Transport Across Membranes: Structure, Function, and Mechanisms

Transport Across Membranes

7.1 Fluid Mosaic Model and Membrane Components

The fluid mosaic model describes the structure of cell membranes as a dynamic arrangement of phospholipids, proteins, and carbohydrates. This model highlights the flexibility and diversity of the membrane's components.

• Components of a Membrane: The main components are phospholipids, proteins (integral and peripheral), cholesterol, and carbohydrates.

• Amphipathic Molecules: Phospholipids are amphipathic, meaning they have both hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. This property is crucial for forming the bilayer structure.

• Membrane Fluidity: Fluidity is influenced by temperature and membrane composition (e.g., the presence of unsaturated fatty acids increases fluidity, while cholesterol modulates fluidity at different temperatures).

• Peripheral vs. Integral Membrane Proteins: Integral proteins span the membrane, while peripheral proteins are attached to the membrane's surface.

• Functions of Membrane Proteins: The six main functions are:

  1. Transport

  2. Enzymatic activity

  3. Signal transduction

  4. Cell-cell recognition

  5. Intercellular joining

  6. Attachment to the cytoskeleton and extracellular matrix (ECM)

• Membrane Carbohydrates: Usually attached to proteins (glycoproteins) or lipids (glycolipids), these play roles in cell recognition and signaling.

Example: The red blood cell membrane contains specific glycoproteins that determine blood type.

7.2 Selective Permeability

Cell membranes are selectively permeable, allowing some substances to cross more easily than others due to their structure.

• Membrane Structure and Selection: The hydrophobic core prevents passage of ions and polar molecules, while small nonpolar molecules can diffuse freely.

• Transport of Hydrophobic Molecules: Small hydrophobic molecules (e.g., O2, CO2) cross membranes by simple diffusion.

• Channel Proteins and Carrier Proteins: Channel proteins form hydrophilic pores for specific molecules or ions, while carrier proteins bind and transport substances across the membrane.

• Aquaporins: Specialized channel proteins that facilitate rapid water transport.

Example: Aquaporins in kidney cells allow efficient water reabsorption.

7.3 Diffusion and Osmosis

Diffusion is the passive movement of molecules from high to low concentration. Osmosis is the diffusion of water across a selectively permeable membrane. Facilitated diffusion uses transport proteins to move substances down their concentration gradient.

• Passive and Spontaneous: These processes do not require energy input.

• Concentration Gradient as Potential Energy: The difference in concentration across a membrane represents stored energy.

• Solution Tonicity:

  • Hypertonic: Higher solute concentration outside the cell; cell loses water.

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

  • Isotonic: Equal solute concentration; no net water movement.

• Regulation of Water Balance: Cells with walls (e.g., plants) resist bursting in hypotonic solutions; animal cells may lyse or shrink depending on the environment.

• Transport Proteins in Facilitated Diffusion: Enable specific molecules (e.g., glucose, ions) to cross membranes efficiently.

Example: Glucose transporters facilitate glucose uptake in muscle cells.

Key Equations:

• Fick's Law of Diffusion: $J = -D \frac{dC}{dx}$

7.4 Active Transport and Electrochemical Gradients

Active transport moves substances against their concentration gradients, requiring energy (usually ATP). This process is essential for maintaining cellular homeostasis.

• Osmosis vs. Facilitated Diffusion vs. Active Transport:

  • Osmosis: Passive movement of water.

  • Facilitated Diffusion: Passive, protein-mediated transport.

  • Active Transport: Requires energy to move substances against gradients.

• Electrochemical Gradient: Created by two forces:

  1. Chemical force (ion concentration difference)

  2. Electrical force (membrane potential)

• Electrogenic Pumps: Transport proteins that generate voltage across membranes. Two examples:

  • Sodium-potassium pump (Na+/K+ ATPase)

  • Proton pump (H+ ATPase)

• Membrane Potential: The voltage difference across a membrane, typically -70 mV in animal cells.

• Cotransport: Coupled transport of two substances; one moves down its gradient, driving the other against its gradient.

Example: The Na+/glucose cotransporter in intestinal cells uses the Na+ gradient to import glucose.

Key Equation:

• Nernst Equation (for equilibrium potential): $E = \frac{RT}{zF} \ln \left( \frac{[\text{ion outside}]}{[\text{ion inside}]} \right)$

7.5 Bulk Transport

Large molecules and particles are transported across membranes via bulk transport mechanisms, which require energy.

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell.

• Phagocytosis: "Cell eating"; the cell engulfs large particles or cells.

• Pinocytosis: "Cell drinking"; the cell takes in extracellular fluid and dissolved solutes.

• Receptor-Mediated Endocytosis: Specific molecules are taken in after binding to receptors on the cell surface.

Example: Cholesterol uptake by cells via receptor-mediated endocytosis of LDL particles.

Key Terms List

• Fluid mosaic model

• Phospholipid bilayer

• Amphipathic

• Integral protein

• Peripheral protein

• Glycoprotein

• Glycolipid

• Selective permeability

• Channel protein

• Carrier protein

• Aquaporin

• Diffusion

• Osmosis

• Facilitated diffusion

• Hypertonic

• Hypotonic

• Isotonic

• Active transport

• Electrochemical gradient

• Electrogenic pump

• Membrane potential

• Cotransport

• Exocytosis

• Phagocytosis

• Pinocytosis

• Receptor-mediated endocytosis

Additional info: Academic context and examples have been added to expand on the original outline and ensure completeness for exam preparation.