Calvin Cycle, C4 and CAM Plants Chapter 8 Calvin Cycle, C4 and CAM Plants

You Must Know How the Calvin cycle uses the energy molecules of the light reactions (ATP and NADPH) to produce carbohydrates (G3P) from CO2. The difference between C3, C4, and CAM plants. (Knowledge of C4/CAM photosynthesis is not required for the AP exam, but it is a nice example of competitive inhibition, an evolutionary relic, and evolutionary adaptations to arid conditions.)

anabolic H2O CO2 Light NADP ADP Calvin Cycle Light Reactions O2 Figure 8.UN03 anabolic H2O CO2 Light NADP ADP Calvin Cycle Light Reactions ATP Concept 8.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar The Calvin cycle is anabolic. It builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH. NADPH O2 [CH2O] (sugar) 3

For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2 Carbon fixation 3  5C 6  3C Calvin Cycle Regeneration of CO2 acceptor 5  3C Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P) For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2 The Calvin cycle has three phases Carbon fixation Reduction Regeneration of the CO2 acceptor Reduction 1 G3P (3C) 4

(Note: for the next three slides, you only need to remember the names of molecules that are marked with a red arrow.)

R u BP? Input 3 as 3 CO2 Phase 1: Carbon fixation Rubisco RuBP Calvin Figure 8.17-1 Input 3 as 3 CO2 Phase 1: Carbon fixation Rubisco 3 P P 3 P P 6 P RuBP 3-Phosphoglycerate Calvin Cycle R u BP? Phase 1, carbon fixation, involves the incorporation of the CO2 molecules into ribulose bisphosphate (RuBP) using the enzyme rubisco 6

phosphorylation reduction Input 3 as 3 CO2 Phase 1: Carbon fixation Figure 8.17-2 Input 3 as 3 CO2 Phase 1: Carbon fixation Rubisco phosphorylation 3 P P 3 P P 6 P RuBP 3-Phosphoglycerate 6 ATP 6 ADP reduction Calvin Cycle 6 P P 1,3-Bisphosphoglycerate 6 NADPH 6 NADP 6 P i Phase 2, reduction, involves the reduction and phosphorylation of 3-phosphoglycerate to G3P. 6 P G3P Phase 2: Reduction Glucose and other organic compounds 1 P G3P Output 7

Phase 1: Carbon fixation Rubisco Figure 8.17-3 Input 3 as 3 CO2 Phase 1: Carbon fixation Rubisco 3 P P 3 P P 6 P RuBP 3-Phosphoglycerate 6 ATP 6 ADP 3 ADP Calvin Cycle 6 P P 3 ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of RuBP 6 NADP 6 P i 5 P Phase 3, regeneration, involves the rearrangement of G3P to regenerate the initial CO2 receptor, RuBP You do not have to recreate the figure from memory, but you should be able to explain the process. G3P 6 P G3P Phase 2: Reduction Glucose and other organic compounds 1 P G3P Output 8

The following two sides contain background Information on why C4 and CAM photosynthesis are adaptive in some environments.

Photosynthetic organisms evolved for roughly 800,000,000 years in a low O2 environment. Life first evolved about 3.5 billion years ago. Very little O2 was available in the atmosphere until about 2.7 billion years ago. The 1st photosynthetic organisms evolved roughly 3.5 billion years ago. The 1st photosynthetic organisms oxygenated the atmosphere. (You won’t be expected to remember the dates from this slide)

What happens when a plant closes its stomata? Water loss decreases Leaf cross section Mesophyll Stomata Chloroplasts Vein CO2 O2 Figure 8.3a What happens when a plant closes its stomata? Water loss decreases CO2 concentrations decreases O2 concentrations increase CO2 enters and O2 exits the leaf through microscopic pores called stomata. Water also exits the leaf through the stomata. Adaptation to dehydration is a problem for land plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis. On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis. The closing of stomata reduces access to CO2 and causes O2 to build up. These conditions favor an apparently wasteful process called photorespiration. H2O 11

competitive inhibition In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two-carbon compound. It uses up ATP without producing any sugar. Evolutionary Relic of low atmosphere competitive inhibition In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate). Photorespiration decreases photosynthetic output by consuming ATP, O2, and organic fuel and releasing CO2 without producing any 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 . Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle. Rubisco

An enzyme in the mesophyll cells has a high affinity for CO2 and can fix carbon into a four-carbon compound even when CO2 concentrations are low. C4 This takes energy! 1 CO2 C4 Mesophyll cell Organic acid CO2 2 C4 plants minimize the cost of photorespiration by incorporating CO2 into a four-carbon compound. An enzyme in the mesophyll cells has a high affinity for CO2 and can fix carbon even when CO2 concentrations are low. These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle. Bundle- sheath cell Calvin Cycle Sugar (a) Spatial separation of steps 13

C4 Plants Sugarcane Corn Switchgrass

CAM Plants This takes energy! CAM plants open their stomata at night, incorporating CO2 into organic acids. Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle. This takes energy! 1 CO2 CAM Organic acid Night CO2 2 Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon. Day Calvin Cycle Sugar (b) Temporal separation of steps 15

CAM Plants CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce water loss, but open at night to collect carbon dioxide. The CO2 is stored as an organic acid , and then used during photosynthesis during the day. The pre-collected CO2 is concentrated around the enzyme rubisco, increasing photosynthetic efficiency.

Advantages? Disadvantages? CAM C3 C4 C4 and CAM plants have a competitive advantage over C3 plants under conditions of drought, high temperature, or CO2 limitation because they lose less water through their stomata when they fix CO2. The disadvantage is that it takes more ATP to make the same amount of G3P. Advantages? Disadvantages?