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Chapter 10 Photosynthesis
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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.
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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 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O © 2011 Pearson Education, Inc.
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How are they connected? Heterotrophs Autotrophs
making energy & organic molecules from ingesting organic molecules glucose + oxygen carbon + water + energy dioxide C6H12O6 6O2 6CO2 6H2O ATP + oxidation = exergonic Autotrophs Where’s the ATP? making energy & organic molecules from light energy 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 + water + energy glucose + oxygen carbon dioxide 6CO2 6H2O C6H12O6 6O2 light energy + reduction = endergonic
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Energy needs of life All life needs a constant input of energy
Heterotrophs (Animals) get their energy from “eating others” eat food = other organisms = organic molecules make energy through respiration Autotrophs (Plants) produce their own energy (from “self”) convert energy of sunlight build organic molecules (CHO) from CO2 make energy & synthesize sugars through photosynthesis consumers producers
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What does it mean to be a plant
Need to… collect light energy transform it into chemical energy store light energy 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 ATP glucose CO2 H2O N P K …
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Plant structure Obtaining raw materials sunlight CO2 H2O nutrients
leaves = solar collectors CO2 stomata = gas exchange H2O uptake from roots nutrients N, P, K, S, Mg, Fe…
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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.
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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.
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Plant structure Chloroplasts Thylakoid membrane contains
ATP thylakoid 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 outer membrane inner membrane granum stroma thylakoid A typical mesophyll cell has 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.
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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
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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.
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A Look at Light The spectrum of color V I B G Y O R
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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.
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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?
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Calvin Cycle Light Reactions [CH2O] (sugar)
Figure H2O CO2 Light NADP ADP + P i Calvin Cycle Light Reactions ATP Figure 10.6 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. NADPH Chloroplast [CH2O] (sugar) O2 17
![Calvin Cycle Light Reactions [CH2O] (sugar)](http://slideplayer.com./slide/12073086/69/images/17/Calvin+Cycle+Light+Reactions+%5BCH2O%5D+%28sugar%29.jpg "Figure H2O. CO2. Light. NADP ADP. + P i. Calvin Cycle. Light Reactions. ATP. Figure 10.6 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle. NADPH. Chloroplast. [CH2O] (sugar) O")
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Slit moves to pass light of selected wavelength. Green light
Figure 10.9 TECHNIQUE Refracting prism Chlorophyll solution Photoelectric tube White light Galvanometer High transmittance (low absorption): Chlorophyll absorbs very little green light. Slit moves to pass light of selected wavelength. Green light Figure 10.9 Research Method: Determining an Absorption Spectrum Low transmittance (high absorption): Chlorophyll absorbs most blue light. Blue light 18
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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.
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Photon (fluorescence)
Figure 10.12 Excited state e Heat Energy of electron Photon (fluorescence) Photon Figure Excitation of isolated chlorophyll by light. Ground state Chlorophyll molecule (a) Excitation of isolated chlorophyll molecule (b) Fluorescence 20
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Photosynthesis Light reactions Calvin cycle 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 It’s not the Dark Reactions!
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A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes
A photosystem consists of a reaction-center complex (protein complex) surrounded by light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center © 2011 Pearson Education, Inc.
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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 best Photosystem I chlorophyll b P700 = absorbs 700nm wavelength red light best 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
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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.
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A photon hits a pigment and its energy is passed among pigment molecules until it excites P680
An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+) © 2011 Pearson Education, Inc.
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P680+ is a very strong oxidizing agent
H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680 O2 is released as a by-product of this reaction © 2011 Pearson Education, Inc.
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Diffusion of H+ (protons) across the membrane drives ATP synthesis
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.
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Electron transport chain
Figure Electron transport chain Primary acceptor Primary acceptor 4 7 Electron transport chain Fd Pq e 2 e 8 e e H2O NADP 2 H Cytochrome complex NADP reductase + H + 3 1/2 O2 NADPH Pc e e P700 5 P680 Light 1 Light 6 ATP Figure How linear electron flow during the light reactions generates ATP and NADPH. Pigment molecules Photosystem I (PS I) Photosystem II (PS II) 28
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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.
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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.
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ETC of Photosynthesis Photosystem II Photosystem I chlorophyll a
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
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Electron transport chain
Figure 10.17 Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE H Diffusion Intermembrane space Thylakoid space Electron transport chain Inner membrane Thylakoid membrane Figure Comparison of chemiosmosis in mitochondria and chloroplasts. ATP synthase Matrix Stroma ADP P i ATP Key Higher [H ] H Lower [H ] 32
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Light Reactions produces ATP produces NADPH
H2O ATP O2 light energy + NADPH H2O sunlight produces ATP produces NADPH releases O2 as a waste product Energy Building Reactions NADPH ATP O2
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STROMA (low H concentration) Cytochrome complex NADP reductase
Figure 10.18 STROMA (low H concentration) Cytochrome complex NADP reductase Photosystem II Photosystem I Light 3 Light 4 H+ NADP + H Fd Pq NADPH 2 Pc H2O 1 1/2 O2 THYLAKOID SPACE (high H concentration) +2 H+ 4 H+ To Calvin Cycle Figure The light reactions and chemiosmosis: the organization of the thylakoid membrane. Thylakoid membrane ATP synthase ADP + P i ATP STROMA (low H concentration) H+ 34
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Concept 10.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 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.
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From Light reactions to Calvin cycle
chloroplast stroma Need products of light reactions to drive synthesis reactions ATP NADPH stroma ATP thylakoid
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Figure 10.19 The Calvin cycle.
Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) 3-Phosphoglycerate 6 ATP 6 ADP 3 ADP Calvin Cycle 6 P P 3 ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP 6 P i Figure The Calvin cycle. 5 P G3P 6 P Glyceraldehyde 3-phosphate (G3P) Phase 2: Reduction 1 P G3P (a sugar) Glucose and other organic compounds Output 37
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To G3P and Beyond! Glyceraldehyde-3-P G3P is an important intermediate
end product of Calvin cycle energy rich 3 carbon sugar “C3 photosynthesis” G3P is an important intermediate G3P glucose carbohydrates lipids phospholipids, fats, waxes amino acids proteins nucleic acids DNA, RNA
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RuBisCo 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 It’s not easy being green! I’m green with envy!
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Accounting The accounting is complicated
3 turns of Calvin cycle = 1 G3P 3 CO2 1 G3P (3C) 6 turns of Calvin cycle = 1 C6H12O6 (6C) 6 CO2 1 C6H12O6 (6C) 18 ATP + 12 NADPH 1 C6H12O6 any ATP left over from light reactions will be used elsewhere by the cell
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Calvin Cycle builds sugars uses ATP & NADPH recycles ADP & NADP CO2
C6H12O6 + NADP ATP NADPH ADP CO2 builds sugars uses ATP & NADPH recycles ADP & NADP back to make more ATP & NADPH ADP NADP Sugar Building Reactions NADPH ATP sugars
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Photosynthesis summary
Light reactions produced ATP produced NADPH consumed H2O produced O2 as byproduct Calvin cycle consumed CO2 produced G3P (sugar) regenerated ADP regenerated NADP ADP NADP
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Putting it all together
CO2 H2O C6H12O6 O2 light energy + H2O CO2 Plants make both: energy ATP & NADPH sugars sunlight ADP NADP Energy Building Reactions Sugar Building Reactions NADPH ATP O2 sugars
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Energy cycle ATP Photosynthesis Cellular Respiration sun CO2 O2 H2O
even though this equation is a bit of a lie… it makes a better story sun Energy cycle Photosynthesis CO2 H2O C6H12O6 O2 light energy + H2O plants CO2 glucose O2 animals, plants CO2 H2O C6H12O6 O2 ATP energy + Cellular Respiration ATP The Great Circle of Life,Mufasa!
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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…not listed in this equation? Where did the CO2 come from? air Where did the CO2 go? carbohydrate (G3P) Where did the H2O come from? soil (xylem) Where did the H2O go? split to donate e- & H+ Where did the energy come from? sun What’s the energy used for? to make ATP in light reactions What will the C6H12O6 be used for? the work of plant life Where did the O2 come from? splitting of H2O Where will the O2 go? air or respiration What else is involved…not listed in this equation? ATP, ADP, NADP, NADPH
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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 CO2 synthesizes 160 billion tons of carbohydrate heterotrophs are dependent on plants as food source for fuel & raw materials
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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 H2O but also limits photosynthesis The closing of stomata reduces access to CO2 and causes O2 to build up These conditions favor an apparently wasteful process called photorespiration © 2011 Pearson Education, Inc.
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Photorespiration: An Evolutionary Relic?
In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate) In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two-carbon compound Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar © 2011 Pearson Education, Inc.
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Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2 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.
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C4 Plants C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells This step requires the enzyme PEP carboxylase PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle © 2011 Pearson Education, Inc.
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Photosynthetic cells of C4 plant leaf PEP carboxylase
Figure 10.20 C4 leaf anatomy The C4 pathway Mesophyll cell Mesophyll cell CO2 Photosynthetic cells of C4 plant leaf PEP carboxylase Bundle- sheath cell Oxaloacetate (4C) PEP (3C) Vein (vascular tissue) ADP Malate (4C) ATP Pyruvate (3C) Bundle- sheath cell Stoma CO2 Calvin Cycle Figure C4 leaf anatomy and the C4 pathway. Sugar Vascular tissue 51
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Photosynthetic cells of C4 plant leaf Bundle- sheath cell
Figure 10.20a C4 leaf anatomy Mesophyll cell Photosynthetic cells of C4 plant leaf Bundle- sheath cell Vein (vascular tissue) Figure C4 leaf anatomy and the C4 pathway. Stoma 52
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The C4 pathway Mesophyll cell CO2 PEP carboxylase Oxaloacetate (4C)
Figure 10.20b The C4 pathway Mesophyll cell CO2 PEP carboxylase Oxaloacetate (4C) PEP (3C) ADP Malate (4C) ATP Pyruvate (3C) Bundle- sheath cell CO2 Calvin Cycle Figure C4 leaf anatomy and the C4 pathway. Sugar Vascular tissue 53
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The effects of such changes are unpredictable and a cause for concern
In the last 150 years since the Industrial Revolution, CO2 levels have risen greatly Increasing levels of CO2 may affect C3 and C4 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.
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CAM Plants Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon CAM plants open their stomata at night, incorporating CO2 into organic acids Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle © 2011 Pearson Education, Inc.
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Calvin Cycle Calvin Cycle
Figure 10.21 Sugarcane Pineapple C4 CAM CO2 CO2 1 CO2 incorporated (carbon fixation) Mesophyll cell Organic acid Organic acid Night Figure C4 and CAM photosynthesis compared. CO2 CO2 Bundle- sheath cell 2 CO2 released to the Calvin cycle Day Calvin Cycle Calvin Cycle Sugar Sugar (a) Spatial separation of steps (b) Temporal separation of steps 56
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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 O2 in our atmosphere © 2011 Pearson Education, Inc.
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H2O CO2 Light NADP ADP + P i Light Reactions: RuBP 3-Phosphoglycerate
Figure 10.22 H2O CO2 Light NADP ADP + P i Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain RuBP 3-Phosphoglycerate Calvin Cycle ATP G3P Figure A review of photosynthesis. Starch (storage) NADPH Chloroplast O2 Sucrose (export) 58
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Electron transport chain Electron transport chain
Figure 10.UN02 Primary acceptor Electron transport chain Primary acceptor Electron transport chain Fd NADP + H H2O Pq NADP reductase O2 Cytochrome complex NADPH Pc Figure 10.UN02 Summary figure, Concept 10.2 Photosystem I ATP Photosystem II 59
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