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Photosynthesis: Energy from Sunlight. 8.1 What Is Photosynthesis?. Photosynthesis : “synthesis from light” The broad outline: Plants take in CO 2 and release water and O 2 Light is required. PHOTOAUTOTROPHS. An organism that transfers radiant (sunlight) energy into organic material/energy.
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8.1 What Is Photosynthesis? Photosynthesis: “synthesis from light” The broad outline: • Plants take in CO2 and release water and O2 • Light is required
PHOTOAUTOTROPHS • An organism that transfers radiant (sunlight) energy into organic material/energy. • Plants, algae, cyanobacteria CHEMOAUTOTROPHS • An organism that transfers chemical energy into organic material/energy. • Usually from CO2 and other dissolved minerals vented by underwater geysers • bacteria/protozoan
HETEROTROPHIC NUTRITION • Organisms that obtain their energy from eating other organisms….. • Basically every heterotroph is dependent on autotrophs for energy. • Therefore EVERY LIVING THING is DEPENDENT UPON PHOTOSYNTHESIS
Photosynthesis occurs in the chloroplasts • Chloroplast structure- two membranes that consist of… • Thylakoids- flattened sacs of membrane • Grana- stacks of thylakoids • Stroma- fluid outside of grana
8.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? Light is a form of electromagnetic radiation, which comes in discrete packets called photons and behave as particles. Light also behaves as if propagated as waves. Energy of a photon is inversely proportional to its wavelength. As wavelength decreases, energy increases.
8.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? When a photon meets a molecule it can be: Scattered -- reflected Transmitted -- pass through the molecule Absorbed—the molecule acquires the energy of the photon. The molecule goes from ground state to excited state.
Figure 8.4 Exciting a Molecule (A) Energy from the photon boosts an electron into another shell
8.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? Photons can have a wide range of wavelengths and energy levels. Molecules that absorb specific wavelengths in the visible range of the spectrum are called pigments. Plants use mainly blue and red light from the visible spectrum, but reflect green light (which is why they appear green to us)
8.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? Several types of pigments absorb light energy used in photosynthesis: Chlorophylls: a and b Accessory pigments: absorb in red and blue regions, transfer the energy to chlorophylls—carotenoids, phycobilins
8.1 What Is Photosynthesis? Photosynthesis is divided into two pathways: Light reactions: occurs on the thylakoid membrane; light energy converted to chemical energy (in ATP and NADPH + H+) Light-independent (dark) reactions: occurs in the stroma; use the ATP and NADPH + H+ plus CO2 to produce sugars/glucose. AKA Calvin cycle.
8.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? Pigments are arranged in photosystems and are packed together on thylakoid membrane proteins. Excitation energy is passed thru pigments and ends up in the reaction center pigment (typically chlorophyll).
Figure 8.8 Energy Transfer and Electron Transport The photon shakes loose “excited” electrons which bounce around until they get absorbed by chlorophyll
8.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? Two systems of electron transport: Noncyclic electron transport—produces NADPH + H+ and ATP Cyclic electron transport—produces ATP only
8.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? Two photosystems are required in noncyclic electron transport. Each photosystem consists of several chlorophyll and accessory pigment molecules. The photosystems complement each other, must be constantly absorbing light energy to power noncyclic electron transport.
Figure 8.9 Noncyclic Electron Transport Uses Two Photosystems
8.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? Noncyclic/Linear electron transport: A photon bounces around PS II until it reaches Chlorophyll a Photon excites electron in chlorophyll a An enzyme catalyzes the splitting of water into two electrons, two hydrogen ions and oxygen. (the electrons replace those lost in step two from chlorophyll, the oxygen combines with another and leaves as oxygen gas and the hydrogens create a gradient.
Noncyclic/Linear electron transport: • The excited electron passes from PSII to PSI through electron transport chain. • The exergonic fall of electrons to lower energy level provides energy for synthesis of ATP. • More light hits PSI exciting more electrons- the electron hole from there is now filled with the electrons from PSII. • The excited electrons from PSI travel down a second electron transport chain. • The enzyme NADP+ reductase catalyzes the transfer of electrons to NADP+ and also a H+
THE LIGHT REACTION: To sum up- The light reactions use solar power to generate ATP and NADPH, which provide chemical energy and reducing power of the carbohydrate-synthesizing reactions of the Calvin cycle. Output NADPH ATP O2 • Input • Radiant energy • NADP+ • ADP • H2O
8.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? Cyclic electron transport: an electron from an excited chlorophyll molecule cycles back to the same chlorophyll molecule. A series of exergonic redox reactions, the released energy creates a proton gradient that is used to synthesize ATP.
Figure 8.10 Cyclic Electron Transport Traps Light Energy as ATP
8.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? Cyclic Electron Flow occurs only in Photosystem I and is a short circuit. • The photon bounces around in PSI. • Electrons are excited to the primary acceptor. • They then continue to travel through the electron transport chain generating ATP. • Then return back to the original chlorophyll A in PSI completing the cycle.
8.2 How Does Photosynthesis Convert Light Energy into Chemical Energy? Photophosphorylation: light-driven production of ATP—a chemiosmotic mechanism. Electron transport is coupled to the transport of H+ across the thylakoid membrane—from the stroma into the lumen.
8.3 How Is Chemical Energy Used to Synthesize Carbohydrates? DARK REACTIONS/LIGHT-INDEPENDENT REACTIONS/CALVIN CYCLE Carbon fixation—CO2 is reduced to carbohydrates using enzymes and the energy in ATP and NADPH.
8.3 How Is Chemical Energy Used to Synthesize Carbohydrates? The pathway of CO2 fixation is called the Calvin cycle and occurs in three main phases. Phase 1: CO2 is first added to a 5-C RuBP (ribulose bisphosphate); the 6-C compound immediately breaks down into two molecules of 3PG (3-phosphoglycerate). The enzyme that catalyzes this rxn is rubisco—the most abundant protein in the world.
Phase 2: Reduction- In this phase electrons are donated from NADPH and phosphates are donated from ATP to rearrange the molecules of the 3GP into a 3 carbon sugar called G3P (glyceraldehyde 3-phosphate). • G3P is a sugar used to build other organic compounds like glucose.
Phase 3: Regeneration of the RuBP- a complex series of reactions that restore the original Carbon acceptor. • The net synthesis of one G3P molecules consumes many CO2, NADPH and ATP molecules. • C3 plants: first product of CO2 fixation is 3PG. • Palisade cells in the mesophyll have abundant rubisco. • Examples: rice, wheat, soybeans
8.4 How Do Plants Adapt to the Inefficiencies of Photosynthesis? EVOLUTIONARY ADAPTATIONS In hot and dry conditions C3 plants will close their stomata to prevent water loss. This also lowers the amount of carbon dioxide. Rubisco fixes Oxygen and releases Carbon dioxide from the conglomeration of carbons in the Calvin cycle and produces no sugar. Photorespiration: Light driven (because ATP from light rxn is used) process that uses oxygen and produces carbon dioxide.
8.4 How Do Plants Adapt to the Inefficiencies of Photosynthesis? C4 plants: first product of CO2 fixation is oxaloacetate, a 4-C compound. C4 plants—corn, sugar cane, tropical grasses—can keep stomata closed on hot days, but photorespiration does not occur. Bundle sheath cells around the veins of leaf that do calvin cycle. Mesophyll cells, that are spaced out in rest of leaf, contain PEP carboxylase.
8.4 How Do Plants Adapt to the Inefficiencies of Photosynthesis? In mesophyll cells, CO2 is accepted by PEP (phosphoenolpyruvate) to form oxaloacetate, a 4 carbon sugar. PEP carboxylase has no affinity for O2. Oxaloacetate diffuses to bundle sheath cells which have abundant rubisco. The oxaloacetate is decarboxylated (PEP returns to mesophyll cells), CO2 enters the Calvin cycle in the bundle sheath cells- all in a condensed area.
Figure 8.18 The Anatomy and Biochemistry of C4 Carbon Fixation (B)
8.4 How Do Plants Adapt to the Inefficiencies of Photosynthesis? CAM plants—crassulacean acid metabolism Occurs in many succulent plants like cacti and pineapples which keep stomata closed during the day to conserve water. Fix CO2 with PEP carboxylase—at night—stomata can open with less water loss. Oxaloacetate is converted to malic acid. Day— when light makes ATP and NAPDH available, malic acid goes to chloroplasts and is decarboxylated—CO2 enters the Calvin cycle.
8.5 How Is Photosynthesis Connected to Other Metabolic Pathways in Plants? Photosynthesis and respiration are closely linked by the Calvin cycle. Glycolysis in the cytosol, respiration in the mitochondria, and photosynthesis in the chloroplasts can occur simultaneously.
8.5 How Is Photosynthesis Connected to Other Metabolic Pathways in Plants? Photosynthesis results in only 5 percent of total sunlight energy being transformed to the energy of chemical bonds. Understanding the inefficiencies of photosynthesis may be important as climate change drives changes in photosynthetic activity of plants.
