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:

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

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:

Then you will conduct one experiment utilizing a living system:

 

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.


 



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