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Q: "First Order Process" ( Answered,   0 Comments )
Subject: "First Order Process"
Category: Science > Biology
Asked by: tedmccall-ga
List Price: $10.00
Posted: 15 Apr 2005 10:15 PDT
Expires: 15 May 2005 10:15 PDT
Question ID: 509694
Please comment on "first order process". I would just like to know
what is it all about, in reference to protein purification or

Subject: Re: "First Order Process"
Answered By: pafalafa-ga on 15 Apr 2005 11:27 PDT
Hello Ted,

Nice to see you back here again.

Your question takes me back to my early days of college biochemistry classes.

A first order process is one that follows a fairly simple dynamic.  As
"A" doubles, "B" doubles.  If "A" is halved, then "B" is halved as
well.  Another way of saying this is that the relationship betwen "A"
and "B" is linear.

"A" and "B" can be a lot of different things, but the term "first
order process" probably finds most use in describing chemical
processes.  And in the context of chemistry (and biochemistry, and
protein separations), first-order processes often refer to the rate of
a reaction, or to the concentration of chemical constituents, which
can make for some complext math, but the overall idea remains the

Here's an example of the use of the term pertaining to protein
purification (not the clearest example, but on point for your


Isoelectric protein purification by orthogonally coupled hydraulic and
electric transports in a segmented immobilized pH gradient.

...Macromolecules and small ions leave the flow chamber at a rate
corresponding to a first order reaction kinetics (the plot of log C
vs. time being linear).

In other words, the "A" and "B" of this situation -- the variables
involved -- are time and protein concentration, and the two follow
first-order process dynamics -- plotting time and log C
(concentration) on a graph gives you a smooth, straight line.

Here are some other references to the term, with some fairly
straightforward definitions:
first-order reaction

A reaction the rate of which is proportional to the concentration of
the single substance undergoing change; radioactive decay is a
first-order process, defined by the equation -(dN/dt)=kN, where N is
the number of atoms subject to decay (reaction), t is time, and k is
the first-order decay (reaction) constant, i.e., the fraction of all
atoms decaying per unit of time.
First-order process

A first order process is a chemical process where the rate is directly
proportional to the amount of chemical present. Any process changing
at a constant fractional rate.

One compartment model: 

Plot of plasma concentration versus time yields a straight line 
Rapidly equilibrate between blood and tissues relative to the rate of elimination 
Assumes that changes occurring in the plasma also reflect changes in
the tissue conc.

Usually a First order process: 
The rate of elimination is directly proportional to the amount of
chemical in the body at that time.

Semilogarithmic plot of plasma concentration versus time yields a
single straight line

Half-life is independent of dose

The concentration of the chemical in plasma and other tissues
decreases by some constant fraction per unit time.



The elimination phase is reached after absorption is largely completed
and after equilibrium between plasma and tissues occurs. For most
drugs, elimination of drug from the body is a first-order process.
This means that the rate of elimination is proportional to the amount
of drug in the body. This rate of elimination is often referred to as
a first-order rate constant (k or ).

The elimination half-life (t1/2) is defined as the length of time it
takes for the plasma concentration to fall by 50% during the
elimination phase. A characteristic of the elimination phase for a
drug that has first-order elimination is that the half-life is
constant. For example, if a drug has a 4-hour half-life, it would take
4 hours for the drug concentration to drop from 80 to 40 mg/L;
following another half-life (4 hours), the concentration would again
decrease by half to 20 mg/L. After 5 half-lives, 97% of the drug would
be eliminated, and this is the basis for the dosing principle that it
takes 5 elimination half-lives to eliminate a drug from the body.

I trust this information offers a useful explanation for you. 
However, please don't rate this answer until you are satisfied that
you have everything you need.

If you'd like more information, just post a Request for Clarification
to let me know, and I'll be sure to get back to you.



search strategy:  Google search on [ (definition OR glossary) "first
order process" ]
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