Biochemistry: Solubility, Concentration and Diffusion

1. Solubility- solubility is the measure of what substances dissolve in.
2. Solute and Solvent
a. The two substances in a solution are:
i. Solute- the substance being dissolved.
1. Example: Kool-aid powder
ii. Solvent- the liquid that the solute dissolves in.
1. Example: Water (this is the most common solvent in body fluids)
3. The most important solubility rule for our course is “like dissolves like” which means solutes
dissolve in solvents that are like themselves.
a. Polar solvents like water dissolve polar molecules, non-polar solvents dissolved non-polar
molecules.
b. Non-polar solutes do not dissolve in polar solvents, Polar solutes do not dissolve in non-
polar solvents.
c. Ions behave like polar solutes and easily dissolved in water.
4. Many substances are named based on whether or not they dissolve in water:
a. Hydrophilic molecules- polar molecules that dissolve in water.
i. Hydrophilic means water loving
ii. Example: Glucose (sugar)
b. Hydrophobic molecules- non-polar molecules that do not dissolve in water.
i. Hydrophobic means water fearing
ii. Example: Lipids/fats like oil
c. Amphipathic molecules- have polar end and a non-polar end.
i. Because of this unique structure amphipathic molecules have a region that dissolves in
water and region that dissolves in non-polar solvents. Amphipathic molecules can be
used as detergents and to dissolve solutes in solvents they otherwise would not be able
to dissolve in. Most importantly for our course amphipathic molecules form biological
membranes like the plasma membrane which are essential for all life.
5. Solute concentration
a. Solute concentration- measure of the amount of solute dissolved in a specific volume of
solvent.
b. Solute concentration is calculated by the formula below:
i. Concentration = amount of solute ÷ volume of solvent
1. More solute = higher concentration, Less solute = lower concentration
2. More solvent = lower concentration, Less solvent = higher concentration
c. There are many ways to calculate concentration with many different units the two ways
that will be important to us in our course are:
i. Molarity
1. Molarity (M)/molar = moles (mol) of solute ÷ one liter (L) of water
2. Example: 1 M glucose = 1 mol glucose ÷ 1 L of water
3. Typical molarities in body fluid are very small and you be measured in millimolar
(mM) or 1/1000 th of 1 molar.
ii. M/V concentration

1. To calculate this, you simply divide the mass of solute by the volume of solvent.
2. Concentrations of this type are presented with the divided units. For example:
g/L, g/mL, or mg/mL.
6. Diffusion
a. Diffusion is the simplest way that solute molecules move in solution.
b. Diffusion- movement of solute molecules from an area of high solute concentration to an
area of low solute concentration until equilibrium.
i. Think of the molecules of solute and solvent (water) like pool balls slowly moving
around colliding with each other and with the walls of the container. When these
molecules collide, they scatter each other and make diffusion happen.
ii. See example if lecture video slide 6
iii. See you tube diffusion video
c. The rate (speed) of diffusion depends on several factors:
i. Temperature alters the rate of diffusion
1. Higher temperatures cause molecules to move faster and the collisions to be
more violent, scattering the molecules faster which increases the rate (speed) of
diffusion. ↑ temperature = faster rate of diffusion.
2. Lower temperatures causes molecules to move slower and the collisions to be
less violent, the molecules do not scatter as quickly which decreases the rate
(speed) of diffusion. ↓ temperature = slower rate of diffusion.
ii. The size of the concentration gradient alters the rate of diffusion
1. The concentration gradient is difference between the high concentration and
the low concentration (Concentration gradient = High concentration – low
concentration).
a. Example A: If the high concentration is 10 M glucose and the low
concentration is 1 M glucose then the concentration gradient is 10-1 = 9.
b. Example B: If the high concentration is 5 M glucose and the low
concentration is 1 M glucose then the concentration gradient is 5-1 = 4.
c. Example A has a larger concentration that example B.
2. Larger concentration gradients cause the collisions between molecules to be
more frequent scattering the molecules faster and a faster rate of diffusion. ↑
concentration gradient = faster rate of diffusion.
3. Smaller concentration gradients causes the collisions between molecules to be
less frequent which slows the scattering of molecules slowing the rate of
diffusion. ↓ concentration gradient = slower rate of diffusion.
iii. Molecular size alters the rate of diffusion
1. Larger molecules encounter more friction with solvent molecules and diffuse
slower.
2. Smaller molecules encounter less friction with solvent molecules and diffuse
faster.
iv. The diffusion distance alters the rate of diffusion
1. Diffusion is faster over short distances and slows as distance increases.