Membrane Structure and Transport – Study Notes for General Biology I

Membrane Structure and Transport

Introduction

Cellular membranes are essential structures that define the boundaries of cells and organelles, regulate the movement of substances, and facilitate communication. Understanding their structure and transport mechanisms is fundamental in biology.

Membrane Structure

Phospholipid Bilayers

The primary structure of biological membranes is the phospholipid bilayer. This bilayer forms the basic barrier that separates the cell from its environment.

• Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• In aqueous environments, phospholipids spontaneously arrange themselves into bilayers, with hydrophilic heads facing outward toward water and hydrophobic tails facing inward, away from water.

• This arrangement creates a hydrophobic core that acts as a barrier to most water-soluble substances.

Micelles are another structure formed by amphipathic lipids, but they consist of a single layer of lipids with hydrophobic tails inward and hydrophilic heads outward. However, micelles cannot function as effective boundaries for living cells because they do not form a stable, two-sided barrier like bilayers do.

• Example: The cell membrane of all living cells is a phospholipid bilayer, not a micelle.

Spontaneous Formation

• Phospholipid bilayers form spontaneously in water due to the hydrophobic effect, requiring no external energy input.

• This self-assembly is driven by the tendency of hydrophobic tails to avoid water and hydrophilic heads to interact with water.

Vesicles

• Small, spherical structures surrounded by lipid bilayers are called vesicles.

• Vesicles are important in cellular transport and compartmentalization.

Membrane Permeability

Selective Permeability

Biological membranes are selectively permeable, allowing some substances to cross more easily than others.

• Small, nonpolar molecules (e.g., O2, CO2) cross membranes easily.

• Large, polar molecules and ions (e.g., glucose, Na+, Cl-) cross slowly or not at all without assistance.

Factors Affecting Permeability

• Phospholipid Composition: Bilayers with unsaturated fatty acid tails (containing double bonds and kinks) are more permeable than those with saturated tails (straight chains).

• Cholesterol: The presence of cholesterol in animal cell membranes decreases permeability by filling spaces between phospholipids and stabilizing the membrane.

• Temperature: Higher temperatures increase membrane permeability by increasing the kinetic energy and movement of lipid molecules.

Phase Transition

• At low temperatures, membranes are in a gel phase (less fluid, less permeable).

• Above a certain temperature (the melting temperature, Tm), membranes transition to a liquid crystalline phase (more fluid, more permeable).

Fluidity vs. Permeability

• Membrane fluidity refers to the ease with which lipid and protein molecules move within the bilayer.

• Increased fluidity generally leads to increased permeability.

• Factors that increase fluidity include unsaturated fatty acids and higher temperatures.

Fluid Mosaic Model

Overview

The fluid mosaic model describes the structure of cell membranes as a dynamic combination of lipids and proteins.

• Phospholipids form a fluid bilayer in which proteins are embedded or associated.

• Integral (transmembrane) proteins span the bilayer and have hydrophobic and hydrophilic regions.

• Peripheral proteins are attached to only one side of the membrane.

• Both lipids and proteins can move laterally within the membrane, contributing to its fluid nature.

Experimental Evidence: Cell fusion experiments (e.g., Frye and Edidin, 1970) demonstrated that membrane proteins can move within the bilayer, supporting the fluid mosaic model.

Membrane Transport Mechanisms

Simple Diffusion

• Diffusion is the spontaneous movement of molecules from regions of higher concentration to regions of lower concentration, driven by kinetic energy.

• Small, nonpolar molecules cross membranes by simple diffusion.

• No energy input is required.

Osmosis

• Osmosis is the diffusion of water 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).

Solution Type	Relative Solute Concentration	Effect on Cell
Hypertonic	Higher outside cell	Cell shrinks (water leaves)
Hypotonic	Lower outside cell	Cell swells (water enters)
Isotonic	Equal inside and outside	No net change

Facilitated Diffusion

• Certain membrane proteins (channels and carriers) enable the passive transport of ions and polar molecules that cannot cross the bilayer unaided.

• Transport occurs down the concentration gradient and does not require energy.

• Examples include aquaporins (water channels) and ion channels (e.g., potassium channels).

Active Transport

• Active transport moves molecules or ions against their concentration or electrochemical gradients.

• This process requires energy, usually in the form of ATP.

• Transmembrane proteins called pumps (e.g., Na+/K+-ATPase) hydrolyze ATP to drive transport.

• Active transport is essential for maintaining concentration gradients and cellular homeostasis.

Summary Table: Membrane Transport Mechanisms

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose, ions via channels
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump

Applications and Examples

• Liposomes (artificial vesicles) can be engineered for drug delivery by incorporating specific proteins into their membranes to control transport or targeting.

• Designing transmembrane proteins for liposomes requires understanding of protein structure, hydrophobic/hydrophilic regions, and function (e.g., channels, anchors, transporters).

Additional info: The notes above include expanded explanations, definitions, and context to ensure completeness and clarity for exam preparation.