Membrane Structure and Properties: Study Notes for General Biology

Membrane Structure and Properties

Introduction

The plasma membrane is a fundamental feature of all 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 can form two main structures:

  • Micelles: Tiny spherical aggregates formed 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:

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

  • Saturated tails pack tightly, decreasing fluidity and permeability.

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

Bond Saturation and Chain Length Table

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

Fatty Acids and Membrane Properties

Saturated vs. Unsaturated Fatty Acids

• Saturated fatty acids: No double bonds, straight chains, pack tightly, solid at room temperature.

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

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

Cholesterol's Role

• Cholesterol increases the density of hydrophobic sections in membranes.

• Reduces membrane permeability by forcing phospholipid tails closer together.

Temperature Effects on Membrane Fluidity

Temperature and Fluidity

• Phospholipids move laterally within the bilayer.

• As temperature drops, membrane fluidity decreases:

  • Molecules move more slowly.

  • Hydrophobic tails pack more tightly.

  • Decreased fluidity leads to decreased permeability.

Adaptations in Different Environments

• Arctic fish (cold environment): More unsaturated fatty acids, less cholesterol, membranes remain fluid and permeable.

• Tropical fish (warm environment): More saturated fatty acids, more cholesterol, membranes are less fluid and less permeable.

Movement Across Membranes

Diffusion

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

• Concentration gradient: Difference in solute concentration across a membrane.

• Net movement: From high to low concentration.

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

• Passive transport: Diffusion across membrane without energy input.

Osmosis

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

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

• Dilutes higher concentration of solute, equalizes concentration on both sides.

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

Membrane-Bound Vesicles and Osmosis

Osmosis Effects

• Osmosis can cause vesicles to shrink (in hypertonic solution) or burst (in hypotonic solution).

• Osmotic balance is crucial for cell survival.

Plasma Membrane Transport Mechanisms

Types of 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.

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

Key Terms and Definitions

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

• Amphipathic: Having both hydrophilic and hydrophobic regions.

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

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

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

• Isotonic: Equal solute concentration inside and outside cell.

• Hypotonic: Lower solute concentration outside cell; water enters cell.

• Hypertonic: Higher solute concentration outside cell; water leaves cell.

Key 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.

Summary Table: Factors Affecting Membrane Fluidity and Permeability

Factor	Effect on Fluidity	Effect on Permeability
Unsaturated fatty acids	Increase	Increase
Saturated fatty acids	Decrease	Decrease
Cholesterol	Decrease	Decrease
High temperature	Increase	Increase
Low temperature	Decrease	Decrease

Example Application

• Red blood cells placed in pure water (hypotonic solution) will swell and burst due to osmosis.

• Arctic fish have membranes with more unsaturated fatty acids to maintain fluidity in cold temperatures.

Additional info: These notes expand on the original slides and handwritten content by providing definitions, tables, and equations for clarity and completeness.