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Photosynthesis

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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

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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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Vascular bundle Stoma

Cuticle

Epidermis

Mesophyll

Chloroplast

Inner membrane

Outer membrane

Cell wall

1.58 mm

Vacuole

Courtesy Dr. Kenneth Miller, Brown University

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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6

O2

Stroma

Photosystem

Thylakoid

NADP+ADP + Pi

CO2

Sunlight

Photosystem

Light-Dependent

Reactions

Calvin

Cycle

Organic

molecules

O2

ATP NADPH

H2O


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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400 nm

Visible light

430 nm 500 nm 560 nm 600 nm 650 nm 740 nm

1 nm0.001 nm 10 nm 1000 nm

Increasing wavelength

Increasing energy

0.01 cm 1 cm 1 m

Radio wavesInfraredX-raysGamma rays

100 m

UV

light

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 a molecule is capable of absorbing

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10

Wavelength (nm)

400 450 500 550 600 650 700

Lig

ht

Ab

so

rbti

on

low

highcarotenoidschlorophyll a

chlorophyll b


– 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

11Pigments in Photosynthesis

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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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

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e–Photon

Photosystem

Thylakoid membrane

Chlorophyll

molecule

Electron

acceptor

Reaction center

chlorophyll

Thylakoid membrane

Electron

donor e–

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

e–

–+–+

Excited

chlorophyll

molecule

Electron

donorElectron

acceptor

Chlorophyll

reduced

Chlorophyll

oxidizedDonor

oxidizedAcceptor

reduced

e–

e– e–

e–

e–

e–

e–

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

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Captu

re o

f lig

ht

energ

y

Cyclic photophosphorylation

– 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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19


En

erg

y o

f ele

ctr

on

s

High

Low

e–

Photon

Photosystem

Excited reaction center

Electron

acceptor

Electron

acceptor

Reaction

center (P870)

b-c1

complex ATPe–

e–

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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En

erg

y o

f ele

ctr

on

s

Photon



Plastoquinone

Plastocyanin

Ferredoxin

Photosystem II

Photosystem I

Photon

b6-f

complex

3. A pair of chlorophylls in the reaction

center absorb two photons. This

excites two electrons that are passed to

NADP+, reducing it to NADPH. Electron

transport from photosystem II replaces

these electrons.

H2O

H+PC

Fd

2H+ + 1/2O2

NADP+ + H+

2

2

2

2

2

1. A pair of chlorophylls in the reaction center absorb

two photons of light. This excites two electrons that

are transferred to plastoquinone (PQ). Loss of

electrons from the reaction center produces an

oxidation potential capable of oxidizing water.

Reaction

center

Proton gradient formed

for ATP synthesis

Reaction

center

e–

e–

PQ

e–

NADP

reductase

NADPHe–

2. The electrons pass through the b6-f

complex, which uses the energy

released to pump protons across

the thylakoid membrane. The proton

gradient is used to produce ATP by

chemiosmosis.

e–

Photosystem II

– Core of 10 transmembrane protein subunits with electron transfer components and two P680 chlorophyll molecules

– Reaction center 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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Photosystem II Photosystem Ib6-f complex

Stroma

Plastoquinone

Proton

gradientPlastocyanin Ferredoxin

H+

H+H+

H+

NADPH

ATPADP

+ NADP+

NADPHNADPATPADP + Pi

Calvin

Cycle

PhotonPhoton

H2O

e–e–

e–

Fd

PC

PQ

1. Photosystem II

absorbs photons,

exciting electrons

that are passed to

plastoquinone (PQ).

Electrons lost from

photosystem II are

replaced by the

oxidation of water,

producing O2

2. The b6-f complex

receives electrons

from PQ and passes

them to plastocyanin

(PC). This provides

energy for the b6-f

complex to pump

protons into the

thylakoid.

3. Photosystem I absorbs

photons, exciting

electrons that are

passed through a

carrier to reduce

NADP+ to NADPH.

These electrons are

replaced by electron

transport from

photosystem II.

4. ATP synthase uses

the proton gradient

to synthesize ATP

from ADP and Pi

enzyme acts as a

channel for protons

to diffuse back into

the stroma using this

energy to drive the

synthesis of ATP.

NADP

reductase

ATP

synthase

1/2O2 2H+

Water-splitting

enzyme

Thylakoid

space

Antenna

complexThylakoid

membrane

Light-Dependent

Reactions

H+

H+

e–22 22

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22

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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4 Pi

12 NADP+

12

12 ADP

NADPHNADP+ADP+ Pi

Light-Dependent

Reactions

Calvin

Cycle

6 molecules of12 molecules of

12 molecules of

1,3-bisphosphoglycerate (3C)

12 molecules of

Glyceraldehyde 3-phosphate (3C) (G3P)

10 molecules of

Glyceraldehyde 3-phosphate (3C) (G3P)

Stroma of chloroplast6 molecules of

Carbon

dioxide (CO2)

12 ATP

6 ADP

6 ATP

Rubisco

Calvin Cycle

Pi

Ribulose 1,5-bisphosphate (5C) (RuBP)3-phosphoglycerate (3C) (PGA)

Glyceraldehyde 3-phosphate (3C)

2 molecules of

Glucose and

other sugars

12 NADPH

ATP

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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