Carbon Reactions: CO 2 is fixed by Rubisco located in the stroma. The molecule that is carboxylated is RuBP. RuBP has 5 carbons and is regenerated in the Calvin cycle. In the Calvin cycle, carbon is conserved, ATP is used and NADPH is used. The result is that some extra triose phosphate is generated that can be exported to the cytosol or used in the chloroplast stroma to make starch. cytosol stroma Photorespiration: This occurs when Rubisco acts as an oxygenase rather than a carboxylase How does Rubisco normally function in C3 photosynthesis? Ribulose bisphosphate carboxylase/oxygenase (Rubisco) Ribulose bisphosphate carboxylase/oxygenase (Rubisco) When rubisco acts as an oxygenase the products are one 3-phosphoglycerate (C3) and one phosphoglycolate (C2). The net cost of oxidation of RuBP is 1 ATP (however 1 C is also lost and the cost of fixing that was at least 1.5 ATP and 1 NADPH) Photorespiration = the recovery of carbon due to oxygenase activity of Rubisco. 1
Photorespiratory cycle: For this class let s just say it is a way for plants to recover carbon after rubisco acts as an oxygenase and that it involves multiple compartments. When is photorespiration a problem? Photorespiration is more predominant at high temperature because the ratio of CO 2 to O 2 in solution decreases. So far we have discussed C 3 photosynthesis, so named because the first stable product in C 3 photosynthesis is 3-phosphoglycerate (3 carbons). Some plants have evolved strategies to avoid photorespiration. These plants are specialized for growth at high temperatures and under dry conditions. These are C 4 and CAM plants. C4 and CAM are add-ons to C3 photosynthesis. C 3 Plant (Poa) C 4 Plant (Saccharum - sugarcane) Kranz anatomy in maize C4 photosynthesis is an add-on to C3 photosynthesis. The purpose is to increase the CO 2 concentration in chloroplasts where the Calvin cycle takes place. PEP carboxylase 2
Differences between C 3 and C 4 photosynthesis C 4 Pathway In C 4, the substrate for primary carboxylation is phosphoenolpyruvate (PEP) (C 3 ). What is the substrate for carboxylation for C 3 photosynthesis? In C 4, the primary carboxylation is done by PEP carboxylase, the first product is oxaloacetate (C 4 ). The first (stable) product of C 3 photosynthesis is 3-phosphoglycerate (C3). The cost of accumulating fixed carbon in bundle sheath cells is 2 ATP per CO 2. C 4 is more efficient than C 3 under conditions where photorespiration would predominate. Figure 10.27b: Diagram of bundle sheath cells. The C 4 pathway imports CO 2 and NADPH (indirectly) into bundle sheath cells where the Calvin cycle takes place. Chloroplasts in bundle sheath mainly use cyclic electron transport (PSI) to make ATP so there is little O 2 evolution. CAM (crassulacean acid metabolism) plants CAM is a form of C 4 photosynthsis. In CAM plants CO 2 is taken up during the night and the first step of C 4 (carbon fixation by PEP carboxylase) occurs. Malate is stored in the vacuole. Quantum yield for C 3 plants depends on temperature. If CO 2 concentration increases how does it affect the efficiency of C 3 vs. C 4 plants. During the day, the stomata are closed. Malate is exported from the vacuole and decarboxylated. CO 2 enter the Calvin cycle. The benefit of CAM photosynthesis is that stomata are open at night when water loss through evaporation is less. However, it is expensive to store carbon as malate during the night, at least 2 ATP per CO 2 fixed. 3
CAM plants Agave deserti Ananas comosus Next topic: Nutrient transport, moving liquids Pathway for water from the soil through the plant and out to the atmosphere. Aquaporins Water potential How does transpiration drive transport in the xylem? What are the main mechanisms of nutrient uptake? How does phloem work? Symplastic pathway vs. apoplastic pathway. (symplasm is within cells, apoplasm is outside cells.) root hair plasmodesma xylem symplastic pathway apoplastic pathway cell wall cytoplasm symplast of endodermis Casparian strip of endodermis epidermis cortex stele water-filled leaf cells substomatal cavity (intercellular space) water-filled xylem in vein cell wall permeated with H 2 O air not saturated cuticle relatively impermeable to H 2 O 4
Negative pressure potential is generated in leaves in response to water evaporation from the cell wall surrounding mesophyll cells. Water remaining in the cell wall is restricted to smaller pores, and due to adhesion, cohesion and surface tension the curvature of the water surface increases. Ψ p = -2T/r T is the surface tension of water, 7.28 x 10-8 MPa m, and r is the radius of curvature in meters. The direction of water transport is determined by water potential Ψ w. Water moves from higher to lower water potential. Three components of water potential Ψ w = Ψ s + Ψ p + Ψ g 1. Ψ s (or Ψ π ) Solute potential depends on solutes in solution (the higher the solute concentration the more negative the solute potential). 2. Ψ p Pressure potential is the hydrostatic pressure and can be negative (in xylem) or positive (turgor pressure). Atmospheric pressure is defined as Ψ p = 0. 3. Ψ g Gravity potential depends on the vertical height, the density of water and gravity. Solute potential in the cell is negative and the pressure potential is positive. The water potential is equal to the surrounding solution. Osmosis: water movement across a semi-permeable membrane. The direction of water movement across the membrane depends on the water potential Ψ w. But the rate across biological membranes depends on aquaporins. The water potential inside the cell and outside the cell always equilibrates because biological membranes are permeable to water. 5
Aquaporins are membrane proteins that transport water. From: Fu and Lu (2007) Molecular Membrane Biology 24: 366-374 Peter Agre discovered aquaporins in 1991, he received the Nobel prize in chemistry for this work in 2003. Peter Agre was born in Northfield MN, and he went to Theodore Roosevelt High School in Minneapolis. He graduated from Augsburg College in 1970 with a BS in chemistry. He is a professor at Johns Hopkins University. 6