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Progress in 20th century Sci 190 E Lecture 14 • • • • • Discovery of DNA Discovery of proteins Application of X-ray diffraction to reveal protein structures Site-specific mutagenesis allows to modify natural systems Quantum physics can explain and predict energy transfer and conversion processes • Modern spectroscopic techniques allow real-time monitoring of the energy transfer and conversion processes Simplified block-diagram of photosynthetic machinery Simplified chemical reaction: hν CO2 + H2O Antenna light energy Antenna captures light and passes its energy to RC Biosynthesis (CH2O) + O2 Reaction current center (RC) Reaction center generates electrical current Chemical battery The electrical current drives chemical reactions creating molecules that store energy in their chemical bonds 1 Photosynthetic apparatus of plants Photosynthetic powerplant is located in thylakoid membranes Photosynthetic apparatus: PS II Photosystem II (PS II) complex uses energy of light to split water and generate electron and proton current. Oxygen molecules are byproduct of this processes The PS II is extremely strong oxidant - it can oxidize water! 2 Photosynthetic apparatus: cytochrome b6f Cytochrome b6f complex transfers electrons (current) from PS II to photosystem I (PS I) complex. In addition, the fraction of this electron current is used to transports protons across the thylakoid membrane. Plastoquinones (Q, QH2) and plastocyanins (PC) serve as intermediate electron carriers Photosynthetic apparatus: PS I Photosystem I gets electrons from PS II via cytochrome b6f and uses light to boost the energy of these electrons and transfer them to mobile ferredoxin (Fd) complex 3 Photosynthetic apparatus: FAD Ferredoxin (Fd) uses the energy of the electron to reduce NADP+ molecule to NADPH and store the energy in chemical bond of NADPH. This reaction is mediated by ferredoxinNADP+ reductase (FAD) Photosynthetic apparatus: proton pumps PS II and cytochrome b6f also pump protons creating elecrtochemical potential across the membrane (like charging a capacitor). That creates strong electric field across the membrane. 4 Photosynthetic apparatus: ATP synthase This strong electric field drives protons through a molecular size ‘turbine’ motor called ATP synthase. The axle of that molecule spins and mechanical force drives chemical reaction transforming ADP molecule to ATP - the energy is stored in extra chemical bond ATP synthase ADP + Pi ATP • • • • ADP→ATP coupled to mechanical motor Motion coupled to proton current Proton current coupled to electron current Electron current coupled to absorption of light http://www.biologie.uni-osnabrueck.de/Biophysik/Junge/overheads.html 5 ATP synthase: visualizing A. Aksimentiev Yoshida et al. Nature Reviews Molecular Cell Biology 2 669-677 2001 http://www.ks.uiuc.edu/~alek/ Photosynthetic apparatus: charging chemical battery Absorption of light causes the synthesis of NADPH and ATP. The chemical energy stored in these molecules is used by other protein ‘machines’ in the cell to synthesize necessary components and fuel life. 6 To understand the molecular processes of photosynthesis we need to know: • • • • • • • What is light? How can light energy be absorbed by a molecule (chlorophyll)? How can this energy initiate charge flow (current)? How can an electron drive a chemical reaction How can a proton spin a large molecule (protein) How can a protein spin and drive chemical reaction …. Nature of Light 7 Light: wave or particles? • 1621: W. Snell - refraction • 1664: R. Hooke - interference: color in thin films • 1665: F. Grimaldi - diffraction • 1677: C. Huygens - wave theory. Light is a wave moving in ‘ether’ analagy: water waves • 1704: I. Newton - light is particles! • 1801-1814: T. Young, Fresnel - it must be a wave! How can light wave interact with matter? Electrical and magnetic forces Between charged objects Between moving charged objects Electromagnetic forces Light is an electromagnetic wave moving in space 8 Electric charges • Two types: positive (+) and negative (-) • Notation: q, Q • Likely charges: repel. Opposite charges: attract Magnitude of charge is measured in Coulombs (C) • Net charge of a system: algebraic sum of all the charges Law: Conservation of charge: The net charge of a closed system never changes The origin of charge charge mass electron -e me proton +e 1836.me neutron 0 1839.me undivisible! Elementary charge: e = 1.602 × 10-19 C C=Coulomb, SI unit of charge me = 9.11 × 10-31 kg Net charge is always a multiple of e: Qnet=Ne Charge is quantized 9 Coulomb’s law Point charge: a point-like object with nonzero charge • The force exerted by one point charge on another acts along the line joining the charges. • The force varies inversely as the square of the distances separating the charges, and is proportional to the product of the charges. • The force is repulsive if the charges have the same sign an attractive if the charges have opposite signs F =k q1 q 2 r2 k = 8.99 × 109 Nm 2 C2 F - force on each charge (N) q - charges measured in C r - distance between charges (m) Electric field Suppose we are inside a black box holding charge q and sense that electric force acts upon it as shown: Can we tell where the other charges outside the box? - No! We can have negative charge on the left side, or F positive on the right side… There is a property at that spot to induce electric force on any charge. This property is called electric field E: E= vector! Fe q Electric field at some point is force per unit ‘probe’ charge placed at that point in space Fe = qE if we know E - can find F on any charge 10 Electric field: analogy with gravity Gravity (Cavendish, 1798) m Fg = G M g= mM r2 Fg m g – gravitational field Fg = mg Gravitational force is always parallel to gravitational field (m>0) Electric force (Coulomb, 1795) qQ r2 Fe = k E= Fe q q Q E – electric field, N/C Fe = qE Electric force can be either parallel (q>0) or antiparallel (q<0) to electric field Electric force versus gravity Gravity (Cavendish, 1798) mm Fg = G 1 2 2 r G = 6.67×10-11 m3/(kg.s2) 3m What is the attraction force? 70 ⋅ 70 Fg = 6.67 × 10 N 32 Fg = 0.000000036 N −11 Electric force (Coulomb, 1795) q q Fe = k 1 2 2 r k = 8.99×109 Nm2/C2 3m q1 = q2 ≈ 1028 ⋅ e = 1.6 × 109 C 1.6 × 109 ⋅ 1.6 × 109 N 32 Fe = 2.6 × 1027 N Fe = 8.99 × 109 Electric forces are much stronger than gravitational forces 11 Magnetic field Permanent magnets and moving charges can produce another kind of field called magnetic field. It is a field concept similar to electric or gravitational fields, but it acts only on moving charges or magnets Magnetic force is perpendicular to both magnetic field B and velocity of the charge on which it acts: Magnetic force on a moving point charge v FB = qv × B vector cross-product v×B Magnitude: q B right-hand rule FB = q v⊥ B = q vB sin θ Note: for negative charge FB will point in the direction opposite to the cross product v × B Electric and magnetic field In the first half of the 19th century it was shown that changing electric field can produce electric field, and changing magnetic field can produce magnetic field. Can one create changing electromagnetic field that will exist without charges? 1831-1879 James Clerk Maxwell IDEA: Changing E will generate changing magnetic field B Changing B will generate changing electric field E 12 Electromagnetic wave ~1864: Maxwell introduced four equations that described all known electro-magnetic phenomena and showed theoretically that electromagnetic pulse or wave moving in space could exist. Maxwell’s equations q ∆Φ E = E⊥ A = ∆Φ B = B⊥ A = 0 E = E||l = − ε0 ∆Φ B ∆t B||∆l = µ0 I + ε 0 ∆Φ E ∆t Surprisingly, he found that this EM-wave must move at a speed of 300,000 km/s - i.e. speed of light! Maxwell suggested that light is electromagnetic wave EM wave characteristics E - electric field λ-w avele ngth B - magnetic field c - speed of light Frequency: at which frequency E-field oscillates at certain point in space Note: λ = c/ν speed of light 13 EM wave characteristics E - electric field B - magnetic field 14