In this unit, students look at the signal transduction pathways that let cells sense and respond to their external and internal environments. The model organism is the free-living unicellular algae, Chlamydomonas reinhardtii. Chlamydomonas is extremely common in freshwater worldwide and can also live in soil, either in a nonmotile form or swimming in the water between soil grains. Students are assigned review articles to read that summarize general knowledge of the signaling pathways that control the behavior of Chlamydomonas. Using these review articles, students develop a hypothesis, then design and execute an experiment to test during the second week of the unit.