Notes on Chapter 12: Transport Across the Membrane

Importance of Ions

• Most transport across cell membranes involves ions.
• Ions are crucial for cellular functions and energy storage.

Overview of Membrane Transport

• Cells exchange molecules with their environment.
• Proteins facilitate transport across the hydrophobic lipid bilayer.
  • Channel Proteins: Create pores for specific ions.
  • Transporter Proteins: Move molecules by changing shape.

Sodium-Potassium Pump

• A key example of a transporter protein.
• Pumps sodium out and potassium into the cell.
• Vital for establishing membrane potential and storing energy.

Membrane Potential

• Defined as the voltage difference across the membrane.
• Ions play a critical role in maintaining this potential.
• Concentration gradients and electrical gradients together create an electrochemical gradient.

Transport Mechanisms

Types of Transport Proteins

1. Transporter Proteins
   • Move molecules by changing shape.
   • Can move molecules against concentration gradients (active transport).
2. Channel Proteins
   • Allow passive diffusion of ions following their concentration gradient.
   • No moving parts; purely passive.

Differences Between Transporters and Channels

• Transporters: Require energy, have moving parts, and change shape to transport molecules.
• Channels: Gates can open or close based on concentration gradients, no energy required.

Ion Concentration and Cell Function

• Sodium and potassium are the key ions in cellular transport.
• Sodium is more concentrated outside the cell; potassium is more concentrated inside.
• Importance of maintaining ion gradients for cell survival and function.

Active vs Passive Transport

• Passive Transport: Molecules move along their concentration gradient without energy.
• Active Transport: Requires energy to move molecules against their gradient (e.g., pumps).

Types of Active Transport

1. Coupled Transporters: Use energy from one gradient to push another against its gradient (symport and antiport).
2. ATP-driven Pumps: Use ATP hydrolysis to drive transport.
3. Light-driven Pumps: Use light to drive transport.

Sodium-Potassium Pump Details

• The pump is crucial for maintaining sodium and potassium gradients.
• Uses approximately 30% of a cell's ATP.
• Pumps sodium out (against gradient) and potassium in (against gradient).

Osmosis and Water Movement

• Water moves in response to ion concentrations.
• Maintaining proper ion concentration prevents cell bursting due to osmotic pressure.
• Plants prevent bursting with cell walls; protists use contractile vacuoles.

Gated Ion Channels

• Ion channels can be gated by:
  • Voltage: Controlled by membrane potential changes.
  • Ligands: Molecules that bind to the channel.
  • Mechanical Stress: Physical pressure or stretch.

Fluctuation of Membrane Potentials

• Membrane potentials fluctuate due to the opening and closing of ion channels.
• Changes in membrane potential can trigger cellular responses and serve as electrical signals.

Summary of Lecture

• Transport proteins are essential for moving molecules across membranes.
• Ions play a pivotal role in cellular function, energy storage, and communication.
• The sodium-potassium pump is a critical example of active transport.
• Membrane potential is influenced by ion flow, which affects overall cell physiology.