Photosynthesis: Life from Light

How are they connected? Heterotrophs and Autotrophs  Autotrophs  making energy & organic molecules from ingesting organic molecules glucose + oxygen  carbon + water + energy dioxide C6H12O6 6O2 6CO2 6H2O ATP  + exergonic Where’s the ATP? Autotrophs So, in effect, photosynthesis is respiration run backwards powered by light. Cellular Respiration oxidize C6H12O6  CO2 & produce H2O fall of electrons downhill to O2 exergonic Photosynthesis reduce CO2  C6H12O6 & produce O2 boost electrons uphill by splitting H2O endergonic making energy & organic molecules from light energy + water + energy  glucose + oxygen carbon dioxide 6CO2 6H2O C6H12O6 6O2 light energy  + endergonic

Photoautotrophs

Plant structure Obtaining raw materials sunlight CO2 H2O Nutrients leaves = solar collectors CO2 stomates = gas exchange Found under leaves H2O uptake from roots Nutrients N, P, K, S, Mg, Fe… 2005-2006

Plant structure Chloroplasts Chlorophyll & ETC in thylakoid membrane double membrane stroma thylakoid sacs grana stacks Chlorophyll & ETC in thylakoid membrane H+ gradient built up within thylakoid sac 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. H+

Pigments of photosynthesis Why does this structure make sense? chlorophyll & accessory pigments “photosystem” embedded in thylakoid membrane structure  function Orientation of chlorophyll molecule is due to polarity of membrane. 2005-2006

Photosynthesis = Light Reactions + Calvin Cycle

Light: absorption spectra Photosynthesis gets energy by absorbing wavelengths of light chlorophyll a (dominant pigment) absorbs best in red & blue wavelengths & least in green other pigments with different structures absorb light of different wavelengths Why are plants green?

Photosynthetic pigments Pigments absorb different λ of light chlorophyll – absorb violet-blue/red light, reflect green chlorophyll a (blue-green): light reaction, converts solar to chemical E chlorophyll b (yellow-green): conveys E to chlorophyll a carotenoids (yellow, orange): photoprotection, broaden color spectrum for photosynthesis Types: xanthophyll (yellow) & carotenes (orange) anthocyanin (red, purple, blue): photoprotection, antioxidants

It’s the Dark Reactions! Photosynthesis Light reactions light-dependent reactions energy production reactions convert solar energy to chemical energy ATP & NADPH Calvin cycle light-independent reactions sugar production reactions uses chemical energy (ATP & NADPH) to reduce CO2 & synthesize C6H12O6 It’s the Dark Reactions!

Light Reactions Summary: Light energy splits H2O to O2 releasing high energy electrons (e-) Movement of e- used to generate ATP Electrons end up on NADP+, reducing it to NADPH

Light reactions Electron Transport Chain (like cell respiration!) membrane-bound proteins in organelle electron acceptor NADPH proton (H+) gradient across inner membrane ATP synthase enzyme Not accidental that these 2 systems are similar, because both derived from the same primitive ancestor. 2005-2006

Photosystem: reaction center & light-harvesting complexes (pigment + protein)

Photosystems 2 photosystems in thylakoid membrane reaction center act as light-gathering “antenna complex” 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.

ETC of Photosynthesis ETC produces from light energy ATP & NADPH NADPH (stored energy) goes to Calvin cycle PS II absorbs light excited electron passes from chlorophyll to “primary electron acceptor” at the REACTION CENTER. splits H2O (Photolysis!!) O2 released to atmosphere ATP is produced for later use

ETC of Photosynthesis Photosystem II 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

Electron Flow A. Linear (noncyclic) electron flow Two routes for electron flow: A. Linear (noncyclic) electron flow B. Cyclic electron flow

Light Reaction (Linear electron flow) Chlorophyll excited by light absorption E passed to reaction center of Photosystem II (protein + chlorophyll a) e- captured by primary electron acceptor Redox reaction  e- transfer e- prevented from losing E (drop to ground state) H2O is split to replace e-  O2 formed

e- passed to Photosystem I via ETC E transfer pumps H+ to thylakoid space ATP produced by photophosphorylation e- moves from PS I’s primary electron acceptor to 2nd ETC NADP+ reduced to NADPH MAIN IDEA: Use solar E to generate ATP & NADPH to provide E for Calvin cycle

Noncyclic Photophosphorylation Light reactions elevate electrons in 2 steps (PS II & PS I) PS II generates energy as ATP PS I generates reducing power as NADPH 1 photosystem is not enough. Have to lift electron in 2 stages to a higher energy level. Does work as it falls. First, produce ATP -- but producing ATP is not enough. Second, need to produce organic molecules for other uses & also need to produce a stable storage molecule for a rainy day (sugars). This is done in Calvin Cycle! 2005-2006

Cyclic photophosphorylation If PS I can’t pass electron to NADP, it cycles back to PS II & makes more ATP, but no NADPH coordinates light reactions to Calvin cycle Calvin cycle uses more ATP than NADPH X 2005-2006

From Light reactions to Calvin cycle Chloroplast stroma Need products of light reactions to drive synthesis reactions ATP NADPH What is there left to do? Make sugar! 2005-2006

Calvin Cycle: Uses ATP and NADPH to convert CO2 to sugar Occurs in the stroma Uses ATP, NADPH, CO2 Produces 3-C sugar G3P (glyceraldehyde-3-phosphate) Three phases: Carbon fixation Reduction Regeneration of RuBP (CO2 acceptor)

From CO2  C6H12O6 CO2 has very little chemical energy fully oxidized C6H12O6 contains a lot of chemical energy reduced endergonic Reduction of CO2  C6H12O6 proceeds in many small uphill steps each catalyzed by specific enzyme using energy stored in ATP & NADPH

Calvin cycle 1C 5C 6C 3C 3C 3C 2x x2 2x CO2 1. Carbon fixation ribulose bisphosphate 1. Carbon fixation 3. Regeneration of RuBP 5C RuBP Rubisco 6C 3 ADP 3 ATP -enzyme that Binds CO2 to RuBP PGAL to make glucose 3C 2x PGA 3C x2 PGAL sucrose cellulose etc. RuBP = ribulose bisphosphate Rubisco = ribulose bisphosphate carboxylase PGA = phosphoglycerate PGAL = phosphoglyceraldehyde 2. Reduction 6 NADP 6 NADPH 6 ADP 6 ATP 3C 2x

Calvin cycle PGAL   important intermediate Six turns of Calvin Cycle = 1 glucose PGAL   glucose   carbohydrates   lipids   amino acids   nucleic acids

Summary Light reactions Calvin cycle produced ATP produced NADPH consumed H2O produced O2 as by product Calvin cycle consumed CO2 produced PGAL regenerated ADP regenerated NADP ADP NADP

Factors that affect Photosynthesis Enzymes are responsible for several photosynthetic processes, therefore, temperature and pH can affect the rate of photosynthesis. The amount and type of light can affect the rate. A shortage of any of the reactants,CO2 and/or H2O, can affect the rate.

Supporting a biosphere On global scale, photosynthesis is the most important process for the continuation of life on Earth each year photosynthesis synthesizes 160 billion tons of carbohydrate heterotrophs are dependent on plants as food source for fuel & raw materials

The Great Circle of Life! Energy cycle sun Photosynthesis glucose O2 H2O CO2 Cellular Respiration The Great Circle of Life! Where’s Mufasa? ATP

Photosynthesis Calvin Cycle Light ENERGY Light Reaction involves both in which stored in CO2 fixed to RuBP organic molecules energized electrons H2O split pass down Reduce NADP+ to C3 phosphorylated and reduced by mechanism of ETC NADPH O2 evolved using to form regenerate RuBP ATP G3P chemiosmosis using in process called glucose & other carbs photophosphorylation

Alternative mechanisms of carbon fixation have evolved in hot, arid climates Photorespiration Metabolic pathway which: Uses O2 & produces CO2 Uses ATP No sugar production (rubisco binds O2  breakdown of RuBP) Occurs on hot, dry bright days when stomata close (conserve H2O) Why? Early atmosphere: low O2, high CO2?

Evolutionary Adaptations Problem with C3 Plants: CO2 fixed to 3-C compound in Calvin cycle Ex. Rice, wheat, soybeans Hot, dry days: partially close stomata, ↓CO2 Photorespiration ↓ photosynthetic output (no sugars made)

C4 Plants: CO2 fixed to 4-C compound Ex. corn, sugarcane, grass Hot, dry days  stomata close 2 cell types = mesophyll & bundle sheath cells mesophyll : PEP carboxylase fixes CO2 (4-C), pump CO2 to bundle sheath bundle sheath: CO2 used in Calvin cycle ↓photorespiration, ↑sugar production WHY? Advantage in hot, sunny areas

C4 Leaf Anatomy

CAM Plants: Crassulacean acid metabolism (CAM) NIGHT: stomata open  CO2 enters  converts to organic acid, stored in mesophyll cells DAY: stomata closed  light reactions supply ATP, NADPH; CO2 released from organic acids for Calvin cycle Ex. cacti, pineapples, succulent (H2O-storing) plants WHY? Advantage in arid conditions

LIGHT REACTIONS Calvin cycle

Mitochondria Chloroplast

Comparison RESPIRATION PHOTOSYNTHESIS Plants + Animals Needs O2 and food Produces CO2, H2O and ATP, NADH Occurs in mitochondria membrane & matrix Oxidative phosphorylation Proton gradient across membrane Plants Needs CO2, H2O, sunlight Produces glucose, O2 and ATP, NADPH Occurs in chloroplast thylakoid membrane & stroma Photorespiration Proton gradient across membrane

Summary of photosynthesis 6CO2 6H2O C6H12O6 6O2 light energy  + Where did the CO2 come from? Where did the CO2 go? Where did the H2O come from? Where did the H2O go? Where did the energy come from? What’s the energy used for? What will the C6H12O6 be used for? Where did the O2 come from? Where will the O2 go? What else is involved that is not listed in this equation? 2005-2006