Transport Across Membranes

Overview

Cells regulate the movement of substances across their membranes using several mechanisms. The physical properties of molecules, the nature of the membrane, and the presence of specific transport proteins determine how substances are transported. Understanding these mechanisms is essential for grasping how cells maintain homeostasis and respond to environmental changes.

Simple Diffusion

Characteristics and Determinants

• Definition: Simple diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration, without the involvement of transport proteins.

• Major Determinants:

  • Size: Smaller molecules diffuse more readily across membranes.

  • Polarity: Nonpolar molecules diffuse more easily than polar or charged molecules.

• Example: Oxygen (O2) and carbon dioxide (CO2) diffuse freely across cell membranes.

Osmosis

• Definition: Osmosis is the diffusion of water across a selectively permeable membrane.

• Directional Flow: Water moves from regions of lower solute concentration (hypotonic) to higher solute concentration (hypertonic).

• Terms:

  • Isotonic: Equal solute concentration on both sides of the membrane; no net water movement.

  • Hypotonic: Lower solute concentration outside the cell; water enters the cell.

  • Hypertonic: Higher solute concentration outside the cell; water leaves the cell.

• Aquaporins: Specialized channel proteins that facilitate rapid water movement across membranes.

Facilitated Diffusion

Mechanism and Protein Types

• Definition: Facilitated diffusion is the passive movement of molecules across membranes via specific transport proteins.

• Carrier Proteins: Bind to the solute and undergo conformational changes to transport it across the membrane.

• Channel Proteins: Form hydrophilic pores allowing specific molecules or ions to pass through.

• Analogy to Enzymes: Transport proteins exhibit specificity and saturation kinetics similar to enzymes.

• Example: Glut1 is a carrier protein that facilitates glucose transport.

• Transport Types:

  • Uniport: Transports a single type of molecule.

  • Symport: Transports two different molecules in the same direction.

  • Antiport: Transports two different molecules in opposite directions.

Kinetics Comparison

• Simple Diffusion: Rate increases linearly with solute concentration.

• Facilitated Diffusion: Rate increases rapidly at low concentrations but plateaus (saturates) at higher concentrations due to limited number of transport proteins.

Active Transport

Mechanism and Energy Source

• Definition: Active transport moves molecules against their concentration gradient, requiring energy input.

• Direct Active Transport: Uses ATP directly to drive transport (e.g., Na+/K+ ATPase pump).

• Indirect Active Transport: Uses energy from the movement of another molecule down its gradient (e.g., Na+/glucose transporter).

• Na+/K+ Pump: Maintains membrane potential by pumping Na+ out and K+ into the cell.

• Membrane Potential: The voltage difference across the membrane, typically measured as the charge inside relative to outside.

Energetics of Transport

Equations and Thermodynamics

• Free Energy Change (\( \Delta G \)): Determines whether transport is energetically favorable.

• Interpretation: A negative \( \Delta G \) means the process can occur spontaneously from outside to inside the cell.

Key Equation for Solute Transport:

• R: Gas constant

• T: Temperature in Kelvin

• [S]_{in}, [S]_{out}: Solute concentrations inside and outside the cell

• z: Charge of the solute

• F: Faraday constant

• \( \Delta V \): Membrane potential (voltage difference)

Summary Table: Transport Mechanisms

Comparison of Simple Diffusion, Facilitated Diffusion, and Active Transport

Additional info: Table entries inferred from standard cell biology sources and Table 8-1 reference.