A mode of action is important in classifying chemicals as it represents an intermediate level of complexity in between molecular mechanisms and physiological outcomes, especially when the exact molecular target has not yet been elucidated or is subject to debate. A mechanism of action of a chemical could be "binding to DNA" while its broader mode of action would be "transcriptional regulation".  However, there is no clear consensus and the term mode of action is also often used, especially in the study of pesticides, to describe molecular mechanisms such as action on specific nuclear receptors or enzymes. 
Prior to attempting to develop a new laxative, pharmacodynamic studies were conducted on the individual active constituents of senna. Sennoside A, sennoside B and rhein were injected intravenously and were introduced into the isolated stomach, small bowel and large bowel of the anaesthetised pig. Blood, urine and bile, collected over 6 hours, were assayed by a novel fluorometric technique. Recoveries and measured oil water partition coefficients indicate that the sennosides are virtually non-absorbed in the stomach and small bowel. Following administrations outside the large bowel, anthracene derivative concentrations in the lumen and wall of the colon were below the limits of detection. Thus, a transport theory involving absorption and resecretion of sennosides into the large bowel is discounted. The results support a mode of action involving direct transport through the alimentary canal to the large bowel where micro-organism break the sugar-anthracene bonds. Derivatives of the liberated lipophilic aglycones are absorbed into the wall of the colon and stimulate the nerve plexuses thereby leading to defaecation. The "colon-specific" mode of action of the natural anthrone glycosides appears to be an ideal method of stimulating bowel action.