Membrane Biochemistry: Structure and Transport Mechanisms

Membrane Biochemistry

Introduction

This section covers the structure and function of cell membranes, focusing on the fluid mosaic model, selective permeability, and the various mechanisms by which substances are transported across biological membranes. Understanding these concepts is fundamental to cell biology and physiology.

Fluid Mosaic Model of Cell Membranes

Structure and Function

• Fluid Mosaic Model: Describes the cell membrane as a dynamic structure composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. The 'fluid' aspect refers to the lateral movement of lipids and proteins within the layer, while 'mosaic' refers to the patchwork of proteins that float in or on the fluid lipid bilayer.

• Phospholipid Bilayer: The fundamental structure of the membrane, consisting of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Membrane Proteins: Include integral proteins (span the membrane) and peripheral proteins (attached to the surface). These proteins serve as channels, carriers, receptors, and enzymes.

• Carbohydrates: Often attached to proteins (glycoproteins) or lipids (glycolipids), playing roles in cell recognition and signaling.

• Cholesterol: Interspersed within the bilayer, cholesterol modulates membrane fluidity and stability.

Example: The sodium-potassium pump is an integral membrane protein that helps maintain cellular ion balance.

Selective Permeability of Membranes

Definition and Importance

• Selective Permeability: The property of biological membranes that allows some substances to pass through while restricting others. This is essential for maintaining homeostasis.

• Factors Affecting Permeability: Size, polarity, and charge of molecules; presence of transport proteins.

• Transport Proteins: Facilitate the movement of specific molecules across the membrane.

Example: Oxygen and carbon dioxide diffuse freely across the membrane, while ions require specific channels.

Passive Transport Mechanisms

Types and Characteristics

• Diffusion: Movement of molecules from an area of higher concentration to lower concentration without energy input.

• Facilitated Diffusion: Movement of molecules across membranes via specific transport proteins, still down their concentration gradient and without energy input.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Tonicity: Describes the effect of a solution on cell volume (isotonic, hypotonic, hypertonic).

Equation:

$J = -P (C_2 - C_1)$ Where $J$ is the flux, $P$ is the permeability coefficient, and $C_2 - C_1$ is the concentration gradient.

Example: Glucose enters red blood cells via facilitated diffusion through GLUT transporters.

Active Transport Mechanisms

Types and Characteristics

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).

• Primary Active Transport: Direct use of ATP to transport molecules (e.g., sodium-potassium pump).

• Secondary Active Transport: Uses the energy from the movement of one molecule down its gradient to drive another molecule against its gradient (e.g., symporters and antiporters).

Equation:

$\Delta G = \Delta G^0 + RT \ln \frac{[C]_2}{[C]_1}$ Where $\Delta G$ is the free energy change, $R$ is the gas constant, $T$ is temperature, and $[C]_2/[C]_1$ is the concentration ratio across the membrane.

Example: The sodium-glucose symporter uses the sodium gradient to import glucose into cells.

Bulk Transport: Endocytosis and Exocytosis

Mechanisms and Functions

• Endocytosis: The process by which cells engulf large particles or fluids. Includes phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis (specific uptake via receptors).

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

• Phagocytosis: Engulfment of large particles or microorganisms by the cell.

• Pinocytosis: Uptake of extracellular fluid and dissolved solutes.

• Receptor-Mediated Endocytosis: Specific uptake of molecules based on receptor-ligand interactions.

Example: White blood cells use phagocytosis to ingest bacteria; neurotransmitter release occurs via exocytosis.

Comparison of Transport Mechanisms

Summary Table

The following table compares key features of passive and active transport mechanisms:

Transport Type	Energy Required?	Direction Relative to Gradient	Examples
Simple Diffusion	No	Down	O2, CO2
Facilitated Diffusion	No	Down	Glucose via GLUT
Osmosis	No	Down (water potential)	Water
Active Transport	Yes (ATP)	Against	Na+/K+ pump
Secondary Active Transport	Indirect (uses gradient)	Against (for one molecule)	Na+/glucose symporter
Endocytosis/Exocytosis	Yes (ATP)	Bulk transport	Phagocytosis, neurotransmitter release

Key Terms and Definitions

• Selective permeability: Ability of the membrane to allow some substances to pass while blocking others.

• Integral proteins: Proteins embedded within the lipid bilayer.

• Peripheral proteins: Proteins attached to the membrane surface.

• Osmosis: Diffusion of water across a membrane.

• Tonicity: The effect of a solution on cell volume.

• Hypotonic: Solution with lower solute concentration than the cell; cell swells.

• Hypertonic: Solution with higher solute concentration than the cell; cell shrinks.

• Isotonic: Solution with equal solute concentration; no net water movement.

• Facilitated diffusion: Passive transport via membrane proteins.

• Active transport: Energy-dependent movement against a gradient.

• Endocytosis: Uptake of materials via vesicles.

• Exocytosis: Release of materials via vesicles.

• Receptor-mediated endocytosis: Specific uptake using membrane receptors.

• Phagocytosis: Engulfment of large particles.

• Pinocytosis: Uptake of fluid and solutes.

• Gated channels: Ion channels that open or close in response to stimuli.

• Electrochemical gradient: Combined effect of concentration and electrical gradients on ion movement.

• Membrane potential: Voltage difference across the membrane.

Additional Resources

• Pearson Modified Mastering Lesson 6 Assignment

• Animations: Functions of the Plasma Membrane, Selective Permeability, Active Transport, Exocytosis and Endocytosis