Photosynthesis: The Process of Converting Light Energy into Chemical Energy, Study notes of Biology

Download Photosynthesis: The Process of Converting Light Energy into Chemical Energy and more Study notes Biology in PDF only on Docsity! 9 Photosynthesis: Capturing Energy Lecture Outline I. Light is composed of particles that travel as waves. A. Light is a small part of the electromagnetic spectrum. B. Radiation varies in wavelength. 1. In the visible spectrum, red has longer wavelengths and violet has shorter wavelengths. C. Light is composed of packets of energy called photons. 1. The energy of a photon is inversely related to its wavelength. 2. Short wavelengths have high energy photons. D. When a molecule absorbs a photon of light energy, one electron is energized. 1. This energized electron may return to the ground state and dissipate the energy as heat, or it will fluoresce (emit light). 2. Alternatively, the electron may leave the atom and be accepted by an electron acceptor. a) Electron acceptors are the basis of photosynthesis. II. Photosynthesis in eukaryotes takes place in chloroplasts. A. Chloroplasts typically contain chlorophyll. B. Mesophyll cells contain numerous chloroplasts. C. The chloroplast has a double membrane. 1. The inner membrane encloses the stroma. 2. Thylakoids are a third set of membranes enclosing the thylakoid interior space. 3. Stacks of thylakoids are known as grana. 4. Chlorophyll and other pigments are embedded in the thylakoid membranes. D. Prokaryotes have no chloroplasts, but thylakoid membranes are formed from in- foldings of the plasma membrane. E. Chlorophyll is found in the thylakoid membrane. 1. Pigments absorb light of particular wavelengths. 2. Chlorophyll absorbs light in the red and blue regions of the spectrum; therefore it “looks” greenish to our eyes. 3. Chlorophyll molecules are composed of a porphyrin ring, which absorbs energy, and a long tail, which embeds the molecule in the thylakoid membrane. 4. At the center of the porphyrin ring is a magnesium atom. 5. Chlorophyll a is the most important in the light-dependent reactions. 6. Chlorophyll b is an accessory pigment that is very similar to chlorophyll a (differs in functional groups of the porphyrin ring). 7. Carotenoids absorb different wavelengths than the chlorophylls. 8. Carotenoids appear yellow and orange to our eyes (think carrots!). docsity.com III. Chlorophyll is the main photosynthetic pigment. A. Absorption spectra plot absorption of light against varying wavelengths. B. Action spectra plot the effectiveness of the varying wavelengths of light. 1. Engelmann in 1883 demonstrated the spectra with Spirogyra. 2. Since the action spectra matched the absorption spectra of chlorophyll, he deduced that chlorophyll was responsible for photosynthesis. 3. Carotenoids widen the action spectrum for photosynthesis. IV. Photosynthesis is the conversion of light energy to chemical bond energy. A. 6 CO2 + 12 H2 O → C6H12O6 + 6 O2 + 6 H2O 1. Water appears as both a reactant and a product. 2. However, there is no net yield of water. 3. Photosynthesis can be divided into the light-dependent and the carbon-fixation reactions (= light independent reactions). 4. This is a redox reaction. B. ATP and NADPH are the products of the light-dependent reactions. 1. Occurs in the thylakoid membranes. 2. Energy from light causes chlorophyll to expel a high-energy electron to an acceptor molecule. 3. The electron is replaced by an electron from water. 4. ATP and NADPH are produced. C. Carbohydrates are produced during the carbon fixation reactions. 1. These reactions are not driven by light; rather, they are driven by the products of the light-dependent reactions. 2. Fixation refers to the conversion of an inorganic atom, in this case carbon in the form of carbon dioxide, to an organic compound, in this case glucose (ultimately). V. The light-dependent reactions convert light energy to chemical energy. A. 12 H2O + 12 NADP+ + 18 ADP + 18 P i → 6 O2 + 12 NADPH + 18 ATP B. This reaction is dependent on light and chlorophyll. C. Photosystems I and II include antenna complexes that trap light. 1. Antenna complexes are aggregations of pigment molecules and electron acceptors. 2. The reaction center is made of a complex of chlorophyll molecules and proteins. a) The reaction centers are characterized by chlorophyll a molecules with slightly different absorption spectra. 3. The chlorophyll a molecules of photosystem I have an absorption peak at 700 nm, called P700. 4. The chlorophyll a molecules of photosystem II have an absorption peak at 680 nm, called P680. D. Noncyclic photophosphorylation produces ATP and NADPH. 1. Both photosystem I and II are involved in this process. docsity.com