Cell Membrane Transport: Passive and Active Mechanisms

Cell Membrane Transport

Introduction

The cell membrane regulates the movement of substances into and out of the cell, maintaining homeostasis. Transport across the membrane can be passive or active, depending on energy requirements and the nature of the substances being moved.

Transport Processes Across the Plasma Membrane

Selective Permeability

The plasma membrane is selectively permeable, allowing only certain molecules to cross. This property is essential for maintaining the internal environment of the cell.

• Passive transport: No energy input is required.

• Active transport: Energy (usually ATP) is required.

Passive Transport

Overview

Passive transport is the movement of substances across the cell membrane without the use of cellular energy. It relies on concentration gradients and includes several mechanisms.

• Simple Diffusion

• Facilitated Diffusion

• Osmosis

Simple Diffusion

Diffusion is the natural movement of molecules from areas of high concentration to areas of low concentration.

• Driven by the random kinetic energy of molecules.

• Rate of diffusion is affected by:

  • Concentration gradient

  • Molecular size

  • Temperature

• Equilibrium is reached when there is no net movement of molecules in one direction.

Example: Movement of oxygen and carbon dioxide across cell membranes.

Facilitated Diffusion

Facilitated diffusion is the passive movement of molecules across the membrane via specific transport proteins.

• Used for polar molecules (e.g., glucose, amino acids) that cannot diffuse through the lipid bilayer.

• Two types:

  • Carrier-mediated facilitated diffusion: Carrier proteins transport specific molecules that are too large for membrane channels.

  • Channel-mediated facilitated diffusion: Channel proteins allow ions and water to pass through water-filled channels.

• Channels are specific based on size and charge of the molecules.

• Types of channels:

  • Leakage channels: Always open.

  • Gated channels: Controlled by chemical or electrical signals.

Example: Movement of ions such as Na+ and K+ through channel proteins.

Osmosis

Osmosis is the passive movement of water (solvent) across a selectively permeable membrane.

• Water moves from areas of low solute concentration (high water concentration) to areas of high solute concentration (low water concentration).

• Occurs through:

  • Lipid bilayer (slowly)

  • Water channels called aquaporins

• Osmolarity measures the concentration of total number of solute particles in solvent.

Example: Water movement in red blood cells placed in different solutions.

Influence of Membrane Permeability on Diffusion and Osmosis

Osmolarity and Water Movement

When solutions of different osmolarities are separated by a membrane permeable only to water, osmosis occurs until equilibrium is reached.

• Low solute side volume increases (high water concentration).

• High solute side volume decreases (low water concentration).

Movement of Water Involves Pressures

• Hydrostatic pressure: Outward pressure exerted on cell side of membrane due to increased volume from osmosis.

• Osmotic pressure: Inward pressure due to tendency of water to be "pulled" into a cell with higher osmolarity.

• Equilibrium is reached when hydrostatic pressure equals osmotic pressure.

Tonicity

Tonicity describes the ability of a solution to change the shape or tone of cells by altering their internal water volume.

Solution Type	Description	Effect on Cell
Isotonic	Same osmolarity as inside the cell	No net water movement; cell shape unchanged
Hypertonic	Higher osmolarity than inside the cell	Water leaves cell; cell shrinks (crenation)
Hypotonic	Lower osmolarity than inside the cell	Water enters cell; cell swells and may burst (lysis)

Active Membrane Transport

Overview

Active transport requires energy (usually ATP) to move solutes against their concentration gradients. It is essential for maintaining cellular homeostasis and function.

• Used when:

  • Solute is too large

  • Solute is not lipid soluble

  • Solute is not able to move down concentration gradient

Primary and Secondary Active Transport

• Primary active transport: Energy from ATP hydrolysis changes the shape of transport protein, pumping solutes across the membrane.

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

Example: Sodium-potassium pump (Na+-K+ ATPase) pumps Na+ out of the cell and K+ into the cell, maintaining electrochemical gradients.

Sodium-Potassium Pump

• Located in all plasma membranes, especially excitable cells (nerves and muscles).

• Maintains resting membrane potential and cell volume.

• Essential for muscle and nerve tissue function.

Vesicular Transport

Overview

Vesicular transport involves the movement of large particles, macromolecules, and fluids across membranes in membrane-bound sacs called vesicles. This process requires cellular energy.

• Endocytosis: Transport into the cell.

• Exocytosis: Transport out of the cell.

Additional Concepts

Membrane Potential and Excitability

• Irritability: Cell's ability to react to a stimulus.

• Depolarization: Change in membrane permeability allowing ion movement; membrane becomes positively charged.

• Action potential: Depolarization of the entire membrane; nerve impulse.

• Repolarization: Return to resting potential.

• Neurotransmitters: Present at synapses; can be excitatory or inhibitory.

• Hyperpolarization: More negative potential; harder to create action potential.

Summary Table: Passive vs. Active Transport

Transport Type	Energy Required?	Direction Relative to Gradient	Examples
Passive	No	Down gradient	Simple diffusion, facilitated diffusion, osmosis
Active	Yes (ATP)	Against gradient	Sodium-potassium pump, endocytosis, exocytosis

Key Equations

• Rate of Diffusion:

  • $\text{Rate} \propto \text{Surface Area} \times \text{Concentration Gradient} \div \text{Distance}$

• Osmotic Pressure:

  • $\Pi = iMRT$

  • Where $\Pi$ = osmotic pressure, $i$ = van 't Hoff factor, $M$ = molarity, $R$ = gas constant, $T$ = temperature

Additional info: These notes expand on the original content by providing definitions, examples, and academic context for each transport mechanism, as well as summarizing key differences and including relevant equations for exam preparation.