Photosynthesis part II – carbon fixation C3 photosynthesis: Calvin cycle with Rubisco C4 photosynthesis: a mechanism to concentrate CO2 for the Calvin cycle

Products of light reactions are used for carbon fixation Thylakoid membranes Chloroplast stroma

Calvin and Benson used radioactive 14CO2 to investigate carbon fixation Calvin’s apparatus Photo by Dr. James A Bassham http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CalvinCycle.html 14CO2 was injected into culture of Chlorella (green alga). After 30 s, algal cells were boiled in ethanol and extract analyzed by chromatography and autoradiography.

The earliest compound to be labeled with 14CO2 is 3-phosphoglycerate The dark spots show the radioactive compounds produced after 10 secs (left) and 2 minutes (right) of photosynthesis by the green alga Scenedesmus. Photo by Dr. James A. Bassham http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CalvinCycle.html

Calvin, Benson and Bassham's conclusions and hypotheses 3PG (3-phosphoglycerate: PGA) is first stable molecule to be labeled, at carboxyl group A cyclic process, because other carbon atoms of 3PG are also labeled at longer times But no 2-carbon compound could be found as acceptor for CO2 Hypothesize that 3PG reduced to G3P (glyceraldehyde-3-phosphate) using ATP & NADPH

Reduction of 3PG (acid) to G3P (aldehyde/sugar) requires ATP and NADPH Purves et al. Life 6th ed. 3PG is labeled with 14CO2 in both light and dark, but G3P is labeled only in light (light reactions produce ATP and NADPH

Ribulose bisphosphate (RuBP) is the CO2 acceptor molecule CO2 + RuBP  [6-carbon intermediate]  2 x 3PG The enzyme rubisco (RuBP carboxylase/oxygenase) may be the most abundant enzyme on Earth!

Calvin-Benson-Bassham cycle ATP is required also for regeneration of RuBP.

Metabolic interactions in plants

Rubisco, key enzyme in carbon fixation Source of most biological carbon on Earth Problem: can oxygenate RuBP (photorespiration) O2 competes with CO2, especially at higher temperature (>28 deg. C), low CO2, high O2 produces glycolate (2-carbon compound) glycolate exits chloroplast, oxidized in peroxisomes, metabolized to CO2 in mitochondria undoes carbon fixation, with no ATP synthesis

In C4 plants, 14CO2 labels a 4-carbon product, instead of 3PG C4 photosynthesis is a CO2 concentration mechanism that reduces photorespiration In C4 plants, 14CO2 labels a 4-carbon product, instead of 3PG C4 plants (corn, sugar cane, crabgrass, etc.) thrive in dry, hot environments plants close stomata to limit water vapor loss, thus limit CO2 availability C4 plants take up CO2 via PEP carboxylase, which has higher affinity for CO2 and no oxygenase reaction

C4 plants concentrate CO2 for Rubisco Mesophyll cells fix CO2 efficiently to 4-carbon malate. Malate shipped to bundle sheath cells decarboxylates to deliver CO2 to Rubisco. Energy cost = 1 ATP per CO2

C4 vs C3 C4 minimizes loss of organic carbon from oxygenase activity of Rubisco C4 plants grow better in hot, dry climates C3 plants grow better in cool, moist climates C3 plants more sensitive to atmospheric CO2 concentrations

Photosynthesis and respiration 6 CO2 + 12 H2O  C6H12O6 + 6 H2O + 6 O2 Carbon and oxygen cycles Chemiosmotic ATP synthesis Organelles with endosymbiotic origins