15 photosynthesis 2014-15

7/24/2019 15 Photosynthesis 2014-15

1/32

Photosynthesis1% sunlight is converted to chemical energy by plants

Fixation of carbon dioxide and evolution of oxygen

Overall reaction: 6CO2+ 6H2O 1 glucose + 6 O2

granum

Light reactions-Light-induced electron transfer-Thylakoid membrane-H2O + 3ADP + 2NADP

+O2+ 3ATP + NADPH

Dark reactions-CO2 fixation in the stroma-Biosynthetic reactions-Requires ATP and NADPH

Two components in photosynthesis:

Ch

loroplast


2/32

Photosynthetic organisms

Green sulfur bacteria

Red tide red algae

Diatoms

Purple

bacteriaCyano-

bacteria

Cyanobacteria

Chlamydomonas


3/32

CHLOROPHYLLS

Light-absorbing pigments in thylakoid membranesGreen pigments: chlorophylls a and bPolycyclic (5-ring system), planar

Similar to hemoglobin


4/32

ACCESSORY PIGMENTS

Absorption spectra of different pigments

other pigments inthylakoid membranes


5/32

Excitation of electrons in pigment molecules

(or other pigment molecules)

a photon

(= single quantum

of light energy)

Reaction

center


6/32

Exciton transfer between neighboring pigment molecules

Reaction center- Photosystem- Light-induced e-flow


7/32

Photosystems in light reactions

- Protein complexes with pigments and electron carriers- Photosystems I and II

(1) Purple bacteriaPS II only

- Cyclic electron flow

Q- quinone


8/32

(2) Green sulfur bacteria PSI only

- H2S as electron donor in non-cyclic flow

Fd: Ferredoxin (an iron-sulfur protein)

FNR: Ferreodoxin reductase


9/32

(3) Cyanobacteria, algae, and plants Both PSI and PSII

Purple bacteria (PSII)

Green sulfur bacteria (PSI)

Cyanobacteria(PS I + II)

Chloroplasts(PS I + II)

Evolution of chloroplasts?


10/32

Cyanobacteria, algae, and plants Both PSI and PSII

OEC oxygen evolving complex

PQ plastoquinone

PC plastocyanin

Fd ferredoxin

FNR ferredoxin reductase


11/32

Photosynthetic electron transfer: Noncyclic vs cyclic pathways

Noncyclic pathway: the Z scheme, unidirectional electron flow


12/32


13/32

Distribution of PSI, PSII, and ATPsynthase in thylakoid membranes


14/32

Photophosphorylation: light-driven ATP synthesis

2H2O + 8 photons + 2NADP++ 3ADP + 3Pi O2+ 2NADPH + 2H++ 3ATP

Non-cyclic photophosphorylation: Z scheme of etransfer involving both PSIand PSII

plastoquinone

ATP synthasecomplex

photophosphorylation

Thylakoid membrane sac


15/32

Cyclic Photophosphorylation

(ferredoxin)

Overall reaction:

ADP + Pi ATP + H2O

light

Cyclic electron flow involving PSI onlySimilar to purple bacteriaNo production of NADPH and O2efrom ferredoxin move back throughthe cyt b6fcomplexProton pumping and phosphorylationRegulation of ATP to NADPH ratio

-1.0


16/32

Lab 3: Hill reaction (Friday Nov 14)

- Investigation of light-induced electron transfer in isolated spinach chloroplasts using a dye

(DCPIP)

- DCPIP (blue) accepts electrons from PQBand is reduced to DCPIPH2(colorless)

- DCPIP replaces NADP+as the final electron acceptor

- Requirements for Hill reaction: light, protein complexes (electron carriers)- DCMU: a herbicide blocking PQA to PQB electron flow

- Ammonia: an uncoupler of ATP formation by elimination of proton gradient

Ph h b h d h


17/32

Photosynthetic carbohydrate synthesis

Photosynthesis might be thought of as the reverse of glycolysis and TCA cycle:

6 CO2+ 6 H2O 1 glucose + 6 O2 (G' = +686 kcal/mol)

Obviously this must be coupled to favorable reactions:18 ATP 18 ADP + 18 Pi12 NADPH 12 NAD+

(i.e. reducing power)

Assimilation of carbon dioxide into biomass in plants


18/32

Three stages of carbon dioxide assimilation: The Calvin cycle

Stroma

(Triose-P)

Transketolase &transaldolase

reactions

1 C b d d f


19/32

Stage 1: Carbon dioxide fixation

Ribulose-1,5-bisphosphate + CO2+ H2O 2 3-phosphoglycerate + 2 H+

C3plants: fixation of CO2into 3-C compoundsEnzyme: ribulose bisphosphate carboxylase/oxygenase (rubisco)

Rubisco: found in stroma, most abundant protein (50% of chloroplast soluble proteins)


20/32

Stage 2: Formation of glyceraldehyde-3-P

Two gluconeogenesis reactions in stroma:

St 3 R ti f ib l 1 5 bi h h t


21/32

Stage 3: Regeneration of ribulose 1,5-bisphosphate

Overall: 5 triose-P 3 pentose-P

*

Carbohydrate

synthesis

*

*


22/32

Stages

1 and 2

Stage 3

Outline of different stages in Calvin cycle

Stage 3:

aldolase

transketolase

aldolase

transketolase

Ribulose 1,5-

bisphosphateRibulose 1,5-

bisphosphate

Glyceraldehyde 3-

phosphate

Carbohydrate synthesis


23/32

Net results of three turns of the Calvin cycle

- Consumption of 9ATPs, 6NADPH- Release of 1 G3P for carbohydrate

synthesis


24/32

Carbohydrate synthesis from Glyceraldehyde 3-Phosphate

Two G3P out of 12 G3P from 6 turns of Calvin cycle are used to make a hexose:


25/32

Starch biosynthesis in chloroplast stroma

Fructose 6-phosphate

- Starch synthase: adds glucose units to existing starch chain (-1,4 linkage)- Glucose 1-P can be released by starch phosphorylase


26/32

Sucrose biosynthesis in cytosol

anomericcarbons

1

2

a non-reducing sugar


27/32

Maple syrup Amyloplasts in potato cells


28/32

Photorespiration

Light-driven reactions with no carbon fixationOxygenaseactivity of Rubisco(ribulose bisphosphate carboxylase/oxygenase)Generation of 3-phosphoglycerate and 2-phosphoglycolate2-Phosphoglycolate: a metabolic wasteful product

Calvin cycle

2 x 3-Phosphoglycerate Calvin cycle

CO2+ H2O

2H+

oxygenase

carboxylase


29/32

Glycolate pathway- Salvages the carbons in 2-phosphoglycolate to form 3-phosphoglycolate- Energy consuming (requires ATP)- Releases CO2

CO2

ATP

Glycolate pathway

(multiple steps)

2 glycolate + 2 Pi

X 2

CHO

COO-

ADP

Why is the process also called photorespiration?

Calvin cycle


30/32

Carbon assimilation in C4plants

CO2fixation into a 4-C compound

Tropical grasses/cereals (e.g. maize,sugarcane vs rice a C3 plant)

Minimization of photorespiration

More ATPs are required

(PEP)

(OAA)

CO assimilation in CAM plants


31/32

CO2assimilation in CAM plants

Succulent plants (e.g. cactus)

Hot and dry habitat

Temporal separation of CO2capture and Rubisco activities

Fixation of CO2into OAA by PEPcarboxylase at night (stomata open)

OAA is converted to malate bymalate dehydrogenase for storagein vacuoles

CO2released from malate by

malate enzyme during the day(stomata closed)


32/32

(PEP)(OAA)

vacuole

15 photosynthesis 2014-15

Documents