ADAPTIONS TO TEMPERATURE AND DROUGHT KEY DIFFERENCE: HOW CO2 IS CAPTURED C3 C4 CAM WHEAT POTATO RICE CORN SUGERCANE PINEAPPLE CACTUS VAST MAJORITY OF PLANTS ≈ 85 % of species) ≈7 % of species, common in deserts ≈3 % of species, most rare THRIVE IN MODERATE TEMPERATURES, PLENTIFUL GROUNDWATER, CO2 > 200 ppm THRIVE IN HIGH TEMPERATURES, DROUGHT THRIVE IN HIGH TEMPERATURES, DROUGHT ADVANTAGE: ENERGY EFFICIENT CHALLENGES: WATER LOSS IN HOT, DRY CONDITIONS ADVANTAGE: MORE DROUGHT RESISTANT CHALLENGES: LESS ENERGY EFFICIENT ADVANTAGE: EXTREMELY DROUGHT RESISTANT CHALLENGES: VERY ENERGY INEFFICIENT

KEY DIFFERENCES IN HOW CO2 IS CAPTURED C4: SEPARATION IN SPACE; CAPTURE CO2 IN MESOPHYLL CELL; CALVIN CYCLE IN SEPARATE CELL CAM: SEPARATION IN TIME CAPTURE CO2 AT NIGHT; CAN USE CO2 DURING THE DAY ENZYME USED FOR CO2 CAPTURE: RUBISCO PEPCO PEPCO

TRANSPIRATION – process of evaporation of water from surface of leaves; helps cool plant Stoma Note: As CO2 is taken in through stoma (opening on surface of leaf), O2 and H2O exit

The addition of CO2 to RuBP to form 3-PGA is catalyzed by the enzyme Rubisco -Very inefficient enzyme Slow rate means that many copies of enzyme are required in each chloroplast

PHOTORESPIRATION: Rubisco can also catalyze the addition of O2 to PGA with disastrous consequences for plants Rubisco evolved at time in earth’s history in which atmospheric oxygen concentration was very low No evolutionary pressure to exclude O2

O2 will outcompete CO2 for active site; PHOTORESPIRATION: O2 CAN OUTCOMPETE CO2 FOR RUBISCO ACTIVE SITE UNDER CERTAIN CONDITIONS Normal Temperature, [CO2] ≥ [O2] If [O2] >> [CO2] , O2 will outcompete CO2 for active site; O2 affinity increases at higher Temp Rubisco normally binds CO2 more tightly than O2

PHOTORESPIRATION O2 is added to RuBP, siphons C out of Calvin cycle H2O escapes from stomata on hot, dry days Hot, dry conditions → Stomata CLOSE to reduce H2O loss. Stomata closed →[CO2] , [O2] O2 outcompetes CO2 to bind to Rubisco O2 is added to RuBP, siphons C out of Calvin cycle Link to Boyer photorespiration animation

PHOTORESPIRATION O2 is added to RuBP, siphons C out of Calvin cycle H2O escapes from stomata on hot, dry days Hot, dry conditions → Stomata CLOSE to reduce H2O loss. Stomata closed →[CO2] , [O2] O2 outcompetes CO2 to bind to Rubisco O2 is added to RuBP, siphons C out of Calvin cycle Link to Boyer photorespiration animation

O2 will outcompete CO2 for active site; PHOTORESPIRATION: O2 CAN OUTCOMPETE CO2 FOR RUBISCO ACTIVE SITE UNDER CERTAIN CONDITIONS Normal Temperature, [CO2] ≥ [O2] If [O2] >> [CO2] , O2 will outcompete CO2 for active site; O2 affinity increases at higher Temp Rubisco normally binds CO2 more tightly than O2

Impact of Photorespiration on Calvin cycle Transfer of O2 ultimately causes loss of carbon, instead of gain of carbon in Calvin cycle

PEPCO HAS VIRTUALLY NO AFFINITY FOR O2 C4 AND CAM PLANTS USE THE ENZYME PHOSPHOENOLPYRUVATE CARBOXYLASE (PEPCO) TO CAPTURE CARBON DIOXIDE TO FORM A 4 CARBON CARBOXYLIC ACID 3C + 1C → 4C REACTION CATALYZED BY PEPCO PEPCO HAS VIRTUALLY NO AFFINITY FOR O2 5C + 1C → 6C → 3C + 3C O2 CAN COMPETE WITH CO2; PHOTORESPIRATION

Fig. 10-19 C4 plants have a different anatomy than C3 plants and capture CO2 initially not in 3-carbon PGA but rather in 4 –carbon oxaloacetate C4 leaf anatomy The C4 pathway Mesophyll cell Mesophyll cell CO2 Photosynthetic cells of C4 plant leaf PEP carboxylase Bundle- sheath cell Oxaloacetate (4C) PEP (3C) Vein (vascular tissue) ADP Malate (4C) ATP Pyruvate (3C) Bundle- sheath cell Stoma CO2 Video link excellent Calvin Cycle Note that Calvin cycle takes place in a completely separate cell, the Bundle –sheath cell, than CO2 capture. Sugar Vascular tissue Video Link to C4

Comparing efficiency of CO2 capture in C3 by RUBSICO vs C4 Plants by PEPCO as a function of [CO2] C4 plants capture CO2 with greater efficiency than C3 at low [CO2]; PEPCO BINDS CO2 MORE TIGHTLY THAN RUBISCO AT LOW [CO2] C3 plants are more efficient at higher [CO2] due to C4 plants reaching saturation at lower [CO2]

HOW DOES TEMPERATURE IMPACT RATE OF PHOTOSYNTHESIS IN C4 VS C3 PLANTS? C4 IS MUCH MORE EFFICIENT AT HIGHER TEMPERATURE; C3 IS MORE EFFICIENT AT LOWER TEMPERATURES

What do you notice about CO2 capture (yellow) and water loss (transpiration) rates (blue) at different times of the day?

What do you notice about CO2 capture (yellow) and water loss (transpiration) rates (blue) at different times of the day? PEPCO RUBISCO ENZYME PEPCO CATALYZES CAPTURE OF CO2 AS A FOUR CARBON ORGANIC ACID; CO2 DELIVERED TO RUBISCO BY FROM 4 C ACID

Figure 10.19 C4 and CAM photosynthesis compared Table Link

Link to Smith Calvin cycle animation Link to University of Arizona Biology Project

Mechanisms of Carbon Fixation KEY DIFFERENCE- HOW CO2 IS CAPTURED

FEATURE C3 C4 CAM

NOTE HOW CO2 IS CAPTURED AS AN ORGANIC ACID, OXALOACETATE IN REACTION CATALYZED BY ENZYME PEP

Link to Boyer photorespiration animation

CAM plants capture CO2 as 4-carbon organic acids, oxaloacetate at night and release CO2 to Calvin cycle during the day

PHOTORESPIRATION: Rubisco can also catalyze the addition of O2 to PGA with disastrous consequences for plants Rubisco evolved at time in earth’s history in which atmospheric oxygen concentration was very low No evolutionary pressure to exclude O2 Rubisco binds CO2 more tightly than O2, but if [O2] is much higher than [CO2] , O2 will out compete CO2 for active site; also higher temperatures increase relative affinity for O2. Transfer of O2 ultimately causes loss of carbon, instead of gain of carbon in Calvin cycle

Photorespiration: An evolutionary relic? Photosynthesis first evolved in the absence of oxygen Oxygen is not detected (as rust in iron-rich rocks) until ~ 2.7 bay. In most plants, initial fixation of CO2, via the enzyme RUBISCO, forms a three-carbon compound In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings