Adaptations to Photosynthesis 2022 PDF

Adaptations to Photosynthesis There are three categories of plants based on method of carbon fixation – C3, C4 and CAM Most plants fit C3 category Name given because first compound to be produced in Calvin cycle (PGA) has 3 carbon atoms  Photorespiration Rubisco can cata...

Adaptations to Photosynthesis There are three categories of plants based on method of carbon fixation – C3, C4 and CAM Most plants fit C3 category Name given because first compound to be produced in Calvin cycle (PGA) has 3 carbon atoms  Photorespiration Rubisco can catalyze two reactions Addition of CO2 to RuBP Addition of O2 to RuBP O2 competes with CO2 for rubisco’s active site When [O2] is greater than [CO2] oxygen binds more often This process is called photorespiration because it occurs in light  Photorespiration in C3 plants Photorespiration ↓ production of sugars by photosynthesis When photosynthesis occurs two PGA molecules are produced when CO2 reacts with RuBP When photorespiration occurs only one PGA molecule is produced along with one 2-carbon phosphoglycolate molecule which is partially converted to CO2 C3 plants lose up to one half of the carbon they fix to photorespiration  Photorespiration and temperature At normal conditions the rate of photosynthesis is 4 times that of photorespiration – about 20% of fixed carbon is lost to photorespiration Photosynthesis optimum temperature is 15-25˚C while photorespiration’s is 30-47 ˚C Hot days increase photorespiration Thus the warm temperatures of tropical climates are a problem  Alternative Mechanisms Some plant species have evolved mechanisms of photosynthesis that concentrates CO2 at the site where rubisco is found This suppresses photorespiration  C4 Plants These plants have a unique leaf anatomy as there are two types of photosynthetic cells Bundle-sheath cells Mesophyll cells Examples: corn sugarcane crabgrass  These plants use PEP carboxylase to add CO2 to a 3 carbon molecule called PEP forming a 4 carbon molecule oxaloacetate  hence C4 plants Oxaloacetate converted to malate and transported to the bundle- sheath cells Malate is decarboxylated  CO2 is then available for Calvin cycle  Bundle-sheath cells are impermeable to CO2 so it gets “stuck” there Pyruvate is left over and transported back to mesophyll cell to be converted back to PEP  CAM Plants Examples include water-storing plants like cacti and pineapple Use carbon fixation method exactly like C4 method but reactions all take place in same cells What is the difference? CAM plants open stomata at night and close them during the day Helps conserve water but prevents CO2 from entering during the day Stomata open at night and CO2 is taken in and incorporated into C4 compounds using PEP carboxylase These compounds are stored in vacuoles until morning when stomata close and CO2 is released to enter the Calvin cycle  Carbon Fixation Methods Criteria C3 Plants C4 Plants CAM Plants Environment Cool, moist Hot, dry Very hot, very dry Stomata Ok to have Not ok to have Stomata only stomata open stomata open all open at night all day – water day – too much to prevent loss is quickly water loss and water loss replaced and lots of during hot day not a lot of transpiration time transpiration Carbon Fixation CO2 + RuBP  PGA CO2 + PEP  oxaloacetate Reaction 1C + 5 C  3C (X2) 1C + 3C  4C Facilitated by rubisco Facilitated by PEP carboxylase How CO2 gets to Directly Oxaloacetate (in mesophyll Reactions to produce Calvin cycle cell)  malate pyruvate + Malate stored in vacuoles CO2 in bundle sheath cell until morning  CO2 to Calvin cycle Predominate process Photorespiration Calvin cycle Calvin cycle in warmer conditions? Example Wheat, barley, potatoes grasses (corn, sugar cane) succulents (cacti, pineapple)