FLOW DOWN GRADIENTS Misconceptions – from survey of Physiology Faculty, Dec. 2011
In the box below, please describe below any misconceptions that your students exhibit about the core concept of “FLOW
DOWN GRADIENTS;” “the transport of ‘stuff’ (ions, molecules, blood, air) is a central process at all levels of
organization in the organism and this transport is described by a simple model.”
For example, many of our students hold the following two misconceptions about flow down gradients:
1) In the student’s words: Higher numbers mean higher flow. In other words, students often don’t consider that it’s
the relative difference between the numbers that matters, so they’re liable to think that there will be greater flow
between regions of 90 and 70 mm Hg than between regions of 50 and 30 mm Hg.
2) Only gradient matters in determining flow. Students often don’t consider resistance.
What other misconceptions do your students exhibit regarding FLOW DOWN GRADIENTS?
. . Students think example given (oxygen flow into the capillaries in the lungs, for example, apply that directionality to
everything. They don't seem to think about what they are dealing with biologically (getting nutrients and oxygen versus
getting rid of wastes and carbon dioxide. 2. I think the resistance issue is also significant. Making the connection with
flow and the reason for the resistance changes.
Many of my students haven't considered these concepts prior to the class and it take them awhile to "get it." The two
examples above are good and many of my students think that way.
In relation to 1), students don't understand that passive transport of ions depends on both the chemical and the electrical
gradient Re 2), they find it difficult to retain the notion that there must be both a gradient AND a pathway (i.e. not infinite
resistance) There are also problems when stuff is carried along by bulk flow vs flow of its individual components along
their individual gradients Re the Starling forces across the capillary wall, students find it hard to imagine osmotic pressure
opposing hydrostatic in determining fluid flow
energy has to be added for stuff to flow down a gradient (the gradient is not enough)
The concept of gradient seems difficult for them to grasp; for example, that it is the difference between pressure at point A
relative to point B that drives flow.
the circulatory system is a one-way highway. the diffusion of one molecule is not dependent on the concentration of a
different molecule. one a molecule is removed from the system (such as O binding to hemoglobin) that the difusion can
continue to occur. students struggle with things that counteract flow such as concentration, hydrostatic pressures and
electrical charges
That the gradient (rather than kinetic energy) drives flow. That flow is in one direction from high to low concentration,
rather than "NET" flow from high to low concentration
They confuse bulk flow with diffusion; I can't think of any other common misconceptions.
The effects of particle charge and dissociation are not well understood
They often think that to "facilitate" movement requires energy. They often do not take into consideration the size of what
is moving.
Sometimes they don't realize that with semipermeable membranes, equilibrium may never be reached.
In addition to the misconceptions you have already indicated, I have found that many often think that transport only occurs
into a cell. However, the most common misconceptions I encounter are that all carrier mediated transport requires ATP and
they often think the secondary transport must be symport and cannot be antiport.
Since I do not know what other questions will follow this one, or whether there will be an opportunity for other comments
at the end, I will be making some extensive comments here. My observations are taken from the perspective of teaching
students who are biology and chemistry majors, students who will take upper level courses to obtain skills and insights they
will use in graduate research and professional programs. Some points may seem trivial, and perhaps they are for 100 level
courses, however, is we are supposed to training critical thinkers, researchers and those who will apply the results of
research. I feel it is important for them to appreciate how the physical world functions, that it is based on real physicalchemical laws, including the laws of mass action and probability. My goal in making these comments is that we as
educators will help upper level students to appreciate how things happen, not simply that they do. Most misconceptions
originate in high schools, but they are perpetuated in colleges, and by well intentioned textbooks at all levels.
I. In
general students think teleologically and anthropomorphically, they have been taught in the parlance of mechanistic
expediency and as if the physical world thinks like humans. In the proper context such thinking may be appropriate (nontechnical settings, introduction of concepts to novices, and speaking with peers with whom one is confident of an accurate
common understanding.) Na+ ions do not enter the cell in order to depolarize the membrane, nor do neurotransmitters
cross the synapse in order to attach to receptors. To allow students to believe this undercuts critical thinking. II. Students
think in terms of small numbers acting purposefully, rather than very large numbers following physical-chemical laws of
mass action and probability. This thinking is perpetuated by simplistic drawings showing a few ions or molecules, usually
one receptor molecule or transport protein, etc. Again, this IS necessary to introduce a topic and a concept, but we should
get beyond this. Now to “Flow Down Gradients”: A. See comments in I and II above. B. Students misconceive
Diffusion and Net-Diffusion. This is routinely made fuzzy in textbooks. Diffusion is NOT, as many students will tell you,
"… the movement of solutes from higher to lower concentration, through a cell membrane." Rather, Diffusion is but one
aspect of several types of molecular movements that collectively are related to molecular kinetic energy, temperature.
(Other movements include vibrations of bonds and rotation around bonds.) “Diffusion” is the RANDOM motion of ions
and molecules in space (3-D). These random motions, coupled with regional differences in concentration, contribute to netdiffusion, which is what “flow down gradients” is all about. C. Many types of gradients are involved. We often do not
do a good job of clarifying the interactions of different types of gradients. Electrical gradients apply if the particles are
charged; they do not apply if particles are not charged. There can be pressure gradients that lead to bulk flow. All of these
contribute to “flow down gradients” (Particularly significant when considering renal filtration, CSF formation, etc.)
Again… Again, benign-misconceptions may be appropriate for a 100-level course, for upper level courses these are tools to
introduce the concept, but should be transcended. D. Membranes are NOT required for net-diffusion. (Flow Down
Gradients.) In fact, any membrane significantly, very significantly reduces the rate of diffusion over an equivalent distance.
E. Net-Diffusion is most relevant over distances of microns or less. Greater distances require considerations of convection,
bulk flow. F. Even at the lowest concentrations, tens-of-thousands to millions of molecules are involved. G. Water
molecules are always present, and are always, by far, present in the greatest numbers. H. Concentrations are not
compared from one “area” to another “area”. Areas are two-dimensional, concentrations are expressed in volumes… at
least say, “one region to another region.” I. When membranes are involved, pores or carrier molecules, there are many,
many more than the one or two shown in the textbook illustration. This is enough for now…. I’m not that sure this will
be read all that closely, and I have final exams to prepare.