Fig. 13.5 General system block diagram.

of feedforward control in an engineering system is to regulate the temperature inside a room by installing a temperature sensor outside the room - if the outside temperature decreases, then a thermostat would begin heating the room before the inside temperature is affected, independent of what is taking place inside the room (i.e., somebody may have turned on the oven). Examples of biological feedforward control systems abound. Much eukaryotic gene expression is regulated in this way, and consequently, the adverse effects of defective genes cannot be ameliorated much by the cell. For instance, the concentration of chloride inside epithelial cells is controlled by production of the protein responsible for its transport to the extracellular matrix, the CFTR (cystic fibrosis transmembrane conductance regulator). The CFTR gene encodes a membrane protein that works as a cyclic AMP-regulated chloride channel in epithelial cells. It also regulates epithelial sodium channels and controls the regulation of other transport pathways. These channels transport chloride ions out of the cell, thus making its surroundings saltier, which in turn draws water out of the cell by osmosis. In the lungs, for example, this fluid acts as disinfectant, inhibiting the growth of microorganisms. Under normal circumstances, cellular control mechanisms lead to the transcription and translation of the gene for this protein in pulmonary epithelial cells. However, as there is no mechanism to check whether this has actually been achieved, in people with the most common mutation causing cystic fibrosis, a defective protein is assembled and directed to the endoplasmic reticulum for degradation instead of to the cell membrane. In consequence, lung cells are unable to secrete water, and clots of mucus obstruct breathing, favouring chronic bacterial and viral infections.

Although, in principle, feedforward control can be made error-free, the fact that it does not verify that the outcome corresponds to the expected effect makes it prone to mistakes. With a good control system, such errors may be few, but they accumulate in time until they eventually destroy the system. The only way to avoid this accumulation is to use feedback control [28].

Feedback control compensates for errors or deviations from the reference value after they have happened. Control actions are determined by the state of the system. In the example of the thermoregulated room, the heating would be switched on after a drop in the temperature inside the room. Basically, the system detects the difference between the desired output and the actual output and corrects this difference. The disadvantage of feedback control is that the actions are taken after the error or deviation has occurred, and therefore this control scheme is by definition imperfect. Nevertheless, if the perturbations are continuous and develop gradually, the controller can intervene at an early stage when the deviation is still small, which makes feedback control effective. This explains why feedback control is one of the most widely used control methods in engineering design and why it is ubiquitously present in biological systems.

Feedback control systems require a reference value - the desired output - which provides a target to aim for. In biological control systems these targets are sometimes obscure. Biological reference values may be genetically determined, for example, via the amino acid sequences of regulatory proteins, which ultimately define their binding constants for allosteric effectors.

Fig. 13.6 System block diagram with feedback.

Fig. 13.6 System block diagram with feedback.

A system is subject to positive feedback if the output that is sent back to the input facilitates and accelerates the transformation in the same direction as the preceding results. If the output instead produces a result in the opposite direction to previous results, the system is said to be subjected to negative feedback. With positive feedback there is exponential growth or decline; with negative feedback there is maintenance of equilibrium (Figure 13.6). In this figure there is a general feedback loop, not specifically labelled as positive or negative. This description is only in the text!

Regulation of the level of glucose in the bloodstream is an example of negative feedback control. When the receptors in the pancreas detect a drop in the level of glucose in the blood, the alpha cells of the islets of Langerhans release the hormone glucagon to initiate a corrective response. Glucagon targets the liver, where it promotes the breakdown of glycogen to glucose. Thus, the lack of glucose in the bloodstream can be corrected by the secretion of glucose produced from glycogen. Conversely, when an increase in the level of glucose in the blood is detected, the beta cells of the islets of Langerhans release the hormone insulin to favour the conversion of glucose to glycogen.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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