Membrane Transport and Intracellular Calcium Regulation Quiz

See Active Transport. Option C is correct because active transport moves a solute against its electrochemical gradient, which means uphill, towards the side where the solute is already more concentrated or where the electrical charge repels it, and this only happens when the movement is coupled to an energy source. The electrochemical gradient combines two forces: the concentration (chemical) gradient and, for charged ions, the electrical gradient set by the membrane potential. Moving a substance against this combined gradient has a positive free energy change (it costs energy), so it cannot happen on its own. The energy can come in two ways. In primary active transport, a pump protein uses energy directly from ATP hydrolysis (splitting ATP to release energy); in secondary active transport, the energy is taken from the electrochemical gradient of another solute, usually sodium. Cells use active transport to build and maintain gradients, for example keeping sodium low and potassium high inside the cell. Memory aid: active means uphill, and uphill needs energy.

• Definition: active transport moves a solute against its electrochemical gradient.
• Energy: the movement must be coupled to an energy source because its free energy change is positive.
• Two types: primary uses ATP hydrolysis directly, while secondary uses the gradient of another solute, usually sodium.