Question: How does a biological system normally respond a perturbation?
About the article "Biology", section "Homeostasis".

Candidate passages:
Homeostasis is the ability of an open system to regulate its internal environment to maintain stable conditions by means of multiple dynamic equilibrium adjustments controlled by interrelated regulation mechanisms.
All living organisms, whether unicellular or multicellular, exhibit homeostasis.
To maintain dynamic equilibrium and effectively carry out certain functions, a system must detect and respond to perturbations.
After the detection of a perturbation, a biological system normally responds through negative feedback.
This means stabilizing conditions by either reducing or increasing the activity of an organ or system.
One example is the release of glucagon when sugar levels are too low.
Perturbation or perturb may refer to:  Perturbation theory, mathematical methods that give approximate solutions to problems that cannot be solved exactly  Perturbation (geology), changes in the nature of alluvial deposits over time  Perturbation (astronomy), alterations to an object's orbit—e.g., caused by gravitational interactions with other bodies  Perturbation theory (quantum mechanics), a set of approximation schemes directly related to mathematical perturbation for describing a complicated quantum system in terms of a simpler one  Perturbation (biology), an alteration of the function of a biological system, induced by external or internal mechanisms  Perturbation (transport), operation of a transport system outside of timetable such that delays in arrival and departure from defined locations are present.
Perturbation function, mathematical function which relates the primal and dual problems.  
In mathematics and physics, a non-perturbative function or process is one that cannot be accurately described by perturbation theory.
An example is the functionformula_1.
The Taylor series at x = 0 for this function is exactly zero to all orders in perturbation theory, but the function is non-zero if "x" ≠ 0.
The implication of this for physics is that there are some phenomena which are impossible to understand by perturbation theory, regardless of how many orders of perturbation theory we use.
Instantons are an example.
Therefore, in theoretical physics, a non-perturbative solution or theory is one that does not require perturbation theory to explicate, or does not simply describe the dynamics of perturbations around some fixed background.
For this reason, non-perturbative solutions and theories yield insights into areas and subjects perturbative methods cannot reveal. 
"Chlamydomonas" is used as a model organism for research on fundamental questions in cell and molecular biology such as:  How do cells move?
How do cells respond to light?
How do cells recognize one another?
How do cells generate regular, repeatable flagellar waveforms?
How do cells regulate their proteome to control flagellar length?
How do cells respond to changes in mineral nutrition?
(nitrogen, sulfur, etc.)
There are many known mutants of "C. reinhardtii".
These mutants are useful tools for studying a variety of biological processes, including flagellar motility, photosynthesis, and protein synthesis.
Since "Chlamydomonas" species are normally haploid, the effects of mutations are seen immediately without further crosses.
In 2007, the complete nuclear genome sequence of "C. reinhardtii" was published.
Channelrhodopsin-1 and Channelrhodopsin-2, proteins that function as light-gated cation channels, were originally isolated from "C. reinhardtii".
These proteins and others like them are increasingly widely used in the field of optogenetics. 
Each of the examples described below shows how a naive perturbation analysis, which assumes that the problem is regular instead of singular, will fail.
Some show how the problem may be solved by more sophisticated singular methods.