B2.1.7 Pump proteins for active transport Notes

Active Transport: The Role of Pump Proteins

1. Cells often need to absorb or expel substances against their concentration gradients, moving particles from a lower to a higher concentration.
2. This process, called active transport, is carried out by specialized pump proteins embedded in the cell membrane.

How Do Pump Proteins Work?

1. Pump proteins are highly specialized and differ from channel proteins in three key ways:
   1. Energy Usage
      1. Pump proteins require energy to function and drive active transport, whereas diffusion through channel proteins is passive and does not use energy.
   2. Directionality
      1. Pump proteins transport particles across the membrane in only one direction.
      2. By contrast, channel proteins allow particles to move in either direction depending on the concentration gradient.
   3. Against the Gradient
      1. Pump proteins typically move particles against their concentration gradient (from low to high concentration).
      2. Facilitated diffusion, in contrast, always occurs down the concentration gradient.

Mechanism of Pump Protein Action

1. Pump proteins operate by switching between two conformations, powered by ATP:
   1. Particle Binding:
      1. In one conformation, the pump protein’s binding site faces the cytoplasm or extracellular space.
      2. This allows the target particle (ion or molecule) to bind.
   2. Energy Use and Conformational Change:
      1. ATP is hydrolyzed (broken down), releasing energy.
      2. This energy alters the pump protein's shape, moving the particle to a central chamber or binding site within the protein.
   3. Transport Across the Membrane
      1. The pump transitions to its second conformation, exposing the particle to the opposite side of the membrane, where it is released.
   4. Resetting the Pump
      1. The protein returns to its original conformation without additional energy, ready to begin the process again.

Example

The sodium-potassium pump (Na⁺/K⁺ pump) in nerve cells moves sodium ions out of the cell and potassium ions in, maintaining electrochemical gradients essential for nerve signaling.

ATP As The Energy Source

1. Most pump proteins rely on ATP (adenosine triphosphate) as their energy source:
   1. ATP Hydrolysis: The breakdown of ATP releases energy, which is harnessed by the pump to change its conformation and move particles across the membrane.
   2. Cellular Respiration: Cells continuously produce ATP during cellular respiration to ensure a steady energy supply for processes like active transport.

Why Is Active Transport Important?

1. Active transport allows cells to maintain concentration gradients that are essential for life.
2. Without it, cells would be unable to:
   1. Absorb vital nutrients against a concentration gradient.
   2. Remove toxic waste products.
   3. Sustain processes like nerve signaling and muscle contraction.