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Photosynthesis

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Photosynthesis

• All cells can break down organic molecules and use the energy that is released to make ATP.

• Some cells can manufacture organic molecules from inorganic substances using energy from light (photosynthesis) or from inorganic chemicals (chemosynthesis).

• Photosynthesis is the ultimate source of almost all organic molecules used by living organisms. It is also the main source of O2 in the atmosphere.

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Chloroplasts: Sites of Photosynthesis in Plants

• Leaves are the major locations of photosynthesis

• Microscopic pores called stomata allow CO2 to enter the leaf and O2 to exit

• The leaf’s green color is from chlorophyll, the green pigment within chloroplasts

• Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast

Leaf cross sectionVein

Mesophyll

Stomata CO2O2

Mesophyll cellChloroplast

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

Thylakoid

GranumStroma

1 µm

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Chloroplasts: Sites of Photosynthesis in Plants

• Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf

• A typical mesophyll cell has 30-40 chloroplasts

• The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana

• Chloroplasts also contain stroma, a dense fluid where carbon fixation reactions occur.

Leaf cross sectionVein

Mesophyll

Stomata CO2O2

Mesophyll cellChloroplast

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

Thylakoid

GranumStroma

1 µm

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Overall Reaction

• Photosynthesis:

6CO2 + 6H2O + light energy → C6H12O6 + 6O2

• During photosynthesis, H is removed from H2O (leaving O2 as a waste product), energized by light, and then used to reduce CO2 to form glucose.

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The Splitting of Water

• Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

6 CO2 12 H2OReactants:

Products: C6H12O66 H2O 6 O2

Figure 10.4

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Photosynthesis• Photosynthesis occurs in 2 stages:• Light dependent stage or energy capturing reactions• Light independent “Dark” stage or carbon fixation reactions (also called

the Calvin cycle)

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

NADP+

CO2

ADPP+ i

CALVINCYCLE

[CH2O](sugar)

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The Nature of Sunlight

• Light is a form of electromagnetic energy, also called electromagnetic radiation

• Like other electromagnetic energy, light travels in waves

• Wavelength ( ) = distance between crests of waves

• Wavelength determines the type of electromagnetic energy

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• The entire range of electromagnetic energy, or radiation • Visible light consists of colors we can see, including

wavelengths that drive photosynthesis

Visible light

Gammarays

X-rays UV Infrared Micro-waves

Radiowaves

10–5 nm 10–3 nm 1 nm 103 nm 106 nm1 m

(109 nm) 103 m

380 450 500 550 600 650 700 750 nm

Longer wavelength

Lower energy

Shorter wavelength

Higher energy

The Electromagnetic Spectrum

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Visible Spectrum

• The portion of the electromagnetic spectrum that we can see

• White light contains all of the visible spectrum

• Colors are the reflection of specific within the visible spectrum not reflected are absorbed

• Composition of pigments affects their absorption spectrum

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Photosynthetic Pigments: The Light Receptors• Pigments are substances that absorb visible light

• Different pigments absorb different wavelengths

• Wavelengths that are not absorbed are reflected or transmitted

• Leaves appear green because chlorophyll reflects and transmits green light

Chloroplast

LightReflected light

Absorbed light

Transmitted light

Granum

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The Absorption Spectra Of 3 Pigments In Chloroplasts

(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments.

Ab

so

rpti

on

of

lig

ht

by

ch

loro

pla

st

pig

me

nts

Chlorophyll a

Wavelength of light (nm)

Chlorophyll b

Carotenoids

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• Chlorophyll a is the main photosynthetic pigment

• Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis

• Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll

Chlorophyll a

C

CH

CH2

CC

CC

C

CNNC

H3C

C

C

C

C C

C

C

C

N

CC

C

C N

MgH

H3C

H

C CH2CH3

H

CH3C

HHCH2

CH2

CH2

H CH3

C O

O

O

O

O

CH3

CH3

CHO

in chlorophyll a

in chlorophyll b

Porphyrin ring:Light-absorbing“head” of molecule;note magnesiumatom at center

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshown

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Excitation of Chlorophyll by Light• When a pigment absorbs light, it goes from a ground state to an excited

state, which is unstable• When excited electrons fall back to the ground state, photons are given

off, an afterglow called fluorescence• If illuminated, an isolated solution of chlorophyll will fluoresce, giving off

light and heat

Excitedstate

En

erg

y o

f e

lect

ion

e–

Heat

Photon(fluorescence)

Chlorophyllmolecule

GroundstatePhoton

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Photosystems (PS)

• A PS is a collection of pigments and proteins found within the thylakoid membrane that harness the energy of an excited electron to do work

• Captured energy is transferred between PS molecules until it reaches the chlorophyll a molecule at the reaction center

• At the reaction center are 2 molecules• Chlorophyll a• Primary electron acceptor

• The chlorophyll a is oxidized as the electron is passed to primary electron acceptor which is reduced

Thylakoid

Photon

Light-harvestingcomplexes

Photosystem

Reactioncenter

STROMA

Primary electronacceptor

e–

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Th

yla

ko

id m

emb

ran

e

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Photosystems• There are two types of photosystems in the thylakoid membrane

• Photosystem II functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm

• Photosystem I is best at absorbing a wavelength of 700 nm

• The two photosystems work together to use light energy to generate ATP and NADPH

P700

+

Photosystem II (PS II)

O2 [CH2O] (sugar)

e

Primary acceptor

2 H+

1⁄2

H2O

e

e

Pq

Cytochromecomplex

Pc

ATP

Primary acceptor

e

Photosystem I (PS I)

Light

66

Light

5

Fd

NADP+

reductaseNADPH

NADP+

+ 2 H+

+ H+

4

P680

O2

e

e

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Photosystems – Electron Flow

ATP

Photosystem II

e–

e–

e–e–

MillmakesATP

e–

e–

e–

Ph

oto

n

Photosystem I

Ph

oto

n

NADPH

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Electron Flow in Photosystems• There two routes for the path of electrons stored in

the primary electron acceptors depending on the photosynthetic organism• Noncyclic electron flow - Plants, algae,

cyanobacteria • Cyclic electron flow - Bacteria other than

cyanobacteria• Both pathways begin with the capturing of photon

energy and utilize an electron transport chain with cytochromes for chemiosmosis

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Noncyclic Electron Flow• Uses both Photosystem II and I

• Electrons from Photosystem II are removed and replaced by electrons donated from water

• Synthesizes ATP and NADPH

• Electron donation converts water into O2 and 2H+

1. Light excites electrons

2. The electrons energize the reaction center as they are passed to the primary acceptor

3. H2O split via enzyme catalysed reaction forming 2H+, 2e-, and O2. Electrons move to fill orbital vacated by removed electron

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2

En

erg

y o

f el

ectr

on

s

O2

e–

e–

+2 H+

H2O

O21/2

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Noncyclic Electron Flow4. Each excited electron is passed along to an electron transport chain

5. ETC produces ATP through chemiosmotic phosphorylation

6. The electron is now lower in energy and enters photosystem I where it is re-energized utilizing sunlight

7. This e- is then passed to a different electron transport system that includes ferridoxin. The enzyme NADP+ reductase assists in the oxidation of ferridoxin and subsequent reduction of NADP+ to NADPH

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

En

erg

y o

f el

ectr

on

s

O2

e–

e–

+2 H+

H2O

O21/2

Pq

Cytochromecomplex

Electron transport chain

Pc

ATP

P700

e–

Primaryacceptor

Photosystem I(PS I)

e–e–

ElectronTransportchain

NADP+

reductase

Fd

NADP+

NADPH+ H+

+ 2 H+

Light

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Cyclic Electron Flow• Uses Photosystem I only

• Electrons from Photosystem I are recycled

• Synthesizes ATP only

1. Electron in Photosystem I is excited and transferred to ferredoxin that shuttles the electron to the cytochrome complex.

2. The electron then travels down the electron chain and re-enters photosystem I

Photosystem I

Photosystem II ATP

Pc

Fd

Cytochromecomplex

Pq

Primaryacceptor

Fd

NADP+

reductase

NADP+

NADPH

Primaryacceptor

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Photosystems – Electron Flow

ATP

Photosystem II

e–

e–

e–e–

MillmakesATP

e–

e–

e–

Ph

oto

n

Photosystem I

Ph

oto

n

NADPH

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MITOCHONDRIONSTRUCTURE

Intermembranespace

MembraneElectrontransport

chain

Mitochondrion Chloroplast

CHLOROPLASTSTRUCTURE

Thylakoidspace

Stroma

ATP

Matrix

ATPsynthase

Key

H+ Diffusion

ADP + PH+

i

Higher [H+]Lower [H+]

Comparison of Chemiosmosis in Chloroplasts & Mitochondria

• Both the Mitochondria and Chloroplast generate ATP via a proton motive force resulting from an electrochemical inbalance across a membrane

• Both utilize an electron transport chain primarily composed of cytochromes to pump H+ across a membrane.

• Both use a similar ATP synthase complex• Source of “fuel” for the process differs• Location of the H+ “reservoir” differs

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STROMA(Low H+ concentration)

Light

Photosystem IICytochrome

complex

2 H+

Light

Photosystem I

NADP+

reductase

Fd

PcPq

H2O O2

+2 H+

1/22 H+

NADP+ + 2H+

+ H+NADPH

ToCalvincycle

THYLAKOID SPACE(High H+ concentration)

STROMA(Low H+ concentration)

Thylakoidmembrane ATP

synthaseATP

ADP+P

H+i

[CH2O] (sugar)O2

NADPH

ATP

ADPNADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

Light

Overview • Water is split by photosystem II on the side of the membrane facing the thylakoid space

• The diffusion of H+ from the thylakoid space back to the stroma powers ATP synthase

• ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place

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The Calvin Cycle

• The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle

• During the carbon fixation reactions (Calvin cycle) energy from ATP and hydrogen from NADPH are used to reduce CO2 and form glucose.

• Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P)

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• The Calvin cycle has three phases:• Carbon Fixation – attached to 5C sugar• Reduction of CO2, NADPH oxidation• Regeneration of the CO2 acceptor (RuBP)

The Calvin Cycle

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

Light Input

CO2

(Entering oneat a time)

Rubisco

3 P P

Short-livedintermediate

Phase 1: Carbon fixation

6 P

3-Phosphoglycerate6 ATP

6 ADP

CALVINCYCLE

3

P P

Ribulose bisphosphate(RuBP)

3

6 NADP+

6

6 NADPH

P i

6 P

1,3-Bisphosphoglycerate

P

6 P

Glyceraldehyde-3-phosphate(G3P)

P1

G3P(a sugar)Output

Phase 2:Reduction

Glucose andother organiccompounds

3

3 ADP

ATP

Phase 3:Regeneration ofthe CO2 acceptor(RuBP) P5

G3P

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Carbon Fixation

• Starts with CO2• A molecule of CO2 is

converted from its inorganic form to an organic molecule (fixation) through the attachment to a 5C sugar (ribulose bisphosphate or RuBP).

• Catalysed by the enzyme RuBP carboxylase (Rubisco).

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Reduction

• The formed 6C sugar immediately cleaves into 3-phosphoglycerate

• Each 3-phosphoglycerate molecule receives an additional phosphate group forming 1,3-Bisphosphoglycerate (ATP phosphorylation)

• NADPH is oxidized and the electrons transferred to 1,3-Bisphosphoglycerate cleaving the molecule as it is reduced forming Glyceraldehyde 3-phosphate

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Regeneration

• The final phase of the cycle is to regenerate RuBP

• Glyceraldehyde 3-phosphate is converted to RuBP through a series of reactions that involve the phosphorylation of the molecule by ATP

• For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2

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Summary

Light

CO2H2O

Light reactions Calvin cycle

NADP+

RuBP

G3PATP

Photosystem IIElectron transport

chainPhotosystem I

O2

Chloroplast

NADPH

ADP+ P i

3-Phosphoglycerate

Starch(storage)

Amino acidsFatty acids

Sucrose (export)

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The Importance of Photosynthesis: A Review

• The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds

• Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells

• In addition to food production, photosynthesis produces the oxygen in our atmosphere

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P700

+

CO2

Photosystem II (PS II)

H2O

Light

LIGHT REACTIONS

CALVIN CYCLE

O2

NADPH

[CH2O] (sugar)

e

Primary acceptor

2 H+

1⁄2

H2O

e

e

1

Ene

rgy

of e

lect

rons

Pq

Cytochromecomplex

Pc

ATP

Electron transport chain

NADP+

Primary acceptor

e

Photosystem I (PS I)

Light

66

2

Light

ADP

ATP

5

Fd

ElectronTransportchain

7

NADP+

reductaseNADPH

NADP+

+ 2 H+

8

+ H+

1

3

4

P680

O2

e

e