 CO 2  6 H 2 O C 6 H 12 O 6  6 O 2 becomes reduced becomes oxidized

AUTOTROPHS VS. HETEROTROPHS Autotrophs Can covert inorganic carbon sources to organic material Are the producers of the biosphere Heterotrophs Obtain their organic material from other organisms Are the consumers of the biosphere

WHERE PHOTOSYNTHESIS OCCURS The leaves of plants Are the major sites of photosynthesis Vein Leaf cross section Mesophyll CO 2 O2O2 Stomata

STRUCTURE OF CHLOROPLASTS Chloroplast Mesophyll 5 µm Outer membrane Intermembrane space Inner membrane Thylakoid space Thylakoi d Granum Stroma 1 µm

THE NATURE OF SUNLIGHT Light Is a form of electromagnetic energy, which travels in waves Wavelength Is the distance between the crests of waves Determines the type of electromagnetic energy The visible light spectrum Includes the colors of light we can see Includes the wavelengths that drive photosynthesis Is in the 380 nm to 750 nm wavelength range

Gamma rays X-raysUVInfrared Micro- waves Radio waves 10 –5 nm 10 –3 nm 1 nm 10 3 nm 10 6 nm 1 m 10 6 nm 10 3 m nm Visible light Shorter wavelength Higher energy Longer wavelength Lower energy ELECTROMAGNETIC SPECTRUM

PHOTOSYNTHETIC PIGMENTS Absorb light waves that are used in photosynthesis or Reflect light waves that are not used in photosynthesis Light Reflected Light Chloroplast Absorbed light Granum Transmitted light Green light is NOT used!

ABSORPTION SPECTRA The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Absorption of light by chloroplast pigments Chlorophyll a Wavelength of light (nm) Chlorophyll b Carotenoids

Photosynthesis consists of two processes Light reactions Occur in the thylakoid membranes Splits water Releases oxygen Produces ATP and NADPH Calvin cycle Occurs in the stroma Forms sugar from carbon dioxide Using ATP (for energy) and NADPH (for reducing power) from the light reactions STAGES OF PHOTOSYNTHESIS

Light Light Reactions Calvin Cycle Chloroplast [CH 2 O] (sugar) ATP NADPH NADP  ADP + P i H2OH2O CO 2 O2O2

EXCITATION OF CHLOROPHYLL BY LIGHT Excited state Energy of election Heat Photon (fluorescence) Chlorophyll molecule Ground state Photon e–e– When a pigment absorbs light It goes from a ground state to an excited state, which is unstable

CHLOROPHYLL IN A PHOTOSYSTEM A photosystem Is composed of a reaction center surrounded by a number of light- harvesting complexes Thylakoid membranes are populated by two types of photosystems, I (P700) and II (P680) Primary election acceptor Photon Thylakoid Light-harvesting complexes Reaction center Photosystem STROMA Thylakoid membrane Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) e–e–

LIGHT REACTIONS ANAOLGY Mill makes ATP e–e– e–e– e–e– e–e– e–e– Photon Photosystem II Photosystem I e–e– e–e– NADPH Photon

Cytochrome complex Primary acceptor H2OH2O O2O2 2 H  + 1/21/2 P680 Light Pigment molecules Photosystem II (PS II ) Photosystem I (PS I ) Pq Pc ATP Electron transport chain P700 Light + H  NADP  NADPH NADP  reductase Fd ee ee ee ee 4 ee ee LIGHT REACTIONS LINEAR (NON-CYCLIC) ELECTRON FLOW

LIGHT REACTIONS CYCLIC ELECTRON FLOW Primary acceptor Pq Fd Cytochrome complex Pc Primary acceptor Fd NADP + reductase NADPH ATP Photosystem II Photosystem I NADP + Occasionally, electrons take a different path In cyclic electron flow Only photosystem I is used Only ATP is produced

Chemiosmosis and Photophosphorylation STROMA (low H  concentration) THYLAKOID SPACE (high H  concentration) Light Photosystem II Cytochrome complex Photosystem I Light NADP  reductase NADP  + H  To Calvin Cycle ATP synthase Thylakoid membrane 2 13 NADPH Fd Pc Pq 4 H + +2 H + H+H+ ADP + P i ATP 1/21/2 H2OH2O O2O2

CALVIN CYCLE The Calvin cycle uses ATP and NADPH from light reactions to convert CO 2 to sugar Sometimes called “dark” reactions or light independent reactions Occurs in the stroma The Calvin cycle has 3 phases 1.Carbon fixation 2.Reduction 3.Regeneration of the CO 2 acceptor

Input 3 (Entering one at a time) CO 2 Phase 1: Carbon fixation Rubisco 3PP P6 Short-lived intermediate 6 6 ADP ATP 6PP Calvin Cycle 6 NADPH 6 NADP  6 P i6 P i 6P Phase 2: Reduction Glyceraldehyde 3-phosphate (G3P) P 5 G3P ATP 3 ADP Phase 3: Regeneration of the CO 2 acceptor (RuBP) 3P P Ribulose bisphosphate (RuBP) 1P G3P (a sugar) Output Glucose and other organic compounds 3 Calvin Cycle

REVIEW OF PHOTOSYNTHESIS Light reactions: Are carried out by molecules in the thylakoid membranes Convert light energy to the chemical energy of ATP and NADPH Split H 2 O and release O 2 to the atmosphere Calvin cycle reactions: Take place in the stroma Use ATP and NADPH to convert CO 2 to the sugar G3P Return ADP, inorganic phosphate, and NADP+ to the light reactions O2O2 CO 2 H2OH2O Light Light reaction Calvin cycle NADP + ADP ATP NADPH + P 1 RuBP Amino acids Fatty acids Starch (storage) Glucose (export) G3P Photosystem II Electron transport chain Photosystem I Chloroplast