Hatch & Slack Pathway or C4 Cycle or Dicarboxylic Acid Pathway

Until 1965, the Calvin (C3) cycle was believed to be the only pathway of CO2 fixation in plants. That year, Kortschak, Hart and Burr, studying sugarcane, observed that the earliest-labelled products of CO2 fixation in that species were not the 3-carbon PGA of the C3 cycle but the 4-carbon acids malate and aspartate instead - an anomaly confirmed and worked out in full mechanistic detail by Hatch and Slack in 1967, whose names the pathway now carries as the Hatch and Slack pathway. This alternative route to CO2 fixation has since been documented in more than 1000 plant species - roughly 300 of them dicots, the rest monocots - concentrated among tropical and sub-tropical grasses and some dicots adapted to hot, dry environments; despite this, C4 plants make up only about 5% of Earth's total plant biomass and just 1% of known plant species, yet, because of their efficiency, they are responsible for roughly 30% of all terrestrial carbon fixation. Because the pathway's first stable product, oxaloacetic acid (OAA), is a 4-carbon compound, the pathway is called the C4 cycle, and because OAA is specifically a dicarboxylic acid, the pathway is also called the dicarboxylic acid pathway. Its defining structural feature is that CO2 is fixed twice, in two physically separate cell types within the leaf - first in mesophyll cells (stage I, section 13.14.1), and then again, via the Calvin cycle, in bundle sheath cells (stage II, section 13.14.2) - an arrangement, described further in section 13.14.3, that concentrates CO2 around RuBisCO and all but eliminates photorespiratory loss. This physical separation into two cell types, ringed one around the other, produces a distinctive leaf anatomy called Kranz anatomy (from the German for "wreath" or "halo"): a central vascular bundle is surrounded by a ring of bundle sheath cells, which is in turn surrounded by an outer ring of mesophyll cells. C4 plants also show dimorphic chloroplasts - the bundle sheath chloroplasts are comparatively large, rich in starch, and have thylakoids that are not organised into grana stacks, while the mesophyll chloroplasts are smaller, contain less starch, and do have their thylakoids organised into normal grana - a difference easily seen by comparing the leaf …