chapter 8: photosynthesis: capturing energy. photosynthesis: – absorb and convert light energy...

Chapter 8: Photosynthesis: Capturing Energy

• Photosynthesis: – absorb and convert light energy into stored

chemical energy of organic molecules

Electromagnetic Spectrum

• Wavelength – all radiation travels in waves• Visible spectrum –

– 760 nm (red) - 380nm (purple)

UV

Fig. 10-6

Visible light

InfraredMicro-waves

RadiowavesX-raysGamma

rays

103 m1 m

(109 nm)106 nm103 nm1 nm10–3 nm10–5 nm

380 450 500 550 600 650 700 750 nm

Longer wavelength

Lower energyHigher energy

Shorter wavelength

Light

• Behaves as waves and particles• Photons = particles/packets of energy• E= hc/λ

2 ways excited electrons can behave (when absorb photon of light)

• 1st shifts to higher-energy orbital, THEN• 1) atom can return to ground state (e- are in

normal, lowest energy levels)– Energy lost as heat or light (fluorescence)

• 2) e- can leave atom and be accepted by e- acceptor molecule– photosynthesis

Fig. 10-11

(a) Excitation of isolated chlorophyll molecule

Heat

Excitedstate

(b) Fluorescence

Photon Groundstate

Photon(fluorescence)

En

erg

y o

f el

ectr

on

e–

Chlorophyllmolecule

Photosynthesis in Chloroplasts

• Chlorophyll - green pigment, in chloroplasts, mesophyll

• Chloroplast – – Outer membrane – Inner membrane – encloses stroma

• Stroma (fluid-filled, enzymes to make carbs.)

• Thylakoids – – in stroma, 3rd sys. Of membranes – forms

interconnected flat, disclike sacs

• Thylakoid lumen – – fluid-filled space inside of thylakoid

• Grana = thylakoid stacks

Fig. 10-3Leaf cross section

Vein

Mesophyll

StomataCO2 O2

ChloroplastMesophyll cell

Outermembrane

Intermembranespace

5 µm

Innermembrane

Thylakoidspace

Thylakoid

GranumStroma

1 µm

Fig. 10-3a

5 µm

Mesophyll cell

StomataCO2 O2

Chloroplast

Mesophyll

Vein

Leaf cross section

Fig. 10-3b

1 µm

Thylakoidspace

Chloroplast

GranumIntermembranespace

Innermembrane

Outermembrane

Stroma

Thylakoid

Chlorophyll

• Thylakoid membrane• Main pigment of photosynthesis• Absorbs mostly blue/red wavelengths• Green – green light is scattered/reflected

Fig. 10-7

Reflectedlight

Absorbedlight

Light

Chloroplast

Transmittedlight

Granum

2 main parts of Chlorophyll

• 1) complex ring = porphyrin ring– Joined smaller rings of C and N– Absorbs light energy– Magnesium in center

• 2) long side chain – Hydrocarbons– Extremely nonpolar

Fig. 10-10

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

in chlorophyll aCH3

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

CHO in chlorophyll b

Types of Chlorophylls• Chlorophyll a

– Most important– Bright green– Initiate light-dependent reactions

• Chlorophyll b– Accessory pigment– Yellow-green– Different functional group on porphyrin ring – shifts λ of

light that is absorbed/reflected

• Carotenoids – Accessory – yellow, orange

Spectrums

• Absorption spectrum – plot of a PIGMENT’S absorption of light of different λ

• Action spectrum – gives relative effectiveness of different λs of light in photosynthesis (PROCESS)– Rate of photosynthesis is measured at each λ for

leaf cells/tissues exposed to monochromatic light

Photosynthesis simplified:

• 6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2O

• Redox– e- transferred from e- donor (reducing agent) to an e-

acceptor (oxidizing agent)

• Many complex steps• 2 parts:

– Light-dependent (photo) – thylakoids– Carbon fixation (synthesis) - stroma

Reactants:

Fig. 10-4

6 CO2

Products:

12 H2O

6 O26 H2OC6H12O6

Light

Fig. 10-5-1

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

Light

Fig. 10-5-2

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

Light

Fig. 10-5-3

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

CalvinCycle

CO2

Light

Fig. 10-5-4

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

CalvinCycle

CO2

[CH2O]

(sugar)

Overview of light-dependent reactions

• Chlorophyll captures light energy• 1 e- moves to higher state• e- transferred to acceptor molecule, replaced by

e- from water• Water is split• Oxygen released• Need some energy for

– ADPATP– NADP+ NADPH

Overview of Carbon fixation

• Fix C atoms from CO2 to existing C skeletons• No direct light needed

– “dark” reactions

• Depends on products of light-reactions

Light-Dependent Reactions - Details

• Light energy chemical energy• Summary equation:

Photosystems I and II• Reaction center + many antenna complexes• Antenna complex (light-harvesting) =

– units of chlorophylls a + b and accessory pigments organized with pigment-binding proteins in thylakoid membranes

– Absorbs light energy and transfers it to reaction center

• Reaction center = – complex of chlorophyll molecules + proteins– Light energy chemical energy by series of e- transfers

• Photosystem I – chlorophyll a – 700 nm (P700)• Photosystem II – chlorophyll a 680 nm (P680)• Pigment absorbs light energy• Energy passed from 1 pigment molecule to

another until it reaches P700 or P680 at reaction center

• e- raised to higher energy level• e- donated to e- acceptor

Fig. 10-12

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

STROMA

e–

Pigmentmolecules

Photon

Transferof energy

Special pair ofchlorophyll amolecules

Th

yla

koid

me

mb

ran

e

Photosystem

Primaryelectronacceptor

Reaction-centercomplex

Light-harvestingcomplexes

Noncyclic electron transport

• Makes ATP and NADPH• Continuous linear process

– 1 way flow of e- from water to NADP+– Water photolysis e- to P680 ETC (e- lose

energy) P700 ETC NADP+

• See diagram

• A photon hits a pigment and its energy is passed among pigment molecules until it excites P680

• An excited electron from P680 is transferred to the primary electron acceptor

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Pigmentmolecules

Light

P680

e–2

1

Fig. 10-13-1

Photosystem II(PS II)

Primaryacceptor

• P680+ (P680 that is missing an electron) is a very strong oxidizing agent

• H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680

• O2 is released as a by-product of this reaction


Photolysis

• “light-splitting”• Catalyzed by manganese-containing enzyme;

breaks water into 2e-, 2p+ and O• Each e- donated to P680• p+ released into thylakoid lumen• 2 water must split to yield 1 O2 atmosphere

• 2H2O O2 + 4H+

Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

Fig. 10-13-2


• Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I

• Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane

• Diffusion of H+ (protons) across the membrane drives ATP synthesis


Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

4

Pq

Pc

Cytochromecomplex

Electron transport chain

5

ATP

Fig. 10-13-3


• In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor

• P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain


Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

4

Pq

Pc

Cytochromecomplex


5

ATP

Photosystem I(PS I)

Light

Primaryacceptor

e–

P700

6

Fig. 10-13-4


• Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)

• The electrons are then transferred to NADP+ and reduce it to NADPH

• The electrons of NADPH are available for the reactions of the Calvin cycle


Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

4

Pq

Pc

Cytochromecomplex


5

ATP

Photosystem I(PS I)

Light

Primaryacceptor

e–

P700

6

Fd


NADP+

reductase

NADP+

+ H+

NADPH

8

7

e–

e–

6

Fig. 10-13-5


Fig. 10-14

MillmakesATP

e–

NADPH

Ph

oto

n

e–

e–

e–

e–

e–

Ph

oto

n

ATP

Photosystem II Photosystem I

e–

Cyclic Electron Transport (simplest light-dependent reaction)

• Makes ATP, no NADPH• Only Photosystem I• Cyclic – energized e- that originate from P700

eventually return to P700• Light – continuous flow of e- through ETC in

thylakoid membrane

• e- passed from 1 acceptor to another, e- lose energy (some energy used to pump protons across thylakoid membranes)

• ATP synthase uses proton gradient to make ATP

• NADPH not made, water not split, O2 not made

Fig. 10-15

ATPPhotosystem II

Photosystem I

Primary acceptor

Pq

Cytochromecomplex

Fd

Pc

Primaryacceptor

Fd

NADP+

reductaseNADPH

NADP+

+ H+

ATP synthesis

• By chemiosmosis• Photosystem II – as e- passed down ETC, some

energy releases (exergonic)• Some energy not released drives synthesis

of ATP (endergonic)• Synthesis of ATP (P +ADP) is coupled with e-

energized by light (photo), process = photophosphorylation

Fig. 10-17

Light

Fd

Cytochromecomplex

ADP +

i H+

ATPP

ATPsynthase

ToCalvinCycle

STROMA(low H+ concentration)

Thylakoidmembrane

THYLAKOID SPACE(high H+ concentration)

STROMA(low H+ concentration)

Photosystem II Photosystem I

4 H+

4 H+

Pq

Pc

LightNADP+

reductase

NADP+ + H+

NADPH

+2 H+

H2OO2

e–

e–

1/21

2

3

Light Reactions

Carbon Fixation

• Requires ATP + NADPH – Energy used to form organic molecules from CO2

• Summary equation:

Calvin Cycle

• Most plants use – C3

• In stroma – 13 reactions• 3 phases:

– CO2 uptake

– Carbon reduction– RuBP regeneration

CO2 Uptake

• One reaction• CO2 + ribulose biphosphate (RuBP) [5-C]

• Enzyme = ribulose biphosphate carboxylase/oxygenase (Rubisco)

• Product = unstable 6-C intermediate• Immediately 2 phosphoglycerate (PGA) (3-

C each)• C3 pathway

Carbon Reduction phase• 2 steps• Energy from ATP and NADPH converts PGA

molecules to glyceraldehyde-3-phosphate (G3P)• For net synthesis of 1 G3P, the cycle must take

place three times, fixing 3 molecules of CO2

• 6C enter as CO2, 6C leave as 2 – G3P (can form glucose or fructose)

• 2 – G3P removed from cycle, 10 G3P remain = 30 C atoms total

RuBP regeneration phase

• 10 reactions• 30 C rearranged into 6 ribulose phosphate

(+P) RuBP (5-C where cycle started)

Fig. 10-18-1

Ribulose bisphosphate(RuBP)

3-Phosphoglycerate

Short-livedintermediate

Phase 1: Carbon fixation

(Entering oneat a time)

Rubisco

Input

CO2

P

3 6

3

3

P

PPP

Fig. 10-18-2


3-Phosphoglycerate




Rubisco

Input

CO2

P

3 6

3

3

P

PPP

ATP6

6 ADP

P P6

1,3-Bisphosphoglycerate

6

P

P6

66 NADP+

NADPH

i

Phase 2:Reduction

Glyceraldehyde-3-phosphate(G3P)

1 POutput G3P

(a sugar)

Glucose andother organiccompounds

CalvinCycle

Fig. 10-18-3


3-Phosphoglycerate




Rubisco

Input

CO2

P

3 6

3

3

P

PPP

ATP6

6 ADP

P P6

1,3-Bisphosphoglycerate

6

P

P6

66 NADP+

NADPH

i

Phase 2:Reduction

Glyceraldehyde-3-phosphate(G3P)

1 POutput G3P

(a sugar)

Glucose andother organiccompounds

CalvinCycle

3

3 ADP

ATP

5 P

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

G3P

Summary of Carbon Fixation

• Inputs:– 6 CO2

– P from ATP– e- (as hydrogen) from NADPH

• End– 6C hexose molecule remaining G3P make RuBP

which combines with more CO2

Calvin Cycle

Photosynthesis Summary Video

C4 and CAM plants

• Initial carbon fixation step differs – precedes Calvin Cycle; does not replace it

C4 Pathway

• Fixes CO2 at low concentration

• 1st - fix CO2 into 4C oxaloacetate– in mesophyll cells (Calvin in bundle sheath cells)

• PEP carboxylase – catalyzes reaction– CO2 + phosphoenolpyruvate (PEP) (3C)

oxaloacetate

• Oxaloacetate +NADPH usually malate (into bundle sheath) decarboxylation pyruvate (3C) + CO2

• Malate + NADP+ Pyruvate + CO2 + NADPH

• CO2 combines with RuBP Calvin Cycle

• C3-C4 pathway – extra energy for pyruvate PEP ( 30 ATPs per hexose)– Increases CO2 conc. – stomate don’t need to be

open as much promotes rapid growth

• C3 alone (18 ATPs per hexose)

Fig. 10-19

C4 leaf anatomy

Mesophyll cellPhotosyntheticcells of C4

plant leafBundle-sheathcell

Vein(vascular tissue)

Stoma

The C4 pathway

Mesophyllcell CO2PEP carboxylase

Oxaloacetate (4C)

Malate (4C)

PEP (3C)ADP

ATP

Pyruvate (3C)

CO2

Bundle-sheathcell

CalvinCycle

Sugar

Vasculartissue

Fig. 10-19a

Stoma

C4 leaf anatomy

Photosyntheticcells of C4

plant leaf

Vein(vascular tissue)

Bundle-sheathcell

Mesophyll cell

Fig. 10-19b

Sugar

CO2

Bundle-sheathcell

ATP

ADP

Oxaloacetate (4C) PEP (3C)

PEP carboxylase

Malate (4C)

Mesophyllcell

CO2

CalvinCycle

Pyruvate (3C)

Vasculartissue

The C4

pathway

CAM plants

• Fix CO2 at night• Xeric plants• Crassulacean acid metabolism (CAM)• NIGHT = Use PEP carboxylase to fix CO2

oxaloacetate malate stored in vacuoles• DAY = CO2 removed from malate and ready

for Calvin cycle

• C3 and C4 – different location• C3 and CAM – different times, same cell

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

• Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits

• In addition to food production, photosynthesis produces the O2 in our atmosphere


You should now be able to:

1. Describe the structure of a chloroplast2. Describe the relationship between an action

spectrum and an absorption spectrum3. Trace the movement of electrons in linear

electron flow4. Trace the movement of electrons in cyclic

electron flow5. Describe the role of ATP and NADPH in the

Calvin cycle


chapter 8: photosynthesis: capturing energy. photosynthesis: – absorb and convert light energy...

Documents

energy slide

monochromatic light

chlorophyll b slide

orange slide

hc slide

scatteredreflected slide

nonpolar slide

light fluorescence