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

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.

Calvin’s apparatusPhoto by Dr. James A Basshamhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CalvinCycle.html

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

3PG is labeled with 14CO2 in both light and dark, but G3P is labeled only in light (light reactions produce ATP and NADPH

Purves et al. Life 6th ed.

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

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

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 plants concentrate CO2 for Rubisco

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