1. Photosynthesis in C4 Plants

2. Building Sugars What products came from the light-dependent reactions? – ATP and NADPH Light-independent reactions use ATP and NADPH to build sugars from CO 2 – ATP  energy source – NADPH  high energy electrons and H + Where do these reactions happen? – Stroma of the chloroplasts Who mapped it out? – Melvin Calvin (called Calvin Cycle)

3. The Calvin Cycle: Part 1 Where do we also find G3P? – Half-way point in glycolysis – Can start glycolysis from here Phase 1: Carbon Fixation 1)CO 2 is fixed to 1,5- bisphosphate (RuBP) by the enzyme RuBP carboxylase/oxygenase (rubisco) creating a 6-C sugar 2)6-C is broken into two 3- phosphateglycerates (3PGA) CO 2 +ATP+NADPH  ADP+P i +NADP + +glyceraldehyde-3-phosphate(G3P )

4. The Calvin Cycle: Part 2 Phase 2: Reduction 3) NADPH and ATP supply the energy, electrons, and H + needed to change 3PGA into G3P How many G3P do we have? –2–2 How many do we need to make glucose? –2–2 Why do plants NOT just make sugar with the 2 G3P? – Need to regenerate RuBP or cycle shuts down Where have we seen this before? – Oxaloacetate in the Krebs cycle

5. The Calvin Cycle: Part 3 Phase 3: Regeneration 4) G3P molecules will be used to build RuBP molecules 3 turns of the cycle will produce 6 G3P but 5 will be used to place the RuBP used; the left over G3P is used to build sugars 3 Turns: - 3 CO 2 and 3 RuBP (5-C)  18 C - 6 G3Ps produced - 5 G3Ps  15 C  3 RuBP - Last G3P builds a sugar How many turns are needed to make glucose? – 6! That is really complex How many ATP are used to make glucose? – 18 ATP (3 ATP/Turn; 6 Turns)

6. Calvin Cycle: Regulation What is used to regulate the cycle? – ATP and NADPH levels What enzyme do they effect? – Rubisco Rubisco is found in all photosynthetic organisms and up to 50% of all proteins in a leaf; most abundant protein on Earth (6kg/person) What does the name RuBP carboxylase/oxygenase tell you about the enzyme? – Reacts with both CO 2 and O 2 – Competitively inhibit each other

7. The Problem with Gas Exchange How does CO 2 and O 2 get in and out of a plant? – Stromata under the leaves What else can escape through a stromata that the plant needs? –H2O–H2O What problems must plants in dry, hot climates deal with? – The plant needs to take in CO 2 and release O 2, but the open stromata will also release H 2 O H 2 O out

8. Photorespiration What happens if O 2 builds in the plant? – RuBP is inhibited RuBP, when bound to O 2, will break down into 3PGA and Phosphogylcolate (2-C) Phosphogylcolate is dephospholated and gylcolate (toxin) is broken down to CO 2 Photorespiration  plants use O 2 to make CO 2 (and H 2 O) Why is this inefficient? – Lose RuPB and only get one 3PGA

9. C 4 Pathway Plants that use 3PGA in their Calvin cycle are C 3 pathway plants C 4 pathway  CO 2 is fixed to phosphoenolpyruvate (PEP) to make oxaloacetate by PEP carboxylase Where is oxaloacetate used? – Starting point of Krebs; can make ATP from this NADPH reduces oxaloacetate to malate (Krebs) and then pyruvate and releases CO 2 Pyruvate can use ATP to recycle PEP

10. C 4 Pathway C 4 plants separate the C 4 cycle from the Calvin cycle – C 4  mesophyll cells – Calvin  bundle sheath cells Malate diffuse through the mesophyll to the bundle sheath to be broken into CO 2 and pyruvate Pyruvate diffuses back to be recycled to PEP Why is C 4 better than photorespiration? – Creates CO 2 without using RuPB and lowering Calvin cycle output – PEP carboxylase is very efficient do stromata do not have to be open long Why is it not used by all plant then? – Extra amount of ATP is needed to recycle PEP

11. CAM Pathway CAM  Crassulacean Acid Metabolism – Water storing plants in desert areas Starts the same as C 4 : – CO 2 fixed to oxaloacetate by PEP carboxylase and malate releases CO 2 for the Calvin cycle In CAM all these reactions occur in the same cells (bundle sheath cells) but at different times – Calvin cycle  Day – Carbon fixation  Night Night: – Stromata open – Malate is made and stored in the large vacuole Day: – Stromata stay closed – CO 2 is released from malate