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 3

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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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• 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

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

Captu

re o

f lig

ht energ

y

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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

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Photosystem II

• Core of 10 transmembrane protein subunits with

electron transfer components and two P680

chlorophyll molecules

– 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 ATP

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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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