3.4: 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 (Figure \(\PageIndex{1}\)). 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 C\(_5\) (RuBP) which turns into PGA and PGAL, becoming glycolate. This glycolate is returned to the Calvin cycle when the cell uses peroxisomes and mitochondria, and spends ATP. The process of photorespiration wastes C\(_5\) and ATP which could be more useful to the plant in other ways.
If concentration of CO\(_2\) is high enough, assimilation will overcome photorespiration. Consequently, to minimize the amount of photorespiration and save their C\(_5\) and ATP, plants employ Le Chatelier’s principle (“ Equilibrium Law ”) and increase concentration of carbon dioxide. They do this by temporarily bonding carbon dioxide with PEP (C\(_3\)) using carboxylase enzyme; this results in C\(_4\) molecules, different organic acids (like malate, malic acid) with four carbons in the skeleton. When plant needs it, that C\(_4\) splits into pyruvate (C\(_3\)) 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 “ C\(_\mathbf{4}\) pathway ” (Figure \(\PageIndex{2}\)).
Plants that use the C\(_4\) pathway waste ATP in their effort to recover PEP, but they still outperform photorespiring C\(_3\)-plants when there is an intensive light and/or high temperature and consequently, high concentration of oxygen . This is why in the tropical climate, C\(_4\)-crops are preferable.
Two groups of plants use the C\(_4\) pathway. Many desert or dryland plants are CAM-plants which drive the C\(_4\) pathway at night. They make a temporal separation 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” C\(_4\) plants drive C\(_4\) pathway in leaf mesophyll cells whereas their C\(_3\) is located in so-called bundle sheath cells . This is a spatial , rather than temporal separation. These C\(_4\)-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 C\(_4\) pathway relate with concentration of carbon dioxide, spatial or temporal (Figure \(\PageIndex{3}\)). Both are called “carbon-concentrated mechanisms”, or CCM.
There are plants which able to drive both C\(_3\) and C\(_4\) pathways (like authograph tree, Clusia ), and plants having both “classic” C\(_4\) and CAM variants (like Portulacaria ).