Lesson 8: Photosynthesis

March 17, 2015

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Photosynthesis Overview• Energy for all life on Earth ultimately comes from

photosynthesis

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

• Oxygenic (produces oxygen) photosynthesis is carried out by– Cyanobacteria– All land plants including algae – chloroplasts

• Anoxygenic (no production of oxygen) photosynthesis is carried out by– Purple bacteria, green-sulfur bacteria

Chloroplast Structure

• Thylakoid membrane – internal membrane of chloroplast– Contains chlorophyll and other photosynthetic pigments– Pigments clustered into photosystems

• Grana – stacks of flattened sacs of thylakoid membrane• Stroma lamella – connect grana• Stroma – semi-liquid surrounding thylakoid membranes– Contains enzymes needed to assemble organic molecules

from CO2

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Chloroplast• Each pigment molecule is capable of capturing

photons from light– Results in the passage of energy from one pigment

to the next (photosystems)• Energy arrives at a chlorophyll molecule that contacts

a membrane bound protein– Transfers electron energy through series of

membrane bound proteins (similar to ETC)– Results in the production of ATP and NADPH• This energy is used to build organic molecules

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Photosynthesis Occurs in Stages

1. Light-dependent reactions– Require light (photons)1.Capture energy from sunlight2.Make ATP and reduce NADP+ to NADPH

2. Light-independent reactions or Carbon Fixation REaction– Does not require light3.Use ATP and NADPH to synthesize organic

molecules from CO2

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Grana

Light-Independent Reactions

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Pigments• Molecules that absorb (and reflect) light energy in the visible

range– Colors that we see are colors not absorbed

• Light is a form of energy• Photon – particle of light energy– Energy content of a photon is inversely proportional to the

wavelength of the light• Shorter the wavelength, higher the energy• Visible light is a small fraction of the electromagnetic spectrum

• Photoelectric effect – removal of an electron from a molecule by light

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Absorption Spectrum

• When a photon strikes a molecule, its energy is either – Lost as heat– Absorbed by the electrons of the molecule• Boosts electrons into higher energy level• Absorption depends on energy of photon and chemical

nature of molecule it strikes• Absorption spectrum – range and efficiency of photons a

molecule is capable of absorbing• Pigments are good absorbers of light in the visible range

• Organisms have evolved a variety of different pigments

• Only two general types are used in green plant photosynthesis– Chlorophylls

• Chlorphyll A• Chlorphyll B

– Carotenoids• In some organisms, other molecules also absorb

light energy

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12Absorption spectra for Chlorophyll and Carotenoid

Chlorophylls

• Two types of chlorophylls in plants– Chlorophyll 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 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

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• Action spectrum– Relative effectiveness of different wavelengths of

light in promoting photosynthesis– Corresponds to the absorption spectrum for

chlorophylls

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• Carotenoids– Carbon rings linked to chains

with alternating single and double bonds

– Can absorb photons with a wide range of energies. Not efficient in transferring energy

– Also scavenge free radicals – antioxidant• Protective role

– β-carotene when split forms Vitamin A• Oxidation of Vitamin A

produces retinal pigment used in vision

– Cause for leaves color change?

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Photosynthesis Flash Animation

• Play till first break, Structure of Chloroplasts

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Photosystem Organization

• Antenna 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

Antenna Complex

• Also called light-harvesting complex• Captures 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 Center

• Transmembrane protein–pigment complex• When 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 Reactions

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 molecule3. Electron transport– Electrons move through carriers to reduce NADP+

4. Chemiosmosis– Produces ATP

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Chloroplasts Contain Two Connected Photosystems

• Oxygenic photosynthesis• Photosystem I (P700)– Functions like sulfur bacteria

• Photosystem II (P680)– Can generate an oxidation potential high enough to oxidize

water

• Working together, the two photosystems carry out a non-cyclic 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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Non-Cyclic Photophosphorylation

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

• Path of electrons not a circle• Photosystems replenished with electrons

obtained by splitting water• Z diagram

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Photosystem II

• Resembles the reaction center of purple bacteria• Core 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 water

• b6-f complex– Proton pump embedded in thylakoid membrane

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Photosystem I

• 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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Photosynthesis Flash Animation

• Play till Section 4 , The Calvin Cycle.

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Chemiosmosis

• Electrochemical gradient can be used to synthesize ATP

• Chloroplast 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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Carbon Fixation – Calvin Cycle

• To build carbohydrates cells need• Energy– ATP from light-dependent reactions– Cyclic and non-cyclic photophosphorylation– Drives endergonic reaction

• Reduction Potential– NADPH from photosystem I– Source of protons and energetic electrons

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Calvin Cycle

• Named after Melvin Calvin (1911–1997)• Also called C3 photosynthesis

• Key step is attachment of CO2 to RuBP to form PGA

• Uses enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco

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Three Phases of the Calvin Cycle

1. 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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Photosynthesis Flash Animation

• Play the remainder

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