BIOL 141 Osmosis Module Practice Material

Pre-Module Extra Credit Assignment, part A


Background Information and Practice Problems
Solutions
You are made of approximately 68% water1. Molecules like salt, amino acids, carbohydrates, enzymes,
or any other polar or charged substances, are dissolved in water and make a solution. Water serves as
the main ingredient in many solutions, especially biological solutions. A solution is a combination of a
solvent, the main medium, and substances dissolved in the solvent, which are called solutes. In our
bodies, the water inside and around our cells is the solvent in which various solutes are dissolved.
Note even the name of intracellular water, cytosol, translates to “cell solvent”.


1. Which of the below best describes the relationship between solvents, solutes and solutions?
a) Solvent = Solute + Solution
b) Solute = Solvent + Solution
c) Solution = Solute + Solvent


Solutions can be described based on how much of the solution is solute and how much of the
solution is solvent. For example, in humans the plasma of blood is a solution composed of about 92%
water as the solvent and 8% solutes like gasses and nutrients (note that the percentages must add up
to 100%).


Solutes vs. non-solutes
Solutes, like sugars and amino acids, interact with water molecules by forming hydrogen bonds
together. Formation of hydrogen bonds between water and solutes is what occurs when a substance
dissolves into water. Molecules that do not form hydrogen bonds do not dissolve, such as lipids and
other non-polar chemicals.


Semipermeable membranes



1 Mitchell, H.H. et al. 1945. J. Biol. Chem. 158:625-637.
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, Cells are surrounded by a non-polar phospholipid bilayer. This lipid bilayer prevents most substances
from entering or leaving the cell. Water is one of the few substances that can freely cross the
membrane. Most polar solutes, like salt, sugar and amino acids, cannot freely cross the membrane
and require help from a protein embedded in the membrane.


Osmosis
Water moving by diffusion across a membrane is called osmosis. Remember, diffusion is the
movement of molecules from a high concentration to a lower concentration, in order to reach
equilibrium. Water will move by osmosis from the side with a higher concentration of water to a side
with a lower concentration of water. In other words, water will move by osmosis from the side with a
lower concentration of solute to the side with a higher concentration of solute. Water is attracted to
solutes.


2. Use the following diagram to answer the questions below. Start by filling in the blanks for the
missing % concentrations for beakers B and C.




Beaker A Beaker B Beaker C

0 % Sugar ___% Sugar 40% Sugar
100 % Water 90% Water __% Water
a) What is the solute concentration of:
○ Beaker A? 0%
○ Beaker B? 10%
b) What is the solvent concentration of Beaker C? 60%
When we think about solutions in human biology, we mostly think about the intracellular cytosol
compared to extracellular solutions like plasma and interstitial fluid. (Remember, we are about 68%
water.) To describe the relative amounts of solutes on either side of a membrane, we use the terms
hypertonic, hypotonic and isotonic. If the solutions have equal concentrations of solute, we say that
they are isotonic. If the concentration of solute is not equal, hypotonic describes the side with a
lower concentration of solute and hypertonic describes the solution with a higher concentration of
solute. Use the questions below to practice using these terms.
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