Cell membrane Study Guide

📖 Core Concepts Plasma membrane – a semipermeable lipid bilayer that separates the cell interior from the outside world. Amphipathic lipids – molecules with a water‑loving head (hydrophilic) and a water‑fearing tail (hydrophobic); they self‑assemble into a bilayer because of the hydrophobic effect (water molecules gain entropy when lipids hide their tails). Fluid Mosaic Model – lipids form a fluid two‑dimensional sheet; proteins float and move laterally like “mosaic tiles.” Selective permeability – the membrane lets small, non‑polar molecules diffuse freely, while ions and larger/charged species need protein carriers. Major protein types Integral (transmembrane) – span the bilayer; include ion channels, pumps, GP‑protein‑coupled receptors. Peripheral – attached to one leaflet; often enzymes or scaffolds. Lipid‑anchored – covalently bound to a lipid tail. Cholesterol & sterols – intercalate between phospholipid tails; stiffen the membrane at high temps, prevent excess rigidity at low temps (homeoviscous adaptation). Lipid rafts / caveolae – cholesterol‑rich microdomains that concentrate signaling proteins. Glycocalyx – outer carbohydrate layer (glycoproteins & glycolipids) that mediates recognition, adhesion, and charge protection. --- 📌 Must Remember 50 % of membrane dry mass = proteins; 50 % = lipids. Phospholipids = >50 % of total membrane lipids; most fatty acids are cis‑unsaturated → kinks → fluidity. Cholesterol: ↑ fluidity when cold, ↓ fluidity when hot. Ion channels = passive, pumps = active (use ATP). Passive diffusion works for small, non‑polar molecules; facilitated diffusion uses specific channels (e.g., aquaporins). Active transport moves substances against concentration gradients, requiring energy (usually ATP). Endocytosis (phagocytosis, pinocytosis, receptor‑mediated) = membrane invagination → vesicle → internalization. Exocytosis = vesicle fusion → release of contents. Apical vs. basolateral membranes in polarized cells have distinct protein/lipid compositions; tight junctions keep them separate. Flippases move specific phospholipids “down” the gradient; scramblases randomize distribution (important for apoptosis). --- 🔄 Key Processes Bilayer Self‑Assembly Amphipathic phospholipids arrange with heads outward, tails inward → hydrophobic effect drives entropy gain of water. Passive Diffusion of a Small Non‑Polar Molecule Molecule → partitions into lipid core → diffuses down concentration gradient → exits on opposite side. Facilitated Diffusion (Aquaporin) Water binds channel → rapid passage → concentration gradient restored. Active Transport (Na⁺/K⁺‑ATPase) ATP hydrolysis → pump changes conformation → 3 Na⁺ out, 2 K⁺ in → establishes membrane potential. Receptor‑Mediated Endocytosis Ligand binds surface receptor → clathrin coat forms → vesicle pinches off → ligand internalized. Exocytosis of Hormone Secretory vesicle traffics to membrane → SNARE proteins mediate fusion → hormone released extracellularly. --- 🔍 Key Comparisons Integral vs. Peripheral proteins Integral: span bilayer, often form channels/pumps. Peripheral: attach to one leaflet, usually enzymes or signaling scaffolds. Cis vs. Trans fatty acids Cis: kinks → increased fluidity. Trans: straighter → tighter packing → decreased fluidity. Gram‑negative outer membrane vs. eukaryotic plasma membrane Gram‑negative: asymmetric leaflets (LPS outer, phospholipids inner), porins for diffusion. Eukaryotic: symmetric phospholipid bilayer, cholesterol, diverse protein content. Passive diffusion vs. Facilitated diffusion Passive: no protein, limited to small, non‑polar molecules. Facilitated: requires specific channel or carrier, faster rate, still down gradient. --- ⚠️ Common Misunderstandings “Membrane is a static barrier.” → It is a dynamic fluid where lipids & proteins move laterally. “All proteins are integral.” → Roughly half are peripheral or lipid‑anchored. “Cholesterol only makes membranes rigid.” → It both stiffens at high temps and prevents excessive rigidity at low temps. “Only ions need pumps.” → Many larger, charged molecules (e.g., sugars, amino acids) also rely on active transporters. --- 🧠 Mental Models / Intuition “Lipid bilayer = oil‑in‑water emulsion” – imagine two layers of oil droplets with water excluded; proteins are “sprinkles” that either sit on the surface or pierce through. “Membrane as a revolving door” – passive diffusion = open door for small guests; facilitated diffusion = a revolving door for specific guests; active transport = a powered door that pushes guests against the crowd. “Cholesterol as a temperature‑regulating spacer” – think of a thermostat: insert more spacers (cholesterol) when it’s cold, fewer when it’s hot. --- 🚩 Exceptions & Edge Cases Very small polar molecules (e.g., water) can cross via aquaporins despite overall bilayer impermeability to polar species. Lipid rafts concentrate certain proteins, making local membrane composition non‑uniform; they can be sites of viral entry or signal clustering. Prokaryotic membranes lack cholesterol but may use other sterol‑like molecules for fluidity control. Flippases act energy‑dependently on specific lipids; scramblases act energy‑independently during apoptosis, exposing phosphatidyl‑serine. --- 📍 When to Use Which Identify molecule size & polarity → Small & non‑polar → passive diffusion. Small polar or charged → facilitated diffusion (channel/ carrier). Large or charged → active transport (pump or co‑transporter). Determine need for specificity → General ion flow → ion channel. Signal‑triggered uptake → receptor‑mediated endocytosis. Assess membrane region → Apical surface of epithelial cell → look for microvilli, transporters for absorption. Basolateral surface → Na⁺/K⁺‑ATPase, basolateral transporters. --- 👀 Patterns to Recognize “Cis‑unsaturated → fluid; saturated → rigid” appears whenever temperature adaptation is discussed. “Cholesterol ↑ → fluid at low temp, ↓ fluid at high temp.” “Porins = water‑filled channels in Gram‑negative outer membrane → small molecule diffusion.” “Tight junctions + polarity = distinct apical vs. basolateral protein sets.” --- 🗂️ Exam Traps Distractor: “Cholesterol only makes membranes less fluid.” – Wrong; it buffers fluidity both ways. Distractor: “All membrane proteins are integral.” – Incorrect; many are peripheral or lipid‑anchored. Distractor: “Passive diffusion can move large charged molecules.” – False; size and charge block diffusion. Distractor: “Lipid rafts are present in all membranes equally.” – Misleading; they are cholesterol‑rich microdomains, not uniform. Distractor: “Gram‑negative outer membrane is identical to eukaryotic plasma membrane.” – Wrong; LPS asymmetry and porins are unique. ---