Photosynthesis: C4 Pathway

C₄ Pathway

Rubisco is the enzyme of extreme importance since it starts the assimilation of carbon dioxide. Unfortunately, Rubisco is “two-faced” since it also catalyzes photorespiration (Fig. 2.9). Photorespiration means that plants take oxygen instead of carbon dioxide. Rubisco catalyzes photorespiration if there is a high concentration of oxygen (which usually is a result of intense light stage). Rubisco oxygenates C₅ (RuBP) which turns into PGA and PGAL, becoming glycolate. This glycolate is returned to the Calvin cycle when the cell uses peroxisomes and mi-tochondria, and spends ATP. The process of photorespiration wastes C₅ and ATP which could be more useful to the plant in other ways.

If concentration of CO₂ is high enough, assimilation will overcome photores-piration. Consequently, to minimize the amount of photorespiration and save their C₅ and ATP, plants employ Le Chatelier’s principle (“Equilibrium Law”) and increase concentration of carbon dioxide. They do this by temporarily bond-ing carbon dioxide with PEP (C₃) using carboxylase enzyme; this results in C₄ molecules, different organic acids (like malate, malic acid) with four carbons in the skeleton. When plant needs it, that C₄ splits into pyruvate (C₃) plus carbon dioxide, and the release of that carbon dioxide will increase its concentration. On the final step, pyruvate plus ATP react to restore PEP; recovery of PEP does cost ATP. This entire process is called the “C₄pathway” (Fig. 2.10).

Plants that use the C₄ pathway waste ATP in their effort to recover PEP, but they still outperform photorespiring C₃-plants when there is an intensive light and/orhigh temperature and consequently, high concentration of oxygen. This is why inthe tropical climate, C₄-crops are preferable.

Two groups of plants use the C₄ pathway. Many desert or dryland plants are CAM-plants which drive the C₄ pathway at night. They make a temporal sepa-ration between the accumulation of carbon dioxide and photosynthesis. CAM-plants make up seven percent of plant diversity, and have 17,000 different species (for example, pineapple (Ananas), cacti, Cactaceae; jade plant, Crassula and their relatives).

“Classic” C₄ plants drive C₄ pathway in leaf mesophyll cells whereas their C₃ is located in so-called bundle sheath cells. This is a spatial, rather than tem-poral separation. These C₄-plants make up three percent of plant biodiversity and have more than 7,000 different species (for example, corn, Zea; sorghum, Sorghum and their relatives).

In all, both variants of C₄ pathway relate with concentration of carbon dioxide, spatial or temporal (Fig. 2.11). Both are called “carbon-concentrated mecha-nisms”, or CCM.

There are plants which able to drive both C₃ and C₄ pathways (like authograph tree, Clusia), and plants having both “classic” C₄ and CAM variants (like Portu-lacaria).