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2.8 Mechanisms of Transport

Syllabus
2025
Topic
2.8
Level
—

Learning objectives

2.8A—Describe the processes that allow ions and other molecules to move across membranesDescribe the processes that allow ions and other molecules to move across membranes.• Metabolic energy (such as that from ATP) is required for active transport of molecules and ions across the membrane and to establish and maintain electrochemical gradients.- i. Membrane proteins are necessary for active transport.- ii. The Na++/K pump and A TPase contribute to the maintenance of the membrane potential.

Active Transport Builds Electrochemical Gradients

Active transport uses metabolic energy and membrane proteins to move ions or molecules in a direction that passive transport alone would not sustain. Because ions cannot freely cross the hydrophobic bilayer, the transport protein provides both a pathway and a mechanism for energy-coupled movement.

  1. ATP supplies energy to an active-transport protein such as an ATPase.
  2. The protein moves selected ions across the membrane.
  3. Unequal ion concentration and charge develop across the membrane.
  4. This electrochemical gradient helps establish or maintain membrane potential.

An electrochemical gradient combines two influences on an ion: its concentration difference and its attraction or repulsion by charge. The Na⁺/K⁺ pump and ATPase activity help maintain these unequal ion distributions and therefore the membrane potential.

Once an ion gradient exists, movement of that ion down its gradient can be coupled to movement of another substance. For example, local question evidence uses a sodium gradient to drive glucose cotransport. The ATP-powered pump establishes the gradient; the coupled transporter uses the gradient's stored potential.

Do not say that every transport protein directly uses ATP. ATP directly powers active pumps; another transporter may instead use an electrochemical gradient that those pumps previously established.