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Photosynthesis Suggested problems from the end of chapter 21: 1,2.3,5,6,8,10,12,14,16,17,19,20. For #2 assume that Eoxidation = +0.816V for oxidation of water into O2 and remember that ΔE= Ereduction + Eoxidation Photosynthesis Earth can be considered an open thermodynamic system, where both heat and matter can be exchanged with the surroundings. Given that earth can exchange energy with its surroundings, it is not surprising that light generated by the sun can enter earth. Some plants and cyanobacteria are able to utilize sunlight for the following overall reaction, in which carbohydrates are synthesized: light CO2 + H2O ⎯ ⎯→ (CH 2O) + O2 This also is called CO2 “fixing” because the carbon is transferred to carbohydrates, which can be used as€ energy sources by the original organism, as well as other systems, living and non-living. The industry that consumes oil, coal, and gas also relies on the above reaction, albeit indirectly, as these carbon sources are thought to have originated from CO2 fixing by ancient organisms. Photosynthesis is estimated to fix ~1011 tons of carbon per year, representing 1018 kJ. 1 Joseph Priestley, 1771; Jan Ingen-Housz, 1779 •The notion that air is changed by combustion and photosynthesis has been appreciated for some time: “On the 17th of August, 1771, I put a sprig of mint into a quantity of air, in which a wax candle had burned out, and found that, on the 27th of the same month, another candle had burned perfectly well in it.” Why did Priestly do this experiment? •Jan Ingen-Housz demonstrated that sunlight-exposed green parts of plants contain “air purifying” powers that correct air that has been “injured” by respiration. •Jan Senebier showed, in 1782, that CO2 is consumed during photosynthesis. •Theodore de Saussure showed, in 1804, that water is taken up during photosynthesis by carefully determining the mass of oxygen, CO2, and the organic matter produced. •In 1842, Robert Mayer concluded that sunlight energy was used in the conversion of water and CO2 into O2 and carbohydrates. Chloroplasts There are about 1-1000 chloroplasts per cell. The chloroplasts are about 5 µm long ellipsoid surrounded by two membranes. The outer membrane is highly permeable, whereas the inner membrane is almost impermeable. The thylakoid consists of grana, and there are about 10-100 grana in a chloroplast. The thylakoid membrane acyl chains are highly unsaturated, which gives the membrane a fluid-like consistency. 2 Chlorophyll The chlorophyll is the principal photosynthesis receptor. It resembles the heme groups of globin and cytochromes, except (1) the central metal is magnesium, and (2) ring V, a cyclopentanone ring, is fused to ring III, a pyrrole ring. Chlorophyll types 3 Absorption of light by matter Electromagnetic radiation can be considered a “stream” of photons, which are quanta of energy. The energy associated with particular photons is given by Plank’s law hc E = hv = λ where h is Plank’s constant, 6.626 x 10-34 J•s, c is the speed of light, 2.998€ x 108 m•s-1 in a vacuum, v is the frequency of the radiation, and λ is the radiation wavelength. For instance, light with a wavelength of 700 nm (red light) has an energy of 171 kJ•mole-1 photons, which is 171 kJ•einstein-1. Absorption of light by matter Every molecule exists as an ensemble of energy amounts (substrates) which depend on the rotational and vibration energies of each atom within the molecule. Although these energy substrates are difficult to calculate for each molecule, it is possible to measure the effect of photon absorption by the molecule. Absorption of light by molecules usually results in the promotion of an electron from a ground state (most stable) orbital to a higher, less stable orbital. Such photon absorption also is called excitation. When a photon is absorbed, the photon energy must equal the energy difference between the ground state electron(s) and the excited state electron(s). Due to this, a given molecule is able to absorb light at distinct wavelengths. 4 Energetic interconversions of excited molecules 1. Internal conversion. Electronic energy is converted to kinetic molecular energy, i.e., heat. Typically, this is a rapid process (complete in <10-11 s). Many molecules relax all the way to the ground state, but chlorophyll usually relaxes only to a lower excited state. 2. Fluorescence. A photon (a quantum of energy) is emitted concurrently with the decay of an excited state electron back to the ground state. Chlorophyll fluorescence life time is ~10-8s, although other molecules have drastically different fluorescence life times. Chlorophyll in solution, where no photosynthesis is possible, fluoresces red. 3. Resonance energy transfer (FRET). Interactions between electron orbitals of neighboring atoms can result in the transfer of energy from the excited to an unexcited atom. If these atoms belong to neighboring molecules, then amount of energy transferred depends on the distance between the molecules, but also the average orientation of the donor atom with respect to the acceptor atom. This can be used to calculate average distances between molecules. 4. Photooxidation. The excited molecule is oxidized when the excited electron is transferred to an acceptor molecule, which is being reduced. This occurs in photosynthesis, where the excited chlorophyll is ionized. The excited (high energy) electron is transferred to the reduced molecule. The photooxidized chlorophyll becomes reduced by another molecule, and thus returns to its ground state. Chlorophyll energy interconversions As discussed in the previous slide, chlorophyll can undergo internal energy interconversion, fluorescence, FRET, and photooxidation. Note that absorption of blue light (high energy, low wavelength) results in a fast internal conversion followed by a slower, red fluorescence interconversion. This is the same overall fluorescence that results from the absorption of a red light (high wavelength, low energy) photon. 5 Absorption of light by matter The amount of light absorbed by a substance is given by Beer’s law (Beer-Lambert’s law) I A = log( i ) = 2 − log(%T) = εcl It where Ii is the intensity of the incident light, It is the intensity of the transmitted light, %T is the percent€ of transmitted light ε is the molar extinction coefficient, c is the molar concentration and l is the length of the light path through the solution. Remember that a given molecule is able to absorb light at distinct wavelengths, and now that we can quantify how much light is being absorbed by a pure solution, we can plot the absorbance as a function of wavelength, which is distinct for a given set of molecules. Light absorption by distinct chlorophyll types 6 Reactions of photosynthesis Photosynthesis reactions traditionally are divided into the “light” and “dark” reactions, which is a terrible nomenclature. Until the early 1930s it was unclear whether carbon dioxide or water served as the source of free oxygen in the photosynthesis reaction: light CO2 + H2O ⎯ ⎯→ (CH 2O) + O2 In 1931 van Niel showed that anaerobic green photosynthetic bacteria generate sulfur: € light CO2 + 2H2S ⎯ ⎯→ (CH 2O) + 2S + H2O What does this suggest about the source of the elemental oxygen product in the top reaction?€ Light and dark reactions of photosynthesis light CO2 + 2H2S ⎯ ⎯→ (CH 2O) + 2S + H2O This can be written, in general, as CO + 2H A ⎯light ⎯→ (CH O) + 2A + H O € 2 2 2 2 where A is oxygen in photosynthetic plants and cyanobacteria; A is sulfur in photosynthetic sulfur bacteria. This suggested€ that the photosynthetic reaction can be composed of two subsequent series of reactions. The first set of reactions requires light, and overall can be written light 2H2A ⎯ ⎯→ 2A + 4[H] where [H] is a reducing agent. The second set of reaction€ does not require light (although it does require the products of the first reaction) and can occur under light or dark conditions. These reactions result in the reduction of CO2 into carbohydrate and water 4[H]+ CO2 → (CH2O) + H2O € 7 Testing van Niel’s notion of two reaction sets 1. The Hill reaction. In 1937 Robert Hill experimented with CO2 - depleted chloroplasts. To 2- these chloroplasts he provided ferrocyanide, Fe(CN)6 , which served as an electron acceptor. 3- The chloroplasts produced O2 and ferricyanide, Fe(CN)6 . Why did this finding support van Niel’s idea? 2. In 1941, Samuel Ruben and Martin Kamen experimented with 18O water. They showed that 18 light 18 CO2 + H2 O ⎯ ⎯→ (CH 2O)+ O2 € The bacterial photosynthetic reaction center In 1952, Louis Duysens observed that purple bacteria underwent slight photobleaching of their absorbance at 870 nm. Duysens suggested that the bacteria underwent photooxidation of a light-absorbing group, later confirmed as the photosynthesis reaction center. The photosynthesis reaction center from Rhodopseudomonas virdis was the first trans-membrane protein to be described by X-ray crystallography. This was accomplished in 1984 by Deisenhofer, Huber, and Michel. This is a view of the Rb. sphaeroids photosynthesis reaction center from the plane of the plasma membrane, such that the cytoplasm is below. The protein complex consists of H (purple), M (blue) and L (orange) subunits. There are also (in yellow): 4 bacteriochlorophyl b, 2 bacteriopheophytin b (Bpheo b, BCh b in which the Mg2+ is replaced with two protons), one Fe2+ ion (in yellow), one ubiquinone one menaquinone. The 11 α helices form a 45 Å trans- membrane barrel with hydrophobic side chains. 8 Ubiquinone and Menaquinone O CH3 CH3 (CH2 C C CH2)8H H O Menaquinone O H3CO CH3 CH3 H3CO (CH2 C C CH2)10H H O Coenzyme Q or ubiquinone Mammalian ubiquinone has 10 isoprenoid units (Q10), whereas other organisms may have only 8 or 6 isoprenoid units Arrangement of the prosthetic groups within the Rps.
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