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Why Circulate?

Why Circulate?

Liquids move and disperse when added to other liquids.
© Baylor College of Medicine\JP Denk.

  • Grades:
  • Length: 60 Minutes


Students investigate and draw conclusions about the movement of dissolved substances, and about the importance of organisms’ internal transport systems.

This activity is from The Science of the Heart and Circulation Teacher's Guide, and was designed for students in grades 6–8. Lessons from the guide may be used with other grade levels as deemed appropriate.

Teacher Background

Have you ever made lemonade and forgotten to stir the mixture? The sweetener and flavoring eventually become distributed within the liquid, but the process, called diffusion, takes time. Diffusion is the random movement of molecules or particles in solution. They bounce against each other, generally moving from regions of higher concentration (where there is more of the dissolved substance) to regions of lower concentration (where there is less of the dissolved substance). Eventually, the mixture becomes evenly distributed. This is the process by which the sweetener and lemonade flavoring become dispersed in the water, even if you don’t stir the mixture.

Single-celled living organisms rely on diffusion to obtain some of the resources necessary for life and to eliminate wastes. It is not a coincidence that almost all unicellular organisms live in water-based environments, where dissolved nutrients are readily available just outside the cell membrane. Single-celled organisms also can move wastes outside the cell membrane into the surrounding water.

What happens in large organisms, such as humans, that consist of many millions of cells? These organisms’ cells are bathed in water, but the cells often are far away from the external environment. Diffusion is not sufficient to provide needed nutrients or to remove waste from distant cells. In addition, most larger, complex organisms carry out important tasks—obtaining nutrients, exchanging gases, removing wastes, etc.—in specialized regions of their bodies (such as the lungs or kidneys in humans). Consequently, most multicellular organisms have specialized systems (such as the circulatory system) to transport nutrients, waste and other materials from one region of the body to another. This activity allows students to investigate the process of diffusion and to consider why many organisms have internal transport systems.

Objectives and Standards

Life Science

  • Living systems at all levels of organization demonstrate the complementary nature of structure and function.

  • All organisms are composed of cells—the fundamental unit of life.

  • Cells carry on many of the functions needed to sustain life. They take in nutrients, which they use to provide energy for the work that cells do and to make the materials that a cell or an organisms needs.

  • The human organism has systems for digestion, respiration, reproduction, circulation, excretion, movement, control and coordination, and for protection from disease.

Physical Science

  • The motion of an object can be described by its position, direction of motion and speed.

Science, Health and Math Skills

  • Measuring

  • Observing

  • Graphing

  • Interpreting data

  • Applying knowledge

Materials and Setup

Materials per Group of Students (see Setup below)

  • 2 sheets of graph paper (0.5-cm grid)

  • Graduated cylinder (100-mL or 250-mL)

  • Lid or bottom of a Petri dish

  • Pencil

  • Small dropper bottle of food coloring (red, blue or green; do not use yellow)

  • Tape

  • Timer, watch or clock

  • Optional: Digital camera for recording observations

Materials per Student

  • Copy of the student sheet (see Lesson pdf)


  1. Place all materials in a central location for each group’s Materials Manager to collect.

  2. Have students work in groups of four.

Procedure and Extensions

  1. Ask students, Have you ever added sugar to lemonade? Follow with questions such as, What did you do after you added the sugar? Was it necessary to stir the mixture? What would happen if you didn’t stir the mixture? Tell students that they will be investigating the movements of a substance when it is dissolved in water.

  2. Have Materials Managers pick up the materials listed above for their groups.

  3. Students will follow the instructions on their student sheets to observe and record the rate at which a drop of food coloring disperses through the water in a Petri dish. A simple way to measure the area reached by the food coloring is to place the dish over a sheet of graph paper before beginning the investigation. Students will make observations every three minutes (or, you may prefer to have students decide upon the frequency of observations). For each observation, students will count the number of squares in which tint from the food coloring is visible. Students should count only every other partial square, or divide the total number of partial squares by two.

  4. Discuss diffusion (the process by which molecules or particles are dispersed randomly through another substance, such as a liquid) with the class. Ask, Based on your observations, do you think diffusion helps to distribute nutrients from one place to another in the body of a living organism, such as an animal? [yes] What are the limitations of diffusion for transporting nutrients and other materials through the body? [very slow, and only moves from regions of higher to lower concentrations] How might organisms transport nutrients more quickly? [with a dedicated transport system, such as the circulatory system in animals]

  5. Have students revisit their concept maps and add any new ideas.

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National Space Biomedical Research Institute

National Space Biomedical Research Institute

This work was supported by National Space Biomedical Research Institute through NASA cooperative agreement NCC 9-58.