Week 5

Core Functions & Homeostatic Roles of the Plasma Membrane

• Homeostasis Maintenance: Cells maintain internal stability (homeostasis) primarily by controlling the movement of substances across the plasma membrane. The plasma membrane is dynamic and fluid rather than rigid.

• Key Functions of the Plasma Membrane:

  • Structural Integrity: Holds the cell together and defines its boundaries.

  • Selective Transport: Controls what enters and leaves the cell through passive mechanisms (simple diffusion, facilitated diffusion, osmosis) and active mechanisms (active transport, vesicular transport).

  • Protection: Shields the cell from the external environment and prevents the entry of harmful substances.

  • Cell Recognition & Immunity: Allows the cell to recognize self and be recognized by other cells or the immune system via cell surface markers.

  • Intercellular Binding: Enables cells to bind to other cells (such as liver cells binding to one another) and to extracellular structures.

  • Catalytic Site for Biochemical Reactions: Serves as a localized platform for membrane-bound enzymes and structured reaction pathways (such as metabolic electron transport chains).

The Phospholipid Bilayer & Membrane Amphipathicity

• Phospholipid Structure:

  • A phospholipid is an amphipathic lipid composed of a hydrophilic (water-attracted) polar head and two hydrophobic (water-repelled) non-polar fatty acid tails.

  • Polar Head: Contains a charged phosphate group connected to a glycerol backbone. It interacts favorably with water molecules in aqueous solutions.

  • Non-Polar Tails: Consist of two long hydrocarbon fatty acid chains. They are uncharged and repelled by water molecules.

• Spontaneous Bilayer Formation:

  • When placed in an aqueous environment, phospholipids exhibit an emergent property: they self-organize to keep their hydrophilic heads wet and their hydrophobic tails dry.

  • Structures Formed in Water:

    • Micelle: A spherical single-layered structure where hydrophobic tails aggregate in the center away from water and polar heads face outward.

    • Liposome: A spherical bilayer surrounding an internal aqueous core.

    • Bilayer Sheet: A two-molecule thick planar sheet where hydrophobic tails face each other internally, away from water, while polar heads point outward toward the cytoplasm or extracellular fluid.

• Membrane Dynamics & Permeability:

  • Phospholipid molecules within the bilayer can flow past one another laterally, giving the membrane its fluid nature. However, individual phospholipids rarely flip-flop vertically from one leaflet to the other.

  • Selective Permeability: The hydrophobic hydrocarbon core acts as a strict physical barrier. Charged ions (such as $Na^+$, $Cl^-$) and polar molecules cannot pass directly through the non-polar tails. Only small, uncharged non-polar molecules (such as $O_2$) diffuse directly across the lipid bilayer.

Structural & Functional Classification of Membrane Proteins

• Types of Membrane Proteins:

  • Integral Proteins: Permanently embedded within the lipid bilayer. They can be polytopic (spanning across the membrane multiple times as transmembrane proteins) or monotopic (penetrating only one surface leaflet).

  • Peripheral Proteins: Temporarily associated with the surface of the membrane. They can be monotopic or non-covalently attached to integral proteins, the cytoskeleton, or the extracellular matrix.

  • Glycoproteins: Proteins with short branched oligosaccharide chains (oligo = few, saccharide = sugar) attached to their extracellular regions. They act as hormone receptors and cell-identity markers for immune recognition.

• Amino Acid Organization in Transmembrane Proteins: Hydrophobic regions of integral proteins consist of non-polar amino acid chains, which often coil into ̑\alpha-helices to interact with the hydrophobic interior of the bilayer.

• Functional Categories of Membrane Proteins (TRACIE Mnemonic):

  • T - Transport: Protein channels for facilitated diffusion and protein pumps for active transport.

  • R - Receptors / Signal Transduction: Binding sites for chemical messengers such as peptide hormones (e.g., insulin, glucagon).

  • A - Anchorage / Attachment: Anchor points for internal cytoskeleton filaments (microfilaments) and external extracellular matrix (ECM) molecules.

  • C - Cell-to-Cell Recognition: Identification tags such as Major Histocompatibility Complex (MHC) proteins and antigens.

  • I - Intercellular Joinings: Structures that hook adjacent cells together, such as tight junctions, gap junctions, and plasmodesmata.

  • E - Enzymatic Activity: Membrane-bound enzymes organized into sequential metabolic pathways (e.g., electron transport chain enzymes).

• Synthesis and Orientation of Membrane Components:

  1. Endoplasmic Reticulum (ER): Membrane proteins and lipids are synthesized in the ER. Carbohydrates are added to transmembrane proteins inside the ER lumen, converting them into glycoproteins.

  2. Golgi Apparatus: Transport vesicles deliver proteins and lipids to the Golgi apparatus, where glycoproteins undergo further carbohydrate modification, and carbohydrates are added to lipids to form glycolipids.

  3. Plasma Membrane Fusion: Vesicles carrying secretable proteins, glycoproteins, and glycolipids fuse with the plasma membrane via exocytosis. As the vesicle fuses, its inner lumen layer becomes continuous with the extracellular face of the plasma membrane, positioning carbohydrate chains on the outside of the cell.

Structural Role of Cholesterol in Membrane Dynamics

• Chemical Structure of Cholesterol:

  • Cholesterol is an amphipathic steroid molecule (not structurally classified as a fat or oil).

  • Hydroxyl Group ($-OH$): Polar and hydrophilic head attracted to the phosphate heads of phospholipids at the edge of the membrane.

  • Carbon Rings: Four rigid, non-polar hydrocarbon rings.

  • Non-Polar Tail: Hydrophobic tail attracted to the fatty acid chains in the center of the bilayer.

• Functions of Cholesterol in the Plasma Membrane:

  • Restricting Fluidity at High Temperatures: Cholesterol intercalates between phospholipids, restricting their physical movement and stabilizing the membrane at moderate to high temperatures.

  • Preventing Solidification at Low Temperatures: It disrupts the regular, tight packing of phospholipid hydrocarbon tails, preventing the membrane from crystallizing, stiffening, or behaving like a solid at low temperatures.

  • Reducing Permeability: It packs tightly into spaces between phospholipid heads, reducing membrane permeability to small hydrophilic, water-soluble molecules and ions such as $Na^+$ and $H^+$.

Models of Membrane Structure: Historical Development & Falsification

• The Davson-Danielli Model (1935):

  • Model Description: Proposed a static "protein-lipid sandwich" model where a central phospholipid bilayer was completely coated on both outer surfaces by continuous layers of globular proteins. It assumed that proteins do not permeate into or across the lipid bilayer.

  • Supporting Evidence at the Time: Early high-magnification transmission electron micrographs showed membranes as two dark outer parallel lines separated by a lighter central region. Because proteins stain dark and lipids stain light in electron micrographs, this was interpreted as protein layers sandwiching a lipid core.

• Falsification of the Davson-Danielli Model:

  • Freeze-Fracture Electron Microscopy: Cells were rapidly frozen and fractured along lines of structural weakness, which split the membrane down the hydrophobic center of the lipid bilayer. Micrographs revealed an irregular, rough surface studded with globular structures inside the bilayer. These structures were identified as transmembrane proteins penetrating into the core.

*   **Biochemical Extraction**: Biochemical analysis showed that membrane proteins vary significantly in size and globular shape (unlike uniform sheets) and contain extensive hydrophobic regions, proving they must be embedded within the lipid bilayer rather than sitting on its outer surface.
*   **Fluorescent Antibody Tagging**: Membrane proteins of human cells were tagged with red fluorescent markers, and membrane proteins of mouse cells were tagged with green fluorescent markers. The two cells were fused into a hybrid cell. Within $40\,minutes$, the red and green markers were completely mixed across the entire membrane surface, proving that membrane proteins move laterally and are not held in fixed peripheral layers.

• The Singer-Nicholson Fluid Mosaic Model (1972):

  • The accepted current model of membrane structure.

  • Fluid: Phospholipid molecules form a continuous bilayer and are free to move laterally past one another.

  • Mosaic: Diverse proteins are embedded within or attached to the lipid bilayer in an irregular pattern, similar to a mosaic.

Thermodynamics & Kinetics of Passive Transport Mechanisms

• Passive Transport Definition: Movement of substances across a cell membrane down their concentration gradient (from high to low concentration) without requiring metabolic energy ($ATP$).

• Simple Diffusion:

  • Passive net movement of particles from an area of higher concentration to an area of lower concentration across a partially permeable membrane.

  • Occurs for small, non-polar molecules like oxygen ($O_2$).

• Factors Affecting the Rate of Diffusion:

  • Concentration Gradient: Steeper gradients result in faster diffusion rates.

  • Surface Area: Larger surface area provides more space for exchange, increasing the diffusion rate.

  • Diffusion Path Length: Shorter diffusion distances increase the diffusion rate. Biological plasma membranes are thin ($7\text{--}10\,nm$).

• Biological Adaptations to Maximize Diffusion:

  • Maintaining High Concentration Gradients: Source-to-sink systems continually add solute to the high-concentration side and remove solute from the low-concentration side (e.g., blood circulation continuously removing oxygen from alveolar capillaries).

  • Maximizing Surface Area: Specialized structures increase surface area, such as alveoli in the lungs, cristae in mitochondria, thylakoid membranes in chloroplasts, root hair cells in plant roots, and villi/microvilli in the small intestine.

  • Minimizing Path Distance: Membranes are extremely thin ($7\text{--}10\,nm$), reducing the distance molecules must travel.

• Facilitated Diffusion:

  • Passive net movement of large, charged, or polar molecules/ions (e.g., $Na^+$, $Cl^-$) across a membrane down their concentration gradient through specific transmembrane transport proteins without energy expenditure.

  • Channel Proteins: Hydrophilic tunnels across the membrane through which water or specific solutes pass.

  • Carrier Proteins: Bind specific solute molecules, undergo a conformational shape change, and release the solute on the opposite side of the membrane.

Osmotic Regulation, Aquaporins, & Cellular Dynamics

• Osmosis Definition: The passive net movement of water molecules across a selectively permeable membrane from an area of lower solute concentration (higher free water molecule concentration) to an area of higher solute concentration (lower free water molecule concentration).

• Aquaporins: Specialized integral channel proteins that form pores in the membrane to accelerate the rate of water diffusion across the lipid bilayer.

• Cellular Response to Solution Tonicity:

  • Hypertonic Solution: Solution has a higher solute concentration (lower water concentration) than the cytoplasm.

    • Animal Cells (Red Blood Cells): Water moves out of the cell; cell shrinks and becomes crenated.

    • Plant Cells: Water leaves the central vacuole; cytoplasm pulls away from the cell wall, causing the cell to become plasmolyzed.

  • Hypotonic Solution: Solution has a lower solute concentration (higher water concentration) than the cytoplasm.

    • Animal Cells (Red Blood Cells): Water moves into the cell; cell swells and undergoes lysis (bursting).

    • Plant Cells: Water enters the vacuole; internal hydrostatic pressure pushes against the rigid cell wall, making the cell turgid (ideal state for plant structural support).

  • Isotonic Solution: Solution has an equal solute concentration relative to the cytoplasm.

    • Animal Cells: Water enters and exits at equal rates; cell maintains normal shape and function.

    • Plant Cells: Water movement is equal in both directions; cell becomes flaccid (lacks turgor pressure).

• Medical Applications of Isotonic Saline Solutions:

  • Intravenous (IV) rehydration drips for blood volume restoration.

  • Rinsing surgical wounds and skin abrasions.

  • Moistening exposed damaged skin before applying skin grafts.

  • Formulating eye drops and contact lens washes.

  • Preserving and transporting donor organs packed in slush/ice mixtures.

Active Transport: Energetics, Primary Pumps, & Secondary Cotransport

• Active Transport Definition: The movement of substances across a biological membrane against their concentration gradient (from an area of low concentration to high concentration). This process requires transmembrane protein pumps and cellular energy ($ATP$).

• ATP Structure & Hydrolysis:

  • Structure: Adenosine molecule bound to three phosphate groups.

  • Hydrolysis: Breaking the terminal phosphate bond via water releases one phosphate group ($P_i$) and a large amount of energy:         $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

  • Cellular respiration recombines $ADP$ and inorganic phosphate ($P_i$) to regenerate $ATP$.

• Primary Active Transport:

  • Directly uses $ATP$ hydrolysis energy to pump solutes against a concentration gradient.

  • Proton Pump: Actively pumps hydrogen ions ($H^+$) out of the cytoplasm into the extracellular space, creating an electrochemical gradient and generating voltage across plant, fungal, and bacterial membranes.

• Secondary Active Transport (Cotransport):

  • Uses energy stored in the concentration gradient of an ion (established by primary active transport) to drive the transport of another solute against its concentration gradient.

  • Uniport: Transports a single type of solute molecule in one direction.

  • Symport: Transports two different solutes simultaneously in the same direction across the membrane (e.g., $H^+/\text{sucrose}$ cotransporter).

  • Antiport: Transports two different solutes in opposite directions across the membrane.

Vesicular Transport Mechanisms: Endocytosis & Exocytosis

• Vesicles: Small spheroidal membrane-bound packages that bud off from the Rough Endoplasmic Reticulum (RER) and Golgi apparatus. They carry newly synthesized proteins to target organelles or to the plasma membrane for secretion.

• Endocytosis: The process by which cells take in external fluid, large molecules, or particles by invaginating the plasma membrane to form an intracellular vesicle. Requires energy ($ATP$) and membrane fluidity.

  • Phagocytosis ("Cell Eating"): Non-specific uptake of solid particles. The membrane extends pseudopodia to engulf particles, forming a phagosome (food vacuole).

  • Pinocytosis ("Cell Drinking"): Non-specific uptake of extracellular fluid and dissolved solutes into small vesicles.

  • Receptor-Mediated Endocytosis: Selective uptake mechanism. Specific extracellular ligands bind to receptors concentrated in coated pits (lined with coat proteins like clathrin), triggering vesicle formation.

• Exocytosis: The process by which membrane-bound vesicles inside the cell fuse with the plasma membrane to discharge their contents into the extracellular space.

  • Constitutive Secretion: Continuous, unregulated secretion occurring in all cells regardless of external signals.

  • Regulated Secretion: Secretion triggered by specific extracellular signals (e.g., $Ca^{2+}$ influx triggering neurotransmitter release at synapses).

• Biophysical Mechanism of Membrane Fusion: Discovered in 2002 by Yang and Hwang using X-ray diffraction images. A vesicle approaches the plasma membrane; the fluid phospholipid layers flow around each other to form a temporary single-bilayer intermediate structure at the point of contact, followed by pore opening without breaking membrane continuity.

Electrophysiology & The Sodium-Potassium Pump ($Na^+/K^+$ ATPase)

• Membrane Potential: Voltage across a cell's plasma membrane created by differences in the distribution of cations ($Na^+$, $K^+$) and anions ($Cl^-$) across the bilayer.

• Neuron Polarization: Resting neurons are polarized, maintaining a net negative internal cytosolic charge relative to the extracellular fluid.

• Electrogenic Pumps: Transport proteins that generate voltage across a membrane. The sodium-potassium pump is the major electrogenic pump in animal cells.

• Operation Steps of the Sodium-Potassium Pump ($Na^+/K^+$ ATPase):

  • Uses 1 $ATP$ per cycle to pump $3\,Na^+$ ions out of the cell and $2\,K^+$ ions into the cell against their concentration gradients through 6 sequential steps:

    1. The interior of the protein pump opens to the intracellular cytoplasm; $3\,Na^+$ ions enter the pump and bind to high-affinity sites.

    2. Binding of $3\,Na^+$ ions activates a protein kinase, which transfers a phosphate group from $ATP$ to the pump (phosphorylation).

    3. Phosphorylation causes a conformational shape change in the pump. The interior closes and the exterior opens, releasing $3\,Na^+$ ions outside the cell.

    4. The new extracellular conformation exhibits high affinity for potassium; $2\,K^+$ ions bind to the pump from the extracellular fluid.

    5. Binding of $2\,K^+$ triggers the loss of the phosphate group (dephosphorylation).

    6. Loss of the phosphate group restores the pump's original shape, opening it to the cytoplasm. $2\,K^+$ ions are released inside the cell, and $Na^+$ binding affinity is restored to repeat the cycle.

Mechanisms of Cell Signaling & Signal Transduction Pathways

• Three Stages of Cell Signaling:

  1. Reception: A chemical signaling molecule (ligand) binds to a specific receptor protein located on the cell surface or inside the cytoplasm.

  2. Transduction: The binding event alters the receptor protein, initiating a signal transduction pathway through a series of relay molecules.

  3. Response: The signal triggers a specific cellular response (e.g., enzyme activation like glycogen phosphorylase, or gene expression).

• Signaling Distances & Types:

  • Direct Contact / Local: Molecules pass directly between adjacent cells through gap junctions (animal cells) or plasmodesmata (plant cells).

  • Paracrine Signaling: A secreting cell discharges local regulators (e.g., growth factors) into the extracellular fluid to act on nearby target cells.

  • Synaptic Signaling: A nerve cell releases neurotransmitters into a synaptic cleft, stimulating the target cell.

  • Hormonal / Endocrine Signaling: Specialized endocrine cells secrete hormones into body fluids (bloodstream) for long-distance transport to target cells.

• Receptor Pathways & Transduction Cascades:

  • G Protein-Coupled Receptors (GPCR): A signaling molecule binds GPCR -> GPCR activates -> G protein binds GTP and becomes active -> active G protein binds and activates adenylyl cyclase -> adenylyl cyclase converts $ATP$ to second messenger cyclic AMP (cAMP) -> cAMP activates Protein Kinase A -> cellular response.

  • Ligand-Gated Ion Channels: Ligand binding opens a protein gate, allowing specific ions to flow through the channel down their gradient, changing the membrane potential or intracellular concentration to trigger a cellular response.

  • Intracellular Steroid Receptors: Hydrophobic steroid hormones (e.g., aldosterone) pass directly through the lipid bilayer -> bind a cytoplasmic receptor protein -> hormone-receptor complex enters the nucleus -> binds specific genes as a transcription factor -> stimulates transcription of mRNA -> mRNA is translated into a specific protein.

  • Phosphorylation Cascade: Relay molecules activate Protein Kinase 1 -> active Protein Kinase 1 uses $ATP$ to phosphorylate and activate Protein Kinase 2 -> active Protein Kinase 2 phosphorylates a protein to induce a response. Protein Phosphatases (PP) catalyze the removal of phosphate groups to deactivate the proteins and turn off the pathway.

  • Nuclear Responses: Activated terminal kinase enters the nucleus -> activates a transcription factor -> transcription factor stimulates gene transcription into mRNA -> directs protein synthesis in the cytoplasm.

Cell Size Limitations & Surface Area-to-Volume ($SA:\text{Vol}$) Dynamics

• Cellular Functions of Life: All living organisms, whether single-celled or multicellular, carry out essential functions of life within cells.

  • Example: Paramecium is a unicellular protist that performs all life functions using cellular organelles (nucleus, contractile vacuole, food vacuole, oral groove, mouth pore, cilia, anal pore).

• Cellular Exchange Requirements:

  • Imports Required: Oxygen ($O_2$), Nutrients (e.g., glucose, amino acids), Water.

  • Exports Required: Carbon Dioxide ($CO_2$), Other Metabolic Wastes, Heat, Secreted Cell Products.

• The Surface Area-to-Volume Ratio ($SA:\text{Vol}$) Principle:

  • As a cell increases in physical size, its volume increases much faster ($\text{Volume} \propto r^3$) than its surface area ($\text{Surface Area} \propto r^2$).

  • Therefore, as cell size increases, the surface area-to-volume ratio decreases.

• Mathematical Example (Cube Model):

  • $1\,\mu m$ Cube: Surface Area = $6\,\mu m^2$; Volume = $1\,\mu m^3$; $SA:\text{Vol} = 6:1$.

  • $2\,\mu m$ Cube: Surface Area = $24\,\mu m^2$; Volume = $8\,\mu m^3$; $SA:\text{Vol} = 3:1$.

  • $4\,\mu m$ Cube: Surface Area = $96\,\mu m^2$; Volume = $64\,\mu m^3$; $SA:\text{Vol} = 1.5:1$.

• Physiological Impact of Large Cell Size:

  • Larger volumes require more nutrient import and waste export per unit time.

  • Because the $SA:\text{Vol}$ ratio decreases as cells grow, exchange pathways across the plasma membrane become less efficient.

  • Diffusion path lengths to the cell interior become too long. If a cell grows too large, diffusion cannot deliver nutrients or remove waste fast enough, leading to cellular failure or cell death.

• Biological Shape Adaptations to Maximize $SA:\text{Vol}$:

  • Membrane Projections: Microvilli, root hairs, and membrane folds increase surface area without significantly increasing volume.

  • Flattened Form: Thin, flat cell shapes (e.g., squamous epithelium).

  • Elongated and Thin Form: Long, thin cell shapes (e.g., nerve cell axons, skeletal muscle fiber cells).

  • Multicellularity: Multicellular organisms consist of many small cells and specialized exchange structures (e.g., circulatory systems, lungs, gills) to deliver materials efficiently.