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 ( 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 prism Chlorophyll solution Photoelectric tube Galvanometer The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. Green light Slit 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 Absorption of light by chloroplast pigments

Action spectrum Rate of photo- synthesis (measured by O 2 release)

● 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 O 2 ● He used aerobic bacteria clustered along the alga as a measure of O 2 production

Engelmann’s experiment 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:

CH 3 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 state Chlorophyll molecule Photon Excitation of isolated chlorophyll molecule Fluorescence Energy of electron e–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-harvesting complexes Photosystem Reaction center STROMA Primary electron acceptor e–e– Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Thylakoid membrane

● 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 H 2 O combine to form O 2. Equation: H 2 O  2H + + ½ O 2

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!)

Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2

Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 e–e– e–e– + 2 H + H2OH2O O2O2 1/21/2

Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 e–e– e–e– + 2 H + H2OH2O O2O2 1/21/2 Pq Cytochrome complex Electron transport chain Pc ATP

Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 e–e– e–e– + 2 H + H2OH2O O2O2 1/21/2 Pq Cytochrome complex Electron transport chain Pc ATP P700 e–e– Primary acceptor Photosystem I (PS I) Light

P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 e–e– e–e– + 2 H + H2OH2O O2O2 1/21/2 Pq Cytochrome complex Electron transport chain Pc ATP P700 e–e– Primary acceptor Photosystem I (PS I) e–e– e–e– Electron Transport chain NADP + reductase Fd NADP + NADPH + H H + Light

ATP Photosystem II e–e– e–e– e–e– e–e– Mill makes ATP e–e– e–e– e–e– Photon Photosystem I Photon NADPH

CYCLIC ELECTRON FLOW: -only photosystem I is used -only ATP is produced -no NADPH produced; no release of O 2

Photosystem I Photosystem II ATP Pc Fd Cytochrome complex Pq Primary acceptor Fd NADP + reductase NADP + NADPH Primary acceptor

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 Membrane Electron transport chain Mitochondrion Chloroplast CHLOROPLAST STRUCTURE Thylakoid space Stroma ATP Matrix ATP synthase Key H+H+ Diffusion ADP +P H+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 II Cytochrome complex 2 H + Light Photosystem I NADP + reductase Fd Pc Pq H2OH2O O2O2 +2 H + 1/21/2 2 H + NADP + + 2H + + H + NADPH To Calvin cycle THYLAKOID SPACE (High H + concentration) STROMA (Low H + concentration) Thylakoid membrane ATP synthase ATP ADP + P H+H+ i [CH 2 O] (sugar) O2O2 NADPH ATP ADP NADP + CO 2 H2OH2O LIGHT REACTIONS CALVIN CYCLE Light