Figure 1. Alternative models for the mechanism of external calcifying band production in the giant-celled alga Chara. In (a), calcification is driven by the diffusion of  H⁺ into the cell in localized regions, producing localized alkalinization of the cell surface and precipitation of  CaCO₃. In an adjacent region,  H⁺ is actively pumped out of the cell producing localized acidification of the cell surface. In (b), calcification is driven by the extrusion of  Ca²⁺ in exchange for  H⁺ in the alkaline zone.  CO₂ diffusion from the cell's interior provides the carbon source for formation and  CaCO₃ precipitation. In both models,  H⁺ extrusion in the acidic zone facilitates the production of  CO₂ from which can be used by photosynthesis in the chloroplasts (green).

Figure 2. Model for fluxes of  Ca^{2 +}, and  H⁺ during calcification in coccolithophores. (a) Scanning electron micrograph of heterococcoliths on the surface of Coccolithus pelagicus cells (cell diameter = 20 m). (b)  Ca^{2 +} and uptake into a Golgi-derived compartment leads to the production of  CaCO₃ and  H⁺ during calcite precipitation.  H⁺ production can be used to counter the alkalinizing effect of  CO₂ production from in the chloroplast (green) and removal of  CO₂ by photosynthesis.

Figure 3. (a) Scanning electron micrograph showing complete silica frustules of the diatom Thallassiosira eccentrica (cell diameter = 50 m). (b) Working model of silica biogenesis in a diatom.  Na⁺-dependent silicate uptake is mediated by specialized silica transporters (SIT) at the plasma membrane. Once inside the cell, the soluble silica is delivered to the silica deposition vesicle (SDV), possibly involving isoforms of the SITs. Silica polycondensation occurs under acidic conditions within the SDV to form insoluble amorphous silica. Soluble silica may be stored in intracellular pools, the size of which is sensitive to silica demand and metabolism. Silifin proteins are thought to play a key role in regulating the formation of insoluble silica within the SDV. Frustulins are a separate group of proteins that together with polysaccharides and lipids are likely to play a role in stabilization of the mature silica wall structure. On maturity, the complete valve is released onto the cell surface by exocytosis.