The light reactions of photosynthesis

The light reactions of photosynthesis Objective of the lecture: 1. To describe the structure of function of chloroplasts. 2. To define the light reactions of photosynthesis. Text book pages: 198-212.

Photosynthesis Chapter 10 of text book Plants use sunlight, carbon dioxide, and water to produce carbohydrate with oxygen as a byproduct. The overall chemical reaction summarizes the process as: CO2 + 2 H2O + light energy  (CH2O)n + H2O + O2 where (CH2O)n stands for carbohydrate. Usually, glucose (C6H12O6) is considered as the carbohydrate made so: 6 CO2 + 12 H2O + light energy  C6H12O6 + 6 H2O + 6 O2 ... this may keep the chemists happy ... but a better summary is of how the process occurs is: Light-dependent reactions Calvin cycle Chemical energy Light energy Chemical energy H2O O2 CO2 (CH2O)n ATP, NADPH Sunlight Thylakoid Reactions Stroma Reactions Dark reactions Light reactions

Plant structure, particularly cell structure (1) makes the reactions possible, (2) enables integration of light and dark reactions. Leaves contain millions of chloroplasts. Cell Chloroplasts Fig. 10.2

Outer membrane Inner membrane Thylakoids Granum Chloroplasts are highly structured, membrane-rich organelles. Stroma Outer membrane Inner membrane Recall that membranes are composed of a lipid bilayer in which are embeded proteins that enable exchange of materials across the membrane. Thylakoids Granum Stroma Fig. 6.13 Phospholipids are in constant lateral motion, but rarely flip to the other side of the bilayer Phospholipid bilayer Membrane proteins Figure 6-18b

There are two processes in photosynthesis that capture light and produce energy rich compounds that are used in carbon fixation. These are termed Photosystem I, and Photosystem II. These processes are linked in what is termed the Z scheme of photosynthesis. The Z refers to changes in redox potential of electrons. Note that PSII comes before PSI in this scheme Wavelength of maximum absorption in the far red Wavelength of maximum absorption in the red

Light reactions occur in the thylakoids (PSII) and stroma lamella (PSI). Dark reactions in occur in the stroma Thylakoid membranes appear stacked like coins but in fact are highly folded and have a well defined interior and exterior with respect to the stroma

Fig. 10.8 Chlorophyll is the most abundant pigment in the chloroplast. All eukaryotic photosynthetic organisms contain both chlorophyll a and chlorophyll b Carotenoids transfer energy from photons to chlorophyll. They also can quench free radicals by accepting or stabilizing unpaired electrons and so protect chlorophyll molecules -carotene Chlorophylls a and b When a photon strikes its energy can be transferred to an electron in the “head” region. The electron is excited, raised to a higher electron shell, with greater potential energy Tail Ring structure in “head” (absorbs light)

Wavelengths (nm) Gamma rays Micro- waves Radio waves Ultra-violet X-rays Infrared The electromagnetic spectrum Shorter wavelength Longer wavelength Visible light nm Higher energy Lower energy

e– Blue photons excite electrons to an even higher energy state Figure 10-9 e– Red photons excite electrons to a high-energy state Photons Energy state of electrons in chlorophyll

Fig. 10.6a Different pigments absorb different wavelengths of light. Chlorophyll b Chlorophylls absorb blue and red light and transmit green light Chlorophyll a Carotenoids absorb blue and green light and transmit yellow, orange, or red light Carotenoids

Fig. 10.6b Pigments that absorb blue and red photons are the most effective at triggering photosynthesis. The oxygen-seeking bacteria congregate in the wavelengths of light where the alga is producing the most oxygen Oxygen- seeking bacteria O2 O2 Filamentous alga

Basic concept of energy transfer during photosynthesis

Three Fates for Excited Electrons in Photosynthesis FLUORESCENCE or RESONANCE or REDUCTION/OXIDATION Electron drops back down to lower energy level; heat and fluorescence are emitted. Energy in electron is transferred to nearby pigment. Electron is transferred to a new compound. Higher Electron acceptor Reaction center Photon Energy of electron Photon Fluorescence e– Heat e– e– Chlorophyll molecule Chlorophyll molecules in antenna complex Reaction center Lower Photochemistry The energy of the excited state causes chemical reactions to occur. The photochemical reactions of photosynthesis are among the fastest known chemical reactions. This extreme speed is necessary for photochemistry to compete with the other possible reactions of the excited state.

Funneling of excitation from antenna system toward reaction center The excited-state energy of pigments increases with distance from the reaction center. Pigments closer to the reaction center are lower in energy than those farther from it. This energy gradient ensures that excitation transfer toward the reaction center is energetically favorable and that transfer back out to the peripheral portions of the antenna is energetically unvavorable.

2-D view of structure of the LHCII antenna complex from higher plants Stroma Thylakoid Lumen

Pheophytin has the structure of chlorophyll but without the Mg in the porphyrin-like ring and acts as an electron acceptor. Higher Pheophytin e– PQ Cytochrome complex Energy of electron Photon Chlorophyll Lower In photosystem II, excited electrons feed an electron transport chain. Electron transport chain 2. Electrons that reach pheophytin are transferred to plastoquinone (PQ), which is lipid soluble, passed to an electron transport chain (quinones and cytochromes) 1. When an electron in the reaction center chlorophyll is excited energetically the electron binds to pheophytin and the reaction center chlorophyll is oxidized 2H2O O2+ 4H+ + 4e-

H2O O2 The oxidized reaction center of the chlorophyll that donated an electron is re-reduced by a secondary donor and the ultimate donor is water and oxygen is produced. Photosystem II Feeds an ETC that Pumps Protons 3. Passage of electrons along the chain involves a series of reduction-oxidation reactions that results in protons being pumped from stroma to thylakoid lumen Plastoquinone carries protons to the inside of thylakoids, creating a proton-motive force. Stroma The ph of the lumen reaches 5 while that of the stroma is around 8 - the concentration of H+ is 1000 times higher in the lumen than the stroma. Photon Stroma H+ Cytochrome complex Photosystem II Antenna complex e– PQ Pheophytin e– An essential component of the reaction is the physical transfer of the electron from the excited chlorophyll. The transfer takes ~200 picoseconds (1 picosecond = 10-12 s). e– Reaction center PQ + Thylakoid Lumen H+ H+ H+ H+ H+ H+ (low pH) H+ H+ H+ H+ H+ H+

Figure 10-14 Photosystem I Higher 2e– ETC NADP+ H+ + Iron and sulphur compounds Ferredoxin NADPH NADP reductase Energy of electron 2 Photons NADPH is an electron carrier that can donate electrons to other compounds and so reduce them. Chlorophyll Lower

The Z scheme linking Photosystem II and Photosystem I Fig. 10.15 4e– ETC 2 NADP+ 2 H+ + Higher Pheophytin ETC Ferredoxin 4e– PQ 2 NADPH 4 Photons Cytochrome complex Energy of electron 4 Photons PC ATP produced via proton-motive force P700 Photosystem I P680 Photosystem II Lower 4e– O2 2 H2O 4 H+ + When electrons reach the end of the Photosystem II electron chain they are passed to a protein plastocyanin that can diffuse through the lumen of the thylakoid and donate electrons to Photosystem I. Shuttle rate of 1000 electrons per second between photosystems.

ATP synthase – only in the stroma lamella and edge of grana stacks Stroma Chemiosmosis Ion concentration differences and electric potential differences across membranes are a source of energy that can be utilized Hydrophilic As a result of the light reactions the stroma has become more alkaline (fewer H+ ions) and the lumen more acid (more H+ ions) The internal stalk and much of the enzyme complex located in the membrane rotates during catalysis. Hydrophobic The enzyme is actually a tiny molecular motor Thylakoid Lumen T

Transfer of electrons and protons in the thylakoid membrane is carried out vectorially Stroma Thylakoid Lumen Protons diffuse to the site of ATP synthase Dashed lines represent electron transfer Solid lines represent proton movement

Organization and structure of the four major protein complexes Stroma LHCI, PSI, and ATP synthase are all in the stroma lamella or on the edge of a grana LHC light harvesting complex

Organization and structure of the four major protein complexes Stroma Thylakoid Lumen

Things you need to know ... 1. The structure of chloroplasts and how the light reactions are distributed and supply ATP and NADPH to the dark reactions 2. The Z scheme of photosynthesis, its photochemical and electro- potential characteristics and its spatial arrangement through the chloroplast membrane system, acidification of the thylakoid lumen and formation of ATP. 3. The energy transfer system during photosynthesis including the role of different pigments, the antenna and reaction center

The light reactions of photosynthesis