Water-oxidizing clock

Eukaryotic photosynthesizing cells possess two photosystems, PSI and PSII, which are connected in series in a mechanism referred to as the Z scheme. The water-oxidizing clock component of PSII generates 02. The protons are used in the synthesis of ATP in a chemiosmotic mechanism. PSI is responsible for the synthesis of NADPH. 

The health rating is provided on the left at the 9 o clock position and is colored blue. The flammability rating is provided at the top or 12 o clock position and is colored red. The reactivity hazard is provided on the right at the 3 o clock position and is colored yellow. The relative rankings for each hazard are indicated in each quadrant. Special hazard identifiers are provided in the bottom quadrant at the 6 o clock position, which is usually white. Special hazard qualifiers generally include radioactivity, explosives, corrosive, water reactive, oxidizer, etc. The NFPA fire hazard identification scheme is somewhat limited as it only identifies relative potential hazards with the individual material. It does not identify the material itself or all of its potential reactions with other materials. See Figure C.3 for a depiction of this placard. See also Chemical Hazard Label Hazardous Materials Identification System (HMIS ) NFPA 704, Standard System for the Identification of the Hazards of Materials for Emergency Response. 

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