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O 2. O 2. O 2. O 2. PHOTORESPIRATION. Definition 1:. O 2. The aberrant use of oxygen by chloroplasts. Definition 2:. O 2. An interference with carboxylation caused by the deviant interaction of RUBISCO with oxygen. A process that leads to only one 3PGA being
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O2 O2 O2 O2 PHOTORESPIRATION Definition 1: O2 The aberrant use of oxygen by chloroplasts Definition 2: O2 An interference with carboxylation caused by the deviant interaction of RUBISCO with oxygen A process that leads to only one 3PGA being produced in the dark reaction in chloroplasts
CH2OP CH2OP CH2OP C-OH HO- HO- C-O-OH C-O-OH H2O CH2OP C-OH C-OH C=O HO- C=O C-OH C-OH C-OH C-OH CH2OP CH2OP CH2OP C-OH CH2OP CH2OP COO COO C-OH PO4 CH2OP CH2OH COO O2 RuBP Phospho- glycolate 3PGA Peroxisomes Glycolate Calvin Cycle Photoefficiency is cut in half
Peroxisome between 2 chloroplasts Chloroplast
Glyoxylate CHNH3 CH2NH3 CH2NH3 CHNH3 CH2OH CHO COO COO COO COO COO COO NH3+ Glycerate CH2OH CH2OH NAD+ NH3+ CH2OH CH2OH NADH CHOH C=O COO COO Hydroxy pyruvate From Chloroplasts O2 Mitochondria Glycolate 2 Glycine CO2 ATP 3PGA Serine Peroxisomes
Why Photorespiration? 1. O2 (21% of air) is more prevalent than CO2 (0.03%) 2. RUBISCO reacts with O2 (Km =200 M) as well as CO2 (Km=20 M) RUBISCO IS BOTH AN OXYGENASE and a CARBOXYLASE 3. Phosphoglycolate can be salvaged. Glycine Serine (mitochondria)releases CO2 Glycolate Glyoxylate (peroxisomes) consumes O2 O2 in CO2 out is respiration
Strategy for Preventing Photorespiration PLAN Avoid RUBISCO Fix CO2 in an environment shielded from O2 Use an enzyme that does not react with O2
Take Home Strategy (cont.) SOLUTION CO2 fixation occurs in Mesophyll cells CO2 fixing enzyme is not RUBISCO CO2 fixing enzyme is PEP carboxylase PEP carboxylase will not react with O2 RUBISCO never changed Instead plant anatomy changed
Anatomy of C3 vs C4 Plants Calvin Cycle Cells
Mesophyll cell Bundle-sheath cell Oxaloacetate Malate Malate Calvin cycle PPi + Pi + CO2 CO2 ATP AMP CO2 Phosphoenol- pyruvate pyruvate pyruvate ATP energy is required to concentrate CO2 in bundle-sheath cells
Hatch-Slack C4 Pathway for concentrating CO2 PEP Carboxylase COO- COO- COO- ATP + Pi AMP + PPi Pi = C=O C-H HO- C~OPO3 CH2 CH2 CH2 COO- COO- NADPH NADP+ Malate C4 COO- C=O CH3 NADP+ NADPH Pyruvate Calvin Cycle 2, 3PGA Mesophyll cell 2Pi Malate dehydrogenase Pyruvate phosphate dikinase CO2 PEP OAA C4 Malic enzyme CO2 RUBISCO Bundle-sheath cell
Key Points 1. Mesophyll cells have no RUBISCO; Bundle Sheath do 2. Mesophyll cells condense CO2 with PEP-C4 cycle 3. OAA and malate are the C4 components 4. In bundle sheath cells malate is oxidatively decarboxylated to pyruvate to release the CO2 5. CO2 released enters the Calvin Cycle-C3 cycle 6. In mesophyll cells, pyruvate is converted to PEP by the enzyme pyruvate-phosphate dikinase 7. Pyruvate-phosphate dikinase uses the equivalent of 2 ATPs to synthesize PEP
CO2 directly RuBP recipient RUBISCO open O2 can interfere Photorespiration likely CO2 indirectly PEP recipient RUBISCO shielded O2 cannot interfere No photorespriation C3 vs C4 PlantsA Lesson in Photoefficiency C3 C4
soybean wheat rice sugar beet alfalfa spinach tobacco sunflower corn sorghum sugar cane millet crab grass Bermuda grass pigweed C3 and C4 Plants C3 C4
What did you Learn? • Photorespiration is to be avoided • RUBISCO is an oxidase/carboxylase • Oxygen cuts photoefficiency in half • C3 and C4 plants differ in anatomy • Mesophyll cells fix CO2 to PEP • Bundle sheath cells have Calvin cycle • C4 plants grow more ferociously
