Transduction of Light Energy in Chloroplasts

Transcription

1 Module : Molecular Biology and Biochemistry of the Cell Lecture 16 Transduction of Light Energy in Chloroplasts Dale Sanders 9 March 2009

2 Objectives By the end of the lecture you should understand 1. How we can express the free energy stored in light; 2. What happens when chlorophyll absorbs a photon, and how the free energy so gained can be lost; 3. How light harvesting chlorophylls funnel energy to reaction centres; 4. That electrons are fed into the chloroplast redox chain via a light-induced decrease in redox potential of the reaction centre chlorophylls; 5. That the chloroplast redox chain comprises two photosystems, and other redox components, arranged in series to oxidize water and reduce NADP +

3 Reading As in my previous lectures, all the topics today are well covered by the standard big biochemistry textbooks. One such text is Voet & Voet (2004) Biochemistry (3 rd Ed.) Chapter 24 and especially pp Also useful for a more in-depth treatment is Nicholls, DG & Ferguson, SJ (2002) Bioenergetics 3 Chapter 6. Buchanan BB et al. (2000) Biochemistry and Molecular Biology of Plants. Chapter 12, pp

4 Energetics of Photosynthesis The basic equation (Van Niel, 1930 s) CO 2 + 2H 2 A light (CH 2 O) + H 2 O + 2A A can be S, but is usually O The relevant reactions (in higher plants) all located in chloroplast. Equation 1 can be split into 4 separate categories: (i) Absorption of light: this lecture (16) (ii) e - transport: next lecture (17) (iii) photophosphorylation: lecture (13) (iv) CO 2 fixation: lecture 18

5 Classes (i) (iii) all occur at THYLAKOID membrane

6 What is Light? Electromagnetic radiation Wavelength (m) Cosmic rays rays X-rays Ultraviolet Infra-red Radio waves Visible nm The wavelength (λ) of light specifies the colour: / nm Blue Green Red

7 Wavelength properties of light are exemplified by interference filters. But, at the molecular level, light has particulate characteristics. (Newton s Corpuscular Theory; Einstein s Quantum Theory) Photon: a particle carrying a quantum of light What is the Energy of a Photon? Energy of a photon is inversely proportional to its wavelength: E = hc (2) λ h = Planck s constant = 6.63 x J.s c = Speed of light = 3.00 x 10 8 m.s -1 e.g. for light of λ = 600 nm E = 3.32 x J

8 Comparison of Light Energy with other forms of Free Energy Like any other molecular particle, we can speak of an Avagadro s Number (N) or mole of photons 1 mole of photons = 6.02 x photons = 1 Einstein i.e. Energy/mol photons, from Eq 2 E = Nhc (3) λ e.g. light of λ = 600 nm, E = 3.32 x x 6.02 x kj/einstein

9 Just as units of chemical potential energy (J/mol) can be converted to units of redox potential energy (V) by F, light energy can be expressed in electrical units: So, for light of λ = 600 nm, E = 200 x 10 3 = 2.07 V E kj. Einstein Inverse relationship of energy and wavelength / nm

10 Pigments, Absorption Spectra and Excited States The primary photosynthetic pigment in higher plants: A very hydrophobic molecule localized exclusively within the membrane. chlorophyll a (chl a)

11 Chlorophyll absorbs blue and red light Absorption spectrum of chl a in acetone: Interpretation

12 Absorption of blue photon pushes e - to 2 nd excited state Only sufficient energy in red photon to get to1 st state [e - concerned is delocalised over tetrapyrrole ring by alternating single and double bands.] Transitions 2 nd 1 st excited state: energy lost as heat 1 st ex state ground state: energy can be lost through emission of photon of longer wavelength = fluorescence.

13 Light Harvesting is Accomplished through a 3 rd form of Energy Loss: Resonance Energy Transfer Neighbouring chl responds to electric field of excited chl (*chl): e - of *chl excites a lower energy e - in neighbouring chl, thereby losing its own energy. Time: s (1 ps). There is a SMALL energy loss in this process (i.e. not quite 100% efficient): Thus, in chl aggregates, excitation tends to pass from species absorbing at shorter wavelengths to those absorbing at longer wavelengths of light.

14 The Biological Importance of Resonance Energy Transfer In practice chl in thylakoid membranes is associated with hydrophobic proteins. Micro-environments offered by the proteins serve to broaden the absorption spectrum. Energy is funnelled to chls absorbing lower energy photons This process is known as light-harvesting

15 Light-Harvesting (Antenna) Chlorophylls A broad spectrum of wavelengths of visible light is capable of funnelling energy to chl absorbing at long wavelengths

16 Antenna and Reaction Centre Chlorophylls The bulk of chlorophyll is in the light-harvesting complexes e.g. 1 protein (LHC IIB): binds: 8 molecules chlorophyll a 7 molecules chlorophyll b Each reaction centre is associated with 300 antenna chlorophyll molecules Each reaction centre has its own special chlorophyll molecules for participation in redox reactions

17 Chloroplasts contain 2 reaction centres: Photosystem I (PSI) & Photosystem II (PSII) Properties: 1. Absorption properties of the 2 photosystems are different PS I: absorbs at 700 nm: Pigment is P700 PSII: absorbs at 680 nm: Pigment is P The 2 reaction centres are spatially-separated: PS I: Stromal lamellae PS II: Granal lamellae

18 Differential Location of the Two Photosystems

19 The 4 th way in which Energy Stored in Excited Chlorophyll (*Chl) can be lost (but only in a reaction centre): A redox reaction: the excited e - is lost from the *chl to some other molecule The redox reaction is facilitated by a change in E' o of chl in the excited state:

20 Redox couple E' o (mv) P680/P highly oxidizing *P680/*P ΔE' o = 1650 [Energy of 680 nm photon: 1820] 91% efficiency P700/P *P700/*P highly reducing ΔE' o = 1550 [Energy of 700 nm photon: 1770] 88% efficiency In both cases, there is a dramatic increase in propensity to donate e - (negative shift in E' o )

21 -1000 *P680 e Acceptor *P700 e Acceptor E' o (mv) 0 Light Light P P680

22 PS II and PS I Act in Series to Catalyse Electron Flow Between H 2 O and NADP + H 2 O ½O 2 +2H + PS II NADP+ + H+ 2e Redox 2e Redox PS I components components NADPH Light + Resonance Energy Transfer Light + Resonance Energy Transfer

23 Evidence for series arrangement of PS II and PS I: Long λ light ( 700 nm: excites PS I): OXIDATION of redox components between PS II and PS I Shorter λ light ( 680 nm: excites mainly PS II): REDUCTION of redox components between PS II and PS I The Initial Redox Reactions in PS II and PS I are Transmembrane Events

24 Summary Energy content of light can be expressed in kj/einstein, and is inversely proportional to wavelength. Energy absorbed by chl in leaf can be funnelled to longer wavelength absorbing forms light harvesting: Leads to reaction centre excitation. Higher plants have 2 photosytems: (a) spatially distinct (b) absorb at different wavelengths 4. Excitation of reaction centre change in E'o 5. This change in E'o powers the thermodynamically unfavourable reduction of NADP+ by H2O 6. Initial redox event at RC chl involves charge separation across thylakoid membrane (estromal side)