BIOL2010 Plant Biology Lecture (Winter 2025) PDF

Instructor: Dr. Junyan Zhang © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION Ø Noncyclic photophosphorylation vs. Cyclic photophosphorylation join.iClicker.com Ø Light-dependent Reactions vs....

Instructor: Dr. Junyan Zhang © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION Ø Noncyclic photophosphorylation vs. Cyclic photophosphorylation join.iClicker.com Ø Light-dependent Reactions vs. Code: ZQJX Carbon-Fixation Reactions BIOL2010 Plant Biology Winter 2025 Ø C3, C4 and CAM Photosynthesis 2 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION Ø PS I and II are spatially separated Ø PSII feeds into PSI Ø Photophosphorylation is the process of utilizing light energy from photosynthesis to convert ADP to ATP. Ø Two types: ü Cyclic Photophosphorylation ü Non-cyclic Photophosphorylation Requires two excitations by light 3 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION Non-cyclic photophosphorylation This process is referred to as non- cyclic photophosphorylation because the lost electrons by P680 of Photosystem II are occupied by P700 of Photosystem I and are not reverted to P680. Here the complete movement of the electrons is in a unidirectional or in a non-cyclic manner 4 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION ü PSI can work independently of PS II in cyclic electron flow, this is called cyclic photophosphorylation ü This process uses the chlorophyll P700 and generates only ATP 5 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION Image source Non-Cyclic Photophosphorylation Cyclic Photophosphorylation 6 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION Two paces of photosynthesis: Light Reaction and Dark Reaction Ø The light reactions occur in the grana of the chloroplast (PSII and PSI) Ø The light reactions convert light energy and the splitting of water to chemical energy as ATP and NADPH and release H and oxygen Ø Notice that one of the main products of photosynthesis is said to be sugars and yet the light reactions don’t form any sugars…. 7 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION Ø Carbon-fixation reactions (also known as light-independent reaction or dark reaction) occur not in the presence of light and in the stroma of the chloroplast Ø Carbon-fixation reactions use the energy from the light-dependent reactions to convert CO2 and H to sugar 8 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION Ø ATP and NADPH generated by the light Stoma closing Stoma opening reaction are used to fix and reduce carbon and to synthesize simple sugars Ø Carbon comes in the form of CO2 in air to plants Ø See stomata, right Ø In water-living photosynthetic organisms (algae and bacteria), CO2 is dissolved into Image source: Nature Plants their surrounding water 9 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION Ø Calvin Cycle (Light-Independent Reaction) Ø The term “dark reaction” is confusing because these reactions can occur in the light or the dark Ø A better term is “light independent” reactions Image source 10 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION Ø The energy-source molecules, ATP and NADPH, which were generated from electron transfer chains, and electrons originally derived from photolysis, which splits H2O molecules are used to make sugars Ø The C3 Calvin cycle starts and ends with ribulose bisphosphate (RuBP), a 5 C sugar with 2 phosphate groups Ø RuBP carboxylase/oxygenase or rubisco which catalyzes the first reaction is the world’s most abundant enzyme Ø Rubisco is BOTH a carboxylase AND an oxygenase 11 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION  Light reactions Dark reactions Harvests photons of light and produces ATP and NADPH used to fix CO2 as energy- ATP and NADPH rich sugars in the Calvin cycle Water is required as a source of The first product is a molecule with 3 C electrons atoms, hence C3 photosynthesis Oxygen is produced as a by-product Produces sugars and starch as a product Doesn’t require light, but happens during This reaction requires light daytime 12 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION The Calvin cycle occurs in the stroma. A. True B. False 13 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION The Calvin cycle consumes ATP and NADPH. A. True B. False 14 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION 15 ü To acquire CO2 for photosynthesis, plants have to open their stomata ü This also leads to increased loss of water ü Under dry conditions, plants close their stomata ü This is a problem for photosynthesis under dry conditions 16 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION ü We have been studying C3 photosynthesis, which is used by most plant species, i.e. approximately 95% of plants. ü C3 plants, C4 plants, and CAM plants each do photosynthesis using different pathways for fixing carbon. ü Each has its own advantages and disadvantages, depending on the environment the plant is in. 17 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION ü C3 plants do it all at once – photosynthesis takes place only in mesophyll cells ü In hot, dry weather, the stomata close to conserve water ü C3 plants do well in moderate temperature with supply of water 18 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION ü An extra set of reactions are inserted between the light dependent and light independent reactions ü These reactions act as a CO2 pump concentrating CO2 at the site of CO2 fixation in the Calvin cycle ü This pathway requires spatial separation of Stage 1: initial fixation of CO2 to form 4- the initial and final fixation of CO2 to function; carbon acids Occurs in mesophyll & bundle sheath Stage 2: release of CO2 to Calvin cycle ü In hot and dry environments C4 photosynthesis is more efficient than C3 19 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION Many grass species use this pathway, including corn, sugarcane and sorghum 20 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION ü Crassulacean Acid Metabolism (CAM) type photosynthesis evolved to reduce loss of water ü Stomata open at night, minimal water loss ü All reactions for photosynthesis occur in mesophyll cells ü Carbon reactions occur only at night, in the cooler, moister air to avoid photorespiration ü Used by plants like this cactus that live in desert conditions 21 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION  22 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION  C4 species à spatial separation of light & dark reactions CAM species à temporal separation of light & dark reactions 23 © JUNYAN ZHANG 2025 NOT FOR SALE OR DISTRIBUTION