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
Light Reflected light Chloroplast Absorbed light Granum Transmitted light
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 0 100 Slit moves to pass light of selected wavelength Green light The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light.
White light Refracting prism Chlorophyll solution Photoelectric tube 0 100 Slit moves to pass light of selected wavelength Blue light The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light.
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
Absorption of light by chloroplast pigments Chlorophyll a Chlorophyll b Carotenoids 400 500 600 700 Wavelength of light (nm) Absorption spectra
Rate of photosynthesis (measured by O 2 release) Action spectrum
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
Aerobic bacteria Filament of algae 400 500 600 700 Engelmann s experiment
PHOTOSYNTHESIS PIGMENTS: 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
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
e Excited state Heat Photon Chlorophyll molecule Photon (fluorescence) Ground state Excitation of isolated chlorophyll molecule Fluorescence
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 membrane Thylakoid Photon Photosystem STROMA Light-harvesting complexes Reaction center Primary electron acceptor e Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)
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 (groundstate 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!)
Energy of electrons H 2 O CO 2 Light LIGHT REACTIONS NADP + ADP ATP CALVIN CYCLE NADPH O 2 [CH 2 O] (sugar) Primary acceptor e Light P680 Photosystem II (PS II)
Energy of electrons H 2 O CO 2 Light LIGHT REACTIONS NADP + ADP ATP CALVIN CYCLE NADPH O 2 [CH 2 O] (sugar) Primary acceptor H 2 O 2 H + + 1 /2 O 2 e Light e e P680 Photosystem II (PS II)
Energy of electrons H 2 O CO 2 Light LIGHT REACTIONS NADP + ADP ATP CALVIN CYCLE NADPH O 2 [CH 2 O] (sugar) H 2 O 2 H + + Primary acceptor e Pq Cytochrome complex 1 /2 O 2 e e Pc Light P680 ATP Photosystem II (PS II)
Energy of electrons H 2 O CO 2 Light LIGHT REACTIONS NADP + ADP ATP CALVIN CYCLE NADPH O 2 [CH 2 O] (sugar) H 2 O 2 H + + Primary acceptor e Pq Cytochrome complex Primary acceptor e Light 1 /2 O 2 e e P680 Pc P700 Light ATP Photosystem II (PS II) Photosystem I (PS I)
Energy of electrons H 2 O CO 2 Light LIGHT REACTIONS NADP + ADP ATP CALVIN CYCLE NADPH O 2 [CH 2 O] (sugar) 1 /2 H 2 O 2 H + + O 2 e e Light Primary acceptor e P680 Pq Cytochrome complex Pc Primary acceptor e P700 Fd e e NADP + reductase Light NADP + + 2 H + NADPH + H + ATP Photosystem II (PS II) Photosystem I (PS I)
ATP e e e NADPH e e e Mill makes ATP e Photosystem II Photosystem I
CYCLIC ELECTRON FLOW: -only photosystem I is used -only ATP is produced -no NADPH produced; no release of O 2
Primary acceptor Fd Primary acceptor Fd Pq Cytochrome complex NADP + reductase NADP + NADPH Pc Photosystem II ATP Photosystem I
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 Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE Intermembrane space H + Diffusion Thylakoid space Membrane Electron transport chain Key Higher [H + ] Matrix ATP synthase Stroma Lower [H + ] ADP + P i H + ATP
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
H 2 O CO 2 Light LIGHT REACTIONS NADP + ADP ATP CALVIN CYCLE NADPH STROMA (Low H + concentration) Light O 2 [CH 2 O] (sugar) Cytochrome Photosystem II complex Light 2 H + Photosystem I Fd NADP + reductase NADP + + 2H + Pq Pc NADPH + H + H 2 O THYLAKOID SPACE (High H + concentration) 1 /2 O 2 +2 H + 2 H + To Calvin cycle STROMA (Low H + concentration) Thylakoid membrane ATP synthase ADP + P i H + ATP