Chapter 10 Photosynthesis Photosynthesis in nature Auautotrophs biotic

Chapter 10~ Photosynthesis

Photosynthesis in nature § Auautotrophs: biotic producers; photoautotrophs; chemoautotrophs; obtains organic food without eating other organisms § Heterotrophs: biotic consumers; obtains organic food by eating other organisms or their byproducts (includes decomposers)

How are they connected? Heterotrophs making energy & organic molecules from ingesting organic molecules glucose + oxygen carbon + water + energy dioxide C 6 H 12 O 6 + 6 O 2 6 CO 2 + 6 H 2 O + ATP oxidation = exergonic Autotrophs making energy & organic molecules from light energy Where’s the ATP? carbon + water + energy glucose + oxygen dioxide 6 CO 2 + 6 H 2 O +light energy reduction = endergonic C 6 H 12 O 6 + 6 O 2

What does it mean to be a plplantant • Need to… – collect light energy ATP • transform it into chemical energy – store light energy glucose • in a stable form to be moved around the plant or stored – need to get building block atoms from the environment • C, H, O, N, P, K, S, Mg – produce all organic molecules needed for growth • carbohydrates, proteins, lipids, nucleic acids CO 2 N K P … H 2 O

The chloroplast § § § § Sites of photosynthesis Pigment: chlorophyll Plant cell: mesophyll Gas exchange: stomata Double membrane Thylakoids, stack-granum Thylakoid membrane contains § chlorophyll molecules § electron transport chain § ATP synthase § , Stroma-fluid-filled interior

Photosynthesis • Light reactions – light-dependent reactions – energy conversion reactions • convert solar energy to chemical energy • ATP & NADPH • Calvin cycle It’s not the Dark Reactions! – light-independent reactions – sugar building reactions • uses chemical energy (ATP & NADPH) to reduce CO 2 & synthesize C 6 H 12 O 6

thylakoid chloroplast Light reactions • Electron Transport Chain • like in cellular respiration – proteins in organelle membrane – electron acceptors • NADPH – proton (H+) gradient across inner membrane • find the double membrane! – ATP synthase enzyme + +H+ H H+ + H+ H+H +H+ H H ATP + +H+ H H+ + H+ H+H H H H

ETC of Photosynthesis Chloroplasts transform light energy into chemical energy of ATP u generates O 2 use electron carrier NADPH

Pigments of photosynthesis How does this molecular structure fit its function? • Chlorophylls & other pigments – embedded in thylakoid membrane – arranged in a “photosystem” • collection of molecules – structure-function relationship

Photosynthetic Pigments § Pigment ~substance that absorbs light § Absorption spectrum~ measures the wavelength of light that absorbed by particular pigment § Accessory pigments~ absorbs energy that chlorophyll a does not absorb ensures that a greater % of incoming photons will stimulate photosynthesis Action spectrum ~plots the efficiency of photosynthesis at various wavelengths

Photosystems § Light harvesting units of the thylakoid membrane § Composed mainly of protein and pigment antenna complexes § Antenna pigment molecules are struck by photons § Energy is passed to reaction centers (redox location) § Excited e- from chlorophyll is trapped by a primary eacceptor

Photosystems of photosynthesis • 2 photosystems in thylakoid membrane – collections of chlorophyll molecules – Photosystem II • chlorophyll a • P 680 = absorbs 680 nm wavelength red light reaction center – Photosystem I • chlorophyll b • P 700 = absorbs 700 nm wavelength red light antenna pigments

chlorophyll a ETC of Photosynthesis Photosystem II chlorophyll b Photosystem I

ETC of Photosynthesis sun 1 e e Photosystem II P 680 chlorophyll a

Inhale, baby! ETC of Photosynthesis thylakoid chloroplast + +H+ H H+ + H+ H+H +H+ H H ATP H+ + +H+ H H+ + H H+ H+ HH Plants SPLIT water! O H H e e O O H e- e e fill the e– vacancy Photosystem II P 680 chlorophyll a e- +H 1 H 2 H+

ETC of Photosynthesis thylakoid chloroplast H+ + +H+ H H+ + H+ H+H H H H e e ATP + +H+ H H+ + H H+ H+ HH 3 2 1 e e H+ 4 ATP H+ H+ H+ Photosystem II P 680 chlorophyll a H+ to Calvin Cycle H+ H+ H ADP + Pi ATP H+ H+ energy to build carbohydrates

e e ETC of Photosynthesis e e y c an e e – e Photosystem II P 680 chlorophyll a e 5 h lt fil e Photosystem I P 700 chlorophyll b e c va sun

ETC of Photosynthesis e e electron carrier 6 e e 5 Photosystem II P 680 chlorophyll a Photosystem I P 700 chlorophyll b $$ in the bank… reducing power! N Cal ADPH vin Cyc to sun le

ETC of Photosynthesis sun e e sun O split H 2 O H+ + H+ H + H+ +H H+ H+ H to Calvin Cycle ATP

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 H 2 O & supplies them to chlorophyll • splits H 2 O • O combines with another O to form O 2 • O 2 released to atmosphere • and we breathe easier!

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 ATP

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 18 ATP + 12 NADPH 1 C 6 H 12 O 6 ATP

Photophosphorylation cyclic photophosphorylation NADP NONcyclic photophosphorylation ATP

You can grow if you Ask Questions! 2007 -2008

Photosynthesis: The Calvin Cycle 2007 -2008

Whoops! Wrong Calvin… The Calvin Cycle 1950 s | 1961

Light reactions • Convert solar energy to chemical energy – ATP energy – NADPH do reducing • What can we now? power build stuff !! photosynthesis

How is that helpful? • Want to make C 6 H 12 O 6 – synthesis – How? From what? What raw materials are available? CO 2 NADPH carbon fixation NADP C 6 H 12 O 6 reduces CO 2 NADP

From CO 2 C 6 H 12 O 6 • CO 2 has very little chemical energy – fully oxidized • C 6 H 12 O 6 contains a lot of chemical energy – highly reduced • Synthesis = endergonic process – put in a lot of energy • Reduction of CO 2 C 6 H 12 O 6 proceeds in many small uphill steps – each catalyzed by a specific enzyme – using energy stored in ATP & NADPH

From Light reactions to Calvin cycle • Calvin cycle – chloroplast stroma • Need products of light reactions to drive synthesis reactions – ATP – NADPH ATP thylakoid stroma

C Calvin cycle 1 C C C 3. Regeneration of Ru. BP C C C C C Ru. BP ribulose bisphosphate starch, sucrose, cellulose & more 3 ATP C= C= C H H H | | | C– C– C C C C CO 2 5 C Ru. Bis. Co glyceraldehyde-3 -P G 3 P 3 C C 6 NADP 6 C PGA phosphoglycerate C C C 3 C C C C 6 ATP 2. Reduction 6 NADPH 1. Carbon fixation C C C ribulose bisphosphate carboxylase 3 ADP used to make glucose 5 C C C 3 C 6 ADP C C C H | H |

To G 3 P and beyond! To G 3 P and Beyond! • Glyceraldehyde-3 -P – end product of Calvin cycle – energy rich 3 carbon sugar – “C 3 photosynthesis” • G 3 P is an important intermediate • G 3 P glucose carbohydrates lipids phospholipids, fats, waxes amino acids proteins nucleic acids DNA, RNA

Ru. Bis. Co • Enzyme which fixes carbon from air – ribulose bisphosphate carboxylase – the most important enzyme in the world! • it makes life out of air! – definitely the most abundant enzyme I’m green with envy! It’s not easy being green!

Photosynthesis summary • Light reactions – produced ATP – produced NADPH – consumed H 2 O – produced O 2 as byproduct • Calvin cycle – consumed CO 2 – produced G 3 P (sugar) – regenerated ADP – regenerated NADP

Light Reactions H 2 O + light energy H 2 O + NADPH + O 2 § produces ATP § produces NADPH § releases O 2 as a waste product sunlight Energy Building Reactions NADPH ATP O 2 ATP

Calvin Cycle CO 2 + ATP + NADPH C 6 H 12 O 6 CO 2 ADP NADP Sugar Building Reactions NADPH ATP sugars + ADP + NADP § builds sugars § uses ATP & NADPH § recycles ADP & NADP § back to make more ATP & NADPH

Putting it all together light CO 2 + H 2 O + energy C 6 H 12 O 6 + O 2 H 2 O CO 2 sunlight ADP NADP Sugar Energy Building Reactions NADPH ATP O 2 sugars Plants make both: § energy § ATP & NADPH § sugars

even though this equation is a bit of a lie… it makes a better story Energy cycle sun Photosynthesis light CO 2 + H 2 O + energy C 6 H 12 O 6 + O 2 plants CO 2 H 2 O animals, plants glucose ATP C 6 H 12 O 6 + O 2 energy + CO 2 + H 2 O Cellular Respiration The Great Circle of Life, Mufasa! ATP O 2

Supporting a biosphere • On global scale, photosynthesis is the most important process for the continuation of life on Earth – each year photosynthesis… • captures 121 billion tons of CO 2 • synthesizes 160 billion tons of carbohydrate – heterotrophs are dependent on plants as food source for fuel & raw materials

If plants can do it… You can learn it! Ask Questions!! 2007 -2008

Photosynthesis: Variations on the Theme 2007 -2008

Controlling water loss from leaves • Hot or dry days – stomates close to conserve water – guard cells • gain H 2 O = stomates open • lose H 2 O = stomates close – adaptation to living on land, but… creates PROBLEMS!

When stomates close… • Closed stomates lead to… – O 2 build up from light reactions – CO 2 is depleted in Calvin cycle • causes problems in Calvin Cycle O 2 xylem (water) CO 2 phloem (sugars) H 2 O The best laid schemes of mice and men… and plants!

Inefficiency of Ru. Bis. Co: CO 2 vs O 2 • Ru. Bis. Co in Calvin cycle – carbon fixation enzyme • • normally bonds C to Ru. BP CO 2 is the optimal substrate reduction of Ru. BP building sugars photosynthesis – when O 2 concentration is high • • Ru. Bis. Co bonds O to Ru. BP O 2 is a competitive substrate oxidation of Ru. BP breakdown sugars photorespiration

Calvin cycle when CO 2 is abundant 1 C ATP Ru. BP ADP G 3 P to make glucose CO 2 5 C Ru. Bis. Co 6 C unstable intermediate 5 C G 3 P 3 C C 3 plants 3 C PGA NADPH ATP NADP 3 C

Calvin cycle when O 2 is high O 2 Ru. BP Hey Dude, are you high on oxygen! It’s so sad to see a good enzyme, go BAD! 5 C Ru. Bis. Co to mitochondria ––––––– lost as CO 2 without making ATP 2 C 3 C photorespiration

Impact of Photorespiration • Oxidation of Ru. BP – short circuit of Calvin cycle – loss of carbons to CO 2 • can lose 50% of carbons fixed by Calvin cycle – reduces production of photosynthesis • no ATP (energy) produced • no C 6 H 12 O 6 (food) produced – if photorespiration could be reduced, plant would become 50% more efficient • strong selection pressure to evolve alternative carbon fixation systems

Reducing photorespiration • Separate carbon fixation from Calvin cycle – C 4 plants • PHYSICALLY separate carbon fixation from Calvin cycle – different cells to fix carbon vs. where Calvin cycle occurs – store carbon in 4 C compounds • different enzyme to capture CO 2 (fix carbon) – PEP carboxylase • different leaf structure – CAM plants • separate carbon fixation from Calvin cycle by TIME OF DAY • fix carbon during night – store carbon in 4 C compounds • perform Calvin cycle during day

C 4 plants • A better way to capture CO 2 – 1 st step before Calvin cycle, fix carbon with enzyme PEP carboxylase • store as 4 C compound corn – adaptation to hot, dry climates • have to close stomates a lot • different leaf anatomy – sugar cane, corn, other grasses… sugar cane

PEP (3 C) + CO 2 oxaloacetate (4 C) C 4 leaf anatomy light reactions O 2 CO 2 PEP carboxylase C 3 anatomy stomate § PEP carboxylase enzyme u bundle sheath cell CO 2 Ru. Bis. Co higher attraction for CO 2 than O 2 § better than Ru. Bis. Co u u fixes CO 2 in 4 C compounds regenerates CO 2 in inner cells for Ru. Bis. Co § keeping O 2 away from Ru. Bis. Co C 4 anatomy

Comparative anatomy Location, location! C 3 C 4 PHYSICALLY separate C fixation from Calvin cycle

CAM (Crassulacean Acid Metabolism) plants § Adaptation to hot, dry climates u separate carbon fixation from Calvin cycle by TIME § close stomates during day § open stomates during night at night: open stomates & fix carbon in 4 C “storage” compounds u in day: release CO 2 from 4 C acids to Calvin cycle u § increases concentration of CO 2 in cells u succulents, some cacti, pineapple It’s all in the timing!

CAM plants cacti succulents pineapple

C 4 vs CAM Summary solves CO 2 / O 2 gas exchange vs. H 2 O loss challenge C 4 plants separate 2 steps of C fixation anatomically in 2 different cells CAM plants separate 2 steps of C fixation temporally = 2 different times night vs. day

Why the C 3 problem? We’ve all got baggage! • Possibly evolutionary baggage – Rubisco evolved in high CO 2 atmosphere • there wasn’t strong selection against active site of Rubisco accepting both CO 2 & O 2 • Today it makes a difference – 21% O 2 vs. 0. 03% CO 2 – photorespiration can drain away 50% of carbon fixed by Calvin cycle on a hot, dry day – strong selection pressure to evolve better way to fix carbon & minimize photorespiration

It’s not so easy as it looks… Any Questions? ? 2007 -2008