PhotosynthesisPhotosynthesis

Chapter 8

Photosynthesis OverviewPhotosynthesis OverviewEnergy for all life on Earth ultimately

comes from photosynthesis

6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

Oxygenic photosynthesis is carried out by◦Cyanobacteria◦7 groups of algae◦All land plants – chloroplasts

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ChloroplastChloroplast

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Thylakoid membrane – internal membrane◦ Contains chlorophyll and

other photosynthetic pigments

◦ Pigments clustered into photosystems

Grana – stacks of flattened sacs of thylakoid membrane

Stroma lamella – connect grana

Stroma – semiliquid surrounding thylakoid membranes

StagesStages

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Light-dependent reactions◦ Require light1.Capture energy from sunlight2.Make ATP and reduce NADP+ to NADPH

Carbon fixation reactions or light-independent reactions◦ Does not require light3.Use ATP and NADPH to synthesize organic molecules

from CO2

Animation

Discovery of Discovery of PhotosynthesisPhotosynthesis

Jan Baptista van Helmont (1580–1644)◦Demonstrated that the substance of the

plant was not produced only from the soilJoseph Priestly (1733–1804)

◦Living vegetation adds something to the airJan Ingen-Housz (1730–1799)

◦Proposed plants carry out a process that uses sunlight to split carbon dioxide into carbon and oxygen (O2 gas)

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F.F. Blackman (1866–1947)◦ Came to the startling conclusion that

photosynthesis is in fact a multistage process, only one portion of which uses light directly

◦ Light versus dark reactions◦ Enzymes involved

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C. B. van Niel (1897–1985)◦Found purple sulfur bacteria do not release

O2 but accumulate sulfur

◦Proposed general formula for photosynthesis CO2 + 2 H2A + light energy → (CH2O) + H2O + 2 A

◦Later researchers found O2 produced comes from water

Robin Hill (1899–1991)◦Demonstrated Niel was right that light

energy could be harvested and used in a reduction reaction

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PigmentsPigmentsMolecules that absorb light energy in

the visible rangeLight is a form of energyPhoton – particle of light

◦Acts as a discrete bundle of energy◦Energy content of a photon is inversely

proportional to the wavelength of the lightPhotoelectric effect – removal of an

electron from a molecule by light

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Absorption spectrumAbsorption spectrumWhen a photon strikes a molecule, its

energy is either ◦Lost as heat◦Absorbed by the electrons of the molecule

Boosts electrons into higher energy levelAbsorption spectrum – range and efficiency

of photons molecule is capable of absorbing

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Organisms have evolved a variety of different pigments

Only two general types are used in green plant photosynthesis◦Chlorophylls◦Carotenoids

In some organisms, other molecules also absorb light energy

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Many pigmentsMany pigments

ChlorophyllsChlorophyllsChlorophyll a

◦Main pigment in plants and cyanobacteria

◦Only pigment that can act directly to convert light energy to chemical energy

◦Absorbs violet-blue and red light Chlorophyll b

◦Accessory pigment or secondary pigment absorbing light wavelengths that chlorophyll a does not absorb

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Structure of chlorophyllStructure of chlorophyll

Porphyrin ring◦ Complex ring structure

with alternating double and single bonds

◦ Magnesium ion at the center of the ring

Photons excite electrons in the ring

Electrons are shuttled away from the ring

Relative effectiveness of different wavelengths of light in promoting photosynthesis

Corresponds to the absorption spectrum for chlorophylls

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Action spectrumAction spectrum

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Other PigmentsOther PigmentsCarotenoids

◦ Carbon rings linked to chains with alternating single and double bonds

◦ Can absorb photons with a wide range of energies

◦ Also scavenge free radicals – antioxidant Protective role

Phycobiloproteins◦ Important in low-light ocean

areas

Photosystem OrganizationPhotosystem OrganizationAntenna complex

◦Hundreds of accessory pigment molecules

◦Gather photons and feed the captured light energy to the reaction center

Reaction center◦1 or more chlorophyll a molecules◦Passes excited electrons out of the

photosystem

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Antenna complexAntenna complexAlso called light-harvesting complexCaptures photons from sunlight and

channels them to the reaction center chlorophylls

In chloroplasts, light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins

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Reaction centerReaction centerTransmembrane protein–

pigment complexWhen a chlorophyll in

the reaction center absorbs a photon of light, an electron is excited to a higher energy level

Light-energized electron can be transferred to the primary electron acceptor, reducing it

Oxidized chlorophyll then fills its electron “hole” by oxidizing a donor molecule

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Light-Dependent Light-Dependent ReactionsReactions

1. Primary photoevent◦ Photon of light is captured by a pigment

molecule

2. Charge separation ◦ Energy is transferred to the reaction

center; an excited electron is transferred to an acceptor molecule

3. Electron transport◦ Electrons move through carriers to reduce

NADP+

4. Chemiosmosis◦ Produces ATP

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Cap

ture

of

light

ene

rgy

Cyclic Cyclic photophosphorylationphotophosphorylation

In sulfur bacteria, only one photosystem is used

Generates ATP via electron transport

Anoxygenic photosynthesis

Excited electron passed to electron transport chain

Generates a proton gradient for ATP synthesis

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Chloroplasts have two connected Chloroplasts have two connected photosystemsphotosystems

Oxygenic photosynthesisPhotosystem I (P700)

◦ Functions like sulfur bacteria

Photosystem II (P680)◦ Can generate an oxidation potential high

enough to oxidize waterWorking together, the two photosystems

carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH

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Photosystem I transfers electrons ultimately to NADP+, producing NADPH

Electrons lost from photosystem I are replaced by electrons from photosystem II

Photosystem II oxidizes water to replace the electrons transferred to photosystem I

2 photosystems connected by cytochrome/ b6-f complex

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The connectionThe connection

Noncyclic Noncyclic photophosphorylationphotophosphorylation

Plants use photosystems II and I in series to produce both ATP and NADPH

Path of electrons not a circlePhotosystems replenished with

electrons obtained by splitting waterZ diagram

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Photosystem IIPhotosystem IIResembles the reaction

center of purple bacteriaCore of 10 transmembrane

protein subunits with electron transfer components and two P680 chlorophyll molecules

Reaction center differs from purple bacteria in that it also contains four manganese atoms◦ Essential for the oxidation of

waterb6-f complex

◦ Proton pump embedded in thylakoid membrane

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Photosystem IPhotosystem I

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Reaction center consists of a core transmembrane complex consisting of 12 to 14 protein subunits with two bound P700 chlorophyll molecules

Photosystem I accepts an electron from plastocyanin into the “hole” created by the exit of a light-energized electron

Passes electrons to NADP+ to form NADPH

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Photophosphorylation (Photophosphorylation.EXE)

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bio13.exe

ChemiosmosisChemiosmosisElectrochemical gradient can be used

to synthesize ATPChloroplast has ATP synthase

enzymes in the thylakoid membrane◦Allows protons back into stroma

Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions

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Production of additional Production of additional ATPATP

Noncyclic photophosphorylation generates◦NADPH◦ATP

Building organic molecules takes more energy than that alone

Cyclic photophosphorylation used to produce additional ATP◦Short-circuit photosystem I to make a

larger proton gradient to make more ATP29

Question 7Question 7Where does the Calvin Cycle take place?A. stroma of the chloroplastB. thylakoid membraneC. cytoplasm surrounding the chloroplastD. chlorophyll moleculeE. outer membranes of the chloroplast

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Carbon Fixation – Calvin Carbon Fixation – Calvin CycleCycle

To build carbohydrates cells useEnergy

◦ATP from light-dependent reactions◦Cyclic and noncyclic photophosphorylation◦Drives endergonic reaction

Reduction potential◦NADPH from photosystem I◦Source of protons and energetic electrons

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Calvin cycleCalvin cycleNamed after Melvin Calvin (1911–

1997)Also called C3 photosynthesisKey step is attachment of CO2 to

RuBP to form PGAUses enzyme ribulose bisphosphate

carboxylase /oxygenase carboxylase /oxygenase or rubisco

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3 phases3 phases1. Carbon fixation

◦ RuBP + CO2 → PGA

2. Reduction◦ PGA is reduced to G3P

3. Regeneration of RuBP◦ PGA is used to regenerate RuBP

3 turns incorporate enough carbon to produce a new G3P

6 turns incorporate enough carbon for 1 glucose

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Output of Calvin cycleOutput of Calvin cycleGlucose is not a direct product of the

Calvin cycleG3P is a 3 carbon sugar

◦Used to form sucrose Major transport sugar in plants Disaccharide made of fructose and glucose

◦Used to make starch Insoluble glucose polymer Stored for later use

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Energy cycleEnergy cyclePhotosynthesis uses the

products of respiration as starting substrates

Respiration uses the products of photosynthesis as starting substrates

Production of glucose from G3P even uses part of the ancient glycolytic pathway, run in reverse

Principal proteins involved in electron transport and ATP production in plants are evolutionarily related to those in mitochondria

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PhotorespirationPhotorespirationRubisco has 2 enzymatic activities

◦Carboxylation Addition of CO2 to RuBP Favored under normal conditions

◦Photorespiration Oxidation of RuBP by the addition of O2 Favored when stoma are closed in hot conditions Creates low-CO2 and high-O2

CO2 and O2 compete for the active site on RuBP

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Types of photosynthesisTypes of photosynthesisC3

◦ Plants that fix carbon using only C3 photosynthesis (the Calvin cycle)

C4 and CAM◦ Add CO2 to PEP to form 4 carbon molecule

◦ Use PEP carboxylase

◦ Greater affinity for CO2, no oxidase activity

◦ C4 – spatial solution

◦ CAM – temporal solution

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CC44 plants plantsCorn, sugarcane, sorghum,

and a number of other grasses

Initially fix carbon using PEP carboxylase in mesophyll cells

Produces oxaloacetate, converted to malate, transported to bundle-sheath cells

Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO2

Carbon fixation then by rubisco and the Calvin cycle 39

C4 pathway, although it overcomes the problems of photorespiration, does have a cost

To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone

C4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C3 pathway alone

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The cost of doing business The cost of doing business this waythis way

CAM plantsCAM plantsMany succulent (water-storing)

plants, such as cacti, pineapples, and some members of about two dozen other plant groups

Stomata open during the night and close during the day◦Reverse of that in most plants

Fix CO2 using PEP carboxylase during the night and store in vacuole

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When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO2

High levels of CO2 drive the Calvin cycle and minimize photorespiration

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CAM plants in actionCAM plants in action

Compare CCompare C44 and CAM and CAMBoth use both C3

and C4 pathwaysC4 – two

pathways occur in different cells

CAM – C4 pathway at night and the C3 pathway during the day

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