LECTURE PRESENTATIONS For CAMPBELL BIOLOGY NINTH EDITION Jane

LECTURE PRESENTATIONS For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson Chapter 10 Photosynthesis Lectures by Erin Barley Kathleen Fitzpatrick © 2011 Pearson Education, Inc.

• Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes • These organisms feed not only themselves but also most of the living world © 2011 Pearson Education, Inc.

Overview: The Process That Feeds the Biosphere • Photosynthesis is the process that converts solar energy into chemical energy • Directly or indirectly, photosynthesis nourishes almost the entire living world 6 CO 2 + 12 H 2 O + Light energy C 6 H 12 O 6 + 6 O 2 + 6 H 2 O © 2011 Pearson Education, Inc.

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 C 6 H 12 O 6 + 6 O 2 energy reduction = endergonic

Energy needs of life • All life needs a constant input of energy – Heterotrophs (Animals) • get their energy from “eating others” consumers – eat food = other organisms = organic molecules • make energy through respiration – Autotrophs (Plants) producers • • produce their own energy (from “self”) convert energy of sunlight build organic molecules (CHO) from CO 2 make energy & synthesize sugars through photosynthesis

What does it mean to be a plant • 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 CO 2 N K P … • carbohydrates, proteins, lipids, nucleic acids H 2 O

Plant structure • Obtaining raw materials – sunlight • leaves = solar collectors – CO 2 • stomata = gas exchange – H 2 O • uptake from roots – nutrients • N, P, K, S, Mg, Fe… • uptake from roots

Concept 10. 1: Photosynthesis converts light energy to the chemical energy of food • Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria • The structural organization of these cells allows for the chemical reactions of photosynthesis © 2011 Pearson Education, Inc.

Chloroplasts: The Sites of Photosynthesis in Plants (Billy Madison Chlorophyll ) • Leaves are the major locations of photosynthesis • Their green color is from chlorophyll, the green pigment within chloroplasts • Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf • Each mesophyll cell contains 30– 40 chloroplasts © 2011 Pearson Education, Inc.

chloroplast H+ Plant structure ATP + + H+ H H+ + H H + H+ H+ H+ + H H thylakoid • Chloroplasts – double membrane – stroma outer membrane inner membrane • fluid-filled interior – thylakoid sacs – grana stacks stroma • Thylakoid membrane contains – chlorophyll molecules – electron transport chain – ATP synthase • H+ gradient built up within thylakoid sac thylakoid granum

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: The Light Receptors • Pigments are substances that absorb visible light • Different pigments absorb different wavelengths • Wavelengths that are not absorbed are reflected or transmitted • Leaves appear green because chlorophyll reflects and transmits green light © 2011 Pearson Education, Inc.

A Look at Light • The spectrum of color V I B G Y O R

• The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation • Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see • Light also behaves as though it consists of discrete particles, called photons © 2011 Pearson Education, Inc.

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 Why are plants green?

Figure 10. 6 -4 CO 2 H 2 O Light NADP +Pi Light Reactions Calvin Cycle ATP NADPH Chloroplast O 2 [CH 2 O] (sugar)

Figure 10. 9 TECHNIQUE Refracting Chlorophyll Photoelectric solution tube White prism Galvanometer light Slit moves to pass light of selected wavelength. Green light High transmittance (low absorption): Chlorophyll absorbs very little green light. Blue light Low transmittance (high absorption): Chlorophyll absorbs most blue light.

Excitation of Chlorophyll by Light • When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable • When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence • If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat © 2011 Pearson Education, Inc.

Figure 10. 12 Energy of electron e Excited state Heat Photon (fluorescence) Photon Chlorophyll molecule Ground state (a) Excitation of isolated chlorophyll molecule (b) Fluorescence

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

A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes • A photosystem consists of a reaction-center complex (protein complex) surrounded by lightharvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center © 2011 Pearson Education, Inc.

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

Concept 10. 2: The light reactions convert solar energy to the chemical energy of ATP and NADPH • Chloroplasts are solar-powered chemical factories • Their thylakoids transform light energy into the chemical energy of ATP and NADPH © 2011 Pearson Education, Inc.

• A photon hits a pigment and its energy is passed among pigment molecules until it excites P 680 • An excited electron from P 680 is transferred to the primary electron acceptor (we now call it P 680+) © 2011 Pearson Education, Inc.

• P 680+ is a very strong oxidizing agent • H 2 O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P 680+, thus reducing it to P 680 • O 2 is released as a by-product of this reaction © 2011 Pearson Education, Inc.

• Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I • Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane • Diffusion of H+ (protons) across the membrane drives ATP synthesis © 2011 Pearson Education, Inc.

Figure 10. 14 -5 Ele ct Primary acceptor 2 H + 1/ O 2 2 H 2 O e 2 ron Pq Primary acceptor 4 tran spo rt c hai n e Cytochrome complex E tra lect ch ns ron ai po n rt 7 Fd e e 8 NADP reductase 3 Pc e e P 700 5 P 680 Light 1 Light 6 ATP Pigment molecules Photosystem II (PS II) Photosystem I (PS I) NADP + H NADPH

Cyclic Electron Flow • Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH • No oxygen is released • Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle © 2011 Pearson Education, Inc.

• Some organisms such as purple sulfur bacteria have PS I but not PS II • Cyclic electron flow is thought to have evolved before linear electron flow • Cyclic electron flow may protect cells from light-induced damage © 2011 Pearson Education, Inc.

chlorophyll a ETC of Photosynthesis Photosystem II chlorophyll b Photosystem I

Figure 10. 17 Chloroplast Mitochondrion CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE H Intermembrane space Inner membrane Matrix Diffusion Electron transport chain Thylakoid membrane ATP synthase Stroma ADP P i Key [H ] Higher Lower [H ] Thylakoid space H ATP

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

Figure 10. 18 STROMA (low H concentration) Photosystem II Light 4 H+ Cytochrome complex Photosystem I Light NADP reductase 3 Fd Pq H 2 O NADPH Pc 2 1 THYLAKOID SPACE (high H concentration) 1/ 2 NADP + H O 2 +2 H+ 4 H+ To Calvin Cycle Thylakoid membrane STROMA (low H concentration) ATP synthase ADP + Pi ATP H+

Concept 10. 3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO 2 to sugar • The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle • The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH © 2011 Pearson Education, Inc.

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

Figure 10. 19 -3 Input (Entering one CO 2 at a time) 3 Phase 1: Carbon fixation Rubisco 3 P Short-lived intermediate P 6 P 3 -Phosphoglycerate P 3 P Ribulose bisphosphate (Ru. BP) 6 ATP 6 ADP 3 Calvin Cycle 6 P P 1, 3 -Bisphoglycerate ATP Phase 3: Regeneration of the CO 2 acceptor (Ru. BP) 6 NADPH 6 NADP 6 Pi P 5 G 3 P 6 P Glyceraldehyde 3 -phosphate (G 3 P) 1 P G 3 P (a sugar) Output Glucose and other organic compounds Phase 2: Reduction

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!

Accounting • The accounting is complicated – 3 turns of Calvin cycle = 1 G 3 P – 3 CO 2 1 G 3 P (3 C) – 6 turns of Calvin cycle = 1 C 6 H 12 O 6 (6 C) – 6 CO 2 1 C 6 H 12 O 6 (6 C) – 18 ATP + 12 NADPH 1 C 6 H 12 O 6 – any ATP left over from light reactions will be used elsewhere by the cell

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

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

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 cyclesun 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

Summary of photosynthesis 6 CO 2 + 6 H 2 O +light C 6 H 12 O 6 + 6 O 2 energy • • • Where did the CO 2 come from? Where did the CO 2 go? Where did the H 2 O come from? Where did the H 2 O go? Where did the energy come from? What’s the energy used for? What will the C 6 H 12 O 6 be used for? Where did the O 2 come from? Where will the O 2 go? What else is involved…not listed in this equation?

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

Concept 10. 4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates • Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis • On hot, dry days, plants close stomata, which conserves H 2 O but also limits photosynthesis • The closing of stomata reduces access to CO 2 and causes O 2 to build up • These conditions favor an apparently wasteful process called photorespiration © 2011 Pearson Education, Inc.

Photorespiration: An Evolutionary Relic? • In most plants (C 3 plants), initial fixation of CO 2, via rubisco, forms a three-carbon compound (3 phosphoglycerate) • In photorespiration, rubisco adds O 2 instead of CO 2 in the Calvin cycle, producing a two-carbon compound • Photorespiration consumes O 2 and organic fuel and releases CO 2 without producing ATP or sugar © 2011 Pearson Education, Inc.

• Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O 2 and more CO 2 • Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle • In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle © 2011 Pearson Education, Inc.

C 4 Plants • C 4 plants minimize the cost of photorespiration by incorporating CO 2 into four-carbon compounds in mesophyll cells • This step requires the enzyme PEP carboxylase • PEP carboxylase has a higher affinity for CO 2 than rubisco does; it can fix CO 2 even when CO 2 concentrations are low • These four-carbon compounds are exported to bundle-sheath cells, where they release CO 2 that is then used in the Calvin cycle © 2011 Pearson Education, Inc.

Figure 10. 20 The C 4 pathway C 4 leaf anatomy Mesophyll cell PEP carboxylase Mesophyll cell Photosynthetic cells of C 4 Bundleplant leaf sheath cell Oxaloacetate (4 C) Vein (vascular tissue) PEP (3 C) ADP Malate (4 C) Stoma Bundlesheath cell CO 2 ATP Pyruvate (3 C) CO 2 Calvin Cycle Sugar Vascular tissue

Figure 10. 20 a C 4 leaf anatomy Photosynthetic cells of C 4 plant leaf Mesophyll cell Bundlesheath cell Vein (vascular tissue) Stoma

Figure 10. 20 b The C 4 pathway Mesophyll cell PEP carboxylase Oxaloacetate (4 C) PEP (3 C) ADP Malate (4 C) Bundlesheath cell CO 2 ATP Pyruvate (3 C) CO 2 Calvin Cycle Sugar Vascular tissue

• In the last 150 years since the Industrial Revolution, CO 2 levels have risen greatly • Increasing levels of CO 2 may affect C 3 and C 4 plants differently, perhaps changing the relative abundance of these species • The effects of such changes are unpredictable and a cause for concern © 2011 Pearson Education, Inc.

CAM Plants • Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon • CAM plants open their stomata at night, incorporating CO 2 into organic acids • Stomata close during the day, and CO 2 is released from organic acids and used in the Calvin cycle © 2011 Pearson Education, Inc.

Figure 10. 21 Sugarcane Pineapple C 4 Mesophyll Organic acid cell CAM CO 2 1 CO 2 incorporated (carbon fixation) Organic acid Calvin Cycle Night CO 2 Bundlesheath cell CO 2 2 CO 2 released to the Calvin cycle Sugar (a) Spatial separation of steps Calvin Cycle Day Sugar (b) Temporal separation of steps

The Importance of Photosynthesis: A Review • The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds • Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells • Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits • In addition to food production, photosynthesis produces the O 2 in our atmosphere © 2011 Pearson Education, Inc.

Figure 10. 22 H 2 O CO 2 Light NADP + Pi Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain Ru. BP 3 -Phosphoglycerate Calvin Cycle ATP NADPH G 3 P Starch (storage) Chloroplast O 2 Sucrose (export)

Figure 10. UN 02 El El Primary acceptor H 2 O O 2 ec tr on ch tr ai an n sp or Pq t Primary acceptor t NADP reductase Cytochrome complex Pc Photosystem II ATP ec tr on ch tr ai an n sp or Fd Photosystem I NADP + H NADPH