PHOTOSYNTHESIS

1. Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes.

2. These organisms feed not only themselves but also the entire living world.

LE 10-2

Plants

Unicellular protist

Multicellular algae Cyanobacteria

Purple sulfurbacteria

10 µm

1.5 µm

40 µm

3. Photosynthesis occurs in the chloroplasts ofgreen plants.

4. General equation:6H2O + 6CO2 ----------------- C6H12O6 + 6O2Chlorophyll

Light

5. Recall that the chloroplast has a double outer membrane, a liquid stroma and an extensive inner membrane system made ofthylakoids that occur in stacks (grana).

LE 10-3

Leaf cross sectionVein

Mesophyll

Stomata CO2O2

Mesophyll cellChloroplast

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

Thylakoid

GranumStroma

1 µm

6. There are 2 basic parts of photosynthesis:light-dependent and light-independentreactions.

LE 10-5_1

H2O

LIGHTREACTIONS

Chloroplast

Light

LE 10-5_2

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

LE 10-5_3

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

NADP+

CO2

ADPP+ i

CALVINCYCLE

[CH2O](sugar)

7. Embedded in the thylakoid membranes arethe photosystems (PS I and PSII).

9. Both types of photosystems are involved innoncyclic photophosphorylation. Only PSIis involved in cyclic photophosphorylation.

8. Photosystem = light harvesting complex +a reaction center. A reaction center = a chlorophyll molecule and a protein.

LE 10-12

Thylakoid

Photon

Light-harvestingcomplexes

Photosystem

Reactioncenter

STROMA

Primary electronacceptor

e–

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Th

ylak

oid

mem

bra

ne

-Each photosystem can absorb a different wavelength of light. PSI absorbs the far-red part of the spectrum best. PSII absorbs the not-so-far red part of the spectrum.

10. Noncyclic photophosphorylation a. Light energizes chlorophyll in PS II exciting

an electron (e-) which moves to the electrontransport chain. Now we see how photosynthesis uses light energy to get started.

QuickTime™ and a decompressor

are needed to see this picture.

LE 10-13_1

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

sO2

b. The chlorophyll that was oxidized (last slide) is reduced by the photolysis (breakdown) of water.

H2O 2 e- + 2 H+ 1 oxygen

These e- replace the ones lost from thechlorophyll.

•Now we see why photosynthesis requires water and why it releases oxygen!

LE 10-13_2

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

sO2

e–

e–

+2 H+

H2O

O21/2

c. Those protons (2 H+)from the split water are added to the lumen. Just like in cellular respiration, we are building a proton gradient. The free energy of the e- is used to power proton pumps transporting protons from the stroma to the lumen.

LE 10-16

MITOCHONDRIONSTRUCTURE

Intermembranespace

MembraneElectrontransport

chain

Mitochondrion Chloroplast

CHLOROPLASTSTRUCTURE

Thylakoidspace

Stroma

ATP

Matrix

ATPsynthase

Key

H+ Diffusion

ADP + P

H+

i

Higher [H+]

Lower [H+]

d. Next, more light energizes electrons in PS I. These excited e- move to the electron transport system.

e. The e- and the free protons in the stroma are used to reduce NADP+ to NADPH.

LE 10-13_3

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

sO2

e–

e–

+2 H+

H2O

O21/2

Pq

Cytochromecomplex

Electron transport chain

Pc

ATP

LE 10-13_4

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

P700

e–

Primaryacceptor

Photosystem I(PS I)

Light

LE 10-13_5

LightP680

e–

Photosystem II(PS II)

Primaryacceptor

[CH2O] (sugar)

NADPH

ATP

ADPCALVINCYCLE

LIGHTREACTIONS

NADP+

Light

H2O CO2E

ner

gy

of

elec

tro

ns

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

LE 10-14

ATP

Photosystem II

e–

e–

e–e–

MillmakesATP

e–

e–

e–

Ph

oto

n

Photosystem I

Ph

oto

n

NADPH

-Our products from noncyclic photophosphorylation are .. .

ATP and NADPH

-These products will be used in the dark reactions of photosynthesis.

11. Cyclic photophosphorylation occurs in PS I. It is not as common as noncyclic.

Excited e- in PS I are cycled back and forthto the electron transport system. Their energyis used to pump protons in the lumen.

-The only product of cyclic is ATP. Thedark cycle use lots of ATP

LE 10-15

Photosystem I

Photosystem II ATP

Pc

Fd

Cytochromecomplex

Pq

Primaryacceptor

Fd

NADP+

reductase

NADP+

NADPH

Primaryacceptor

-For both cyclic and nocyclic:

A proton gradient is then established. The protons pass down the gradient andgo through ATP synthases embeddedin the membrane. Their energy hereis used to join ADP and Pi to formATP (This process is calledchemiosmosis).

LE 10-17

STROMA(Low H+ concentration)

Light

Photosystem IICytochrome

complex

2 H+

Light

Photosystem I

NADP+

reductase

Fd

PcPq

H2O O2

+2 H+

1/2

2 H+

NADP+ + 2H+

+ H+NADPH

ToCalvincycle

THYLAKOID SPACE(High H+ concentration)

STROMA(Low H+ concentration)

Thylakoidmembrane ATP

synthase

ATP

ADP+P

H+i

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

Light

12. Light Independent Reactions – A.K.A. –THE DARK REACTIONS!

A.The light dependent reactions producedATP’s and NADPH’s. These are used to power the dark reactions.

B. The dark reactions occur in the stroma. Theyoccur in a cycle called the Calvin cycle.

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

D. The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH.

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

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

G. The Calvin cycle has three phases:

*Carbon fixation (catalyzed by rubisco)

*Reduction

*Regeneration of the CO2 acceptor (RuBP)

LE 10-18_1

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHTREACTIONS

CALVINCYCLE

LightInput

3

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)

LE 10-18_2

[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-BisphosphoglycerateP

6 P

Glyceraldehyde-3-phosphate(G3P)

P1

G3P(a sugar)Output

Phase 2:Reduction

Glucose andother organiccompounds

LE 10-18_3

[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-BisphosphoglycerateP

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

-The majority of the 3-carbon molecules are actually recycled to keep the Calvin cyclegoing.

The G3P are used to make sugars. Thesesugars are stored or used by the mitochondriain the process of cellular respiration.