Nature

The vector stencils library "Citric acid cycle (TCA cycle)" contains 26 symbols of metabolites for drawing metabolic pathway maps and biochemical shematic diagrams of the citric acid cycle (TCA cycle, tricarboxylic acid cycle, Krebs cycle) and diagrams of metabolism processes.
"The citric acid cycle - also known as the tricarboxylic acid cycle (TCA cycle), or the Krebs cycle, - is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetate derived from carbohydrates, fats and proteins into carbon dioxide and chemical energy in the form of adenosine triphosphate (ATP). In addition, the cycle provides precursors of certain amino acids as well as the reducing agent NADH that is used in numerous other biochemical reactions. Its central importance to many biochemical pathways suggests that it was one of the earliest established components of cellular metabolism and may have originated abiogenically.
The name of this metabolic pathway is derived from citric acid (a type of tricarboxylic acid) that is consumed and then regenerated by this sequence of reactions to complete the cycle. In addition, the cycle consumes acetate (in the form of acetyl-CoA) and water, reduces NAD+ to NADH, and produces carbon dioxide as a waste byproduct. The NADH generated by the TCA cycle is fed into the oxidative phosphorylation (electron transport) pathway. The net result of these two closely linked pathways is the oxidation of nutrients to produce usable chemical energy in the form of ATP." [Citric acid cycle. Wikipedia]
The shapes example "Design elements - TCA cycle" is included in the Biology solution from the Science and Education area of ConceptDraw Solution Park.