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Cell Membrane Transport - Biology 201
Section (Pages 1-8)
Key Concepts
Fluid Mosaic Model: The cell membrane is composed of a phospholipid bilayer with embedded proteins that can move laterally, creating a fluid and dynamic structure. The “mosaic” refers to the diverse mix of proteins scattered throughout the lipid layer(p. 3)
Selective Permeability: Cell membranes allow certain molecules to pass through while blocking others based on size, polarity, and charge. Small nonpolar molecules like O₂ and CO₂ cross freely, while large polar molecules and ions require transport proteins(p. 5)
Amphipathic Nature of Phospholipids: Each phospholipid has a hydrophilic head group containing a phosphate and a hydrophobic tail made of two fatty acid chains. This dual nature drives the spontaneous formation of the bilayer in aqueous environments(p. 2)
Membrane Asymmetry: The two leaflets of the bilayer differ in lipid composition and protein orientation. The outer leaflet is enriched in sphingomyelin and glycolipids, while the inner leaflet contains more phosphatidylserine and phosphatidylethanolamine(p. 4)
Definitions
Phospholipid Bilayer: A two-layered arrangement of phospholipid molecules forming the basic structure of cell membranes, with hydrophilic heads facing outward and hydrophobic tails facing inward, shielded from water(p. 2)
Integral (Transmembrane) Proteins: Proteins that span the entire thickness of the membrane, often forming channels or carriers for molecular transport. Their hydrophobic midsection anchors them within the lipid bilayer(p. 6)
Peripheral Proteins: Proteins loosely attached to the inner or outer surface of the membrane rather than embedded within it. They often serve as enzymes or structural anchors connecting the membrane to the cytoskeleton(p. 6)
Glycocalyx: A carbohydrate-rich layer on the extracellular surface formed by glycoproteins and glycolipids. Functions in cell-cell recognition, protection, and adhesion(p. 7)
Cholesterol: A steroid molecule interspersed among phospholipids in animal cell membranes that moderates membrane fluidity, preventing it from becoming too rigid at low temperatures or too fluid at high temperatures(p. 4)
Glycoprotein: A membrane protein with covalently attached carbohydrate chains on the extracellular side. Plays roles in cell signaling, immune recognition, and forming the glycocalyx(p. 7)
Lipid Raft: A microdomain within the membrane enriched in cholesterol and sphingolipids that forms a more ordered, tightly packed region. Concentrates specific proteins involved in signaling and transport(p. 5)
Membrane Fluidity: The viscosity of the lipid bilayer, influenced by temperature, cholesterol content, and the ratio of saturated to unsaturated fatty acid tails. Unsaturated tails with kinks increase fluidity(p. 4)
Examples
Red Blood Cell Membrane: Contains roughly 50% protein by mass including Band 3 (anion transporter) and glycophorin. The spectrin network on the cytoplasmic side gives the cell its flexible biconcave shape(p. 8)
Section (Pages 9-18)
Key Concepts
Simple Diffusion: The net movement of molecules from a region of higher concentration to lower concentration. Does not require energy or transport proteins. Rate depends on the steepness of the concentration gradient, temperature, and molecular size(p. 9)
Facilitated Diffusion: Passive transport of polar molecules and ions across the membrane through specific channel or carrier proteins. No ATP is consumed. Transport is still down the concentration gradient but is faster and more selective than simple diffusion(p. 12)
Osmosis: The net movement of water molecules across a selectively permeable membrane from a region of lower solute concentration to higher solute concentration. Critically important for maintaining cell volume and turgor pressure in plant cells(p. 10)
Tonicity and Cell Volume: The ability of a surrounding solution to cause a cell to gain or lose water. Determines whether cells swell, shrink, or remain stable. Unlike osmolarity, tonicity accounts only for solutes that cannot cross the membrane(p. 11)
Definitions
Concentration Gradient: The difference in concentration of a substance between two regions. Molecules naturally move down their gradient (high to low) unless energy is used to move them against it(p. 9)
Hypertonic Solution: A solution with a higher solute concentration relative to the cell interior, causing water to leave the cell by osmosis. In animal cells this leads to crenation; in plant cells the plasma membrane pulls away from the cell wall (plasmolysis)(p. 11)
Hypotonic Solution: A solution with a lower solute concentration relative to the cell interior, causing water to enter the cell. Animal cells may lyse (burst); plant cells become turgid as the central vacuole fills and pushes the membrane against the cell wall(p. 11)
Isotonic Solution: A solution with the same solute concentration as the cell interior. No net movement of water occurs, so cell volume remains stable. Cells are in osmotic equilibrium with their surroundings(p. 11)
Aquaporins: Specialized channel proteins that allow rapid transport of water molecules across the membrane. A single aquaporin can transport up to 3 billion water molecules per second(p. 13)
Channel Proteins: Transmembrane proteins that form hydrophilic pores allowing specific ions or small molecules to pass through. Many are gated, opening or closing in response to voltage changes, ligand binding, or mechanical stimulation(p. 12)
Carrier Proteins: Transport proteins that bind a specific solute and undergo a conformational change to shuttle it across the membrane. Slower than channels but can move larger or polar molecules like glucose and amino acids(p. 13)
Plasmolysis: The shrinkage of a plant cell’s cytoplasm away from the cell wall when placed in a hypertonic environment. The central vacuole loses water and the plasma membrane detaches from the rigid cell wall(p. 14)
Formulas
: Total water potential equals the sum of solute potential (Ψs, always negative or zero) and pressure potential (Ψp, positive in turgid cells). Water moves from regions of higher Ψ to regions of lower Ψ(p. 15)
: The diffusion flux (J) is proportional to the diffusion coefficient (D) and the concentration gradient (dC/dx). The negative sign indicates movement is from high to low concentration(p. 16)
Examples
IV Saline Solutions: Hospitals use 0.9% NaCl (isotonic saline) for intravenous fluids because it matches the osmolarity of blood plasma, preventing red blood cells from swelling or shrinking(p. 11)
GLUT Transporters: GLUT1 through GLUT4 are carrier proteins that facilitate glucose diffusion into cells. GLUT4 is insulin-regulated and primarily found in muscle and adipose tissue(p. 14)
Section (Pages 19-28)
Key Concepts
Primary Active Transport: Direct use of chemical energy (ATP hydrolysis) to move molecules against their concentration gradient. The Na⁺/K⁺ ATPase is the most prominent example, pumping 3 Na⁺ out and 2 K⁺ in per ATP consumed, maintaining the resting membrane potential(p. 20)
Secondary Active Transport (Cotransport): Uses the electrochemical gradient established by primary active transport to drive movement of a second substance. No direct ATP use, but depends indirectly on ATP-powered pumps to maintain the driving gradient(p. 22)
Electrochemical Gradient: The combined force of the concentration gradient and the electrical potential difference across the membrane. For charged ions, both components must be considered when determining the direction and magnitude of net ion movement(p. 21)
Endocytosis and Exocytosis: Bulk transport mechanisms for moving large molecules or particles across the membrane using vesicles. Endocytosis brings material in by membrane invagination; exocytosis releases material by vesicle fusion with the plasma membrane(p. 26)
Definitions
Na⁺/K⁺ ATPase (Sodium-Potassium Pump): A transmembrane enzyme that hydrolyzes one ATP to pump three sodium ions out and two potassium ions in, generating a net negative charge inside the cell. Consumes roughly 25% of a cell’s total ATP budget(p. 20)
Symport (Cotransporter): A membrane transport protein that moves two different substances in the same direction simultaneously. Example: SGLT1 couples sodium influx with glucose uptake in intestinal epithelial cells(p. 23)
Antiport (Exchanger): A transport protein that moves two substances in opposite directions across the membrane. Example: the Na⁺/Ca²⁺ exchanger uses the inward sodium gradient to pump calcium out of cardiac muscle cells(p. 23)
Electrogenic Pump: Any ion pump that generates a net electrical current across the membrane by moving unequal charges. The Na⁺/K⁺ ATPase is electrogenic because it moves 3 positive charges out but only 2 in per cycle(p. 21)
Phagocytosis: A form of endocytosis in which the cell extends pseudopodia to engulf large particles such as bacteria or dead cells, forming a phagosome that fuses with lysosomes for digestion(p. 26)
Receptor-Mediated Endocytosis: A highly selective form of endocytosis where specific ligands bind to receptors concentrated in clathrin-coated pits on the membrane surface. Allows the cell to import specific macromolecules in bulk(p. 27)
Clathrin: A protein that assembles into a polyhedral lattice on the cytoplasmic side of the membrane, helping to shape and pinch off vesicles during receptor-mediated endocytosis(p. 27)
Proton Pump (H⁺ ATPase): An active transport protein that uses ATP to pump hydrogen ions across a membrane. Found in stomach lining cells (maintains acidic pH), lysosomes, and plant cell vacuolar membranes(p. 24)
Formulas
: Calculates the equilibrium potential for a single ion species across the membrane. R is the gas constant, T is temperature in Kelvin, z is the ion’s valence, and F is Faraday’s constant(p. 24)
: Extends the Nernst equation to account for multiple ions and their relative permeabilities (P). Predicts the resting membrane potential when several ion species contribute(p. 25)
Examples
Glucose Absorption in the Small Intestine: SGLT1 on the apical surface uses the Na⁺ gradient to pull glucose into epithelial cells (symport). GLUT2 on the basolateral surface then releases glucose into the bloodstream by facilitated diffusion(p. 23)
Cholesterol Uptake via LDL Receptors: Low-density lipoprotein particles bind to LDL receptors in clathrin-coated pits. The resulting vesicle delivers cholesterol to the cell. Defective LDL receptors cause familial hypercholesterolemia(p. 28)
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