Ch 10 Photosynthesis--> To make with light!

LE 10-2

Plants

Unicellular protist

Multicellular algae Cyanobacteria

Purple sulfurbacteria

10 µm

1.5 µm

40 µm

Photoautotrophs:

Self feeders, producers

Use light and inorganic

molecules to make own

organic molecules.

• Heterotrophs (food from others): -Consumers

-Obtain organic material from other organisms

-Dependent on photoautotrophs for food and oxygen

Photosynthesis: conversion of light energy into chemical energy

Simplified rxn:

6CO2 + 6H2O + light --> C6H12O6 + 6O2

Reaction:

6CO2 + 12H2O + light --> C6H12O6 + 6O2 + 6H2Oglucose

Simplest rxn:

CO2 + H2O + light --> [CH2O] + O2

carbohydrate

LE 10-3

Leaf cross sectionVein

Mesophyll

Stomata CO2 O2

Mesophyll cellChloroplast

5 µm

Outermembrane

IntermembranespaceInner

membrane

Thylakoidspace

ThylakoidGranumStroma

1 µm

6CO2 + 6H2O + light --> C6H12O6 + 6O2

Gas enters through stomata

Enters through roots

Exits throughstomata

or used

in respiration

Organic moleculefor fuel or other

Two major reactions in photosynthesis

Light-dependent (in thylakoid)

Light-independent aka dark or Calvin cycle (in stroma)

LE 10-7

Chloroplast

Light

Reflected light

Absorbed light

Transmitted light

Granum

Stroma

Chlorophyll in thylakoid membranes

LE 10-10

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

LE 10-9a

Chlorophyll a

Chlorophyll b

Carotenoids

Wavelength of light (nm)

Absorption spectra

Ab

sorp

tio

n o

f lig

ht

by

chlo

rop

last

pig

men

ts

400 500 600 700

How do we know that absorption of certain wavelengths of light by plants stimulates a chemical reaction in plants?

Specifically how do we know that O2 is a product?

LE 10-9c

Engelmann’s experiment (1883): Action spectrum

400 500 600 700

Aerobic bacteria

Filament of algae

What would be an important control experiment?

• Chlorophyll a:– main photosynthetic pigment

• Accessory pigments– chlorophyll b and carotenoids absorb excessive light that

would damage chlorophyll– broaden the spectrum used for photosynthesis

• When a pigment absorbs light– departs from a ground state to an excited state

--> unstable Draw

– excited electrons fall back to the ground state, give off photons (glow)--

>fluorescence

Light-Induced Excitation:

LE 10-11

Excitedstate

Heat

Photon(fluorescence)

GroundstateChlorophyll

molecule

Photon

Excitation of isolated chlorophyll molecule Fluorescence

En

erg

y o

f el

ectr

on

e–

LE 10-5_1

H2O

LIGHTREACTIONS

Chloroplast

Light

Light-dependent rxn: in thylakoid

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)

Calvin cycle: in stroma

Photosynthesis as a Redox Process

• Water is oxidized (e- are removed).• Carbon dioxide is reduced (e- are gained).

Two major reactions in photosynthesis

Light dependent (in thylakoid):

Creates ATP and an electron carrier, NADPH

Electrons supplied through splitting and oxidation of H2O

Light -independent (aka dark or Calvin cycle)(in stroma):

Synthesis of organic molecules from CO2

Reduction reactions

Endergonic: requires ATP

Light Reaction:Consists of 2 photosystems

Occurs at two different reaction centerseach surrounded by light harvesting complexes

Light harvesting complex funnels energy to reaction center

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

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

Once P680 is oxidized(gives up e-), is it functional?

How is it restored to functionality?

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

Splitting of H2O yields e- that fill e-”hole” in oxidized P680

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

After P700 is oxidized by light energy in PS Iare its missing electrons replaced?

If so what is the electron source?

What would be the effect on photosynthesis if P700 were not reduced to its original state i.e. if the e- hole were not filled?

Electron Flow

• Noncyclic electron flow– involves both photosystems (II & I)

– produces ATP and NADPH

LE 10-14

ATP

Photosystem II

e–

e–

e–e–

MillmakesATP

e–

e–

e–

Ph

oto

n

Photosystem I

Ph

oto

n

NADPH

Cyclic Electron Flow

- Uses only photosystem I - Produces only ATP, no NADPH

- Generates surplus ATP– to satisfy demand in the Calvin cycle

LE 10-15

Photosystem I

Photosystem II ATP

Pc

Fd

Cytochromecomplex

Pq

Primaryacceptor

Fd

NADP+

reductase

NADP+

NADPH

Primaryacceptor

How is ATP made?

By chemiosmosis

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

• Current chemiosmotic model:– H+ (protons) accumulate in thylakoid space

• 1. Through splitting of water

• 2. By translocation into thylakoid when e- are transported

• 3. By removal of H+ from stroma due to bonding with NADPH

– H+ diffuses from thylakoid space --> stroma through membrane enzyme, ATP synthase

– Movement activates ATP synthase

– ATP synthesized on stromal face where the Calvin cycle takes place

Products from light reactions power Calvin cycle!

What are the light reaction products?

ATP: energy carrierNADPH: electron carrier

What is the product of the Calvin cycle?

Glucose (fuel)

What additional molecule must enter

the Calvin cycle to make sugar?

CO2

• Calvin cycle – 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

I had no idea Icould do these things!

Alternative mechanisms of carbon fixation in hot, dry climates

• How to avoid dehydration during day?

close stomata

conserves water

but also blocks CO2 uptake

Consequences? Positive & Negative

Overall: reduces rate of photosynthesis

LE 10-20

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

CAM

CO2

CO2

CALVINCYCLE

Sugar

Spatial separation of steps Temporal separation of steps

Sugar

Day

Night

CAM: Crassulacean acid metabolism

CAM Plants

• CAM plants open stomata at night, incorporating CO2 into organic acids

• Stomata closed during the day

• CO2 released from organic acids and used in the Calvin cycle

• Photosynthesis can occur during day!

The Importance of Photosynthesis: A Review

• sunlight stored as chemical energy in organic compounds by chloroplasts

• Sugar supplies chemical energy and carbon skeletons to synthesize other organic molecules

• Production of food and atmospheric oxygen

LE 10-21

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)