Figure 2.

Basic Control Design of a System Manipulating Many Aspects of Plant Behavior and Incorporating Negative Feedback.

The primary element involves an assessment of output by a feedback loop using a comparator that assesses current input against a predetermined reference signal. In a simple metabolic system or pathway, the end product uses the first enzyme in the sequence as the reference signal and controls metabolic flux through inhibition. In more complex cellular and tissue systems, the reference signal is currently unknown. Note that in growth and tissue development there are feedback loops operating continuously because growth in turn continually alters the external environment that feeds back into the system.