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:

Plants use sunlight, carbon dioxide, and water to produce carbohydrate with oxygen as a byproduct. The overall chemical reaction summarizes the process as: CO H 2 O + light energy  (CH 2 O)n + H 2 O + O 2 where (CH 2 O)n stands for carbohydrate. Photosynthesis Chapter 10 of text book... but a better summary is of how the process occurs is: Light energy Sunlight H2OH2OO2O2 Light-dependent reactions ATP, NADPH Chemical energy CO 2 Calvin cycle (CH 2 O) n Chemical energy... this may keep the chemists happy Thylakoid Reactions Light reactions Stroma Reactions Dark reactions Usually, glucose (C 6 H 12 O 6 ) is considered as the carbohydrate made so: 6 CO H 2 O + light energy  C 6 H 12 O H 2 O + 6 O 2

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

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

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. Wavelength of maximum absorption in the red Wavelength of maximum absorption in the far red The Z refers to changes in redox potential of electrons. Note that PSII comes before PSI in this scheme

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

Chlorophylls a and b Ring structure in “head” (absorbs light)  -carotene Tail Fig Chlorophyll is the most abundant pigment in the chloroplast. All eukaryotic photosynthetic organisms contain both chlorophyll a and chlorophyll 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 Carotenoids transfer energy from photons to chlorophyll. They also can quench free radicals by accepting or stabilizing unpaired electrons and so protect chlorophyll molecules

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

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

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

Oxygen- seeking bacteria Pigments that absorb blue and red photons are the most effective at triggering photosynthesis. Filamentous alga O2O2 O2O2 The oxygen-seeking bacteria congregate in the wavelengths of light where the alga is producing the most oxygen Fig. 10.6b

Basic concept of energy transfer during photosynthesis

Three Fates for Excited Electrons in Photosynthesis Reaction center Fluorescence Heat Photon e–e– e–e– Electron acceptor Chlorophyll molecules in antenna complex Reaction center Chlorophyll molecule Lower Higher Energy of electron e–e– FLUORESCENCE Electron drops back down to lower energy level; heat and fluorescence are emitted. REDUCTION/OXIDATIONor Electron is transferred to a new compound. RESONANCEor Energy in electron is transferred to nearby pigment. 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

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

Photosystem II Feeds an ETC that Pumps Protons Cytochrome complex PQ e–e– e–e– e–e– Pheophytin Antenna complex Reaction center Photosystem II Stroma Photon H+H+ H+H+ (low pH) H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Stroma Thylakoid Lumen 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. An essential component of the reaction is the physical transfer of the electron from the excited chlorophyll. The transfer takes ~200 picoseconds (1 picosecond = s). 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. + 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. H2OH2O O2O2

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

4e – 4 Photons Energy of electron 2 H + 2 NADP + 2 NADPH Lower Higher Photosystem I Ferredoxin ETC + 4e – 4 Photons ETC 4e – Photosystem II 4 H + PQ PC P700 ATP produced via proton-motive force Cytochrome complex Pheophytin P680 + O2O2 2 H 2 O Fig The Z scheme linking Photosystem II and Photosystem I 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.

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

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 LHC light harvesting complex LHCI, PSI, and ATP synthase are all in the stroma lamella or on the edge of a grana

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

Things you need to know 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