Chapter 7nMembrane Structure and Function-REVIEW

Membrane Structure and Function

Fluid Mosaic Model of Cell Membranes

The fluid mosaic model describes the structure of cell membranes as a dynamic arrangement of phospholipids, proteins, and carbohydrates. The membrane is 'fluid' because its components can move laterally within the layer, and 'mosaic' because of the patchwork of proteins embedded in or attached to the bilayer.

• Phospholipid bilayer: Forms the fundamental structure; amphipathic molecules with hydrophilic heads (facing outward) and hydrophobic tails (facing inward).

• Integral proteins: Span the membrane; involved in transport and signaling.

• Peripheral proteins: Loosely attached to the membrane surface; often involved in signaling or maintaining cell shape.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids); important for cell recognition.

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

• Extracellular matrix (ECM): Network outside animal cells; provides structural support.

• Cytoskeleton: Network inside the cell; anchors membrane proteins and maintains cell shape.

Amphipathic molecules have both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions. In phospholipids, the phosphate head is hydrophilic, and the fatty acid tails are hydrophobic.

Membrane Fluidity

Membrane fluidity is essential for proper function, affecting permeability and the movement of membrane proteins.

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

• Cholesterol: Acts as a 'fluidity buffer.' At high temperatures, it stabilizes the membrane and reduces fluidity; at low temperatures, it prevents tight packing of phospholipids, increasing fluidity.

• Phospholipid composition: Unsaturated fatty acids (with double bonds) increase fluidity due to kinks in the tails; saturated fatty acids (no double bonds) decrease fluidity by allowing tight packing.

Functions of Membrane Proteins

Membrane proteins perform a variety of essential functions:

• Transport: Move substances across the membrane (channels, carriers, pumps).

• Enzymatic activity: Catalyze specific reactions at the membrane surface.

• Signal transduction: Relay signals from outside to inside the cell.

• Cell-cell recognition: Allow cells to identify each other (important in immune response).

• Intercellular joining: Connect adjacent cells (e.g., gap junctions, tight junctions).

• Attachment to cytoskeleton and ECM: Maintain cell shape and stabilize membrane proteins.

Selective Permeability

Selective permeability refers to the membrane's ability to allow some substances to cross more easily than others. This property arises from the lipid bilayer and the specific proteins embedded within it.

• Lipid bilayer: Hydrophobic core prevents passage of ions and polar molecules; small nonpolar molecules (e.g., O2, CO2) diffuse easily.

• Transport proteins: Facilitate movement of ions, polar molecules, and large substances.

Transport Across Membranes

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

Passive Transport

• Diffusion: Movement of molecules from high to low concentration, down their concentration gradient. No energy required. Example: O2 entering a cell.

• Osmosis: Diffusion of water across a selectively permeable membrane. Driven by differences in solute concentration.

• Facilitated diffusion: Passive movement of molecules via transport proteins (channels or carriers). Example: Glucose transport into red blood cells.

Active Transport

• Moves substances against their concentration gradient (from low to high concentration).

• Requires energy, usually from ATP.

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

Comparison Table: Passive vs. Active Transport

Feature	Passive Transport	Active Transport
Energy Required?	No	Yes (usually ATP)
Direction	Down concentration gradient	Against concentration gradient
Examples	Diffusion, osmosis, facilitated diffusion	Sodium-potassium pump, proton pump

Osmosis and Tonicity

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

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

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

• Hypotonic: Lower solute concentration outside the cell; water enters the cell.

Animal cells:

• Isotonic: Normal

• Hypertonic: Cell shrivels

• Hypotonic: Cell may burst (lyse)

Plant cells:

• Isotonic: Flaccid

• Hypertonic: Plasmolyzed (membrane pulls away from cell wall)

• Hypotonic: Turgid (normal, firm)

Key terms: Flaccid (limp), turgid (firm), burst (lysed), plasmolyzed (shrunken cytoplasm).

Bulk Transport Mechanisms

Cells move large molecules or particles across the membrane via bulk transport, which requires energy.

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell. Example: Secretion of neurotransmitters.

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

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

  • Pinocytosis: 'Cell drinking'; cell engulfs extracellular fluid and dissolved solutes.

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

Additional Academic Context

• Concentration gradient: The difference in concentration of a substance across a space or membrane. Movement 'down a concentration gradient' means from high to low concentration.

• Principle driving osmosis: Water moves to dilute higher solute concentrations, seeking equilibrium.

• Predicting water movement: In a U-tube with a selectively permeable membrane, water moves toward the side with higher solute concentration.

• Macromolecule behavior: Hydrophobic molecules (e.g., lipids) integrate into membranes; hydrophilic molecules (e.g., proteins with polar side chains) interact with aqueous environments.

Key Equations

• Osmotic potential (Ψs):

• $\Psi_s = -iCRT$

• Where:

  • i = ionization constant

  • C = molar concentration

  • R = pressure constant

  • T = temperature in Kelvin