All organisms need to maintain internal stability to survive. We need a constant stream of food, oxygen, energy, waste removal, and more to stay alive.  A lack of just one of these (in fact, even an excess of one of these) throws an organism out-of-balance, and will ultimately lead to death.  There are some external factors that need to be controlled as well; sunlight (too much and you cook, not enough and you die); temperature (organisms live in a narrow band of temperatures), pH (the acidity/baseness of our environment; all of these factors must remain stable for an organism to survive.  We'll look at these and see how organisms maintain or create that stability.


All organisms need to establish stable internal conditions to stay alive. Human beings maintain a pretty constant temperature of 98.6F, regardless of external temperature, because that is the temperature that cells in our body work best at.

An Example:

You can take your temperature and find your core body temperature is roughly 98.6F. (Core Body Temperature is the temperature the center of your body and brain maintain.)  If you then go into a hotbox or sauna and stay there until you are uncomfortably warm,  your core temperature will again still be roughly the same (98.6F).  If you swim in cold water until you are very cold (your lips turn blue), and your core temperature is measured again, we will still find it to be roughly 98.6F.  In fact, humans can only stay alive if their body temperature remains between 95F and 101F, and these extremes are not good for us.  That is a very narrow band of temperature.  The human organism does all it can to maintain a constant temperature, sweating when we are hot (to cool us down evaporatively), and shivering when we are cold (to warm us by muscle movement). We use other strategies to maintain our body temperature:

  • the hair on our arms stand up when we are cold (piloerection)

  • our blood vessels consrict to conserve heat (vasoconstriction)

  • our blood vessels dilate (expand) when we are hot (vasodilation)

  • our body sweats to cause evaporative cooling.

Our body does its best to maintain a constant temperature for the most important parts

(heart, brain, lungs) so these organs can continue to function.  This helps keep these

organs functioning, maintaining life.


The Definition:

Homeostasis is the tendency for a living organism to maintain a constant internal environment that best meets the needs of the organism.  Our energy level stay pretty constant during the day without us having to eat constantly.

The energy is metered out to our cells as they need it, always maintaining a constant energy level.  Our temperature remains fairly constant throughout the day regardless of our surrounding temperature as described above.  The chemical levels in our cells remains pretty constant - allowing our cells to live and function.


Its Roots:

Homeostasis has its roots in Greek:

  • Homoios  – similar

  • Stasis  – standing still

and is the property of a system in which variables are regulated so that internal conditions remain stable and relatively constant. Examples of homeostasis include the regulation of temperature and the balance between acidity and alkalinity (pH). It is a process that maintains the stability of the human body's internal environment in response to changes in external conditions.


The History:
The concept was described by  Claude Bernard in 1865 and the word was coined by Walter Cannon  in 1926. Although the term was originally used to refer to processes within living organisms, it is also applied to automatic control systems, such as thermostats. Homeostasis in automatic control systems require a sensor to detect changes in the condition to be regulated, an effector mechanism that can vary that condition; and a negative feedback  connection between the two.  This is similar to life processes that happen inside an organism.


Outline Activity:

Take your time to watch these videos and outline the facts you learn.





















































In chemistry, pH is a measure of the acidity or basicity of an aqueous solution (a solution of water).  Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline.  Pure water has a pH very close to 7.

pH measurements are important in medicine, biology, chemistry, agriculture, water treatment & purification, and many other applications.

An Explanation:
Mathematically, pH is the negative logarithm of the activity of the dissolved hydrogen ion, more often expressed as the measure of the hydrogen ion concentration.  






















pH is a measure of the Hydrogen Ion concentration in a (water based) liquid.  Water molecules are chemically H2O.  The atoms that make up water can dissociate (to split into separate smaller atoms, ions, or molecules, especially reversibly) into a Hydrogen Ion (a proton with no associated electrons), and a Hydroxyl Ion (an Oxygen atom combined with a Hydrogen Atom with the electron taken from the Hydrogen Ion).  This is taking a neutrally charged water molecule and splitting it into two IONS (atoms or molecules in which the total number of electrons is not equal to the total number of protons, giving the atom a net positive or negative charge).  This dissociation is reversible, and there is always an equal number of Hydrogen Ions and Hydroxyl ions in pure water.







Acids have more Hydrogen Ions than Hydroxyl Ions, while Bases have more Hydroxyl Ions than Hydrogen Ions. The measure of pH is a measurement of the concentration of Hydrogen Ions.  As the difference between these two ions increases, the acidity (or basicity) changes accordingly.



Aqueous solutions (mixtures of chemicals that are dissolved in water) each have a measurable pH.  Here are some common liquids and their measured pH:


















pH Explained: a video

























A buffer is an Aqueous Liquid solution that can resist pH change when acids or bases are added to it.   The buffer neutralizes small amounts of added acid or base, maintaining a constant pH. This is important for processes and/or reactions which require specific and stable pH ranges, like inside living organisms. Buffer solutions have a working pH range (only working for certain pH) and capacity (only able to neutralize a certain amount of acid or base).  That is to say that a buffer has a limit to how much acid/base can be neutralized before pH changes, and the amount by which it will change.


A graph of a buffered solution with added base is here. Notice that the pH doesn't change even after 10ml of base was added.  This buffer stabilized the solution, and maintained the pH at a fairly constant level.



















We can see that the pH doesn't change as we add base to the solution.  The pH is stable (at the pH = pKa line), showing the narrow range of the buffer. In fact, adding 10 mL of base causes no real change to the pH of the solution (shown between the dashed lines).  This is the capacity of  the buffer.  Buffers cannot neutralize an infinite amount of acid/base, and are limited to a specific pH.


Buffer Video: