Light Reactions Takes place in the Thylakoids of chloroplasts

in eukaryotes Captures solar energy and converts it to Energy

storage molecules ATP and NADPH along with O2

NADP+=nicotinamide adenine dinucleotide phosphate

NADP+ + H++ 2e- --->NADPH Method of storing high energy electrons

Chlorophyll

Contained in Thylakoids Similar to Heme in structure Two main types chlorophyll a and chlorophyll b

Structurally similar, but slight differences

Absorbs light at red and blue wavelengths Reflects light at green wavelenghts

Spectral absorption of light Absorbed light is used as energy Chlorophyll a and b have slight

differences in absorption Carotenoids can absorb light

energy Red, yellow and orange pigments Sometimes function in

photosynthesis but are usually for “sunscreen”

In fall, chlorophyll production drops and carotnoids remain

Fig. 10-11

(a) Excitation of isolated chlorophyll molecule

Heat

Excitedstate

(b) Fluorescence

Photon Groundstate

Photon(fluorescence)

En

erg

y o

f el

ectr

on

e–

Chlorophyllmolecule

How solar power works

Excites electrons to a higher energy state Electrons remain at excited state for very short periods

of time When electron returns to ground state a photon of light

is given off This photon may excite an electron on another

chlorophyll molecule Process continues

Photosystems Act as “dish antennae”

Many photon collectors Only one reaction center

Reaction center is adjacent to Primary electron acceptor

Electron acceptor captures excited electron from reaction center before it is able to return to ground state

Fig. 10-12

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

STROMA

e–

Pigmentmolecules

Photon

Transferof energy

Special pair ofchlorophyll amolecules

Th

yla

koid

me

mb

ran

e

Photosystem

Primaryelectronacceptor

Reaction-centercomplex

Light-harvestingcomplexes

Photsystems I and II

Photosystem I Discovered first Acts second Has P700 chlorophyll at

reaction center Yields NADPH

Photosystem II Discovered second Acts first in Light Cycle Has P680 chlorophyll at

reaction center Yields ATP + O2

Noncyclic electron flow

Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

4

Pq

Pc

Cytochromecomplex

Electron transport chain

5

ATP

Photosystem I(PS I)

Light

Primaryacceptor

e–

P700

6

Fd

Electron transport chain

NADP+

reductase

NADP+

+ H+

NADPH

8

7

e–

e–

6

Fig. 10-13-5

Photosystem II(PS II)

Noncyclic electron flow

Noncyclic electron flow 2 Photosystem II collects photon and activated

electron is harvested by Primary e acceptor Reaction center chloropyll requires electron

Enzyme degrades H2O to obtain 2e-, 2H+ and O Electron is transferred to reaction center

Electron is transferred through electron transport chain Energy levels decrease while released energy is used to

make ATP Plastoquinone, Cytochrome complex and plastocyanin

Noncyclic electron flow 3 Newly grounded electron is used to fill void in P700

following electron loss Electron is excited and captured by Primary acceptor Transferred through another electron transport chain

Ferredoxin, then NADP+ reductase

NADP+ reductase takes 2H+ produced earlier plus 2 high energy electrons + NADP+ and produces NADPH + H+

NADPH stores activated electrons for later use

Noncyclic electron flow summary

Cyclic electron flow

Ferredoxin transfers electron back to Cytochrome complex Electron is recycled and ATP is produced instead of NADPH

Results in more ATP than NADPH (required in Calvin)

Fig. 10-15

ATPPhotosystem II

Photosystem I

Primary acceptor

Pq

Cytochromecomplex

Fd

Pc

Primaryacceptor

Fd

NADP+

reductaseNADPH

NADP+

+ H+