Photosynthesis Dr.Samih Tamimi Biology 304101

Prof.DR. Samih Tamimi Bio304101 Structures Photosynthesis occurs only in plants and a small number of single-celled organisms (like algae). To be able to photosynthesize, you must have a specific organelle: the Chloroplast. Prof.DR. Samih Tamimi Bio304101

chloroplasts in plant cell chloroplasts contain chlorophyll absorb sunlight & CO2 cross section of leaf leaves CO2 chloroplasts in plant cell chloroplasts contain chlorophyll chloroplast make energy & sugar Prof.DR. Samih Tamimi Bio304101

Prof.DR. Samih Tamimi Bio304101 Plant structure chloroplast H+ Chloroplasts double membrane stroma fluid-filled interior thylakoid sacs grana stacks Thylakoid membrane contains chlorophyll molecules electron transport chain ATP synthase H+ gradient built up within thylakoid sac ATP thylakoid outer membrane inner membrane granum stroma thylakoid A typical mesophyll cell has 30-40 chloroplasts, each about 2-4 microns by 4-7 microns long. Each chloroplast has two membranes around a central aqueous space, the stroma. In the stroma are membranous sacs, the thylakoids. These have an internal aqueous space, the thylakoid lumen or thylakoid space. Thylakoids may be stacked into columns called grana. Prof.DR. Samih Tamimi Bio304101

Prof.DR. Samih Tamimi Bio304101 Photosynthesis Light reactions light-dependent reactions energy conversion reactions convert solar energy to chemical energy ATP & NADPH Calvin cycle light-independent reactions sugar building reactions uses chemical energy (ATP & NADPH) to reduce CO2 & synthesize C6H12O6 Prof.DR. Samih Tamimi Bio304101

Prof.DR. Samih Tamimi Bio304101 A Look at Light The spectrum of color V I B G Y O R Prof.DR. Samih Tamimi Bio304101

Pigments of photosynthesis Chlorophylls & other pigments embedded in thylakoid membrane arranged in a “photosystem” collection of molecules structure-function relationship Prof.DR. Samih Tamimi Bio304101

Photosynthesis – light absorption • pigments: • chlorophyll a -energy-absorbing ring -hydrocarbon tail • accessory pigments - chlorophyll b - carotenoids - photoprotective

Light: absorption spectra Photosynthesis gets energy by absorbing wavelengths of light chlorophyll a absorbs best in red & blue wavelengths & least in green accessory pigments with different structures absorb light of different wavelengths chlorophyll b, carotenoids, xanthophylls Prof.DR. Samih Tamimi Bio304101

Photosystems of photosynthesis 2 photosystems in thylakoid membrane collections of chlorophyll molecules act as light-gathering molecules Photosystem II chlorophyll a P680 = absorbs 680nm wavelength red light Photosystem I chlorophyll b P700 = absorbs 700nm wavelength red light reaction center Photons are absorbed by clusters of pigment molecules (antenna molecules) in the thylakoid membrane. When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule = the reaction center. At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a. This starts the light reactions. Don’t compete with each other, work synergistically using different wavelengths. antenna pigments Prof.DR. Samih Tamimi Bio304101

Photosynthesis – light absorption Pigments are held by proteins in the thylakoid membranes light harvesting complex • energy absorbed from light - to pigments • to reaction center - two special chlorophyll a - proteins - 1° electron acceptor • light harvesting complex & reaction center = photosystem (PS)

Photosynthesis – energy transfer • PSII: absorbs 680 nm, splits water, powerful ETC, ATP made • PS I: absorbs 700 nm, (less energy) e- from PSII, short ETC, NADPH made

Photosynthesis – energy transfer • e- in PS II, from split H20 • e- from PS II electron transport chain (ETC) PS I • e- from PS I 2nd ETC e- carrier: NADP+  NADPH

Prof.DR. Samih Tamimi Bio304101 ETC of Photosynthesis chlorophyll a Photosystem II chlorophyll b Photosystem I Two places where light comes in. Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy. Need to look at that in more detail on next slide Prof.DR. Samih Tamimi Bio304101

Photosystem II P680 chlorophyll a ETC of Photosynthesis sun 1 e PS II absorbs light Excited electron passes from chlorophyll to the primary electron acceptor Need to replace electron in chlorophyll An enzyme extracts electrons from H2O & supplies them to the chlorophyll This reaction splits H2O into 2 H+ & O- which combines with another O- to form O2 O2 released to atmosphere Chlorophyll absorbs light energy (photon) and this moves an electron to a higher energy state Electron is handed off down chain from electron acceptor to electron acceptor In process has collected H+ ions from H2O & also pumped by Plastoquinone within thylakoid sac. Flow back through ATP synthase to generate ATP. Photosystem II P680 chlorophyll a Prof.DR. Samih Tamimi Bio304101

Photosystem II P680 chlorophyll a ETC of Photosynthesis thylakoid chloroplast H+ H+ ATP Plants SPLIT water! 1 O H 2 e O O H e- H+ +H e e fill the –e vacancy Photosystem II P680 chlorophyll a Prof.DR. Samih Tamimi Bio304101

energy to build carbohydrates Photosystem II P680 chlorophyll a ETC of Photosynthesis thylakoid chloroplast H+ H+ ATP e H+ 3 1 2 e H+ ATP 4 to Calvin Cycle H+ ADP + Pi energy to build carbohydrates Photosystem II P680 chlorophyll a ATP Prof.DR. Samih Tamimi Bio304101

Photosystem I P700 chlorophyll b Photosystem II P680 chlorophyll a ETC of Photosynthesis e e sun fill the e– vacancy e 5 Need a 2nd photon -- shot of light energy to excite electron back up to high energy state. 2nd ETC drives reduction of NADP to NADPH. Light comes in at 2 points. Produce ATP & NADPH e e Photosystem I P700 chlorophyll b Photosystem II P680 chlorophyll a Prof.DR. Samih Tamimi Bio304101

ETC of Photosynthesis e e electron carrier 6 NADPH to Calvin Cycle 5 sun NADPH to Calvin Cycle Need a 2nd photon -- shot of light energy to excite electron back up to high energy state. 2nd ETC drives reduction of NADP to NADPH. Light comes in at 2 points. Produce ATP & NADPH Photosystem I P700 chlorophyll b Photosystem II P680 chlorophyll a $$ in the bank… reducing power! Prof.DR. Samih Tamimi Bio304101

Prof.DR. Samih Tamimi Bio304101 ETC of Photosynthesis sun sun e e H+ Two places where light comes in. Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy. Need to look at that in more detail on next slide O to Calvin Cycle split H2O ATP Prof.DR. Samih Tamimi Bio304101

Prof.DR. Samih Tamimi Bio304101 ETC of Photosynthesis ETC uses light energy to produce ATP & NADPH go to Calvin cycle PS II absorbs light excited electron passes from chlorophyll to “primary electron acceptor” need to replace electron in chlorophyll enzyme extracts electrons from H2O & supplies them to chlorophyll splits H2O O combines with another O to form O2 O2 released to atmosphere and we breathe easier! Prof.DR. Samih Tamimi Bio304101

Photophosphorylation cyclic photophosphorylation NADP NONcyclic photophosphorylation ATP Prof.DR. Samih Tamimi Bio304101

Prof.DR. Samih Tamimi Bio304101

The Calvin cycle uses ATP and NADPH to convert CO2 to sugar Is similar to the citric acid cycle in mitochondria Occurs in the stroma Prof.DR. Samih Tamimi Bio304101

Prof.DR. Samih Tamimi Bio304101 The Calvin cycle has three phases 1. Carbon fixation 2. Reduction 3. Regeneration of the CO2 acceptor (RuBP) Prof.DR. Samih Tamimi Bio304101

Enzyme RUBISCO catalyzes reaction The Calvin Cycle Steps CARBON FIXATION 1. CO2 enters cycle and attached to a 5-carbon sugar called ribulose biphosphate (RuBP) forming 6-C molecule (unstable) Enzyme RUBISCO catalyzes reaction 2. Unstable 6-C molecule immediately breaks down to 3 3-C molecules called 3-phosphoglycerate (3-PGA) Prof.DR. Samih Tamimi Bio304101

Prof.DR. Samih Tamimi Bio304101 REDUCTION 3. Each 3-phosphoglycerate (3-PGA) gets an additional phosphate from ATP (from LIGHT RXN)  becomes 1,3 phosphoglycerate 4. NADPH reduces 1,3 phosphoglycerate to Glyceraldehyde-3-phosphate (G3P) G3P = a sugar that stores potential energy Every 3 CO2  yields 6 G3P’s BUT only 1 can be counted in net gain for carbohydrate (GLUCOSE) production Prof.DR. Samih Tamimi Bio304101

REGENERATION OF CO2 ACCEPTOR (RuBP) 5. The C- skeletons of 5 G3P molecules are rearranged into 3 RuBP molecules ATP is used !!!! Prof.DR. Samih Tamimi Bio304101

MORE ATP is needed than NADPH!! The Calvin cycle NOTE: MORE ATP is needed than NADPH!! Phase 1: Carbon fixation Phase 3: Regeneration of the CO2 acceptor (RuBP) Phase 2: Reduction Also known as PGAL Prof.DR. Samih Tamimi Bio304101

Prof.DR. Samih Tamimi Bio304101 Calvin Cycle Overview For 1 G3P molecule made 9 ATP molecules are used 6 NADPH molecules are used G3P (aka PGAL)= starting material to make other organic molecules (glucose, starch, etc.) Prof.DR. Samih Tamimi Bio304101