1. The Dark Reaction - - light-independent - - energy stored in ATP and NADPH (from light reaction) is used to reduce CO 2 to sugar

4. Three independent ways to reduce CO 2 to make sugar: 1. the Calvin cycle (C3), 2. C4 photosynthesis, 3. crassulacean acid metabolism (CAM).

5. The Calvin Cycle 1. CO 2 is fixed by rubisco 1.CO 2 + RuBP  unstable C6  2 PGA 2. Reduction of CO 2 to make G3P - Uses ATP and NADPH - G3P is exported to cytoplasm to make starch, sucrose, oils 3. Regenerating RuBP 1.- for enery 12 molecules of G3P made in the Calvin cycle two are “released” 2.- the Calvin cycle needs to “turn” 6 times to make one glucose!!!

8. G3P - - one-third forms starch - - two-thirds are converted to sucrose and then hydrolyzed in other parts of plant into glucose and fructose - - Ultimately used as a source of C for nucleic acids, amino acids, fats…

9. Rubisco ● Ribulose bisphosphate carboxylase oxygenase ● fixes CO 2 & O 2 ● Enzyme in Calvin Cycle (1 st step) ● Most abundant protein on Earth – About 50% total plant protein!

10. Transpiration: water loss  Leaf transpiration occurs through stomata  Stomata open to allow the diffusion of carbon dioxide gas from the air for photosynthesis  Transpiration also cools plants and enables mass flow of mineral nutrients from roots to shoots

11. Stomata - lungs  openings on the surface of the leaf that allow the exchange of gases between air spaces in the leaf and the atmosphere  Guard cells –control the size of the stoma in response to environmental conditions

12. ● The size of the guard cell changes when water moves into or out of the cell ● K + ions are actively pumped into the guard cell and water follows by osmosis ● Light, and CO 2 concentration affect the movement of K + ions into the cells ● Generally stomata are open during the day and closed at night

13. Photorespiration ● the reaction of RuBP with oxygen, reduces the efficiency of photosynthesis ● rubisco is inefficient: “fixes” O 2, as well as CO 2 ● C3 plants lose 20% of their energy to fix one CO 2 ● this gets worse with heat!

14. - Under hot and dry conditions (daytime) plants will close their stomata to prevent water loss - This causes a build up of oxygen since CO 2 can’t enter…so MORE photorespiration

15. Avoiding Photorespiration ● C3 – The majority of plants ● C4 – CO 2 temporarily stored as 4-C organic acids resulting in more more efficient C exchange rate – Advantage in high light, high temperature, low CO 2 – Many grasses and crops (e.g., corn, sorghum, millet, sugar cane) ● CAM – Stomata open during night – Advantage in arid climates – Many succulents (e.g. cacti)

18. Fig. 10.21

19. Comparison of Photosynthesis in C 3 and C 4 Plants VARIABLEC 3 PLANTSC 4 PLANTS Photorespirati on ExtensiveMinimal Perform Calvin cycle? Yes Primary CO 2 acceptor RuBPPEP CO 2 -fixing enzyme Rubisco (RuBP carboxylase/oxyge nase) PEP carboxylase and rubisco First product of CO 2 fixation 3PG (3-carbon compound) Oxaloacetate (4- carbon compound) Affinity of carboxylase for CO 2 ModerateHigh Photosynthetic cells of leaf MesophyllMesophyll + bundle sheath Classes of chloroplasts OneTwo

20. - C3 photosynthesis: about 3.5 billion years ago, - C4 plants appeared about 12 million years ago. - A possible factor in the emergence of the C4 pathway is the decline in atmospheric CO2

21. CAM PLANTS - - CAM is similar to C4 - - CO2 is fixed to a 4-carbon compound - - Separated by time rather than space: At night   cooler and water loss is minimized   stomata open and CO2 is fixed in mesophyll cells to form the 4- carbon oxaloacetate, which is converted into malic acid. During the day   when the stomata close to reduce water loss, the accumulated malic acid is shipped to the chloroplasts to form sugars

22. Fig. 10.22

23. CAM Plants

24. Global Environmental Change & Photosynthesis: C3 vs. C4 vs. CAM ● Increasing CO2 ● Increasing chronic and acute temperatures ● Changes in water

27. *At high CO2, C3 more efficient than C4 at all temps. (photosynthesis only, not other processes)