CH 7 Pearson Study Guide from Lecture Outline

Fluid Mosaic Model and Cell Membrane Structure

Overview of the Fluid Mosaic Model

The fluid mosaic model describes the structure of the plasma membrane as a dynamic and flexible layer composed of various molecules, including lipids, proteins, and carbohydrates. This model highlights the fluidity of the lipid bilayer and the mosaic arrangement of proteins embedded within it.

Components of the Cell Membrane

Phospholipids

• Structure: Phospholipids form a bilayer, with hydrophobic (water-repelling) fatty acid tails facing inward and hydrophilic (water-attracting) phosphate heads facing outward.

• Hydrophobic: The fatty acid tails are nonpolar and avoid water.

• Hydrophilic: The phosphate head is polar and interacts with water.

• Amphipathic: Phospholipids have both hydrophobic and hydrophilic regions, allowing them to form bilayers in aqueous environments.

• Function: They create a semi-permeable barrier that separates the cell from its environment and regulates the movement of substances in and out of the cell.

• Example: The plasma membrane of all living cells is primarily composed of a phospholipid bilayer.

Sterols (Cholesterol)

• Structure: Cholesterol molecules are interspersed within the phospholipid bilayer.

• Function: Cholesterol modulates membrane fluidity and stability, preventing the membrane from becoming too rigid or too fluid at different temperatures.

Glycoproteins and Glycolipids

• Structure: Glycoproteins are proteins with carbohydrate chains attached; glycolipids are lipids with carbohydrate chains.

• Function: These molecules are involved in cell recognition, signaling, and adhesion.

• Examples of Glycoprotein Function in Cell Recognition:

  • Blood type antigens on red blood cells

  • Major histocompatibility complex (MHC) proteins in immune response

Extracellular Matrix (ECM)

• Structure: A network of proteins and carbohydrates outside the cell membrane.

• Functions: Provides structural support, mediates cell signaling, and anchors cells in tissues.

Membrane Proteins

Types of Membrane Proteins

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

• Peripheral Proteins: Attached to the membrane surface; involved in signaling and maintaining cell shape.

Functions of Membrane Proteins

• Transport: Channel and carrier proteins move substances across the membrane.

• Enzymatic Activity: Some proteins catalyze reactions at the membrane surface.

• Signal Transduction: Receptor proteins transmit signals from outside to inside the cell.

• Cell Recognition: Glycoproteins serve as identification tags.

• Cell Junctions: Proteins form connections between adjacent cells (e.g., tight junctions, gap junctions).

• Attachment to Cytoskeleton and ECM: Integrins anchor the membrane to internal and external structures.

Factors Affecting Membrane Fluidity

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

• Cholesterol Content: Cholesterol buffers fluidity, making the membrane less fluid at high temperatures and more fluid at low temperatures.

• Fatty Acid Composition: Unsaturated fatty acids increase fluidity; saturated fatty acids decrease it.

Transport Across the Plasma Membrane

Selective Permeability

The plasma membrane allows some substances to cross more easily than others, maintaining homeostasis.

• Molecules that move freely: Small, nonpolar molecules (e.g., O2, CO2), and some small polar molecules (e.g., H2O).

• Molecules unable to cross passively: Large, polar molecules and ions (e.g., glucose, Na+, K+).

Types of Transport

• Passive Transport: Movement of substances down their concentration gradient without energy input.

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

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

Passive Transport Mechanisms

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

• Facilitated Diffusion: Movement of molecules via channel or carrier proteins.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Osmosis and Tonicity

• Aquaporins: Channel proteins that facilitate water movement.

• Water Movement: Water moves from a region of lower solute concentration (hypotonic) to higher solute concentration (hypertonic).

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

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

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

• Importance in Plant Cells:

  • Turgor Pressure: Pressure of the cell membrane against the cell wall; maintains plant rigidity.

  • Plasmolysis: Shrinking of the cytoplasm due to water loss in a hypertonic environment.

• Importance in Animal Cells:

  • Hemolysis: Bursting of red blood cells in a hypotonic solution.

  • Crenation: Shrinking of red blood cells in a hypertonic solution.

Active Transport Mechanisms

• Electrochemical Gradient: Combined effect of concentration and electrical gradients across the membrane.

• Membrane Potential: Voltage difference across the membrane, important for nerve and muscle function.

• Electrogenic Pump: A transport protein that generates voltage across a membrane (e.g., sodium-potassium pump, proton pump).

• Cotransport: Coupled transport of two substances across the membrane (e.g., symport and antiport systems).

Bulk Transport Mechanisms

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

• Endocytosis: Uptake of materials by forming vesicles from the plasma membrane.

• Phagocytosis: "Cell eating"; engulfment of large particles.

• Pinocytosis: "Cell drinking"; uptake of extracellular fluid.

Summary Table: Methods of Transport Across the Membrane

Key Equations

• Fick's Law of Diffusion:

• Where J is the rate of diffusion, D is the diffusion coefficient, and \frac{dC}{dx} is the concentration gradient.

• Osmotic Pressure Equation:

• Where \Pi is osmotic pressure, i is the van 't Hoff factor, M is molarity, R is the gas constant, and T is temperature in Kelvin.

Examples and Applications

• Example of Simple Diffusion: Oxygen entering cells from the bloodstream.

• Example of Active Transport: Sodium-potassium pump maintaining ion gradients in nerve cells.

• Example of Bulk Transport: White blood cells engulfing bacteria via phagocytosis.

Additional info: Some explanations and examples have been expanded for academic completeness and clarity.