CH 7: Biological Membranes: Structure, Transport, and Cellular Communication

Core Functions and Organization of Biological Membranes

• Cell membranes serve as fundamental organizers and regulators of all cellular processes through two primary mechanisms:

  • Compartmentalization: They provide distinct boundaries to keep biological reactions separate, preventing interference between metabolic pathways.

  • Selective Permeability: They control the influx and efflux of substances, ensuring the internal environment remains stable.

• Membranes are essential for cellular communication:

  • Vesicle transport allows for the movement of molecules within the cell and their release via membrane fusion.

  • Membrane-bound receptors detect external signals and trigger appropriate intracellular responses.

• Membranes are active regulators, not just passive barriers; they ensure communication occurs with spatial and temporal precision for maximum efficiency.

• There are five specific functional roles of biological membranes:

  1. Boundary and Permeability Barrier: The plasma membrane acts as a selective barrier. The phospholipid bilayer prevents the free passage of charged molecules, such as sodium ions ($\text{Na}^+$), which instead require specialized transport proteins.

  2. Organization and Localization of Function: Membranes create specialized internal compartments. Organelles like the Golgi apparatus, Endoplasmic Reticulum (ER), and mitochondria possess distinct chemical environments that support their specific enzymatic functions.

  3. Transport Processes: Membrane proteins regulate the movement of key molecules. For instance, the sodium-potassium pump maintains ion balances necessary for nerve signaling.

  4. Signal Detection: Specific membrane proteins act as receptors for external stimuli (e.g., hormones). Unlike transport proteins, these do not move molecules across the membrane but instead initiate internal signaling cascades.

  5. Cell-to-Cell Interactions: Certain proteins link neighboring cells to allow the direct exchange of materials. This is vital in cardiac cells for synchronized contractions and in plant cells for nutrient distribution.

The Fluid Mosaic Model and Membrane Composition

• The Fluid Mosaic Model describes the membrane as a dynamic, asymmetric assembly of three main components:

  • Lipids: Primarily phospholipids and cholesterol, which provide the structural foundation and regulate fluidity.

  • Proteins: Includes integral and peripheral proteins that facilitate transport and communication.

  • Carbohydrates: Glycolipids and glycoproteins used for cell-to-cell recognition and signaling.

• Membrane Asymmetry (Sidedness):

  • E face (Extracellular side): Rich in glycolipids and glycoproteins for external communication.

  • P face (Cytoplasmic/Protoplasmic side): Contains specific phospholipids involved in internal signaling pathways.

• Synthesis and Maintenance of Sidedness:

  • Membrane asymmetry begins in the ER, where proteins are synthesized and modified with carbohydrate groups to become glycoproteins.

  • Further processing and sorting occur in the Golgi apparatus.

  • Asymmetry is preserved during vesicle transport; the side facing the cytoplasm remains on the inside as the vesicle moves. When the vesicle fuses with the plasma membrane, its internal face becomes the extracellular face of the surface membrane.

Phospholipid Structure and Diversity

• Phospholipids are amphipathic, containing both hydrophilic and hydrophobic regions:

  • Hydrophilic Head: Composed of a phosphate group, glycerol, and a variable head group.

  • Hydrophobic Tails: Consist of two non-polar fatty acid chains.

• Common phospholipid head groups include Phosphatidylcholine, Phosphatidylinositol, and Phosphatidylserine.

• Sphingomyelin is a distinct phospholipid that utilizes sphingosine instead of glycerol as its backbone.

• Composition varies significantly across different membranes and species:

  • Rat liver plasma membranes differ from rat liver mitochondrial membranes.

  • Human myelin sheaths have a distinct profile compared to the plasma membrane of $E. coli$.

• This diversity allows membranes to be specifically tailored to the functional requirements of their respective cells or organelles.

Membrane Fluidity: Movement and Experimental Proof

• Membranes are held together by weak hydrophobic interactions, making them flexible rather than rigid.

• Types of lipid movement within the bilayer:

  • Lateral Diffusion: Side-to-side movement within the same monolayer (highly frequent).

  • Rotation: Spinning in place, which facilitates proper molecular packing.

  • Transverse Diffusion ("Flip-Flop"): Switching between the two monolayers. This is rare and energetically unfavorable because the polar head must pass through the hydrophobic core.

• Experimental Evidence for Protein Movement:

  • Mouse-Human Cell Fusion: Researchers fused mouse cells (labeled with green fluorescent markers) and human cells (labeled with red markers) into a heterokaryon. Initially, the colors were separate, but after 40 minutes, the proteins were completely intermixed, proving lateral mobility.

• FRAP Experiment (Fluorescence Recovery After Photobleaching):

  • Method: Membrane molecules are labeled with fluorescent dye. A high-intensity laser "bleaches" a small spot, turning it dark. Over time, the spot disappears as fluorescent molecules diffuse into the bleached area.

  • Findings: This confirms that lipids and many proteins move laterally. However, some proteins are immobile because they are anchored to the Extracellular Matrix (ECM) or cell junctions.

Regulatory Factors of Membrane Fluidity

• Fatty Acid Saturation:

  • Unsaturated fatty acids contain cis double bonds, creating "kinks" that prevent tight packing, leading to higher fluidity.

  • Saturated fatty acids lack double bonds, allow for close packing, and result in a more viscous (rigid) membrane.

  • Trans fats are structured straight like saturated fats, thus reducing fluidity.

• Cholesterol as a Fluidity Buffer:

  • In animal cells, cholesterol stabilizes the membrane. At high temperatures, it restricts phospholipid movement to prevent excessive fluidity. At low temperatures, it prevents solidification by disrupting the regular packing of phospholipids.

• Fatty Acid Tail Length:

  • Longer tails increase hydrophobic interactions, leading to higher viscosity.

  • Shorter tails decrease interactions, increasing fluidity.

• Temperature: Increased thermal energy enhances lipid movement, while low temperatures can cause the membrane to transition into a gel-like or solid state.

• Homeoviscous Adaptation: Organelles or organisms modify their lipid composition to maintain constant viscosity in different temperatures. For example, $Micrococcus$ bacteria shorten their fatty acid chains to stay fluid in cold environments.

Membrane Protein Classification and Specialized Functions

• Membrane proteins are categorized by how they associate with the bilayer:

  • Integral Membrane Proteins: Embedded in the hydrophobic core; removal requires detergents.

    • Monotopic: Inserted into only one side.

    • Transmembrane: Span the entire bilayer. These can be single-pass or multi-pass, or consist of multiple subunits.

  • Peripheral Membrane Proteins: Attached to the surface via non-covalent bonds (hydrophilic interactions); easily dissociated.

  • Lipid-Anchored Proteins: Covalently bonded to a lipid (e.g., GPI-anchored or fatty acid-anchored proteins) that sits in the bilayer.

• Functional Roles of Membrane Proteins:

  1. Transport: Channels and pumps (e.g., ATP-powered pumps).

  2. Enzymatic Activity: Facilitating chemical reactions at the membrane surface.

  3. Signal Transduction: Receptors binding to signaling molecules like hormones.

  4. Cell-Cell Recognition: Glycoproteins acting as identification tags.

  5. Intercellular Joining: Forming junctions between cells.

  6. Attachment: Anchoring the cytoskeleton and connecting to the ECM.

• Case Study: HIV Infection:

  • HIV requires two specific membrane proteins to enter host cells: the CD4 receptor and the CCR5 co-receptor.

  • Individuals born without CCR5 are resistant to HIV infection because the virus cannot attach and enter the cell.

Principles of Selective Permeability

• Selective permeability is influenced by both membrane composition and the chemical nature of the passing molecules.

• Hierarchy of Permeability:

  1. Small, Nonpolar Molecules: $\text{O}_2$, $\text{CO}_2$, and $\text{N}_2$ pass the fastest because the hydrophobic core does not resist them.

  2. Small, Uncharged Polar Molecules: $\text{H}_2\text{O}$ and glycerol have moderate permeability.

  3. Large, Uncharged Polar Molecules: Glucose and sucrose are mostly blocked and require transport proteins.

  4. Ions: $\text{Na}^+$, $\text{Cl}^-$, and $\text{K}^+$ are completely blocked because their charge and hydration shells are incompatible with the hydrophobic core.

  5. Exception: Large, nonpolar molecules (e.g., steroid hormones like testosterone) can diffuse because they interact well with lipids.

• Aquaporins: These tetrameric transmembrane proteins facilitate the rapid movement of water. The channel is narrow ($0.3 \text{ nm}$), allowing water molecules to move in a single-file line via hydrogen bonding while preventing the passage of ions such as protons ($\text{H}^+$).

Passive Transport: Diffusion and Osmosis

• Passive transport occurs down a concentration gradient without energy input ($\text{ATP}$ free).

• Simple Diffusion: Small or nonpolar molecules move from high to low concentration until equilibrium is reached.

• Facilitated Diffusion: Uses transport proteins to move solutes down their gradient.

  • Channel Proteins: Provide a hydrophilic corridor for specific ions or water.

  • Carrier Proteins: Undergo conformational (shape) changes to move solutes. These include uniporters (one solute), symporters (two solutes, same direction), and antiporters (two solutes, opposite directions).

• Osmosis: The diffusion of water across a selectively permeable membrane toward the side with higher solute concentration.

• Tonicity and Water Balance:

  • Hypotonic (lower external solute): Water enters the cell. Animal cells may undergo lysis (burst). Plant cells become turgid (normal/firm) due to the cell wall.

  • Isotonic (equal solute): No net movement. Animal cells are normal. Plant cells become flaccid (limp).

  • Hypertonic (higher external solute): Water leaves the cell. Animal cells shrivel (crenation). Plant cells undergo plasmolysis, where the plasma membrane pulls away from the cell wall.

Active Transport and Membrane Potential

• Active Transport: Moves solutes against their concentration gradient, requiring carrier proteins and energy ($\text{ATP}$).

• Sodium-Potassium Pump ($\text{Na}^+/\text{K}^+$ Pump): An electrogenic pump crucial for nerve signaling.

  • Step 1: Three $\text{Na}^+$ ions bind to the pump from the cytoplasm side.

  • Step 2: ATP hydrolysis phosphorylates the pump, causing a shape change.

  • Step 3: $\text{Na}^+$ is released to the extracellular fluid; two $\text{K}^+$ ions bind.

  • Step 4: Dephosphorylation occurs, causing the pump to return to its original shape.

  • Step 5: $\text{K}^+$ is released into the cytoplasm.

• Membrane Potential: The voltage difference across a membrane created by the separation of charges. This results in an electrochemical gradient, combining chemical force (concentration) and electrical force (charge attraction).

• Secondary Active Transport (Cotransport): Indirectly uses ATP. A primary pump (e.g., $\text{H}^+$ pump) creates an ion gradient. A cotransporter then uses the energy of that ion moving down its gradient to pull another solute (e.g., sucrose) against its own gradient.

Bulk Transport: Endocytosis and Exocytosis

• Bulk transport moves large molecules using vesicles and energy.

• Exocytosis: Exporting materials by fusing internal vesicles with the plasma membrane.

• Endocytosis: Inward folding of the membrane to bring in materials. There are three types:

  1. Phagocytosis ("Cell Eating"): Pseudopodia surround a large particle, forming a food vacuole that often fuses with a lysosome.

  2. Pinocytosis ("Cell Drinking"): Non-specific intake of extracellular fluid and dissolved solutes into coated vesicles.

  3. Receptor-Mediated Endocytosis: Specific uptake where ligands bind to receptors on the membrane, triggering vesicle formation.

• Hypercholesterolemia: A genetic condition caused by defective LDL (Low-Density Lipoprotein) receptors. This prevents the receptor-mediated endocytosis of cholesterol, leading to its accumulation in the blood and significantly increased risk of heart disease.