Alternative mechanisms of carbon fixation in plants

C4 and CAM pathway

Evolutionary purpose

• Allows for more diversification of species, higher survival rate because can produce their own food in different climates than other plants

• Less dependence on H20• Based off C3

C3 pathway

• Metabolic pathway converting CO2 and RuBP into 3 phosphoglycerate

• Occurs in all plants as first step of calvin cycle

• Plants that solely survive on this pathway tend to live where sunlight and temperatures are moderate, and H2O is abundant

C4 plants

• those that have to survive in high day temperatures, intense sunlight, drought like conditions

• Allow for stomata to remain closed more to retain H2O

• Draws CO2 from malate, an organic compound• -C4 evolved from C3 plants in order to effectively

fix CO2 at low concentrations• C4 plants have a unique leaf anatomy

cactus

Kranz Anatomy of C4 plants

• two different types of photosynthetic cells:• -Bundle-sheath: arranged into tightly packed

sheaths around the veins of the leaves• -Mesophyll cells: between the bundle sheath and

the leaf surface– pump CO2 into the bundle sheath, keeping the

concentration high enough for RUBISCO to bind to CO2 rather than O2 , tendency to waste energy is thus minimized

– The enzyme responsible for this is PEP carboxylase.

Process within C4 plants

• 1. CO2 is fixed effeciently to phosphoenolpyruvate to produce oxaloacetate by PEP carboxylase, The oxaloacetate is then converted to another four-carbon compound called malate in a step requiring the reducing power of NADPH.

• 2. The malate then exits the mesophyll cells and enters the chloroplasts of specialized cells called bundle sheath cells. Here the four-carbon malate is decarboxylated to produce CO2, a three-carbon compound called pyruvate, and NADPH. The CO2 combines with ribulose bisphosphate and goes through the Calvin cycle.

• 3. The pyruvate re-enters the mesophyll cells, reacts with ATP, and is converted back to phosphoenolpyruvate, the starting compound of the C4 cycle.

CAM pathway

• Crassulacean acid metabolism• Named after plants family Crassuleae, where it was

first discovered• discovered in the 1940s by Ranson and Thomas• Second photosynthetic adaptation to arid conditions• Evolved in succulent (water-storing) plants

– Jade, many cacti, pineapples, other families

pinapple

Carbon fixation within CAM plants

• Plants open their stomata during night and close them during the day– Opposite of what other plants do• Closing stomata during day conserves H20

and prevents CO2 from entering

Process of CAM at night

• Plants open stomata during night

• Take in CO2 and incorporates it into a variety of organic acids

• Mesophyll cells store organic acids in vacuoles until morning

Process of CAM during the day

• Plants close stomata at morning

• Light reaction supply ATP and NADPH for calvin cycle

• CO2 is released from organic acids to become incorporated into sugars in chloroplasts

Comparisons to C3 and C4

• Similar to C4 because CO2 first incorporated into organic intermediates before entering calvin cycle

• Different from C4 because the 2 initial steps of carbon fixation occur at separate times within the same cell, but in C4, the 2 steps are separated structurally

• All 3 pathways eventually use calvin cycle to make sugar from CO2

• “I lost an electron!”

• “Are you sure?”

• “I’m positive!”