What is diffusion?

BIOL 1406

PreLab 5.1

What is diffusion?

Yellow food coloring diffusing through water. The glass on the left contains hot water, while the glass on the right contains cold water. The food coloring was added to the cold water slightly before the coloring was added to the hot water, yet after a few seconds it has diffused more thoroughly through the hot water. The frames are roughly 1 second apart (so the animation is roughly 2x real-time).

Cell survival depends on the movement of substances (such as enzymes, and chemical messengers) within the cell. In addition, certain materials (such as nutrients) must enter the cell, and other materials (such as waste products) must leave. In order to enter or leave a cell, materials must cross the plasma membrane. The movement of substances within the cell or across the plasma membrane is called cell transport. Mechanisms of cell transport can be divided into 2 main categories: active transport and passive transport.

Both active and passive transport require energy. With active transport, the cell must expend some of its own energy, stored in molecules of ATP. With passive transport, on the other hand, the energy comes from an external source such as random molecular motion.

In this lab, you will study various types of passive transport. To understand how passive transport works, you need to understand the kinetic theory of matter. According to this theory, all atoms and molecules are in constant random motion. This gives them energy of motion, also called kinetic energy. The kinetic energy of atoms and molecules is detected by humans as heat. As a substance is heated, the atoms and molecules move faster, and their kinetic energy increases. Furthermore, the constant random motion of atoms and molecules causes adjacent substances to become evenly mixed together, even if the substances are undisturbed by outside forces.

As an example, suppose you gently place a few drops of yellow food coloring on top of some water in a glass. Even if the water is left totally undisturbed, each water molecule in the glass is actually moving in a straight line until it bumps into a molecule of food coloring, the glass, or another water molecule; then it ricochets off in another direction. At the same time, each food coloring molecule is also moving in a straight line until it bumps into a water molecule, the glass, or another food coloring molecule; then it ricochets off in another direction.

In this situation, it is important to note that the water and food coloring molecules move at random. However, when the food coloring is first placed on top of the water, more food coloring molecules will be moving down towards the bottom of the glass than are moving back up towards the top of the glass. Therefore, we say movement of the food coloring molecules is random, but net movement is from the area where the concentration of food coloring is higher to the area where the concentration of food coloring is lower (i.e. down a concentration gradient.) Likewise, the water molecules in the glass also move at random, but more water molecules will be moving up toward the top of the glass (where the water concentration is lower) than are moving down towards the bottom of the glass. Therefore, net movement of the water molecules is toward the top of the glass. Note that each substance shows net movement down its own concentration gradient. As a result, net movement of water and food coloring are in opposite directions. This process continues until the food coloring and water molecules are evenly mixed – a point called equilibrium. At equilibrium, the food coloring and water molecules continue to move at random but there is no longer any net movement of either substance.

The net movement of a substance from a region where it has a higher concentration to a region where it has a lower concentration, due to random molecular motion, is called diffusion. Diffusion is a widespread and important process which occurs in both living and non-living systems. Because diffusion occurs under a variety of conditions, scientists have adopted the following terms to specify particular types of diffusion:

Simple diffusion refers to diffusion of substances without the help of transport proteins.
Facilitated diffusion refers to diffusion of substances across a cell membrane with the help of transport proteins.
Dialysis refers to the diffusion of solutes across a selectively permeable membrane. A selectively permeable membrane (also called a semipermeable membrane) is a membrane that allows some substances to pass through easily while other substances pass through very slowly or not at all.
Osmosis, on the other hand, refers to the diffusion of the solvent across a selectively permeable membrane. Because water is the solvent in all living systems, biologists usually define osmosis as the diffusion of water across a selectively permeable membrane.

Although various types of diffusion have been recognized, all share the following characteristics:

Net movement of each substance is caused by random molecular motion.
Net movement of each substance involves passive transport.
Net movement of each substance is down its own concentration gradient.
At equilibrium, random molecular motion continues but there is no longer any net movement.

In this lab, you will study several different types of diffusion using both non-living and living systems. First, you will conduct two experiments utilizing non-living systems:

You will study the diffusion of 2 dyes, potassium permanganate and methylene blue, through a non-living, gel-like material called agar. Agar is prepared by boiling a complex polysaccharide (called agarose) in water until it dissolves and then allowing the solution to cool. As the solution cools, it turns into a semi-solid gel.
You will study both dialysis and osmosis across a membrane made of dialysis tubing. Dialysis tubing is a commercially prepared, artificial membrane made of cellulose. The membrane contains microscopic pores that allow ions and molecules to pass through. The size of the pores in the membrane determines the size of the ions and molecules that can pass through.

Then you will conduct one experiment utilizing a living system:

You will study the process of osmosis across the plasma membrane of living cells found in the leaves of an aquatic plant called Elodea.

Your Turn
Describe the difference between active transport and passive transport.	Hint	Check your answer.
Describe the kinetic theory of matter.	Hint	Check your answer.
During diffusion of sucrose in water, the direction of movement for individual sucrose molecules is .	Hint	Check your answer.
During diffusion of sucrose in water, the net movement of sucrose is .	Hint	Check your answer.
Diffusion of a substance across the plasma membrane of a cell with the help of specific transport proteins is called .	Hint	Check your answer.
Diffusion of a substance without the help of specific transport proteins is called .	Hint	Check your answer.
Diffusion of water across a selectively permeable membrane is called .	Hint	Check your answer.
Diffusion of solutes across a selectively permeable membrane is called .	Hint	Check your answer.
Net movement of a substance from a region where it has a higher concentration to a region where it has a lower concentration, due to random molecular motion, is called .	Hint	Check your answer.

Close this browser window to return to Blackboard and complete the practice quiz and assessment quiz.