Membrane Structure and Function: Study Guide

Membrane Structure and Function

Overview of Membrane Structure

The plasma membrane is a thin, flexible boundary that separates the internal environment of the cell from the external environment. It is primarily composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates, forming a dynamic structure known as the fluid mosaic model.

• Fluid Mosaic Model: Describes the membrane as a mosaic of protein molecules drifting in a fluid bilayer of phospholipids.

• Phospholipids: Amphipathic molecules with hydrophilic (polar) heads and hydrophobic (non-polar) tails, forming the bilayer.

• Proteins: Integral and peripheral proteins serve various functions such as transport, signaling, and structural support.

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

• Junctions: Specialized structures that connect adjacent cells (e.g., tight junctions, desmosomes, gap junctions).

Function: The plasma membrane controls the movement of substances into and out of the cell, maintaining homeostasis and enabling communication with the environment.

Amphipathic Nature of Phospholipids

• Amphipathic Molecule: Contains both hydrophilic and hydrophobic regions (e.g., phospholipids).

• Lipid Bilayer: Hydrophilic heads face outward toward aqueous environments; hydrophobic tails face inward, away from water.

• Movement: Phospholipids move laterally within the layer rapidly (about 107 times per second); flip-flop between layers is rare.

• Forces: Hydrophobic interactions hold the membrane together.

Factors Influencing Membrane Fluidity

• Temperature: Higher temperatures increase fluidity; lower temperatures decrease fluidity.

• Lipid Composition: Unsaturated fatty acids increase fluidity (kinks prevent tight packing); saturated fatty acids decrease fluidity.

• Cholesterol: At high temperatures, reduces fluidity by restraining movement; at low temperatures, prevents tight packing, maintaining fluidity.

Additional info: Other factors include the length of fatty acid tails and the presence of glycolipids.

Membrane Proteins

• Integral Proteins: Span the membrane; have hydrophobic regions that interact with the bilayer's core. Often function as transporters or receptors.

• Peripheral Proteins: Loosely bound to the membrane surface; often interact with integral proteins or phospholipid heads. Function in signaling or maintaining cell shape.

Comparison: Integral proteins are embedded within the membrane, while peripheral proteins are attached to the surface. Integral proteins are involved in transport and signaling; peripheral proteins are involved in support and communication.

Functions of Membranes

1. Selective permeability

2. Transport of substances

3. Cell signaling and communication

4. Cell recognition

5. Attachment to the cytoskeleton and extracellular matrix

6. Enzymatic activity

Selective Permeability of Membranes

What Can and Cannot Cross the Membrane

• Cannot Cross Easily:

  1. Ions (e.g., Na+, K+, Cl-) – due to charge

  2. Large polar molecules (e.g., glucose)

  3. Proteins

  4. Nucleic acids

  5. Charged molecules

• Can Cross Easily: Small nonpolar molecules (e.g., O2, CO2), small uncharged polar molecules (e.g., H2O, though water moves slowly without channels).

Driving Force: Movement is driven by concentration gradients (diffusion) and, for ions, electrochemical gradients.

Synthesis and Sidedness of Membranes

Membrane Protein Insertion

• Proteins are synthesized in the rough endoplasmic reticulum (ER), inserted into the membrane, and transported to the plasma membrane via vesicles.

• Orientation is maintained during transport.

Additional info: Glycoproteins and glycolipids are modified in the Golgi apparatus before reaching the membrane.

Transport Across Membranes

Transport Proteins

• Channel Proteins: Provide hydrophilic channels for specific molecules or ions to pass (e.g., aquaporins for water).

• Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane.

Passive Transport

• Movement of substances down their concentration gradient without energy input.

• Includes simple diffusion and facilitated diffusion.

• Energy comes from the inherent kinetic energy of molecules.

Diffusion

• Random movement of molecules from high to low concentration.

• Net movement is directional for a population of molecules.

Key Principle: In the absence of other forces, a substance diffuses from where it is more concentrated to where it is less concentrated, down its concentration gradient.

Osmosis and Tonicity

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

• Water moves from a region of lower solute concentration to higher solute concentration until equilibrium is reached.

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

Term	Definition	Effect on Animal Cell	Effect on Plant Cell
Isotonic	Solute concentration is equal inside and outside	No net water movement; cell remains normal	Flaccid
Hypertonic	Higher solute concentration outside	Cell shrinks (crenates)	Plasmolyzed
Hypotonic	Lower solute concentration outside	Cell swells and may burst (lyse)	Turgid (normal)

Facilitated Diffusion

• Transport proteins speed the passive movement of molecules across the membrane.

• Does not require energy; moves substances down their concentration gradient.

Active Transport

• Moves substances against their concentration gradient.

• Requires energy, usually from ATP hydrolysis.

• Examples: Sodium-potassium pump, proton pump, cotransporters.

Sodium-Potassium Pump

• Maintains high K+ and low Na+ inside the cell.

• Pump uses ATP to move 3 Na+ out and 2 K+ in per cycle.

$\text{ATP} + 3\,\text{Na}^+_{\text{in}} + 2\,\text{K}^+_{\text{out}} \rightarrow \text{ADP} + 3\,\text{Na}^+_{\text{out}} + 2\,\text{K}^+_{\text{in}}$

Proton Pump

• Actively transports H+ ions out of the cell, creating a proton gradient.

• Establishes membrane potential (voltage across the membrane).

Electrochemical Gradient: Combination of concentration gradient and electrical gradient across the membrane; stores potential energy for cellular work (e.g., cotransport).

Cotransport

• Active transport of one solute indirectly drives transport of another solute.

• Example: Sucrose-H+ cotransporter in plants uses the proton gradient to import sucrose.

Vesicular Transport

• Large molecules (e.g., proteins, polysaccharides) cross the membrane in bulk via vesicles.

• Requires energy (active process).

Exocytosis and Endocytosis

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell (e.g., neurotransmitter release, hormone secretion).

• Endocytosis: Cell takes in materials by forming vesicles from the plasma membrane.

  • Phagocytosis: "Cell eating" – ingestion of large particles.

  • Pinocytosis: "Cell drinking" – ingestion of extracellular fluid.

  • Receptor-mediated endocytosis: Specific uptake of molecules via receptor proteins.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction Relative to Gradient	Example
Simple Diffusion	No	Down	O2, CO2
Facilitated Diffusion	No	Down	Glucose via GLUT transporter
Active Transport	Yes (ATP)	Against	Sodium-potassium pump
Cotransport	Indirect (uses gradient)	Against (for one solute)	Sucrose-H+ symporter
Vesicular Transport	Yes	Bulk movement	Exocytosis, endocytosis