Membrane Structure and Properties: Study Notes for General Biology

Membrane Structure and Properties

Introduction

The plasma membrane is a fundamental feature of all living cells, serving as a selective barrier that separates life from nonlife. Its structure and properties are essential for maintaining cellular integrity, regulating transport, and facilitating chemical reactions necessary for life.

Phospholipids and Membrane Formation

Phospholipid Structure

• Phospholipids consist of a glycerol backbone linked to a phosphate group and two hydrocarbon chains.

• Fatty acid tails are found in Bacteria and Eukarya; isoprenoid tails are found in Archaea.

• The primary role of phospholipids is to form cell membranes.

Amphipathic Nature

• Amphipathic molecules have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

• Example: Cholesterol is also amphipathic.

• The amphipathic nature of phospholipids drives the spontaneous formation of the plasma membrane.

Phospholipid Bilayers

• Amphipathic lipids do not dissolve in water; hydrophilic heads interact with water, hydrophobic tails do not.

• Phospholipids form two main structures in water:

  • Micelles: Tiny spherical aggregates, form from free fatty acids.

  • Lipid bilayers: Created when lipid molecules align in paired sheets, forming spontaneously in water.

Membrane Permeability and Selectivity

Selective Permeability

• Phospholipid bilayers allow small or nonpolar molecules to move across quickly.

• Charged or large polar substances cross slowly, if at all.

• Examples:

  • Oxygen (O2): Small nonpolar molecule, moves quickly across bilayer.

  • Glucose: Large polar molecule, moves slower across bilayer.

Permeability Ranking Table

Substance	Relative Permeability
Oxygen (O2)	Most permeable
Water (H2O)	Second most permeable
Glucose	Third most permeable
Sodium ion (Na+)	Least permeable

Factors Affecting Membrane Permeability

• Length of hydrocarbon tails: Longer tails decrease permeability.

• Saturation of hydrocarbon tails:

  • Saturated tails pack tightly, decreasing permeability.

  • Unsaturated tails (with double bonds) create kinks, increasing permeability.

• Presence of cholesterol: Increases density of hydrophobic section, reducing permeability.

Bond Saturation and Chain Length Table

Bilayer Type	Permeability
Short, unsaturated tails	Higher permeability
Long, saturated tails	Lower permeability

Fatty Acids and Physical State

• Saturated fatty acids: No double bonds, solid at room temperature.

• Unsaturated fatty acids: One or more double bonds, liquid at room temperature.

• Foods with unsaturated lipids (e.g., vegetable oil) are liquid; saturated lipids (e.g., butter) are solid.

Cholesterol's Role

• Cholesterol increases the density of the hydrophobic section of the membrane.

• Reduces membrane permeability by packing phospholipid tails closer together.

Temperature Effects

• Membrane fluidity decreases as temperature drops.

• Molecules in bilayer move more slowly; hydrophobic tails pack together more tightly.

• Decreased fluidity leads to decreased permeability.

Example: Adaptation in Fish

• Arctic fish (cold environment): More unsaturated fatty acids, less cholesterol for increased fluidity and permeability.

• Tropical fish (warm environment): More saturated fatty acids, more cholesterol for decreased fluidity and permeability.

Transport Across Membranes

Diffusion

• Diffusion: Spontaneous movement of molecules and ions due to thermal energy.

• Occurs down a concentration gradient (from high to low concentration).

• Equilibrium: Molecules are randomly distributed, but no net movement occurs.

• Passive transport: Diffusion across membrane without energy input.

Osmosis

• Osmosis: Special case of diffusion; water moves across selectively permeable membranes.

• Water moves from regions of low solute concentration to high solute concentration, diluting the higher concentration.

• Equalizes concentration on both sides of the bilayer.

Osmosis Table

Solution Type	Effect on Cell
Isotonic	No net water movement; cell remains unchanged
Hypotonic	Water enters cell; cell may burst (lyse)
Hypertonic	Water leaves cell; cell shrivels

Osmosis in Red Blood Cells

• Red blood cells in isotonic plasma remain unchanged.

• Placed in pure water (hypotonic): Water rushes in, cell swells and may burst.

Osmosis and Vesicles

• Osmosis can shrink or burst membrane-bound vesicles depending on the solute concentration inside and outside.

• Hypertonic solution: Water leaves vesicle, vesicle shrinks.

• Hypotonic solution: Water enters vesicle, vesicle swells or bursts.

• Isotonic solution: No net flow, vesicle remains unchanged.

Membrane Transport Mechanisms

Types of Membrane Transport

• Passive transport: Small molecules move without energy, may involve transport proteins.

• Active transport: Requires energy (ATP) and transport proteins to move molecules against concentration gradients.

• Bulk transport:

  • Exocytosis: Large molecules exit cell via vesicles fusing with plasma membrane.

  • Endocytosis: Large molecules enter cell as plasma membrane pinches inward, forming vesicles.

Summary Table: Membrane Transport Types

Transport Type	Energy Required?	Example
Passive (diffusion, osmosis)	No	O2 diffusion, water osmosis
Active	Yes	Na+/K+ pump
Bulk (endo/exocytosis)	Yes	Hormone secretion, nutrient uptake

Key Terms and Definitions

• Phospholipid: Amphipathic molecule forming the basic structure of cell membranes.

• Amphipathic: Having both hydrophilic and hydrophobic regions.

• Selective permeability: Property of membranes allowing some substances to cross more easily than others.

• Diffusion: Movement of molecules from high to low concentration.

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

• Isotonic: Solution with equal solute concentration as the cell.

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

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

Equations

• Diffusion Rate Equation: $J = -D \frac{dC}{dx}$ Where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Additional info:

• Membrane fluidity and permeability are crucial for cell survival and adaptation to environmental changes.

• Transport proteins and aquaporins facilitate movement of specific molecules across membranes.