Molecular Movement
Understanding the ways that molecules move and are transported through the body is vital to understanding nearly every single physiological process. As such, we shall have a look at a few of these in detail here. Though some of the concepts could perhaps be considered to come under chemistry or physics headings, given that we are considering them as part of cellular activity we have put them in our biochemistry chapter.
Diffusion
Diffusion is the movement of a substance from a high concentration to a low concentration
This is a pretty basic scientific concept that I'm sure you all know pretty well. However, we can go into the topic in some detail to help us understand it's impact on physiological systems.
The first thing to clarify is that diffusion is different from flow (the mass movement of a substance due to a pressure difference) as it is a passive process.
Diffusion results from the random motion of molecules.
There is no driving force and nothing pulling or pushing to molecules from the high density to low density location.
It is simply a result of random movement.
The first thing to clarify is that diffusion is different from flow (the mass movement of a substance due to a pressure difference) as it is a passive process.
Diffusion results from the random motion of molecules.
There is no driving force and nothing pulling or pushing to molecules from the high density to low density location.
It is simply a result of random movement.
Though this movement is random, its results in the molecules distributing themselves equally through the space available to them (a solute in a solution for example).
This is because the molecules in a higher concentration have a greater probability of randomly moving towards the area of low density (because there are more of them).
This is because the molecules in a higher concentration have a greater probability of randomly moving towards the area of low density (because there are more of them).
You might be able to see how diffusion is simply an extrapolation of kinetic theory.
Factors that affect kinetic movement therefore affect diffusion.
The most obvious of these is temperature. At a higher temperature the molecules (on average) have a higher kinetic energy and will therefore diffuse faster.
Other factors which impact on this include molecular size, and the substances viscosity.
Factors that affect kinetic movement therefore affect diffusion.
The most obvious of these is temperature. At a higher temperature the molecules (on average) have a higher kinetic energy and will therefore diffuse faster.
Other factors which impact on this include molecular size, and the substances viscosity.
Graham's Law
Graham's law is an expansion on this idea of kinetic theory in diffusion of gases.
If you think about it molecular movement is going to be less if the molecule is bigger (more massive if we're thinking in chemical terms).
It's therefore intuitive to think that the rate of diffusion will be inversely proportional to the mass of the molecule.
Indeed that is what we find, except that rate of diffusion is actually inversely proportional to the square root of the molecular mass.
If you think about it molecular movement is going to be less if the molecule is bigger (more massive if we're thinking in chemical terms).
It's therefore intuitive to think that the rate of diffusion will be inversely proportional to the mass of the molecule.
Indeed that is what we find, except that rate of diffusion is actually inversely proportional to the square root of the molecular mass.
In gases we can expand on this and say that rate of diffusion is inversely proportional to the square root of the density of the substance (as through Avagadro's law, the density of a mass is directly proportional to its molecular weight.
Diffusion Across Membranes
Fick's Law of Diffusion
Fick describes a bit more about the factors that affect the rate of diffusion, this time in relation to diffusion across a membrane.
This is a very common scenario in human physiology given the huge number of membranes throughout the body.
This is a very common scenario in human physiology given the huge number of membranes throughout the body.
Let's break it down.
As we have said, diffusion is a process based on random movement of molecules, and we can see how factors that affect this random movement will affect the rate of diffusion.
The first part of Fick's Law we already know about; the rate of diffusion is proportional to the concentration gradient (the difference in the concentrations of the substance on either side).
This is simply because if there is more of a molecule on one side, it is has a higher probability of randomly moving to the other side.
As we have said, diffusion is a process based on random movement of molecules, and we can see how factors that affect this random movement will affect the rate of diffusion.
The first part of Fick's Law we already know about; the rate of diffusion is proportional to the concentration gradient (the difference in the concentrations of the substance on either side).
This is simply because if there is more of a molecule on one side, it is has a higher probability of randomly moving to the other side.
The second part relates to the surface are; the rate of diffusion is proportional to the surface area of the membrane.
The greater the area between the two different concentration again means that there is a higher probability that the molecules will randomly move from one side to the other.
The greater the area between the two different concentration again means that there is a higher probability that the molecules will randomly move from one side to the other.
Thirdly, we can see that the greater the distance between the two concentration, the lower the probability that a molecule will randomly move from one side to the other.
As such, the rate of diffusion is inversely proportional to the thickness of the membrane.
As such, the rate of diffusion is inversely proportional to the thickness of the membrane.
Other Features
There are a couple of other important but more specific features that will also affect the rate of diffusion across a membrane.
A pressure gradient across a membrane is an important determinant on diffusion of a substance.
A pressure gradient will oppose movement of a substance against it (think about it simply as the pressure pushing the molecules away.
Indeed a pressure gradient is often used to measure the strength of diffusion movement across a membrane (e.g. the oncotic pressure).
It is simply the pressure that needs to be applied to prevent diffusion occuring.
We will cover this a bit more when covering osmosis.
A pressure gradient will oppose movement of a substance against it (think about it simply as the pressure pushing the molecules away.
Indeed a pressure gradient is often used to measure the strength of diffusion movement across a membrane (e.g. the oncotic pressure).
It is simply the pressure that needs to be applied to prevent diffusion occuring.
We will cover this a bit more when covering osmosis.
For molecules that have an associated charge, the electrical potential across a membrane has a strong influence on diffusion.
Positively charged molecules are repelled by a positive electrical potential and attracted down a negative potential.
The opposite is true of negatively charged molecules.
This attraction or repulsion can go against the concentration gradient of the molecules themselves.
The strength of the impact is something that is measured by the Nernst Equation.
Positively charged molecules are repelled by a positive electrical potential and attracted down a negative potential.
The opposite is true of negatively charged molecules.
This attraction or repulsion can go against the concentration gradient of the molecules themselves.
The strength of the impact is something that is measured by the Nernst Equation.
Facilitated Diffusion
When we think of diffusion, we tend to think of the molecules moving freely through a solvent medium.
Facilitated diffusion refers to a common mechanism of molecular movement in cells where these molecules must cross a membrane.
In the case of cells it is the lipid cell membrane which acts as a significant barrier to most substances unless they are highly lipid soluble.
This would present an obstruction for molecular movement, even if there was a concentration gradient for it to flow down.
Facilitated diffusion refers to a common mechanism of molecular movement in cells where these molecules must cross a membrane.
In the case of cells it is the lipid cell membrane which acts as a significant barrier to most substances unless they are highly lipid soluble.
This would present an obstruction for molecular movement, even if there was a concentration gradient for it to flow down.
