Cells: The Living Units

Introduction

Cells are the fundamental units of structure and function in the human body. Understanding how substances move across the cell membrane is essential for comprehending cellular physiology and the maintenance of homeostasis.

Active Membrane Transport

Overview of Active Membrane Transport Processes

• Active transport and vesicular transport are processes that require cellular energy (usually ATP) to move substances across the plasma membrane.

• These processes are necessary when solutes are:

  • Too large for membrane channels

  • Not lipid soluble

  • Unable to move down their concentration gradient

Active Transport

Active transport is the movement of solutes against their concentration gradient (from low to high concentration) using energy, typically in the form of ATP.

• Requires carrier proteins (also called solute pumps).

• Carrier proteins bind specifically and reversibly to the substances they transport.

• Some carrier proteins can transport more than one substance at a time:

  • Antiporters: Transport one substance into the cell while moving a different substance out.

  • Symporters: Transport two different substances in the same direction across the membrane.

• Active transport always requires energy (ATP).

Types of Active Transport

• Primary active transport: Energy is derived directly from the hydrolysis of ATP.

• Secondary active transport: Energy is derived indirectly from ionic gradients created by primary active transport.

Primary Active Transport

In primary active transport, the hydrolysis of ATP causes a conformational change in the transport protein, allowing solutes (usually ions) to be pumped across the membrane.

• Examples of primary active transport pumps include:

  • Calcium pumps

  • Hydrogen (proton) pumps

  • Sodium-potassium (Na+-K+) pumps

Example: Sodium-Potassium Pump (Na+-K+ ATPase)

• The most studied active transport pump.

• Pumps three Na+ ions out of the cell and two K+ ions into the cell per ATP molecule hydrolyzed.

• Located in all plasma membranes, especially active in excitable cells such as nerves and muscles.

• Maintains electrochemical gradients essential for functions such as nerve impulse transmission and muscle contraction.

Equation:

Summary Table: Types of Active Transport

Example Application

• The Na+-K+ pump is crucial for maintaining the resting membrane potential in nerve and muscle cells, enabling the propagation of electrical signals.

Additional info: Secondary active transport often couples the movement of one solute down its gradient (e.g., Na+ moving into the cell) to the uphill transport of another solute (e.g., glucose or amino acids).