LAB The Movement of Substances
into and out of Cells
4
Introduction
The movement of substances into and out of cells is accomplished largely by diffusion, the
movement of molecules of a given substance so that they spread out evenly to occupy the available
space. Additionally, much of the movement of substances within organisms and, specifically, within
cells, is also accomplished by diffusion. Substances tend to move, or diffuse, from areas of higher
concentration to areas of lower concentration of molecules of the same type. In other words, a
substance will diffuse from where it is most concentrated to where it is less concentrated until the
concentration reaches equilibrium.
In lecture you learned that the cytoplasm of Eukaryotic cells is bounded by a plasma membrane, that
it contains numerous internal membranes, and that organelles within the cytoplasm are bounded by
membranes and often contain internal membranes themselves. All of these membranes display
selective permeability, such that dissolved substances do not cross the barrier indiscriminately.
Whereas water molecules tend to pass through biological membranes freely, dissolved ions and
molecules such as sugars and salts pass through at slower and variable rates. In biological systems,
all substances that enter the cell (solutes) are dissolved in water (solvent).
Diffusion of water across a selectively permeable membrane is called osmosis. Because of the
selectively permeable properties of cell membranes there is a tendency for water to move across
these membranes from the region of lower solute concentration toward that of higher solute
concentration, until the concentrations on both sides of the membrane are equal. Water potential is
a measure of the amount of free energy per mole of water molecules. Water potential is affected by
the amount solutes dissolved in the water. The more solute molecules dissolved in a given amount
of water, the lower the water potential. Pure water, which lacks any dissolved solutes, has the
highest water potential.
To see how solutions with different solute concentrations interact, imagine a U-shaped tube with a
selectively permeable membrane separating two solutions with differing solute concentrations. Pores
in the membrane allow water (solvent) molecules to pass through freely, but are too small to allow
the solute molecules to pass through. Water molecules on the side of the membrane with the higher
solute concentration tend to be clustered around the solute molecules, such that there are fewer free
water molecules present. On the side of the membrane with lower solute concentration, fewer water
molecules are interacting with the solute molecules, and more of them are free. The free water
molecules of the side of the membrane with lower solute concentration will diffuse across the
membrane until the solute concentrations on both sides of the membrane are equal. It is this
difference in concentration of free water molecules that determines water potential. A higher
concentration of solute molecules results in fewer free water molecules, and therefore, lower water
potential.
As you will learn in lecture and your readings, substances are also able to enter cells by other
means, including active transport, which uses energy to move solutes against their concentration
gradients. In lab today however, we will investigate passive transport as it occurs by diffusion and
osmosis.
, Exercise 1: Osmosis
In this exercise you will investigate the movement of water molecules through a selectively
permeable membrane by osmosis. As a rule, water molecules will always diffuse from an area
higher water potential to an area of lower water potential.
Water potential (Ψ) results from the combined actions of osmotic potential (Ψπ), which is dependent
on solute concentration in a solution, and pressure potential (Ψp), which results from exertion of
positive pressure or negative pressure on a solution. This relationship can be expressed as:
Ψ = Ψπ + Ψp
water osmotic pressure
potential potential potential
Differences in water potential are a measure of the tendency of water to leave one area in favor of
another. The addition of a solute to water lowers the osmotic potential of a solution (i.e., makes Ψπ
negative) and, therefore, lowers the water potential of a solution. In the absence of other factors,
such as pressure, osmosis results in the net movement of water from an area of lower solute
concentration (higher Ψ) to an area of higher solute concentration (lower Ψ). Water potential on two
sides of a selectively permeable membrane will eventually become equal if there are not limits to
membrane and cell expansion.
However, if a barrier such as a cell wall prevents expansion, then pressure builds up inside the cell
into which water is moving. An increase in positive pressure raises the pressure potential (Ψp), and
therefore the water potential inside the cell. This decreases the water potential gradient (difference)
between the solution outside the cell and the solution inside the cell. Water continues to flow into the
cell until the difference is offset by increased pressure and a dynamic equilibrium is reached. Then,
the net flow of water molecules from outside to inside stops, although individual water molecules
continue to travel back and forth.
The pressure required to stop the net osmotic movement of water into the cell is known as osmotic
pressure. In a simple system, such as that we will set up in lab, it is a measure of the difference in
osmotic potentials of a solution inside a compartment (i.e., cell) and the solution outside.
Part 1: Measuring osmotic pressure with an osmometer.
The osmometer (see setup in lab) provides a way for us to measure osmotic pressure, and
therefore, osmotic potential. Water will move into a cell (or bag in our experiment) and the solution
will rise in the tube until equilibrium is reached, that is, until the water potential on both sides of the
membrane is equal. At this point, the solution in the tube exerts enough positive hydrostatic
pressure, or osmotic pressure, on the contents of the bag to equal the negative osmotic potential of
the sucrose solution.
In the osmometer set up in lab, a dialysis bag containing 40% sucrose solution, to which red dye has
been added, will be suspended in a beaker of distilled water. The bag is semi-permeable – allowing
the passage of water molecules but not the passage or sucrose or red dye molecules.
At 15-minute intervals during the laboratory period, observe the changes in the osmometer on
demonstration. Record the time and the height, in millimeters, of the solution in the tube. Record
your observations in the table below, and plot the data on the graph provided. Place time on the
into and out of Cells
4
Introduction
The movement of substances into and out of cells is accomplished largely by diffusion, the
movement of molecules of a given substance so that they spread out evenly to occupy the available
space. Additionally, much of the movement of substances within organisms and, specifically, within
cells, is also accomplished by diffusion. Substances tend to move, or diffuse, from areas of higher
concentration to areas of lower concentration of molecules of the same type. In other words, a
substance will diffuse from where it is most concentrated to where it is less concentrated until the
concentration reaches equilibrium.
In lecture you learned that the cytoplasm of Eukaryotic cells is bounded by a plasma membrane, that
it contains numerous internal membranes, and that organelles within the cytoplasm are bounded by
membranes and often contain internal membranes themselves. All of these membranes display
selective permeability, such that dissolved substances do not cross the barrier indiscriminately.
Whereas water molecules tend to pass through biological membranes freely, dissolved ions and
molecules such as sugars and salts pass through at slower and variable rates. In biological systems,
all substances that enter the cell (solutes) are dissolved in water (solvent).
Diffusion of water across a selectively permeable membrane is called osmosis. Because of the
selectively permeable properties of cell membranes there is a tendency for water to move across
these membranes from the region of lower solute concentration toward that of higher solute
concentration, until the concentrations on both sides of the membrane are equal. Water potential is
a measure of the amount of free energy per mole of water molecules. Water potential is affected by
the amount solutes dissolved in the water. The more solute molecules dissolved in a given amount
of water, the lower the water potential. Pure water, which lacks any dissolved solutes, has the
highest water potential.
To see how solutions with different solute concentrations interact, imagine a U-shaped tube with a
selectively permeable membrane separating two solutions with differing solute concentrations. Pores
in the membrane allow water (solvent) molecules to pass through freely, but are too small to allow
the solute molecules to pass through. Water molecules on the side of the membrane with the higher
solute concentration tend to be clustered around the solute molecules, such that there are fewer free
water molecules present. On the side of the membrane with lower solute concentration, fewer water
molecules are interacting with the solute molecules, and more of them are free. The free water
molecules of the side of the membrane with lower solute concentration will diffuse across the
membrane until the solute concentrations on both sides of the membrane are equal. It is this
difference in concentration of free water molecules that determines water potential. A higher
concentration of solute molecules results in fewer free water molecules, and therefore, lower water
potential.
As you will learn in lecture and your readings, substances are also able to enter cells by other
means, including active transport, which uses energy to move solutes against their concentration
gradients. In lab today however, we will investigate passive transport as it occurs by diffusion and
osmosis.
, Exercise 1: Osmosis
In this exercise you will investigate the movement of water molecules through a selectively
permeable membrane by osmosis. As a rule, water molecules will always diffuse from an area
higher water potential to an area of lower water potential.
Water potential (Ψ) results from the combined actions of osmotic potential (Ψπ), which is dependent
on solute concentration in a solution, and pressure potential (Ψp), which results from exertion of
positive pressure or negative pressure on a solution. This relationship can be expressed as:
Ψ = Ψπ + Ψp
water osmotic pressure
potential potential potential
Differences in water potential are a measure of the tendency of water to leave one area in favor of
another. The addition of a solute to water lowers the osmotic potential of a solution (i.e., makes Ψπ
negative) and, therefore, lowers the water potential of a solution. In the absence of other factors,
such as pressure, osmosis results in the net movement of water from an area of lower solute
concentration (higher Ψ) to an area of higher solute concentration (lower Ψ). Water potential on two
sides of a selectively permeable membrane will eventually become equal if there are not limits to
membrane and cell expansion.
However, if a barrier such as a cell wall prevents expansion, then pressure builds up inside the cell
into which water is moving. An increase in positive pressure raises the pressure potential (Ψp), and
therefore the water potential inside the cell. This decreases the water potential gradient (difference)
between the solution outside the cell and the solution inside the cell. Water continues to flow into the
cell until the difference is offset by increased pressure and a dynamic equilibrium is reached. Then,
the net flow of water molecules from outside to inside stops, although individual water molecules
continue to travel back and forth.
The pressure required to stop the net osmotic movement of water into the cell is known as osmotic
pressure. In a simple system, such as that we will set up in lab, it is a measure of the difference in
osmotic potentials of a solution inside a compartment (i.e., cell) and the solution outside.
Part 1: Measuring osmotic pressure with an osmometer.
The osmometer (see setup in lab) provides a way for us to measure osmotic pressure, and
therefore, osmotic potential. Water will move into a cell (or bag in our experiment) and the solution
will rise in the tube until equilibrium is reached, that is, until the water potential on both sides of the
membrane is equal. At this point, the solution in the tube exerts enough positive hydrostatic
pressure, or osmotic pressure, on the contents of the bag to equal the negative osmotic potential of
the sucrose solution.
In the osmometer set up in lab, a dialysis bag containing 40% sucrose solution, to which red dye has
been added, will be suspended in a beaker of distilled water. The bag is semi-permeable – allowing
the passage of water molecules but not the passage or sucrose or red dye molecules.
At 15-minute intervals during the laboratory period, observe the changes in the osmometer on
demonstration. Record the time and the height, in millimeters, of the solution in the tube. Record
your observations in the table below, and plot the data on the graph provided. Place time on the
