Photosynthesis in C4 Plants

Building Sugars• What products came from the

light-dependent reactions?– ATP and NADPH

• Light-independent reactions use ATP and NADPH to build sugars from CO2

– 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)

The Calvin Cycle: Part 1• Where do we also find G3P?

– Half-way point in glycolysis– Can start glycolysis from here

Phase 1: Carbon Fixation1) CO2 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)

CO2+ATP+NADPHADP+Pi+NADP++glyceraldehyde-3-phosphate(G3P)

The Calvin Cycle: Part 2Phase 2: Reduction3) NADPH and ATP supply the energy, electrons, and H+ needed to change 3PGA into G3P• How many G3P do we have?

– 2• How many do we need to make

glucose?– 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

The Calvin Cycle: Part 3Phase 3: Regeneration4) 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 CO2 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)

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 CO2 and O2

– Competitively inhibit each other

The Problem with Gas Exchange• How does CO2 and O2 get

in and out of a plant?– Stromata under the

leaves• What else can escape

through a stromata that the plant needs?– H2O

• What problems must plants in dry, hot climates deal with?– The plant needs to take

in CO2 and release O2, but the open stromata will also release H2O

H2O out

Photorespiration• What happens if O2 builds in

the plant?– RuBP is inhibited

• RuBP, when bound to O2, will break down into 3PGA and Phosphogylcolate (2-C)

• Phosphogylcolate is dephospholated and gylcolate (toxin) is broken down to CO2

• Photorespiration plants use O2 to make CO2 (and H2O)

• Why is this inefficient?– Lose RuPB and only get one

3PGA

C4 Pathway• Plants that use 3PGA in their

Calvin cycle are C3 pathway plants

• C4 pathway CO2 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 CO2

• Pyruvate can use ATP to recycle PEP

C4 Pathway• C4 plants separate the C4 cycle from the

Calvin cycle– C4 mesophyll cells– Calvin bundle sheath cells

• Malate diffuse through the mesophyll to the bundle sheath to be broken into CO2 and pyruvate

• Pyruvate diffuses back to be recycled to PEP

• Why is C4 better than photorespiration?– Creates CO2 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

CAM Pathway• CAM Crassulacean Acid

Metabolism– Water storing plants in desert areas

• Starts the same as C4:– CO2 fixed to oxaloacetate by PEP

carboxylase and malate releases CO2 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– CO2 is released from malate