Positive-feedback cycle

As shown in Fig. 24, the mechanism of the instability is elucidated as follows At the portion where dissolution is accidentally accelerated and is accompanied by an increase in the concentration of dissolved metal ions, pit formation proceeds. If the specific adsorption is strong, the electric potential at the OHP of the recessed part decreases. Because of the local equilibrium of reaction, the fluctuation of the electrochemical potential must be kept at zero. As a result, the concentration component of the fluctuation must increase to compensate for the decrease in the potential component. This means that local dissolution is promoted more at the recessed portion. Thus these processes form a kind of positive feedback cycle. After several cycles, pits develop on the surface macroscopically through initial fluctuations. 

Hypercarbia depresses the excitability of the cerebral cortex and increases the cutaneous pain threshold through a central action. In patients who are hypoventilating from narcotics or anesthetics, increasing Pco may result in further CNS depression, which in turn may worsen the respiratory depression. This positive-feedback cycle can be deadly. 

Commonly in chemistry, we do not teach about feedback cycles in this way, but they can be a helpful way to think about an abstract explanation with older students. The feedback cycle offers a useful tool for thinking about systems that link across the sciences. Homeostasis in the body depends upon such cycles. In physics there are a number of phenomena that can be described with the same very simple feedback structure as in Figure 3.1 (cooling of a hot object, radioactive decay, capacitor discharge, for example). Feedback cycles are also very useful in thinking about aspects of Earth and environmental sciences (for example the possibility of a positive feedback cycle when atmospheric warming leads to warmer seas in which the greenhouse gas carbon dioxide is less soluble). 

Operation of such an interactive, positive feedback cycle for environmental lead, while helpful for effective societal responses to lead s hazards, is nevertheless challenged by some of the environmental and toxicological characteristics of human lead exposures. 

This adds up to a positive feedback cycle oxygen feeds life, life breaks down rocks, life fills the cracks and makes more oxygen. Fitting with this story, aluminum-silicon clays like smectite and kaolinite are found increasingly before the Cambrian explosion. As these cracks formed, minerals dissolved in the ocean, especially phosphate, sulfate, and calcium. The first two are food for life and further expanded the algal mats, assisted at each step by oxygen. The last one, calcium, was the opposite of food and was kept on life s outside and put to use there. 

Our result is reasonable, but low. Mechanistic insight into the reaction may explain some of the losses in quantification since it is possible to form species that do not decompose immediately to yield the necessary Br" to establish the complete positive feedback cycle. A possible route for the oxidation of 00(4 ) by Br2 is shown in Equations 16.23 through 16.25 ... 

J. R. Pomerening, S. Y. Kim, and J. E. Eerrell, Jr. Systems-level dissection of the cell-cycle oscillator Bypassing positive feedback produces damped oscillations. Cell 122, 565-578 (2005). 

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