Membrane Structure and Function: Study Notes

Membrane Structure & Function

Phospholipid Bilayer & Fluid Mosaic Model

The plasma membrane is a dynamic structure essential for cell integrity and function. It is primarily composed of a phospholipid bilayer with embedded proteins and carbohydrates, described by the fluid mosaic model.

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

• Bilayer Formation: Phospholipids spontaneously arrange with hydrophobic tails inward and hydrophilic heads outward, forming a bilayer.

• Fluid Mosaic Model: The membrane is a fluid structure with a "mosaic" of various proteins embedded or attached to the bilayer.

• Membrane Fluidity:

  • Unsaturated fatty acids increase fluidity.

  • Saturated fatty acids decrease fluidity.

  • Cholesterol modulates fluidity by restraining movement at high temperatures and preventing solidification at low temperatures.

Membrane Proteins & Carbohydrates

Membrane proteins and carbohydrates play critical roles in cell function, communication, and structure.

• Integral Proteins: Span the membrane, often involved in transport and signaling.

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

• Functions of Membrane Proteins:

  • Transport of molecules across the membrane

  • Signal transduction (cell signaling)

  • Cell-cell recognition

  • Intercellular joining

  • Attachment to the cytoskeleton and extracellular matrix (ECM)

• Glycoproteins & Glycolipids: Membrane molecules with carbohydrate chains important for cell recognition.

• Medical Relevance: For example, HIV binds to CD4 and CCR5; individuals lacking CCR5 are resistant to infection.

Membrane Synthesis & Sidedness

Membrane components are synthesized and assembled in a specific, directional manner, resulting in membrane asymmetry.

• Components are synthesized in the endoplasmic reticulum (ER), modified in the Golgi apparatus, and delivered to the plasma membrane via vesicles.

• Membranes are asymmetrical, with distinct inside (cytoplasmic) and outside (extracellular) faces.

Selective Permeability

The plasma membrane controls the movement of substances in and out of the cell, maintaining homeostasis.

• Selective Permeability: Only certain molecules can cross freely; others require transport proteins.

• Small, nonpolar molecules (e.g., O2, CO2) diffuse easily across the membrane.

• Hydrophilic substances (e.g., ions, glucose) require transport proteins (channels or carriers).

• Establishes concentration and electrochemical gradients across the membrane.

Transport Types

Cells use various mechanisms to move substances across membranes, classified as passive or active transport.

• Osmosis: Diffusion of water across a selectively permeable membrane from low to high solute concentration.

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

  • Hypotonic: Water enters the cell; cell may swell.

  • Hypertonic: Water leaves the cell; cell shrinks.

  • Isotonic: No net water movement.

• Facilitated Diffusion: Transport proteins assist hydrophilic substances to cross the membrane (e.g., aquaporins for water, glucose carriers).

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

  • Sodium-Potassium Pump: Moves Na+ out and K+ in, maintaining gradients and membrane potential.

  • Electrochemical Gradient: Combination of concentration and electrical gradients.

  • Co-transport: Active transport indirectly drives another solute's transport (e.g., sucrose-H+ symporter in plants).

• Bulk Transport (Large Molecules): Involves vesicles for moving large substances.

  • Exocytosis: Vesicles fuse with the membrane to export molecules.

  • Endocytosis: Cell takes in materials via vesicles.

    1. Phagocytosis: Cell engulfs large particles.

    2. Pinocytosis: Cell engulfs extracellular fluid.

    3. Receptor-mediated endocytosis: Specific uptake via membrane receptors.

Key Terms and Definitions

• Amphipathic Molecule: Contains both hydrophilic and hydrophobic regions.

• Fluid Mosaic Model: Describes the structure of the plasma membrane as a mosaic of components in a fluid bilayer.

• Integral Protein: Protein embedded within the membrane.

• Peripheral Protein: Protein attached to the membrane surface.

• Glycoprotein/Glycolipid: Membrane molecules with carbohydrate chains for cell recognition.

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

• Passive Transport: Movement of substances across the membrane without energy input.

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

• Osmosis: Diffusion of water across a membrane.

• Tonicity: Effect of a solution on cell water balance.

• Hypotonic/Hypertonic/Isotonic: Terms describing water movement states relative to the cell.

• Facilitated Diffusion: Protein-assisted passive transport.

• Active Transport: Energy-requiring movement against a gradient.

• Sodium-Potassium Pump: Membrane pump for Na+/K+ gradients.

• Electrochemical Gradient: Combined effect of voltage and ion concentration.

• Co-transport: Coupled transport of solutes.

• Exocytosis: Vesicle-mediated export.

• Endocytosis: Vesicle-mediated import.

• Phagocytosis: Engulfing of particles.

• Pinocytosis: Fluid intake.

• Cholesterol: Modulates membrane fluidity.

• Aquaporin: Channel protein for water transport.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose, ions via channels
Osmosis	No	Down water potential gradient	Water via aquaporins
Active Transport	Yes (ATP)	Against gradient	Sodium-potassium pump
Bulk Transport (Exo/Endocytosis)	Yes	Varies	Phagocytosis, exocytosis

Key Equations

• Osmosis (Water Potential): $\Psi = \Psi_s + \Psi_p$ Where $\Psi$ is water potential, $\Psi_s$ is solute potential, and $\Psi_p$ is pressure potential.

• Electrochemical Gradient: $\Delta G = RT \ln \left( \frac{[C]_\text{in}}{[C]_\text{out}} \right) + zF\Delta \Psi$ Where $\Delta G$ is free energy change, $R$ is gas constant, $T$ is temperature, $[C]$ is concentration, $z$ is ion charge, $F$ is Faraday's constant, and $\Delta \Psi$ is membrane potential.