Chapter 07: Membrane Structure and Function

Introduction

The plasma membrane is a fundamental component of all living cells, serving as a selective barrier that regulates the movement of substances into and out of the cell. Understanding its structure and function is essential for comprehending many cellular processes, including transport, communication, and homeostasis.

Structure of the Plasma Membrane

Phospholipid Bilayer

• Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• In aqueous environments, phospholipids spontaneously arrange into a bilayer, with hydrophobic tails facing inward and hydrophilic heads facing outward toward water.

• This bilayer forms the basic structure of the plasma membrane, providing a semi-permeable barrier.

• Example: The classic "fluid mosaic model" describes the membrane as a dynamic structure with proteins and other molecules embedded within or attached to the bilayer.

Membrane Proteins

• Integral proteins penetrate the hydrophobic core of the bilayer and often span the membrane (transmembrane proteins).

• Peripheral proteins are loosely bound to the surface of the membrane, often attached to integral proteins or phospholipids.

• Membrane proteins perform various functions, including transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.

• Example: Aquaporins are channel proteins that facilitate water transport across the membrane.

Carbohydrates in the Membrane

• Carbohydrates are attached to proteins (glycoproteins) or lipids (glycolipids) on the extracellular surface of the membrane.

• They play a key role in cell-cell recognition and communication.

Membrane Fluidity

Factors Affecting Fluidity

• Temperature: Higher temperatures increase membrane fluidity, while lower temperatures decrease it, potentially causing the membrane to solidify.

• Fatty Acid Composition: Membranes rich in unsaturated fatty acids are more fluid than those with saturated fatty acids, due to kinks in unsaturated tails preventing tight packing.

• Cholesterol: In animal cells, cholesterol acts as a "fluidity buffer," reducing fluidity at moderate temperatures and preventing solidification at low temperatures.

• Example: Fish living in cold environments have membranes with a higher proportion of unsaturated fatty acids to maintain fluidity.

Adaptations in Membrane Lipid Composition

• Organisms adapt their membrane lipid composition in response to environmental conditions.

• Plants like winter wheat increase unsaturated phospholipids in autumn to prevent membranes from solidifying in cold temperatures.

The Fluid Mosaic Model

The fluid mosaic model describes the plasma membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. Proteins are not randomly distributed; they often form specialized patches that carry out specific functions.

• Key Features: Lateral movement of lipids and proteins, occasional "flip-flop" of phospholipids, and dynamic organization.

• Experimental Evidence: Fusion of mouse and human cells shows rapid intermixing of membrane proteins, supporting membrane fluidity.

Membrane Transport

Selective Permeability

• The plasma membrane allows some substances to cross more easily than others.

• Hydrophobic (nonpolar) molecules, such as hydrocarbons, O2, and CO2, can dissolve in the lipid bilayer and pass through rapidly.

• Hydrophilic (polar) molecules, such as ions and sugars, pass through slowly or require transport proteins.

Types of Transport

• Passive Transport: Movement of substances across the membrane without energy input. Includes diffusion, osmosis, and facilitated diffusion.

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

• Bulk Transport: Movement of large molecules or particles via vesicles (endocytosis and exocytosis).

Passive Transport

Diffusion

• Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration (down their concentration gradient).

• At equilibrium, molecules move across the membrane in both directions at equal rates.

• Equation: Fick's Law of Diffusion:

Osmosis

• Osmosis is the diffusion of free water across a selectively permeable membrane.

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

• Example: If a cell is placed in a solution with higher solute concentration, water will leave the cell, causing it to shrink.

Tonicity

• Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water.

• Isotonic Solution: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypertonic Solution: Solute concentration is higher outside the cell; cell loses water and shrivels.

• Hypotonic Solution: Solute concentration is lower outside the cell; cell gains water, swells, and may burst (lyse).

Facilitated Diffusion

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

• Channel proteins provide hydrophilic pathways for specific molecules or ions (e.g., aquaporins for water).

• Carrier proteins undergo conformational changes to move solutes across the membrane.

Active Transport

Primary Active Transport

• Uses energy from ATP to move ions or molecules against their concentration gradient.

• Sodium-potassium pump (Na+/K+ ATPase): Moves 3 Na+ ions out and 2 K+ ions into the cell per ATP hydrolyzed, maintaining electrochemical gradients.

• Equation: (energy released drives the pump)

Membrane Potential and Electrogenic Pumps

• Membrane potential is the voltage difference across a membrane, created by the unequal distribution of ions.

• Electrogenic pumps generate membrane potential (e.g., sodium-potassium pump in animals, proton pump in plants, fungi, and bacteria).

• The electrochemical gradient combines the chemical gradient (ion concentration) and electrical gradient (membrane potential).

Secondary Active Transport (Cotransport)

• Uses the energy stored in the electrochemical gradient of one molecule (often H+ or Na+) to drive the transport of another molecule against its gradient.

• Example: In plants, a proton pump creates an H+ gradient, which is used to drive the uptake of sucrose via a cotransporter.

Bulk Membrane Transport

Exocytosis

• Process by which cells export large molecules (e.g., proteins, polysaccharides) by fusing vesicles with the plasma membrane.

• Example: Pancreatic cells secrete insulin via exocytosis.

Endocytosis

• Process by which cells import large molecules by engulfing them in vesicles formed from the plasma membrane.

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

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

• Receptor-mediated endocytosis: Specific molecules are taken up after binding to receptors on the cell surface.

• Example: Uptake of cholesterol via LDL receptors.

Table: Types of Endocytosis

Clinical Connection: Receptor-Mediated Endocytosis and Health

• Defects in receptor-mediated endocytosis can lead to diseases such as familial hypercholesterolemia, where defective LDL receptors cause cholesterol buildup in the blood, increasing the risk of heart disease and stroke.