10.2 - The light reactions convert

solar energy to the chemical energy

of ATP and NADPH

● Chloroplasts are solar-powered chemical

factories

● The conversion of light energy into chemical

energy occurs in

the THYLAKOIDS.

PROPERTIES OF LIGHT:

● form of electromagnetic

energy (radiation)

● light behaves like a wave;

● Wavelength = distance

between crests of waves

● wavelengths of light important

to life = visible light (380-750

nm)

PROPERTIES OF LIGHT:

● The electromagnetic spectrum is the

entire range of electromagnetic energy, or

radiation

PROPERTIES OF LIGHT:

● light also behaves as though it consists of

discrete bundles of energy called

PHOTONS

(amt. of energy in 1 photon is inversely

proportional to wavelength)

● Blue & red light/wavelengths are most

effectively absorbed by chlorophyll & other

pigments

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

Chloroplast

Light

Reflected

light

Absorbed

light

Transmitted

light

Granum

EXPERIMENTAL EVIDENCE:

● A spectrophotometer measures a

pigment’s ability to absorb various

wavelengths

● This machine sends light through

pigments and measures the fraction of

light transmitted at each wavelength

White

light

Refracting

prismChlorophyll

solution

Photoelectric

tube

Galvanometer

The high transmittance

(low absorption)

reading indicates that

chlorophyll absorbs

very little green light.

Green

lightSlit moves to

pass light

of selected

wavelength

0 100

White

light

Refracting

prism

Chlorophyll

solution

Photoelectric

tube

The low transmittance

(high absorption)

reading indicates that

chlorophyll absorbs

most blue light.

Blue

light

Slit moves to

pass light

of selected

wavelength

0 100

● An absorption spectrum is a graph plotting a

pigment’s light absorption versus wavelength

● The absorption spectrum of chlorophyll a

suggests that violet-blue and red light work

best for photosynthesis

● An action spectrum profiles the relative

effectiveness of different wavelengths of

radiation in driving a process

Chlorophyll a

Chlorophyll b

Carotenoids

Wavelength of light (nm)

Absorption spectra

Ab

so

rpti

on

of

lig

ht

by

ch

loro

pla

st

pig

men

ts

400 500 600 700

Action spectrum

Rate

of

ph

oto

-

syn

thesis

(m

ea

su

red

by O

2re

leas

e)

● The action spectrum of photosynthesis was first demonstrated in 1883 by Thomas Engelmann

● In his experiment, he exposed different segments of a filamentous alga to different wavelengths

● Areas receiving wavelengths favorable to photosynthesis produced excess O2

● He used aerobic bacteria clustered along the alga as a measure of O2 production

Engelmann’s experiment

400 500 600 700

Aerobic bacteria

Filament

of algae

● Chlorophyll a is the main photosynthetic

pigment

● Accessory pigments, such as chlorophyll b,

broaden the spectrum used for photosynthesis

● Accessory pigments called carotenoids (yellows

and oranges) absorb excessive light that would

damage chlorophyll (PHOTOPROTECTION)

*as chlorophyll and other pigments absorb photons

of light, electrons become excited and move from

ground state to excited state…

PHOTOSYNTHESIS PIGMENTS:

CH3

CHO

in chlorophyll a

in chlorophyll b

Porphyrin ring:

light-absorbing

“head” of

molecule; note

magnesium atom

at center

Hydrocarbon tail:

interacts with

hydrophobic

regions of proteins inside

thylakoid membranes of

chloroplasts; H atoms not

shown

Excitation of Chlorophyll by Light

● When a pigment absorbs light, it goes from

a ground state to an excited state, which is

unstable

● When excited electrons fall back to the

ground state, photons are given off, an

afterglow called fluorescence

● If illuminated, an isolated solution of

chlorophyll will fluoresce, giving off light and

heat

Excited

state

Heat

Photon

(fluorescence)

Ground

stateChlorophyll

molecule

Photon

Excitation of isolated chlorophyll molecule Fluorescence

e–

PHOTOSYSTEM = an organized group of

pigment molecules and proteins

embedded in the thylakoid membrane

Photosystem I: P700 (absorbs 700 nm)

Photosystem II: P680 (absorbs 680 nm)

A Photosystem: A Reaction Center

Associated with Light-Harvesting

Complexes

● A photosystem consists of a reaction

center surrounded by light-harvesting

complexes

● The light-harvesting complexes

(pigment molecules bound to proteins)

funnel the energy of photons to the

reaction center

● A primary electron acceptor in the

reaction center accepts an excited

electron from chlorophyll a

● Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

● In a chloroplast, excited electrons are passed from molecule to molecule until it reaches the REACTION CENTER (the part of the antenna that converts light energy into chemical energy…the pigment molecule here is always chlorophyll-a)

Thylakoid

Photon

Light-harvestingcomplexes

Photosystem

Reactioncenter

STROMA

Primary electronacceptor

e–

Transferof energy

Specialchlorophyll amolecules

Pigmentmolecules

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

Th

yla

ko

id m

em

bra

ne

● There are two types of photosystems in the thylakoid membrane: PS-II and PS-I

● Photosystem II functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm

● Photosystem I is best at absorbing a wavelength of 700 nm

* light drives ATP and NADPH production

by energizing the 2 photosystems

* energy transformation occurs by electron

flow, which can be: CYCLIC or NONCYCLIC

Noncyclic Electron Flow

(a.k.a. “Linear Electron Flow”)

● Noncyclic electron flow, the primary

pathway, involves both photosystems

and produces ATP and NADPH

● also called LINEAR ELECTRON FLOW

NONCYCLIC ELECTRON FLOW:

1) Photosystem absorbs LIGHT (ground-

state electrons are “excited”); excited

electrons in photosystem II are passed

to the chlorophyll-a molecule in the

reaction center;

2) an enzyme splits water, extracting

electrons which fill the electron “hole” of

chlorophyll; the oxygen atoms from the

split H2O combine to form O2.

Equation: H2O 2H+ + ½ O2

3) electrons flow from photosystem II to photosystem I

via an electron transport chain

4) the E.T.C. uses chemiosmosis to drive ATP

formation (NONCYCLIC PHOTOPHOSPHORYLATION)

-the ATP generated here will be used to drive the

Calvin cycle!

5) as electrons reach the end of the E.T.C. they fill the electron “hole” of P700 of photosystem I;

6) the reaction center of photosystem I passes photoexcited

electrons down a

second E.T.C. which

transmits them to

NADP+, reducing it

and forming NADPH

(which is also used to run the Calvin Cycle!)

LightP680

e–

Photosystem II

(PS II)

Primary

acceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVIN

CYCLELIGHT

REACTIONS

NADP+

Light

H2O CO2

En

erg

y o

f e

lec

tro

ns

O2

LightP680

e–

Photosystem II

(PS II)

Primary

acceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVIN

CYCLELIGHT

REACTIONS

NADP+

Light

H2O CO2

En

erg

y o

f e

lec

tro

ns

O2

e–

e–

+

2 H+

H2O

O21/2

LightP680

e–

Photosystem II

(PS II)

Primary

acceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVIN

CYCLELIGHT

REACTIONS

NADP+

Light

H2O CO2

En

erg

y o

f e

lec

tro

ns

O2

e–

e–

+

2 H+

H2O

O21/2

Pq

Cytochromecomplex

Pc

ATP

LightP680

e–

Photosystem II

(PS II)

Primary

acceptor

[CH2O] (sugar)

NADPH

ATP

ADP

CALVIN

CYCLELIGHT

REACTIONS

NADP+

Light

H2O CO2

En

erg

y o

f e

lec

tro

ns

O2

e–

e–

+

2 H+

H2O

O21/2

Pq

Cytochromecomplex

Pc

ATP

P700

e–

Primary

acceptor

Photosystem I

(PS I)

Light

LightP680

e–

Photosystem II

(PS II)

Primary

acceptor

[CH2O] (sugar)

NADPH

ATP

ADPCALVIN

CYCLELIGHT

REACTIONS

NADP+

Light

H2O CO2E

nerg

y o

f ele

ctr

on

s

O2

e–

e–

+

2 H+

H2O

O21/2

Pq

Cytochrome

complex

Pc

ATP

P700

e–

Primary

acceptor

Photosystem I

(PS I)

e–e–

NADP+

reductase

Fd

NADP+

NADPH

+ H+

+ 2 H+

Light

ATP

Photosystem II

e–

e–

e–e–

Mill

makes

ATP

e–

e–

e–

Photosystem I

NADPH

CYCLIC ELECTRON FLOW:-only photosystem I is used

-only ATP is produced

-no NADPH produced; no release of O2

Photosystem I

Photosystem II ATP

Pc

Fd

Cytochromecomplex

Pq

Primaryacceptor

Fd

NADP+

reductase

NADP+

NADPH

Primaryacceptor

A Comparison of Chemiosmosis in

Chloroplasts and Mitochondria:

● chloroplasts and mitochondria generate

ATP by chemiosmosis, but use different

sources of energy

● mitochondria transfer chemical energy from food to ATP;

● chloroplasts transform light energy into the chemical energy of ATP

● The spatial organization of chemiosmosis

differs in chloroplasts and mitochondria

MITOCHONDRION

STRUCTURE

Intermembrane

space

MembraneElectron

transport

chain

Mitochondrion Chloroplast

CHLOROPLAST

STRUCTURE

Thylakoid

space

Stroma

ATP

Matrix

ATP

synthaseKey

H+ Diffusion

ADP + P

H+

i

Higher [H+]

Lower [H+]

● The current model for the thylakoid membrane is based on studies in several laboratories

● Water is split by photosystem II on the side of the membrane facing the thylakoid space

● The diffusion of H+ from the thylakoid space back to the stroma powers ATP synthase

● ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place

STROMA

(Low H+ concentration)

Light

Photosystem IICytochrome

complex

2 H+

Light

Photosystem I

NADP+

reductase

Fd

PcPq

H2OO2

+2 H+

1/2

2 H+

NADP+ + 2H+

+ H+NADPH

To

Calvin

cycle

THYLAKOID SPACE

(High H+ concentration)

STROMA

(Low H+ concentration)

Thylakoid

membrane ATP

synthase

ATP

ADP

+

PH+

i

[CH2O] (sugar)O2

NADPH

ATP

ADP

NADP+

CO2H2O

LIGHT

REACTIONS

CALVIN

CYCLE

Light