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CHAPTER 8LECTURE

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Photosynthesis

Chapter 8

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Photosynthesis Overview

• Energy 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

Chloroplast

• 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

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Stages

• Light-dependent reactions– Require light

1.Capture energy from sunlight

2.Make ATP and reduce NADP+ to NADPH

• Carbon fixation reactions or light-independent reactions– Does not require light

3.Use ATP and NADPH to synthesize organic molecules from CO2

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Discovery of Photosynthesis

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

was not produced only from the soil

• Joseph Priestly (1733–1804)– Living vegetation adds something to the air

• Jan Ingen-Housz (1730–1799)– Proposed plants carry out a process that uses

sunlight to split carbon dioxide into carbon and oxygen (O2 gas)

• 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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Pigments

• Molecules that absorb light energy in the visible range

• Light is a form of energy

• Photon – particle of light– Acts as a discrete bundle of energy– Energy content of a photon is inversely

proportional to the wavelength of the light

• 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 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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Chlorophylls

• 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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Pigments

Pigments:

• 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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• Carotenoids– 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 areas18

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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 molecule

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

4. Chemiosmosis– Produces ATP

Cap

ture

of

light

ene

rgy

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• In sulfur bacteria, only one photosystem is used

• Generates ATP via electron transport

• Excited electron passed to electron transport chain

• Generates a proton gradient for ATP synthesis

Cyclic photophosphorylation

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Chloroplasts have 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 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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Noncyclic 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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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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Production of additional ATP

• 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 ATP35

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Carbon Fixation – Calvin Cycle

• To build carbohydrates cells use

• Energy– 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 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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3 phases

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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Output of Calvin cycle

• Glucose is not a direct product of the Calvin cycle

• G3P 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 cycle

• Photosynthesis 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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Photorespiration

• Rubisco 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 photosynthesis

• C3

– 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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C4 plants

• Corn, 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

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• 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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CAM plants

• Many 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

• 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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Compare C4 and CAM

• Both use both C3 and C4 pathways

• C4 – two pathways occur in different cells

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

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