Membrane Structure and Function: Study Notes for General Biology

Membrane Structure and Function

Introduction

The plasma membrane is a fundamental structure in all living cells, serving as a barrier that separates the internal environment of the cell from the external surroundings. Its unique composition and properties allow it to regulate the movement of substances in and out of the cell, maintaining homeostasis and enabling communication with the environment.

Plasma Membrane: Structure and Properties

• Definition: The plasma membrane is a selectively permeable boundary that encloses the cell, controlling the passage of materials.

• Selective Permeability: Only certain substances can cross the membrane freely, while others require assistance or cannot cross at all.

• Phospholipid Bilayer: The main structural component of the membrane, consisting of two layers of phospholipids.

• Phospholipids: Amphipathic molecules with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Fluid Mosaic Model: Describes the membrane as a dynamic structure with proteins and other molecules embedded in or attached to a fluid phospholipid bilayer.

Example: The plasma membrane of animal cells separates the cytoplasm from the extracellular fluid, allowing the cell to maintain a stable internal environment.

Phospholipid Bilayer

• Hydrophilic Heads: Face outward toward the aqueous environments inside and outside the cell.

• Hydrophobic Tails: Face inward, away from water, forming the interior of the membrane.

• Arrangement: This orientation creates a semi-permeable barrier that restricts the passage of most water-soluble substances.

Example: The bilayer structure is visible in electron micrographs and is essential for membrane function.

Membrane Fluidity

• Fluidity: The membrane is not rigid; lipids and proteins can move laterally within the layer.

• Temperature Effects: Lower temperatures decrease fluidity, making the membrane more rigid.

• Fatty Acid Composition: Membranes rich in unsaturated fatty acids are more fluid than those with saturated fatty acids.

• Cholesterol: Acts as a fluidity buffer, increasing fluidity at low temperatures and decreasing it at high temperatures.

Additional info: Organisms can adjust membrane lipid composition in response to environmental temperature changes to maintain optimal fluidity.

Membrane Proteins

• Integral Proteins: Span the membrane and are involved in transport, signal transduction, and cell recognition.

• Peripheral Proteins: Loosely attached to the membrane surface, often involved in signaling or maintaining cell shape.

• Transmembrane Proteins: A type of integral protein that completely crosses the membrane, with regions exposed on both sides.

Functions of Membrane Proteins

• Transport: Move substances across the membrane (channels and carriers).

• Enzymatic Activity: Catalyze chemical reactions at the membrane surface.

• Signal Transduction: Relay signals from outside to inside the cell.

• Cell-Cell Recognition: Allow cells to identify each other, important in immune response.

• Intercellular Joining: Connect adjacent cells.

• Attachment: Anchor the membrane to the cytoskeleton and extracellular matrix.

Membrane Carbohydrates

• Location: Found on the extracellular surface of the plasma membrane.

• Function: Play a key role in cell-cell recognition and communication.

• Glycolipids and Glycoproteins: Carbohydrates covalently bonded to lipids or proteins.

Selective Permeability of Membranes

• Permeable to: Nonpolar molecules (e.g., lipids, O2, CO2), small uncharged molecules, and some gases.

• Impermeable to: Large polar molecules (e.g., sugars, polysaccharides) and ions (e.g., K+, Na+).

• Transport Proteins: Facilitate the movement of hydrophilic substances across the membrane.

Types of Membrane Transport

1. Passive Transport

• Diffusion: Movement of molecules from an area of higher concentration to lower concentration, down their concentration gradient.

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

• Facilitated Diffusion: Passive movement of molecules via transport proteins (channels or carriers).

2. Active Transport

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

• Ion Pumps: Transport proteins that move ions, creating a membrane potential (voltage difference across the membrane).

• Cotransport: Active transport of one solute indirectly drives the transport of another solute.

3. Bulk Transport

• Exocytosis: Movement of large molecules out of the cell via vesicles.

• Endocytosis: Movement of large molecules into the cell by engulfing them with the plasma membrane.

• Types of Endocytosis:

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

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

  • Receptor-mediated Endocytosis: Uptake of specific molecules via receptor proteins.

Osmosis and Tonicity

• Osmosis: Water moves from areas of low solute concentration to high solute concentration.

• Tonicity: The ability of a solution to cause a cell to gain or lose water.

Solution Type	Animal Cell	Plant Cell
Hypotonic	Lysed (bursts)	Turgid (normal)
Isotonic	Normal	Flaccid
Hypertonic	Shriveled	Plasmolyzed

Key Equations

• Diffusion: No energy required; molecules move down their concentration gradient.

• Osmosis: Water moves to balance solute concentrations across the membrane.

• Membrane Potential: $V_m = V_{inside} - V_{outside}$

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Bulk Transport	Yes	Varies	Endocytosis, exocytosis