Sperm cells contain a variety of ion channels that control the movement of ions and protons (H+) into and out of the cell (Mannowetz et al., 2013). As a sperm cell moves up the fallopian tube, the CatSper ion channel (right), which controls the movement of calcium ions (Ca2+), is partially activated as a result of alkalinization inside the cell (caused by protons leaving through the Hv1 ion channel) and low levels of progesterone outside the cell. As the sperm gets closer to the egg, the increased levels of progesterone inhibit the Slo1 ion channel, causing potassium ions (K+) to leave the cell. This hyperpolarizes the cell membrane and leads to full activation of the CatSper ion channel. The resulting influx of large numbers of calcium ions leads to hyperactivation of the sperm—the vigorous tail thrashing motion that is a prerequisite of fertilization. Protons and calcium ion can also move through the Ca2+ ATPase transporter (left).