Membrane Structure and Function: Passive and Active Transport

Vocabulary and Key Concepts

This section introduces essential terminology for understanding membrane transport processes in cells.

• Tonicity: Refers to the relative concentration of solutes in two solutions separated by a semipermeable membrane. Types include hypotonic, isotonic, and hypertonic.

• Osmosis: The passive movement of water molecules across a selectively permeable membrane.

• Endocytosis: The process by which cells take in substances by engulfing them in a vesicle.

• Exocytosis: The process by which cells expel materials in vesicles that fuse with the plasma membrane.

• Phagocytosis: A type of endocytosis where large particles or cells are engulfed.

• Pinocytosis: A type of endocytosis involving the uptake of fluids and dissolved solutes.

• Hypercholesterolemia: A condition characterized by high levels of cholesterol in the blood, often linked to receptor-mediated endocytosis.

• Passive Transport: Movement of substances across membranes without energy input.

• Active Transport: Movement of substances against their concentration gradient, requiring energy input.

Learning Outcomes

Students should be able to:

• Explain and compare passive and active transport across a selectively permeable membrane.

• Define osmosis and predict cell behavior in solutions of different tonicity.

• Identify and describe types of transport: direct passage, mediated passage, vesicular transport.

• Explain the function and mechanism of specific transport systems, including receptor-mediated endocytosis and LDL receptors.

• Describe the mode of action of specific transport mechanisms, such as pumps and cotransporters.

Traffic Across the Plasma Membrane

Selectively Permeable Membrane

The plasma membrane is selectively permeable, allowing certain substances to cross while restricting others. This property is essential for maintaining cellular homeostasis.

• What can cross? Small nonpolar molecules (e.g., O2, CO2), some small polar molecules, and water can cross directly. Larger or charged molecules require specialized transport mechanisms.

• Mechanisms mediating crossing: Direct passage, protein-mediated transport, and vesicular transport.

Types of Membrane Transport

Transport across the cell membrane occurs via three main mechanisms:

• Direct passage through lipid bilayer: Simple diffusion of small, nonpolar molecules.

• Passage mediated by transport proteins: Includes facilitated diffusion and active transport.

• Vesicular (Bulk) Transport: Movement of large molecules or bulk material via endocytosis and exocytosis.

Passive vs. Active Transport

Comparison of Transport Types

Passive and active transport differ in energy requirements and directionality relative to concentration gradients.

Direct Passage Through Lipid Bilayer

Small, nonpolar molecules such as oxygen and carbon dioxide diffuse directly through the lipid bilayer.

• Simple Diffusion: Movement of molecules from an area of higher concentration to lower concentration.

• Equation:

Where J is the flux, D is the diffusion coefficient, and is the concentration gradient.

Facilitated Diffusion

Transport proteins assist the movement of molecules across the membrane down their concentration gradient.

• Channel proteins: Form pores for specific molecules (e.g., aquaporins for water, ion channels).

• Carrier proteins: Undergo conformational changes to transport molecules.

• No energy required: Movement is passive.

Osmosis and Tonicity

Osmosis

Osmosis is the passive diffusion of water across a selectively permeable membrane.

• Water moves from regions of low solute concentration to high solute concentration.

• Equation:

Where is the water potential, is the solute potential, and is the pressure potential.

Tonicity

Tonicity describes how a solution affects cell volume due to osmosis.

• Hypotonic: Lower solute concentration than the cell; water enters the cell, causing swelling or lysis.

• Isotonic: Equal solute concentration; no net water movement, cell volume remains stable.

• Hypertonic: Higher solute concentration than the cell; water leaves the cell, causing shrinkage.

Example: Red blood cells in a hypotonic solution swell and may burst (lyse), while in a hypertonic solution they shrink (crenate).

Vesicular (Bulk) Transport

Endocytosis and Exocytosis

Large molecules and particles are transported via vesicles in processes requiring energy.

• Endocytosis: Uptake of material into the cell via vesicle formation.

• Phagocytosis: "Cell eating"; engulfment of large particles.

• Pinocytosis: "Cell drinking"; uptake of fluids and dissolved solutes.

• Receptor-mediated endocytosis: Specific uptake of molecules via receptor binding (e.g., LDL cholesterol uptake).

• Exocytosis: Expulsion of materials from the cell via vesicle fusion with the plasma membrane.

Example: Neurotransmitter release at synapses is mediated by exocytosis.

Summary Table: Types of Membrane Transport

Additional info:

• Hypercholesterolemia is often caused by defects in receptor-mediated endocytosis of LDL cholesterol.

• Electrogenic pumps (e.g., Na+/K+ ATPase) create membrane potential by moving ions against their gradients.