C4 and CAM Plants

– O₂ competes with CO₂ for rubisco’s active site
– rubisco catalyzes two reactions:

1) addition of carbon to RuBP; forms 2 PGA molecules (photosynthesis)
2) addition of oxygen to RuBP; forms 1 PGA and one glycolate (photorespiration)

– photorespiration decreases the production of carbohydrates by removing PGA from the Calvin cycle
– glycolate is partially converted to CO₂ -> thus carbon fixation is reduced
– C₄ adaptations work to reduce photorespiration

C₄ Plants:

-several thousand species of plants undergo C₄ photosynthesis
-enzyme PEP carboxylase catalyzes the addition of a CO₂ molecule to a three-carbon molecule, forming a four-carbon molecule; hence being called C₄

Structure:

-the leaves of C₄ plants contain two types of photosynthetic cells: bundle-sheath cells surrounding a vein and mesophyll cells

How it works:

-in mesophyl cells, PEP carboxylase fixes CO₂; PEP + CO₂ -> OAA -> malate
-malate diffuses into bundle-sheath cells and CO₂ portion is removed
-CO₂ from malate enters the C₃ Calvin cycle (the 2^nd fixation reaction)

Purpose:

-continually pumps CO₂ molecules into bundle-sheath cells, where rubisco brings them into the  Calvin cycle
-keeps the CO₂ concentratoin in the bundle sheath cells high so that CO₂ outcompetes O₂ for rubisco’s active site

Cost:

-costs the plant 2 ATP per molecule of CO₂ transported; 12 extra ATP per molecule of glucose

CAM Plants

-occurs in water storing plants, such as cacti and pineapples

Structure:

-open their stomata at night and close them during the day; reverse of other plants
-closing stomata during the day helps conserve water but prevents CO₂ from entering the leaves

How it works:

-in the dark, plants take in CO₂ and make C₄ organic acids
-organic acids are stored in vacuoles until morning
-organic acids release CO₂ molecules that then enter the C₃ Calvin cycle