Chapter 7: Membrane Structure & Transport

Introduction to Cell Membranes

The cell membrane is a fundamental boundary that separates the living cell from its external environment. It regulates the movement of materials into and out of the cell, maintaining homeostasis and enabling communication with other cells.

• Cell membrane: Also known as the plasma membrane, it is a selectively permeable barrier.

• Selective permeability: Allows some substances to cross more easily than others.

Membrane Structure

Cell membranes are primarily composed of phospholipids, proteins, and carbohydrates, each contributing to the membrane's structure and function.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails. They self-assemble into a bilayer in aqueous environments.

• Bilayer arrangement: Hydrophilic heads face outward toward water, while hydrophobic tails are buried inside, away from water.

• Membrane proteins: Embedded within or attached to the bilayer. Two main types:

  • Integral proteins: Span the membrane, with regions exposed on both sides.

  • Peripheral proteins: Loosely associated with the surface of the membrane.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids) on the external side of the membrane, involved in cell recognition.

Fluid Mosaic Model

The fluid mosaic model describes the dynamic nature of the cell membrane, where lipids and proteins can move laterally within the layer, creating a flexible and varied structure.

• Fluid: Refers to the ability of membrane components to move laterally.

• Mosaic: Indicates the patchwork of proteins embedded in or attached to the lipid bilayer.

• Experimental evidence: Fusion of mouse and human cells shows mixing of membrane proteins over time.

Membrane Fluidity

Membrane fluidity is essential for proper function and is influenced by lipid composition, temperature, and the presence of cholesterol.

• Unsaturated fatty acids: Increase fluidity due to kinks that prevent tight packing.

• Saturated fatty acids: Decrease fluidity due to straight chains that pack tightly.

• Cholesterol: Stabilizes membrane fluidity by preventing extremes (too fluid or too rigid).

• Adaptation: Organisms adjust membrane lipid composition in response to temperature changes (e.g., more unsaturated fatty acids in cold environments).

Selective Permeability

The cell membrane's structure determines its selective permeability, allowing certain substances to pass while restricting others.

• Permeable substances: Small nonpolar molecules (e.g., O2, CO2) and some small polar molecules (e.g., H2O) can cross the membrane.

• Impermeable substances: Ions (e.g., Na+, H+) and large polar molecules (e.g., glucose, amino acids) require transport proteins.

Membrane Transport Mechanisms

Diffusion

Diffusion is the passive movement of particles from an area of high concentration to an area of low concentration, driven by the kinetic energy of molecules.

• Simple diffusion: Movement of small, nonpolar molecules directly through the lipid bilayer.

• Facilitated diffusion: Movement of larger or charged molecules via transport proteins (channel or carrier proteins).

Osmosis and Tonicity

Osmosis is the diffusion of water across a selectively permeable membrane. Tonicity describes the effect of a solution on cell volume.

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

• Hypertonic solution: Higher solute concentration; cells lose water and shrink.

• Hypotonic solution: Lower solute concentration; cells gain water and may burst (animal cells) or become turgid (plant cells).

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

Facilitated Diffusion

Facilitated diffusion uses transport proteins to move substances down their concentration gradient without energy input.

• Channel proteins: Provide hydrophilic corridors for specific molecules or ions.

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

Active Transport

Active transport moves substances against their concentration gradient, requiring energy (usually from ATP).

• Primary active transport: Direct use of ATP to pump ions (e.g., Na+/K+ pump).

• Secondary active transport: Uses the energy from an ion gradient established by primary active transport to move other substances.

• Membrane potential: Voltage across the membrane due to differential distribution of ions; typically, the inside of the cell is negative relative to the outside.

• Electrochemical gradient: Combination of concentration gradient and membrane potential that drives ion movement.

Equation for membrane potential:

Bulk Transport

Bulk transport moves large particles or quantities of substances across the membrane via vesicles, requiring energy.

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

• Endocytosis: Membrane engulfs material to bring it into the cell.

  • Phagocytosis: "Cellular eating" of solid particles.

  • Pinocytosis: "Cellular drinking" of extracellular fluid.

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

Summary Table: Types of Membrane Transport

Key Terms and Definitions

• Phospholipid bilayer: Double layer of phospholipids forming the basic structure of cell membranes.

• Integral protein: Protein embedded within the membrane, often spanning it.

• Peripheral protein: Protein attached to the surface of the membrane.

• Glycoprotein: Protein with carbohydrate attached, involved in cell recognition.

• Glycolipid: Lipid with carbohydrate attached.

• Osmosis: Diffusion of water across a membrane.

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

• Membrane potential: Electrical potential difference across the membrane.

• Electrochemical gradient: Combined effect of concentration gradient and electrical charge.

Examples and Applications

• Gas exchange in lungs: O2 and CO2 diffuse across alveolar membranes by simple diffusion.

• Glucose uptake: Glucose enters cells via facilitated diffusion through carrier proteins.

• Na+/K+ pump: Maintains ion gradients essential for nerve impulse transmission.

• Phagocytosis: White blood cells engulf bacteria via endocytosis.

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard biology curriculum.