Moving materials across the cell membrane sometimes requires energy, called active transport. This tutorial covers both primary and secondary active transport with good clear animations.

An image of "The Calling of St Peter and St Andrew", created by Jacob Willemsz de Wet the Elderz (Oil on panel, 37 x 52 cm).

A map of Massachusetts Bay and Cape Cod showing the area of early Norse exploration in America. "In 986 A.D. a Norseman named Herjulfson, who was voyaging from Iceland to Greenland, was driven by a gale far out of his reckoning to the W....