Project 3
Student id: s25
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<DOC>
<docHead>
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<title>WP3 Edutainment Blog Post</title>
<author xml:id="s25">Leo Stolov</author>
<version n="1" date="2016-08-18"/>
</docHead>
<blog_post>
<title>Traveling at the microscopic level</title>
<background>
<question>How is it that bacteria travel from one place to another?</question>
<misconception>To many, the answer may seem trivial, since motion is a problem solved by
nature time and time again, when in reality, there are many concepts at play which
complicate this problem. To truly understand the mechanisms for travel and why they
are the way they are requires an understanding about microscopic things like
bacteria, specifically the world they live in.</misconception>
<technical_principle>The first step is to understand the <important_idea>Reynolds
number.</important_idea>
</technical_principle>
</background>
<answer>
<section_header>What is the Reynolds number?</section_header>
<technical_principle n="1">Mathematically, the Reynolds number can be expressed like
this: <visual type="other"
url="http://ffden-2.phys.uaf.edu/104_2012_web_projects/Daniela_Wilner/Images/ReynoldsNumber.png"
/> The Reynolds number (Re in the equation above) is defined as the ratio of
inertial forces to viscous forces. What does that mean? Well the first thing that
should be noted is that it is a ratio. It has no units attached. The Reynolds number
can be 46, it can<note type="instructor" resp="#kgs">be</note> 46 million. It’s just
a number. But it is a very useful number, especially for engineers. Engineers use
the Reynolds number to characterize how a fluid (liquid or gas) flows. It
characterizes the flow of a liquid, allowing engineers to make certain assumptions
and what equations to use. </technical_principle>
<section_header>What does the Reynolds number depend on?</section_header>
<technical_principle n="2">Here’s the breakdown: ρ is density of the fluid, V is
velocity, D is diameter, or the size of the volume being analyzed, and μ is
viscosity, which is a fluid's resistance to movement <analogy>(molasses is much more
viscous than water).</analogy>
</technical_principle>
<section_header>So what is low for a Reynolds number?</section_header>
<analogy>Let's take a person swimming in water. Let's call him Pichael Melps. <visual
type="drawing"
url="http://www.how-to-draw-funny-cartoons.com/image-files/cartoon-swimmer-007.jpg"
/>Pichael swims at an average speed of 2 mph or .894 m/s in water which has a
density of 1000 kg/m^3 and a viscosity of about 10^-3 Pa*s. We can approximate
Pichael's size, or diameter, as 1.5 ft or .457 m. Put these numbers into the
equation and we get a Reynolds number of about 409000. Now, lets take a look at an
E. coli cell. <visual type="drawing"
url="http://hyperphysics.phy-astr.gsu.edu/hbase/biology/imgbio/cellprokaryote6.gif"
/>A typical cell might be 2 μm or 2*10^-6 m and travel in water at a speed of 30
μm/s or 3*10^-7 m/s. In this case your Reynolds number would be 0.00006 . That's
several magnitudes smaller than Pichael's situation. If Pichael wanted to experience
the same Reynold's number as an E. coli cell he would have to move at a speed of
about 1.3*10^-10 m/s. If he were to move at that speed for a whole year he would
travel a staggering 4 millimeters.</analogy>
<section_header>What does this tell us?</section_header>
<important_idea>It tells us fluid flow has different properties at this level. It tells
us life is different at low Reynolds numbers. </important_idea><technical_principle
n="3">To understand this further we have to go back to the definition of Reynolds
number: the ratio of inertial forces to viscous forces. The lower your Reynolds
number, the lower your inertia is compared to the viscous forces of the fluid around
you.</technical_principle>
<analogy>What does this mean? Imagine you get in the water and push Pichael. He will
move while he’s being pushed, and then inertia will carry him so he floats a little
bit farther away from you. Do the same in molasses, and he’ll stop almost
immediately once the force of your push stops acting on him. <technical_principle
n="4">His movement in the molasses only depends on a current force acting on
him, not on the forces acting on him before. In this case, his movement is not
dependent on time. It is said to be <important_idea>time
invariant.</important_idea>
</technical_principle>
</analogy>
<section_header>Scallop Theorem </section_header>
<specific_topic>This concept is the cornerstone of Scallop Theorem, a theorem developed
by physicist Edward Mills Purcell in his paper "Life at low Reynolds
number."</specific_topic><technical_principle n="5">”. It states that to travel
at low Reynolds number a swimmer has to move or deform in a way that is not
reciprocal. </technical_principle>
<analogy>. A great example of this is, of course, the scallop. A scallop travels in
water by slowly opening its shell to take in water, and quickly squirting it out to
propel itself. <visual type="drawing"
url="https://upload.wikimedia.org/wikipedia/commons/4/40/Scallop_jump.svg"
/><technical_principle n="6">While this works great in a scallop’s environment,
it would be useless at low Reynolds numbers, due to the fact that the movement
is reciprocal. Since motion at low Reynolds flow is time invariant, the fact
that the scallop opened and closed its shell slowly and quickly does not matter.
In this case, the scallop would simply move one way when the shell opens, and
move right back to the same spot when it closes.
</technical_principle></analogy>
<section_header>How does this apply to microorganisms?</section_header>
<explanation>Due to these constraints, bacteria have developed interesting ways to move
around. The most common of these methods is the use of flagella, which are whip-like
appendages connected to the main cell body by a hook.<visual type="drawing"
url="http://i.imgur.com/hDHJp6l.jpg"/> The hook is connected to a biological
motor, which spins the flagella. The spinning of these typically helical flagella
works since it is not reciprocal; it is different when done in forward or in
reverse. <visual type="drawing" url="http://i.imgur.com/l2bQzNs.png"/> The use of
flagella is fairly common in the world of bacteria. E coli. use flagella. <visual
type="photo"
url="http://knowledgeinsider.com/wp-content/uploads/2016/02/e-coli-streptococci.jpg"
/> So does Helicobacter pylori, a bacterium found in the stomach. <visual
type="photo"
url="http://userfiles.steadyhealth.com/userfiles/4540/articles/H.pylori.gif"/>
Other bacteria such as the Spirochaete have a spiral shape and move by whole-body
undulations. Their spiral shape allow their movement to be non-reciprocal. <visual
type="photo"
url="https://microbewiki.kenyon.edu/images/thumb/f/f4/Figure1.png/400px-Figure1.png"
/>
</explanation>
<section_header>Conclusion</section_header>
<conclusion>Given their microscopic size, it's quite fascinating to see such developed
forms of movement. A naturally occurring rotating element and motor are certainly
surprising characteristics to find in puny bacteria since these are things that
cultured humans like Pichael need lots of brainpower and resources to create.
Nevertheless, it makes sense, since these traits were the response to a completely
different set of rules on the microscopic level. </conclusion><visual type="comic"
url="https://s-media-cache-ak0.pinimg.com/564x/cb/32/6c/cb326c2a1112a7ed8b9484651ae3a3e6.jpg"
/>
</answer>
<sources>
<section_header>Sources</section_header>I highly recommend reading <ref type="source"
url="http://www.uam.es/personal_pdi/ciencias/agrait/nico_archivos/docencia/fisica%20de%20fluidos/Life%20at%20Low%20Reynolds%20Number,%20EM%20Purcell%201973.htm"
> Life at Low Reynolds Number </ref> for a more in depth look at this topic. <ref
type="source" url="http://www.damtp.cam.ac.uk/user/lauga/papers/114.pdf">This
paper</ref> on bacterial hydrodynamics provides a closer look on bacteria mechanism
for movement, especially the use of rotating flagella. Additionally, <ref type="source"
url="https://www.amherst.edu/users/D/ndarnton/extra/hydrodynamics">this post</ref>
on hydrodynamics has great analogies and examples relevant to the Reynolds number.
</sources>
</blog_post>
<docReview>
<reviewer type="peer" xml:id="d13"/>
<report type="peer" resp="#d13">
<p>For project 3 Leo composed an edutainment blog about the movement of bacteria and the
challenges of moving anything at a microscopic level. He focuses on three main
scientific topics, the Reynolds number, Microscopic movement and how bacteria
overcome their difficult environment to move around. He begins with an explanation
of the Reynolds number and how it is calculated and then discusses what happens to
the Reynolds number at a microscopic level before focusing on the movement of
bacteria.</p>
<p>This blog post is well thought out and fits well with the schema created by group 1.
In particular, the visuals are very effective aides alongside the explanation of
complex scientific and engineering concepts. The explanation of the Reynolds number
was easy to understand and I believe that your intended audience will be able to
grasp the concepts outlined in this blog post. All the required technical
information is explained well, and the intended audience of the high school educated
population will be able to comprehend it.</p>
<p>As mentioned in you writer's memo, I do believe you have yet to decide on what the
main topic of this blog post should be. The title of the post is "How do bacteria
move", but this is only addressed very briefly at the end of the post. This post
could equally be about the principle of the Reynolds number in general, what happens
to movement at a microscopic level or how bacteria overcome the challenges of their
environment, specifically related to movement. I think you have all the content
necessary to pursue any one of these three topics, the post would just need to be
edited to use that information to focus on the answer to the main question. Of the
three topics, I believe you will have the most success by focusing on movement at
the microscopic level. The information about the Reynolds number would serve as a
good lead in to the microscopic environment and the challenges it presents. In
particular, the idea that no movement is possible at the Reynolds numbers seen at
microscopic levels is theoretically impossible as it would be inherently reversible
should be expanded upon. By establishing the conditions of movement at the
microscopic level, you could then explain how nature has found ways to overcome
these challenges(the scallops or flagella).</p>
<p>Overall, I believe this post is very well constructed and has a good balance of
scientific information and the analogies and pictures that help explain the complex
topics. The post adheres very well to the schema created by group one and is both
educational and interesting to read. My main advice is to make minor changes
throughout the piece that focuses the information towards answering one main
question, rather than three at once.</p>
</report>
<reviewer type="instructor" xml:id="kgs"/>
<report type="instructor" resp="#kgs">
<p>This post is a great example of the genre--an appropriate topic, tone, and level of
complexity for a non-specialist (me) to easily understand. I also think your use of
images was helpful throughout, and the extended Pichael Melps analogy worked well.
Nice work.</p>
</report>
</docReview>
</DOC>