General biology unit 7

Concept 7.1: Structure and Properties of Biological Membranes

General Chemical Structure of a Phospholipid

Phospholipids are the fundamental building blocks of biological membranes, providing both structural integrity and selective permeability.

• Head: Composed of glycerol + phosphate, often with a polar "head group" (e.g., choline, serine, ethanolamine); hydrophilic.

• Tails: Two fatty acid chains (can be saturated or unsaturated); hydrophobic.

• Structure: Amphipathic nature—hydrophilic head and hydrophobic tails.

• Sketch: $\Box$ (head) — $\vert\vert$ (tails)

Amphipathic Molecule

Phospholipids possess both hydrophilic and hydrophobic regions, allowing them to form bilayers in aqueous environments.

• Other amphipathic examples: cholesterol, detergents, some membrane proteins.

How Amphipathic Structure Drives Bilayer Formation

Phospholipids spontaneously arrange into bilayers in water, with hydrophobic tails shielded from water and hydrophilic heads exposed.

• Hydrophobic effect: Driven by van der Waals forces and tail-tail interactions.

• Bilayer formation minimizes free energy.

• Vesicle formation: Seals to avoid exposing edges (lowest free energy).

Fluid-Mosaic Model

The fluid-mosaic model describes the dynamic and heterogeneous nature of biological membranes.

• Fluid: Lipids and many proteins move laterally within the bilayer; flip-flop of lipids is rare.

• Mosaic: Membranes are a patchwork of lipids, proteins, and carbohydrates.

• Proteins are not randomly distributed—often organized into microdomains (rafts, scaffolds, cytoskeletal anchors).

Membrane Fluidity (Definition & Determinants)

Fluidity refers to the ease of lateral movement of lipids and proteins within the bilayer.

• Fatty acid composition: Unsaturated (cis) tails increase fluidity; saturated tails decrease fluidity.

• Cholesterol: Acts as a "fluidity buffer"—reduces fluidity at high temperatures, prevents solidification at low temperatures.

• Mnemonic: "Chol chills chaos at heat, fights freeze when it’s neat."

Membrane Proteins: Types

Membrane proteins are essential for transport, signaling, and structural support.

• Integral membrane proteins: Embedded in bilayer.

  • Transmembrane proteins: Span membrane (often α-helices; β-barrels in some bacteria/mitochondria); extracellular & cytosolic domains.

  • Lipid-anchored proteins: Covalently attached to lipids (e.g., myristoyl, palmitoyl, prenyl on inner leaflet; GPI-anchored on outer leaflet).

• Peripheral proteins: Loosely attached to membrane surface via interactions/hydrogen bonds with lipids or integral proteins; removed by salt/pH.

Carbohydrates in Membranes

Carbohydrates are added to proteins and lipids in the ER & Golgi (glycosylation), then delivered to the extracellular surface.

• Functions: Cell recognition, adhesion, protection.

Glycolipids vs. Glycoproteins

• Glycolipids: Carbohydrate covalently attached to lipid (commonly sphingolipids); cell recognition/stability.

• Glycoproteins: Carbohydrate attached to proteins (N-linked on Asn; O-linked on Ser/Thr); receptors, adhesion molecules, immune markers.

Membrane Sidedness (Asymmetry)

Membrane leaflets differ in lipid and protein composition.

• Lipid composition differs between inner vs. outer leaflets (e.g., phosphatidylserine mainly inner).

• Carbohydrates face outside only.

• Membrane proteins are oriented (e.g., receptor binding site outside; enzyme site inside).

• Asymmetry is built in the ER/Golgi and preserved by vesicular trafficking.

Concept 7.2: Membrane Permeability and Transport

Selective Permeability

Biological membranes allow some substances to cross more readily than others, based on size, polarity, and charge.

• Determinants of permeability:

  • Size & polarity: Small, nonpolar pass best; large or polar pass poorly.

  • Charge: Ions are strongly excluded without channels.

  • Membrane fluidity & thickness.

  • Presence/specificity of transport proteins.

Pass Easily

• Small nonpolar molecules: O2, CO2, N2, ethanol.

• Small uncharged polar molecules: water (via aquaporins).

Do Not Pass Easily

• Ions and charged molecules: Na+, K+, Cl-, Ca2+, HCO3-.

• Large polar molecules: glucose, amino acids, nucleotides.

Transport Proteins

• Channels: Hydrophilic pores; fast, selective; may be gated (voltage, ligand, mechanical).

• Carriers/transporters: Undergo conformational change; specific, saturable (Michaelis-Menten-like kinetics).

• Aquaporins: Water channels; massively increase water permeability while excluding ions.

Concepts 7.3 & 7.4: Membrane Transport Mechanisms

Transport Mechanisms

Cells use various mechanisms to move substances across membranes, either passively or actively.

• Simple diffusion: Movement down concentration gradient through lipid bilayer; no protein, no energy.

• Facilitated diffusion: Movement down gradient via protein (channel or carrier); no energy; specific and saturable.

• Active transport: Movement against gradient; requires energy (ATP hydrolysis or ion gradient); mediated by pumps or coupled transporters.

Types of Facilitated Transport

• Channel-mediated (e.g., ion channels).

• Carrier-mediated (e.g., glucose transporters).

• Rate plateaus for carriers (saturable kinetics).

Electrochemical Gradients

Charged solutes feel both chemical and electrical gradients; the sum is the electrochemical gradient, which determines direction and magnitude of passive movement.

Sodium-Potassium Pump (Na+/K+-ATPase)

The Na+/K+-ATPase is a primary active transporter that maintains membrane potential and cell volume.

• Cycle (per ATP): Pumps 3 Na+ out / 2 K+ in; electrogenic (net + out).

• Steps simplified:

  1. Pump (E1) binds 3 Na+ inside.

  2. ATP phosphorylates pump + conformational change (E2).

  3. 3 Na+ released outside.

  4. 2 K+ bind outside.

  5. Dephosphorylation + conformational reset.

  6. 2 K+ released inside.

• Roles: Maintains membrane potential, cell volume, and gradients for secondary transport (e.g., Na+/glucose symport in intestines).

• Other electrogenic pumps: H+-ATPase (plants/fungi), Ca2+-ATPase (ER/SR), H+/K+-ATPase (stomach).

Parameters Influencing Diffusion Across Membranes

• Nonpolarity: Faster (hydrocarbons > O2, CO2).

• Smaller size: Faster.

• Lower charge/ionization: Faster (ions do not pass without channels).

• Higher lipid solubility (partition coefficient): Faster.

• Membrane fluidity & thickness: Also modulate rate.

Concept 7.5: Bulk Transport (Vesicular Transport)

Bulk Transport & Cargo Types

Bulk transport moves large molecules or particles via vesicles, requiring energy and cytoskeletal involvement.

• Exocytosis: Fusion of vesicles with plasma membrane; secretion of large or hydrophilic cargo (e.g., neurotransmitters, peptide hormones, ECM proteins); also adds membrane.

• Endocytosis:

  • Phagocytosis ("cell eating"): Engulfment of large particles/cells; phagosome formation (e.g., macrophages ingest bacteria).

  • Pinocytosis ("cell drinking"): Uptake of extracellular fluid and solutes; non-specific.

  • Receptor-mediated endocytosis: Selective uptake via receptors and clathrin-coated pits (e.g., LDL cholesterol uptake via LDL receptor).

• Bulk transport is energy-dependent (cytoskeleton, coat proteins) and moves cargo too big for channels/transporters.

Micro-mnemonics

• FLUIDITY: "Unsaturated Undosolidifies; Saturated Stiffens; Cholesterol Calibrates."

• Na+/K+ pump numbers: "Pump 3 Na+ out, 2 K+ in 1 ATP" (3-2-1).

• Endocytosis trio: "Phag—Big; Pino—sip; Receptor—specific."

Additional info: These notes expand on the original bullet points with academic context, definitions, and examples for clarity and completeness.