Instruction is the process during which input signals exert an influence on one or several output signals according to the rules of the system. If a system captures a quantity continuously and aligns it to a set point by, then it is spoken of regulation
The definition given by the Deutsche Industrienorm (DIN 19226)
(German Industrial Standard) says:
Instruction is the process occurring in a system where one or several input quantities exert an influence on other output quantities according to the specific rules of the system.'
Instruction does thus describe a directed triggering of a process and corresponds exactly to what has been said in the section above about the properties of a transitional system. Here, too, numerous examples can be given:
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The one gene - one enzyme hypothesis is a directed process (an algorithm). A gene instructs the generation of an enzyme. The enzyme again catalyzes a specific reaction where a substance B is produced from a substance A. By selection of suitable varieties, the yield of cultivated plants can be enhanced. |
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The choice of suitable extern conditions may increase the production of plants. |
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Light controls the development of plants. |
An instruction has always a certain intention. It is assumed that the system functions in a specific way. It is completely neglected which consequences this may have. Instruction describes therefore only parts of living systems and is never able to cover living systems completely because they are thought to be controlled, i.e. consequences arising from reactions are recognized, settled and used as a basis for future decisions.
DIN 19226 says that
"Regulation is the process during which a quantity, the quantity that is to be controlled (regulated) is continuously captured, compared to another quantity, the commanding element, and is, depending on the result of this comparison, influenced in order to be aligned to the commanding element. The resulting course of effects occurs in a closed circuit, the feedback loop."
A simple, linearly closed circuit consists of a number of elements that are assembled in an appropriate way.
Linear transitional systems carry through mathematical operations. They form a net of mutual dependencies. A control process serves usually to stabilize the state of a system against the influence of unforeseen disruptions. To perform this task , the system does need information about its actual state and information about possible counter-reactions. The most important elements of a feedback loop are:
The sensor: The sensor, in biological systems also often called receptor, is a gadget for measuring or registering that measures the real or actual value of the quantity that is to be controlled.
The controlled variable The state or process that is to be kept constant is called controlled variable or set point. The real value tallies only rarely with the set point, it usually oscillates periodically around the set point instead. The frequency of the oscillation is dependent on the system's speed of reaction, the amplitude on the power capacity.
The disruptive element: All systems are exposed to disruptive factors that have to be processed. The disruptions have to be able to be corrected (changes of the commanding element). If they exceed the system's control capacity, a control catastrophe will happen and the system will break down.
The regulator or controller: The controller is the place where the actual or real value (Istwert) and the set point (Sollwert) are compared and tuned. The actual value enters into the tuning with a negative sign, it is hence also spoken of a negative feedback. A positive feedback, as it occurs for example with growth functions does either lead to effects of self-amplification or to a breakdown of the system.
There exist numerous examples in biology that show how a feedback loop works and how disruptive elements can be processed. A well-known example is the so-called inhibition of the final product (end-product inhibition) that is used to regulate metabolic pathways. In this process, the final product of a biosynthesis pathway inhibits its own synthesis. If enough of the product has been generated, the synthesis is stopped for a while. Another example is the repression of genes. Most of the time most genes of a cell are in an inactive state. They are activated exclusively during certain phases of the cell cycle or the organism's development. Both extern factors and gene products of other genes may be involved.
A stable system can only exist, if it receives enough
energy from its surrounding. The
fact alone that energy is always a limiting factor and that
uninhibited growth is described by an exponential equation indicates
that a breakdown of the system will inevitably occur, if no steps are
taken to transform uninhibited into controlled growth and thus bring
it into line with the supply of energy. In reality, hence only a
controlled system has a chance of survival.
Disruptions of natural systems have repeatedly led to explosive political discussions during the last years. Remember alone the topics polluted rivers, acid rain and the dumping of dilute acid in the North Sea. Inshore waters becoming polluted mean that the uptake capacity of the system has been exceeded so that it breaks down.
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