Begin this section with Mini Activity: Transport of Nutrients: Exploring Diffusion.
Draw students' attention to the key ideas by using posters and / or overhead transparencies.
Activity 4-1: Making a Capillary Bed Model is the pivotal activity in this section because of the critical role of the capillaries in exchanging nutrients, gases, and wastes. Ask students to use the model to demonstrate the role of capillaries in the diffusion of materials.
Use a video clip to show the flow of blood through capillaries.
Enrichment 4-2: Transport of Materials-Exploring Diffusion helps students learn about the process of diffusion and builds on the concepts presented in Mini Activity: Transport of Nutrients: Exploring Diffusion.
Review the Apply Your Knowledge and the Review Questions.
Review the vocabulary terms listed for this section.
Throughout and at the end of the section refocus students' attention to the key ideas.
Diffusion can be defined as the movement of materials from a region of greater concentration to a region of lesser concentration. The energy required for this movement comes from molecular motion of the materials involved. This energy is available to move materials even in systems above −273∘C. Diffusion will continue as long as there is a difference in concentration and there is molecular motion within the system. Diffusion will stop when equilibrium is reached. But molecular motion continues once equilibrium is reached even though diffusion stops.
Temperature is a net measurement of the molecular motion of the substances in a given system. For example, water molecules in an ice cube vibrate as they would in any solid. The water molecules in liquid water are moving. They move slower at colder temperatures than at warmer ones, and they move randomly. As a gas, water vapor molecules are moving very rapidly and at greater distances from each other.
Altering the rate of molecular motion can change the rate of diffusion. Molecular motion can be increased by adding more energy to the system in the form of heat energy (raising the temperature) or mechanical energy (stirring). Removing heat from the system (cooling) can slow the rate of diffusion.
The rate of diffusion also can be influenced by concentration. For example, students could change the rate of diffusion by increasing or decreasing the dissolved substance added or the amount of liquid in the system.