Photosynthesis Life on Earth is solar powered. Photosynthesis => conversion of light energy to chemical energy (stored in sugars and other organic molecules). Organisms obtain organic compounds by one of two major modes: autotrophic nutrition or heterotrophic nutrition. 1) Autotrophs => self feeders => producers of the biosphere Photoautotrophs => use light as a source of energy to synthesize organic compounds. ex. plants, algae, some protists, some prokaryotes Chemoautotrophs => use inorganic substances (sulfur and ammonia) as a source of energy Exclusive to prokaryotes. 2) Heterotrophs => feed on organic compounds produced by other organisms => consumers of the biosphere. ex. animals, fungi (decomposers) 10.1 Photosynthesis Overview For most plants, leaves are the major site of photosynthesis. The color of a leaf comes from chlorophyll, the green pigment in the chloroplasts which absorbs light energy. Chloroplasts are found mainly in mesophyll => cells forming the interior tissues of the leaf O2 exits and CO2 enters the leaf through microscopic pores called stomata. Veins transport water and sugar. Review the structure of a chloroplast Photosynthesis-1
Photosynthetic prokaryotes lack chloroplasts. Instead, their photosynthetic membranes arise from infolded regions of the plasma membrane. The equation describing the process of photosynthesis is: 6CO2 + 12H2O + light energy C6H12O6 + 6O2+ 6H2O Equation simplified: 6CO2 + 6H2O + light energy C6H12O6 + 6O2 History: Before the 1930s, it was believed that carbon dioxide was split and then water was added to the carbon: Step 1: CO2 C + O2 Step 2: C + H2O CH2O C. B. van Niel challenged this hypothesis. The bacteria that he was studying, used hydrogen sulfide (H2S), not water. Sulfur was produced as a waste, not oxygen. CO2 + 2H2S [CH2O] + H2O + 2S He applied his idea to plants: CO2 + 2H2O [CH2O] + H2O + O2 Conclusion: water was split (not carbon dioxide). Confirmed by other scientists twenty years later, using 18 O, a heavy isotope, as a tracer. Experiment 1: CO2 + 2H2O [CH2O] + H2O + O2 Experiment 2: CO2 + 2H2O [CH2O] + H2O + O2 Finding: 18 O label appeared in O2 when water was the source. Hydrogen extracted from water is incorporated into sugar, and oxygen is released to the atmosphere. Photosynthesis-2
Photosynthesis is a redox reaction (involves transfer of electrons). Oxidation => loss of electron(s) (loss of energy) Reduction => gain of electron(s) (gain of energy) Water is split (and oxidized) and electrons transferred with H + from water to CO2, reducing it to sugar. Two stages of photosynthesis: 1. The light reactions (photo) convert solar energy to chemical energy => occur in thylakoids => results in ATP and NADPH => provides power for #2. 2. The Calvin cycle AKA light-independent reactions (synthesis) uses energy from the light reactions to incorporate CO2 from the atmosphere into organic substance (carbon fixation) => occurs in the stroma => results in sugar. 10.2 The Light Reactions Properties of light: Form of electromagnetic radiation. Travels in rhythmic waves. Wavelength => distance between crests (nm-gamma; km-radio) Part of electromagnetic spectrum => entire range of radiation Also behaves like discrete particles => photons => have fixed quantities of energy inversely related to its wavelength. ex. shorter wavelengths pack more energy Photosynthesis-3
When light meets matter, it may be reflected, transmitted, or absorbed. pigment => substance that absorbs visible light chlorophyll => absorbs red and blue light, while transmitting and reflecting green light => photosynthetic pigment spectrophotometer => measures pigment s ability to absorb various wavelengths of light. Different wavelengths pass through solution of pigment and measure light transmitted. Sets up an absorption spectrum (light absorption vs wavelength) action spectrum => rate of photosynthesis vs. wavelength Thomas Engelmann s experiment with filamentous algea Photosynthesis-4
Different types of pigments in chloroplasts: 1. Chlorophyll a => dominant pigment => absorbs best in red and violet-blue, least in the green => participates directly in the light reaction 2. Chlorophyll b => accessory pigment => funnels the energy from additional wavelengths to chlorophyll a. 3. Carotenoids => accessory pigment and photoprotection from excessive light. When a molecule absorbs a photon, one of that molecule s electrons is elevated to an orbital with more potential energy (The photon absorbed matches exactly the energy difference between the ground state and excited state). Excited electrons are unstable. Generally, they drop to their ground state releasing heat (some emit light as well - fluorescence). In the thylakoid membrane, chlorophyll is organized along with proteins and smaller organic molecules into photosystems => reaction center surrounded by a light-harvesting complex (contains pigments bound to proteins). photon absorbed transferred to multiple pigments picked up by chlorophyll a in reaction center electron boosted to primary electron acceptor (redox reactions begin) Photosynthesis-5
Two types of photosystem: 1. Photosystem II (PS II) => absorption peak at 680 nm 2. Photosystem I (PS I) => absorption peak at 700 nm Chlorophyll a the same but different protein association. Two possible routes for electron flow: cyclic and noncyclic. Noncyclic (Linear) Electron Flow (predominant route) 1. Photosystem II absorbs photon of light. Electrons of P680 excited to a higher energy state. 2. Electrons are captured by primary electron acceptor (redox). 3. Some energy and an enzyme splits water (photolysis) 2H + + (1/2)O2 + 2e -. Electrons are given to the oxidized reaction center. O2 released as waste product. 4. Photoexcited electrons pass along an electron transport chain (plastoquinone, cytochrome complex, plastocyanin), harnessing their energy to produce ATP. 5. Meanwhile, light energy has excited electron of PS I s P700 reaction center PS I s primary electron acceptor. PS II electron replaces PS I missing electron. 6. Photoexcited electrons are passed from PS I s primary electron acceptor down a second electron transport chain (ferredoxin). 7. The enzyme NADP + reductase transfers electrons from Fd to NADP +. Two electrons are required for NADP + s reduction to NADPH. Purpose of light dependent reactions: solar power converted to chemical energy in ATP and reducing power in NADPH to drive Calvin Cycle. Photosynthesis-6
Animation from text: https://www.youtube.com/watch?v=bk_cjd6evcw Short video from Pearson includes both parts: https://www.youtube.com/watch?v=hjbtjjaln7s Alternate: https://www.youtube.com/watch?v=yed9idmcx0w Cyclic Electron Flow Excited electrons primary acceptor electron transport chain (Fd) Cytochrome complex P700. This generates ATP but no NADPH and no release of oxygen. Necessary because: Calvin cycle consumes more ATP than NADPH. Chloroplasts and mitochondria generate ATP by chemiosmosis => energy stored in the form of H + gradient across a membrane used to drive cellular work (ATP synthesis). ETC pumps protons into thylakoid space (from stroma) as electrons are passed along. This stores energy as a proton-motive force (H + gradient) across the membrane. H + gradient also contributed by photolysis of water AND removal of H + from stroma by NADP +. ATP synthase molecules use the H + gradient to make ATP as H + diffuses back into stroma. (Note: Chemiosmosis also occurs in mitochondria. We ll cover those details in respiration chapter.) Photosynthesis-7
10.3 The Calvin Cycle The Calvin cycle is anabolic, using energy to build sugar. Carbon enters the cycle as CO2 and leaves as sugar. ATP provides energy and NADPH provides the reducing power of electrons. The actual sugar product of the Calvin cycle is not glucose, but a three-carbon sugar, glyceraldehyde-3-phosphate (G3P). Each turn of the Calvin cycle fixes one carbon. One glucose needs six cycles (6CO2). Three phases: 1) Carbon fixation => Enzyme RuBP carboxylase (rubisco) attaches CO2 (3 enter one at a time) to a five-carbon sugar, ribulose bisphosphate (RuBP) to form a six-carbon intermediate that is very unstable. Immediately splits in half to form two molecules of 3-phosphoglycerate. 2) Reduction => each 3-phosphoglycerate receives another phosphate group from ATP to form 1,3-bisphosphoglycerate. A pair of electrons from NADPH reduces each 1,3- bisphosphoglycerate to G3P. These (one) can exit and build carbs or continue to phase 3 (remaining five). 3) Regeneration => five G3P (15C) remain in the cycle to regenerate three RuBP. For the net synthesis of one G3P molecule, the Calvin cycle consumes nine ATP and six NADPH (regenerated by light reactions). Photosynthesis-8
10.4 Alternative Mechanisms of Carbon Fixation The stomata are the major route for gas exchange (CO2 in and O2 out), and also for the loss of water (transpiration). On hot, dry days, most plants (C3 plants) close their stomata. This causes problems for photosynthesis. Why??? CO2 levels drop, O2 levels rise. Rubisco can add O2 to RuBP which results in a 3-C and a 2-C fragment. The 2-C leaves the chloroplast, enters peroxisome or mitochondria and released as CO2=> photorespiration => produces NO ATP but consumes it and removes organic material from the cell. Why is it in existence? Evolutionary baggage => early Earth had little to no oxygen, so no biggie. In some cases, plays a protective role: those that are impaired in ability for photorespiration suffer more damage from excess light. Adaptations that minimize photorespiration: C4 plants => first fix CO2 into a four-carbon compound. Mesophyll cells are loosely arranged between the bundle sheath (tightly packed around vein) and the leaf surface. Limited to cyclic electron flow here Phosphoenolpyruvate (PEP) carboxylase (which has no affinity for O2) adds CO2 to phosphoenolpyruvate (PEP) to form oxaloacetate (4-C). These are pumped into bundle-sheath cells in form of malate through plasmodesmata. A CO2 molecule is stripped from this 4-C compound, and the 3-C portion returns to the mesophyll cells (to regenerate PEP). The CO2 molecule enters the Calvin cycle via rubisco. ex. sugarcane and corn => do best in intense sunlight Photosynthesis-9
CAM plants => crassulacean acid metabolism => open their stomata during the night and close them during the day. CO2 is fixed into a variety of organic acids in mesophyll cells and stored in vacuoles until day when photosynthesis resumes. (Leaf structure is the same as a C3 plant) ex. succulent plants, cacti, pineapples Importance of photosynthesis: plant supplied with chemical energy fuel for respiration and raw materials for anabolic pathways provide fuel and raw materials for heterotrophs presence of oxygen Photosynthesis-10