Membrane Channels and Pumps

Introduction

Biological membranes are selectively permeable barriers that regulate the movement of molecules and ions into and out of cells. Specialized proteins embedded in the membrane facilitate this transport, ensuring cellular homeostasis and communication.

Three Classes of Membrane Transporter Proteins

Main Types of Transporters

• Pumps: Proteins that use energy (often from ATP hydrolysis) to move substances against their concentration gradients.

• Carriers: Proteins that bind specific molecules and undergo conformational changes to transport them across the membrane, sometimes using energy indirectly.

• Channels: Proteins that form pores allowing passive movement of ions or molecules down their concentration gradients.

Transport of Molecules Across Membranes: Active and Passive Mechanisms

Simple Diffusion

• Lipophilic molecules can pass through cell membranes by dissolving in the lipid bilayer.

• Simple diffusion is the process by which these molecules move across the membrane from regions of high to low concentration, down their concentration gradient.

Facilitated Diffusion (Passive Transport)

• Facilitated diffusion is the process by which polar molecules are transported across the membrane via specific channels, down their concentration gradient.

• This is a form of passive transport because the energy driving the movement comes from the gradient itself, not from cellular energy sources.

Active Transport

• Active transport moves molecules against their concentration gradient, requiring input of energy (often from ATP hydrolysis).

• This process is essential for maintaining concentration differences of ions across membranes.

Energetic Cost of Solute and Ion Transport Across Membranes

Free-Energy Change and Membrane Potential

• Transport of solutes and ions across membranes imposes an energetic cost, which can be quantified by changes in free energy ().

• Electrochemical potential (membrane potential) is the sum of concentration and electrical terms.

• The free energy required to move a charged species from side 1 to side 2 of a membrane is given by:

• Where Z is the electrical charge of the species, ΔV is the potential voltage across the membrane, F is the Faraday constant (96.5 kJ V-1mol-1), R is the gas constant, and T is temperature in Kelvin.

Active vs. Passive Transport Energetics

• Active transport has a positive free energy change (requires energy input).

• Passive transport has a negative free energy change (spontaneous).

ATP-Driven Membrane Transport Proteins

Role of ATP Hydrolysis

• Most animal cells maintain high intracellular K+ and low Na+ concentrations compared to the external medium.

• Pumps are membrane proteins that catalyze the active transport of ions against their electrochemical gradients, using energy from ATP hydrolysis.

Examples of ATP-Driven Pumps

• Na+-K+ ATPase: Hydrolyzes ATP to generate sodium and potassium gradients across the membrane. Transports 3 Na+ out and 2 K+ in per cycle. Requires Mg2+ as a cofactor.

• P-Type ATPases: Family of ATPases that form a phosphorylated aspartate intermediate. Includes Na+-K+ ATPase, sarcoplasmic reticulum Ca2+ ATPase (SERCA), and gastric H+-K+ ATPase.

Patch Clamp Technique

Measuring Channel Activity

• Patch clamp recording is a method using a glass pipette to create a high-resistance seal with a small patch of cell membrane, allowing measurement of ion conductance through individual channels.

• This technique provides direct evidence of channel opening and closing events, as shown by current traces over time.

Summary Table: Types of Membrane Transport Proteins

Additional info: The notes above expand on the brief points in the slides, providing definitions, examples, and equations relevant to biochemistry students. The table summarizes the main differences between pumps, carriers, and channels for clarity.