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Lecture #9 Photosynthesis

Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

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Page 1: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Lecture #9

Photosynthesis

Page 2: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Photosynthesis

6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O

1.Light Reactions: light + water = O22.Stroma Reactions - Calvin Cycle: CO2 + ATP + NADPH = sugar

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

NADP+

CO2

ADPP+ i

CALVINCYCLE

[CH2O](sugar)

Page 3: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Photosynthesis• 6 CO2 + 12 H2O + Light energy C6H12O6 + 6 O2 + 6 H2 O

• redox process• requires the reduction of carbon – converting it into

carbohydrate• this will require 4 electrons and a good source of energy to

reduce the carbon• electrons come from water• energy comes from light• water and light do not act directly on CO2

– rather they create the intermediates ATP and NAPDH via light-dependent reactions

– the ATP and NADPH then interact with CO2 in the stroma reactions (formerly the dark reactions) to produce carbohydrates

Page 4: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Light• light is a small segment of the electromagnetic radiation spectrum

– from gamma rays to radio waves

• the radiation can be thought of as a set of waves or as a set of energized particles called photons

– each wave has a specific wavelength and photons with specific energy levels

• in photosynthesis – specialized pigments are present to absorb wavelengths of radiation in the visible range

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

Page 5: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

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

• the pigments of photosynthesis are located in the chloroplast

Chloroplast

LightReflected light

Absorbed light

Transmitted light

Granum

• photosynthetic pigments: chlorophylls & carotenoids

– chlorophyll a & chlorophyll b

• transfer absorbed light energy to electrons that then enter chemical reactions

Page 6: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Chlorophyll a

Chlorophyll b

Carotenoids

Wavelength of light (nm)

Absorption spectra

Abs

orp

tion

of

light

by

chlo

rop

last

pig

men

ts

400 500 600 700

• chlorophylls do not absorb light at short wavelengths (e.g. 400nm or less) – and little photosynthesis occurs at those wavelengths

• as wavelengths get longer – absorption increases and so does photosynthesis

• chlorophyll a: peak absorptions at 425nm and 650nm• the accessory pigments – the carotenoids and chlorophyll b–

absorb in wavelengths not covered by the chlorophyll a– the absorbed energy is then passed on to chlorophyll a – broadens the

absorption spectrum of chlorophyll a

Page 7: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Chlorophyll a

Chlorophyll b

Carotenoids

Wavelength of light (nm)

Absorption spectra

Abs

orp

tion

of

light

by

chlo

rop

last

pig

men

ts

400 500 600 700

• the accessory pigments – the carotenoids and chlorophyll b• carotenoid:s peak absorption from 480nm – 500nm• chlorophyll b: peak absorption at 480nm and 680nm• the shorter wavelengths of light have more energy to transfer to the

electron in the chlorophylls – they excite the electron to a higher “state”

• and the electrons emit more energy as they return to the “ground” state

Page 8: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Plastids

• Plastids: group of organelles that perform many functions– synthesis, storage and export– storage plastids for sugar = amyloplasts– plastids with bright red and yellow

pigments = chromoplasts• like mitochondria – plastids are

comprised of an outer and inner membrane– plus an inner fluid = stroma– also have ribosomes and DNA

Page 9: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Photosynthesis:The Chloroplast

• plastids that undergo photosynthesis = chloroplasts

– known as the green plastids due to the presence of chlorophylls

• earliest chloroplasts are called proplastids– once exposed to light – mature into

chloroplasts• like mitochondria – the inner

membrane of the chloroplast is extensively folded to increase surface area for the enzymes of photosynthesis– these folded membranes are called

thylakoid membranes– a stack of thylakoid membranes =

granum• photosynthetic pigments are located in

the thylakoid membranes

Page 10: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

• thylakoid membrane of the chloroplast is the site for the photosynthetic pigments and enzymes of photosynthesis

• PS pigments are the chlorophylls and caretenoids

• chlorophylls have a specific structure• they are amphipathic:

– 1. porphyrin ring for absorbing light• Mg atom at the center surrounded by

numerous N and C rings• only one difference in the porphyrin ring of

chlorophyll a and b – CH3 vs. CHO– 2. hydrocarbon tail for interaction with

the thylakoid membrane

Chlorophyll

CH3

CHO

in chlorophyll a

in chlorophyll b

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

Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes of chloroplasts; H atoms not shown

Page 11: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

• chlorophyll pigments & associated enzymes make up two Photosystems (named in order of the discovery NOT their functional order)– photosystem I – occurs after PSII– photosystem II– each PS has a characteristic reaction center, special chlorophyll

a molecules and specific associated proteins– PSII chlorophyll a = P680– PSI chlorophyll a = P700– absorbed light energizes these two photosystems and induces a

flow of electrons through these photosystems and other molecules built into the thylakoid membrane

– known as the light reactions – there are two possible routes for this electron flow:

• noncyclic• cyclic

Page 12: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Photosystems

Thylakoid

Photon

Light-harvestingcomplexes

Photosystem

Reactioncenter

STROMA

Primary electronacceptor

e–

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Th

yla

koid

me

mb

rane

• embedded in the thylakoid membranes are light harvesting complexes

• light harvesting complexes: proteins and photosynthetic pigments that surround a reaction center

– pigments – chlorophyll b, carotenoids, xanthophylls

– reaction center – pair of chlorophyll a molecules

• act to focus the energy attained from photons (absorbed by the pigments) to the reaction center

– through a process called resonance energy transfer

– when an electron is excited – as it returns to its ground state – can transfer some of its energy to a neighboring molecule

Page 13: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Photosystems

Thylakoid

Photon

Light-harvestingcomplexes

Photosystem

Reactioncenter

STROMA

Primary electronacceptor

e–

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Th

yla

koid

me

mb

rane

• the reaction center contains a pair of chlorophyll a molecules that are different from the light harvesting complexes

– in photosystem II = P680– in photosystem I = P700

• the energy of light (photon) excites the electrons of P680 or P700

• electrons are transferred by electron acceptors located in the thylakoid membrane

• electrons are eventually transferred to a final acceptor = NADP+ reducing it to NADPH

• both photosystems run at the same time since light is absorbed by both photosystems

Page 14: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Light Reactions: Non cyclic electron flow

• 1. a photon of light strikes the PS pigments in the thylakoid membrane (i.e. light-harvesting complex) - the energy is relayed via excited electrons to the two P680 chlorophyll a molecules in the reaction center of PSII– an electron of P680 is excited to a higher

energy state (P680+)• 2. the excited electron from P680+ is

captured by a primary electron acceptor in the reaction center– called phaeophytin

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

Pq

Cytochromecomplex

Electron transport chain

Pc

ATP

since two electrons are created from water – this happens twice

Page 15: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Light Reactions: Non cyclic electron flow

• 3. IN ADDITION: water is split into two H+, two electrons and an oxygen atom– these electrons are transferred to P680 to

replace the electrons it has lost to the primary electron acceptor

– oxygen atoms combine to form O2• 4. each excited electron passes from the

primary electron acceptor of PSII to the reaction center of PSI via an electron transport chain comprised of a cytochrome complex and two cofactors called Pq (plastoquinone) and Pc (plastocyanin) Light

P680

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

Pq

Cytochromecomplex

Electron transport chain

Pc

ATP

Page 16: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

• 5. the exergonic “fall” of an electron to its lower energy state through the electron transport chain provides energy for the creation of ATP

• 6. light energy gets transferred to the PSI complex • 7. WHILE PSII IS ABSORBING LIGHT – SO IS PSI

– photons are absorbed by the light-harvesting complex of the PSI system and this excites an electron within P700 (P700+)

– this electron is captured by the primary acceptor of PSI & creates a “hole” in p700

– the hole in P700 is filled by the electrons that have reached the bottom of the ETC of PSII

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

Ene

rgy

of e

lect

rons

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

Page 17: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

• 8. each photoexcited electron is passed from PSI down a second ETC through a cofactor called ferredoxin (Fd) and ultimately to NADP+ reductase

• 9. NADP+ reductase takes electrons from Fd and passes them to NADP+ (2 electrons) reducing it to NADPH• this requires two electrons (which originally were provided by the splitting of water)

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

Ene

rgy

of e

lect

rons

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

Page 18: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

LE 10-14

ATP

Photosystem II

e–

e–

e–e–

MillmakesATP

e–

e–

e–

Ph

oto

n

Photosystem I

Ph

oto

n

NADPH

Page 19: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

STROMA(Low H+ concentration)

Light

Photosystem II Cytochromecomplex

2 H+

LightPhotosystem I

NADP+

reductaseFd

PcPq

H2O O2

+2 H+

1/22 H+

NADP+ + 2H+

+ H+NADPH

ToCalvincycle

THYLAKOID SPACE(High H+ concentration)

STROMA(Low H+ concentration)

Thylakoidmembrane ATP

synthase

ATPADP

+P

H+i

[CH2O] (sugar)O2

NADPH

ATP

ADPNADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

Light

• as electrons pass from one carrier to another, H+ ions are pumped from the stroma and are deposited in the thylakoid space

• these H+ ions stored in the thylakoid space create a proton gradient• when H+ flows back down its gradient – an enzyme (ATP synthase) uses this energy to

create ATP from ADP

Chemiosmosis

Page 20: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

SOUND FAMILIAR?

MITOCHONDRIONSTRUCTURE

Intermembranespace

MembraneElectrontransport

chain

Mitochondrion Chloroplast

CHLOROPLASTSTRUCTURE

Thylakoidspace

Stroma

ATP

Matrix

ATPsynthase

Key

H+ Diffusion

ADP + P

H+

i

Higher [H+]

Lower [H+]

Page 21: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesis

Page 22: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Cyclic Electron flow• under certain conditions – the cyclic electron flow path is an alternative – short-circuit

path• uses PSI but not PSII• electrons instead of continuing on from ferroredoxin/Fd to NADP+reductase - cycle back

to the cytochrome complex and “re-excite”the P700 chlorophyll a molecules• no production of NADPH and no release of O2• but cyclic flow does generate ATP – since electrons pass through the cytochrome

complex• function??

– noncyclic flow produces NADPH and ATP is roughly equal amounts– the Calvin cycle consumes more ATP than NADPH – creates an ATP “debt”– cyclic electron flow “pays” this ATP debt – makes up the difference– concentration of NADPH may regulate which pathway is taken

Photosystem IPhotosystem II ATP

Pc

Fd

Cytochromecomplex

Pq

Primaryacceptor

Fd

NADP+

reductase

NADP+

NADPH

Primaryacceptor

Page 23: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Non-cyclic and cyclic flow animations

• http://www.mcgrawhill.ca/school/applets/abbio/ch05/phothospo_cyclic_and_no.swf

Page 24: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Stroma Reactions

• light reactions – electron flow pushes electrons from water (low potential energy) to NAPDH (high potential energy)

• so at the end of the light reactions – produced two potential energy sources– ATP– NADPH

• NADPH and ATP shuttle this energy to the Calvin cycle for the production of sugar

• reactions are performed in the stroma of the chloroplast• used to be called the dark reactions – no involvement of light

– happens in the dark

Page 25: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Calvin cycle

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)

• similar to the citric acid cycle – starting material is regenerated after molecules enter and leave the cycle

– citric acid cycle is catabolic: breakdown– oxidizes acetyl CoA and releases energy

– Calvin cycle is anabolic: synthesizes– builds sugar from smaller molecules and requires energy

• spends ATP as a energy source and consumes NAPDH as an electron sourc

• performed by C3 plants – since the first organic product made is a 3 carbon sugar

sugar produced = glyceraldehyde-3-phosphate

Page 26: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Calvin cycle

sugar produced = glyceraldehyde-3-phosphate

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)

• has three phases: – Carbon fixation– Carbon reduction– Regeneration of the CO2 acceptor

Page 27: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Carbon Fixation1. Carbon Fixation: incorporation of CO2 into a 5-carbon sugar called ribulose bisphosphate (RuBP)

• 3 CO2 molecules are attached one at a time to RuBP

• done by the enzyme rubisco – the most abundant protein on Earth??

• so 3 molecules of rubisco are required

• produces a 6 carbon intermediate that is very short lived

• immediately broken down into two molecules of a 3 carbon sugar called

3-phosphoglycerate

[CH2O] (sugar)

NADPH

ATP

ADPNADP+

CO2

CALVINCYCLE

Input

3 CO2

(Entering oneat a time)

Rubisco

P

Short-livedintermediate

Phase 1: Carbon fixation

P6 molecules3-Phosphoglycerate 6 ATP

6 ADP

CALVINCYCLE

P P3 molecules

Ribulose bisphosphate(RuBP)

6 NADP+

6

6 NADPH

Pi

P6 molecules1,3-Bisphosphoglycerate

P

P

6 moleculesGlyceraldehyde-3-phosphate

(G3P)

P1 molecule

G3P

Output

Phase 2:Reduction

Glucose andother organiccompounds

3

3 ADP

ATP

Phase 3:Regeneration ofthe CO2 acceptor(RuBP) P

5 moleculesG3P

Page 28: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

2. Carbon Reduction: -each 3-phosphoglycerate receives an

additional phosphate group from ATP = 1,3-bisphosphoglycerate

• requires 6 molecules of ATP• next - a pair of electrons from

NADPH reduces 1,3-BPG to make the 3 carbon end-product called glyceraldehye 3-phosphate (G3P)

• this consumes 6 molecules of NADPH

• the aldehyde group of G3P stores more potential energy than the bonds of 1,3-BPG

• 1,3-BPG & G3P are the same intermediates produced during glycolysis

Carbon Reduction

[CH2O] (sugar)

NADPH

ATP

ADPNADP+

CO2

CALVINCYCLE

Input

3 CO2

(Entering oneat a time)

Rubisco

P

Short-livedintermediate

Phase 1: Carbon fixation

P6 molecules3-Phosphoglycerate 6 ATP

6 ADP

CALVINCYCLE

P P3 molecules

Ribulose bisphosphate(RuBP)

6 NADP+

6

6 NADPH

Pi

P6 molecules1,3-Bisphosphoglycerate

P

P6 molecules

Glyceraldehyde-3-phosphate(G3P)

P1 molecule

G3P

Output

Phase 2:Reduction

Glucose andother organiccompounds

3

3 ADP

ATP

Phase 3:Regeneration ofthe CO2 acceptor(RuBP) P

5 moleculesG3P

Page 29: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

3. Regeneration of the CO2 acceptor:

- a series of complex steps that requires the carbon skeletons of 5 molecules of G3P

- converts these G3P molecules into three molecules of ribulose bisphosphate

• RuBP is the carbon acceptor of carbon fixation

• cycle spends three more molecules of ATP

Regeneration of Ribulose BP

-for the net synthesis of one G3P sugar – the Calvin cycle consumes 9 ATP and 6 molecules of NAPDH and makes 1 molecule of sugar

[CH2O] (sugar)

NADPH

ATP

ADPNADP+

CO2

CALVINCYCLE

Input

3 CO2

(Entering oneat a time)

Rubisco

P

Short-livedintermediate

Phase 1: Carbon fixation

P6 molecules3-Phosphoglycerate 6 ATP

6 ADP

CALVINCYCLE

P P3 molecules

Ribulose bisphosphate(RuBP)

6 NADP+

6

6 NADPH

Pi

P6 molecules1,3-Bisphosphoglycerate

P

P6 molecules

Glyceraldehyde-3-phosphate(G3P)

P1 molecule

G3P

Output

Phase 2:Reduction

Glucose andother organiccompounds

3

3 ADP

ATP

Phase 3:Regeneration ofthe CO2 acceptor(RuBP) P

5 moleculesG3P

Page 30: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

http://www.science.smith.edu/departments/Biology/Bio231/calvin.html

Page 31: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Arid plants and photosynthesis

• in most plants the initial fixation of carbon occurs by rubisco = C3 plants– e.g. rice, wheat and corn– during a dry, hot day - their stomata are partially closed

• these plants produce less sugar at this point due to declining levels of CO2 in the leaf (starves the Calvin cycle)

• instead, rubisco can bind O2 in place of CO2 – results in a two carbon compound that exits the chloroplast

• the peroxisomes and mitochondria rearrange this 2 carbon compound to regenerate CO2 = photorespiration

– photorespiration – consumes O2 and produces CO2 & occurs in the light– photorespiration in C3 plants does NOT generate ATP and does NOT produce

sugar – so why do it???• may be evolutionary baggage – relic from an earlier time when the

atmosphere has less O2 and more CO2 than it does today• not known currently whether photorespiration benefits the plant

Page 32: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Arid plants and photosynthesis: C4 plants

• in C4 plants the Calvin cycle is prefaced with an alternate mode of carbon fixation and this results in a 4-carbon product

– C4 plants have a unique leaf anatomy– two distinct types of photosynthetic cells: bundle-sheath cells and mesophyll

cells– bundle-sheath cells are arranged as sheaths around the vascular bundles with

mesophyll cells in between these BS cells and the leaf surface– sugar is produced in a three step process:

C4 leaf anatomy

Photosyntheticcells of C4 plantleaf

Mesophyll cell

Bundle-sheathcell

Vein(vascular tissue)

Stoma

Page 33: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

Arid plants and photosynthesis: C4 plants

Bundle-sheathcell

Pyruvate (3 C)

CO2

Sugar

Vasculartissue

CALVINCYCLE

PEP (3 C)

ATP

ADP

Malate (4 C)

Oxaloacetate

CO2PEP carboxylaseMesophyllcell

• 3 step process in C4 plants:– 1. CO2 enters the mesophyll cells of

the leaf and is added to a 3 carbon substrate called PEP (phosphoenolpyruvate) to eventually generate a 4 carbon sugar (malate)

• done by the enzyme called PEP carboxylase

• CO2 addition to PEP produces a 4 carbon compound called oxaloacetate which is then converted into a 4 carbon sugar called malate

– 2. malate enters the bundle sheath cells & is converted back into a 3 carbon sugar called pyruvate

– 3. this results in the liberation of CO2 which then enters the Calvin cycle for the production of 3-glyceraldehyde phosphate

– 4. the pyruvate is converted back into PEP (requires ATP)

in arid climates the mesophyll cells bring CO2 into the cell to keep the CO2 levels high in the leaf and ensure an efficient Calvin cycle

Page 34: Lecture #9 Photosynthesis. 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 1.Light Reactions: light + water = O2 2.Stroma Reactions

• CAM plants – succulents, many cacti, pineapples

– open their stomata at night only

– at night - incorporate the CO2 into a variety of 4-C organic acids through the crassulacean acid metabolic (CAM) pathway

– the mesophyll cells store these organic acids they make during the night in vacuoles

– in the morning - the stomata close and ATP and NAPDH are made by the light reactions

– the organic acids then release the CO2 so it can enter the Calvin cycle

– Calvin cycle happens in the mesophyll cells of CAM plants

– C4 plants: organic acid synthesis and Calvin cycle happen in different cells (mesophyll and bundle-sheath)

(not at a particular time of the day)

Bundle-sheathcell

Mesophyllcell

Organic acid

C4

CO2

CO2

CALVINCYCLE

Sugarcane Pineapple

Organic acidsrelease CO2 toCalvin cycle

CO2 incorporatedinto four-carbonorganic acids(carbon fixation)

Organic acid

CAMCO2

CO2

CALVINCYCLE

Sugar

Spatial separation of steps Temporal separation of steps

Sugar

Day

Night