Experiment 7: Diffusion and Osmosis
Learning Objectives
1. Define diffusion and predict how molecular mass affects the rate of diffusion.
2. Define solute, solvent, and solution.
3. Define hypotonic, hypertonic, and isotonic in terms of relative concentrations of water and solute.
4. Predict the movement of water across a selectively permeable membrane in solutions with different tonicities.
5. Compare and contrast diffusion and osmosis
Introduction
Cell membranes, which separate living cells from their non-living surroundings, regulate the flow of particles of
substances into and out of the cell. They are selectively permeable barriers, meaning they allow some particles to pass
through while blocking others. Membrane transport can occur either passively or actively. The driving force for passive
transport is the concentration gradient, where particles move from an area of higher concentration to an area of lower
concentration. In contrast, active transport requires an additional input of energy, often in the form of ATP (the energy
currency of cells), to move particles against their concentration gradient.
The concentration gradient also provides the driving force for diffusion, a natural process where particles move down
their concentration gradients. This movement occurs due to the random motion and collisions of particles, driven by
their intrinsic kinetic energy (the energy of motion). The collective effect of this molecular motion results in the even
distribution of particles throughout the available space. Diffusion continues until the concentration gradient is
dissipated, although the motion of individual particles persists.
Several factors influence the rate of diffusion. Temperature is one such factor, as heat increases the movement of
particles, speeding up diffusion. Another factor is the molecular size or mass of the particle: smaller particles tend to
move faster than larger ones, thus diffusing more quickly. Additionally, the concentration gradient plays a role: the
greater the concentration difference between two areas, the faster diffusion will occur, as particles move more quickly
to equalize the concentration.
The diffusion of water across a selectively permeable membrane is called osmosis. This process is influenced by the
relative solute concentrations on either side of the cell membrane. The cytosol (the fluid inside the cell) and the
extracellular fluid (the fluid outside the cell) are aqueous solutions, homogeneous mixtures composed of water (the
solvent) and numerous organic and inorganic molecules (the solutes or dissolved substances).
The direction of osmosis is always from an area of lower solute concentration (hypotonic) to an area of higher solute
concentration (hypertonic). Osmosis continues until an isotonic state is reached, where the solute concentration is
equal on both sides of the membrane. At this point, there is no net movement of water.
Activity 7.1: Diffusion (to be completed before lab)
Observe the diffusion of gases through air (watch this video), record your observations, and provide explanations.
The process of diffusion can be demonstrated using the length of glass tubing with cotton plugs inserted into either end.
1. Two cotton plugs are prepared by soaking them in two different solutions:
Cotton Plug A: Soaked in ammonia hydroxide solution (NH 4OH). The NH4OH solution releases colorless
ammonia gas (NH3), which has a molecular weight of 17.0 Daltons.
Cotton Plug B: Soaked in concentration hydrochloric solution (HCl). The HCl solution releases colorless
hydrogen chloride gad (HCl), which has a molecular weight of 36.5 Daltons.
2. The cotton plugs are inserted into opposite ends of the glass
tubing simultaneously. Each plug releases its respective gas, and
the gases diffuse through the tube towards the opposite end.
3. When the two gases meet somewhere along the length of the tube, they react to form a white precipitate called
ammonium chloride: NH3 (g) + HCl (g) NH4Cl (s).
4. Observations: The reaction occurs closer to the HCI (HCl/NH3) end of the glass tube. This is because it has a HIGHER
(higher/lower) molecular weight, thus it moves SLOWER (faster/slower).
1
Learning Objectives
1. Define diffusion and predict how molecular mass affects the rate of diffusion.
2. Define solute, solvent, and solution.
3. Define hypotonic, hypertonic, and isotonic in terms of relative concentrations of water and solute.
4. Predict the movement of water across a selectively permeable membrane in solutions with different tonicities.
5. Compare and contrast diffusion and osmosis
Introduction
Cell membranes, which separate living cells from their non-living surroundings, regulate the flow of particles of
substances into and out of the cell. They are selectively permeable barriers, meaning they allow some particles to pass
through while blocking others. Membrane transport can occur either passively or actively. The driving force for passive
transport is the concentration gradient, where particles move from an area of higher concentration to an area of lower
concentration. In contrast, active transport requires an additional input of energy, often in the form of ATP (the energy
currency of cells), to move particles against their concentration gradient.
The concentration gradient also provides the driving force for diffusion, a natural process where particles move down
their concentration gradients. This movement occurs due to the random motion and collisions of particles, driven by
their intrinsic kinetic energy (the energy of motion). The collective effect of this molecular motion results in the even
distribution of particles throughout the available space. Diffusion continues until the concentration gradient is
dissipated, although the motion of individual particles persists.
Several factors influence the rate of diffusion. Temperature is one such factor, as heat increases the movement of
particles, speeding up diffusion. Another factor is the molecular size or mass of the particle: smaller particles tend to
move faster than larger ones, thus diffusing more quickly. Additionally, the concentration gradient plays a role: the
greater the concentration difference between two areas, the faster diffusion will occur, as particles move more quickly
to equalize the concentration.
The diffusion of water across a selectively permeable membrane is called osmosis. This process is influenced by the
relative solute concentrations on either side of the cell membrane. The cytosol (the fluid inside the cell) and the
extracellular fluid (the fluid outside the cell) are aqueous solutions, homogeneous mixtures composed of water (the
solvent) and numerous organic and inorganic molecules (the solutes or dissolved substances).
The direction of osmosis is always from an area of lower solute concentration (hypotonic) to an area of higher solute
concentration (hypertonic). Osmosis continues until an isotonic state is reached, where the solute concentration is
equal on both sides of the membrane. At this point, there is no net movement of water.
Activity 7.1: Diffusion (to be completed before lab)
Observe the diffusion of gases through air (watch this video), record your observations, and provide explanations.
The process of diffusion can be demonstrated using the length of glass tubing with cotton plugs inserted into either end.
1. Two cotton plugs are prepared by soaking them in two different solutions:
Cotton Plug A: Soaked in ammonia hydroxide solution (NH 4OH). The NH4OH solution releases colorless
ammonia gas (NH3), which has a molecular weight of 17.0 Daltons.
Cotton Plug B: Soaked in concentration hydrochloric solution (HCl). The HCl solution releases colorless
hydrogen chloride gad (HCl), which has a molecular weight of 36.5 Daltons.
2. The cotton plugs are inserted into opposite ends of the glass
tubing simultaneously. Each plug releases its respective gas, and
the gases diffuse through the tube towards the opposite end.
3. When the two gases meet somewhere along the length of the tube, they react to form a white precipitate called
ammonium chloride: NH3 (g) + HCl (g) NH4Cl (s).
4. Observations: The reaction occurs closer to the HCI (HCl/NH3) end of the glass tube. This is because it has a HIGHER
(higher/lower) molecular weight, thus it moves SLOWER (faster/slower).
1
