Membrane & Transport — Comprehensive Study Notes Summary & Study Notes

🧩 Membrane Overview

The plasma membrane separates the cell interior from the external environment and is semipermeable, allowing some substances to pass while excluding others. Its core structure is the phospholipid bilayer, which provides fluidity and flexibility that support cell shape and movement.

🧪 Phospholipid Bilayer & Fluid‑Mosaic Model

Phospholipids have amphipathic properties (hydrophilic heads, hydrophobic tails), so they spontaneously form bilayers in water. The fluid‑mosaic model describes a dynamic membrane with embedded proteins (integral/transmembrane and peripheral) moving within the lipid matrix.

🔬 Evidence & Experimental Tools

The freeze‑fracture technique and electron microscopy revealed proteins within the bilayer, supporting the fluid‑mosaic model. Detergents (amphipathic) are used to isolate membrane proteins by forming detergent‑protein complexes.

⚖️ Diffusion vs Osmosis

Diffusion is net movement of molecules down a concentration gradient (high → low); it increases entropy and is spontaneous. Osmosis is the diffusion of water across a selectively permeable membrane: water moves from low solute concentration toward high solute concentration to equalize solute levels.

💧 Tonicity Effects

• Hypertonic (outside > inside): water leaves the cell → shrinks.
• Hypotonic (outside < inside): water enters the cell → swells.
• Isotonic: no net water movement → cell volume stable.

✅ How to Predict Movement

Compare overall solute concentrations (not specific solutes) for osmosis. For diffusion, compare the concentration of the solute itself across the membrane.

🎯 Learning Objectives Summary

Be able to: sketch a phospholipid, explain bilayer formation, predict diffusion/osmosis directions, describe how membrane proteins facilitate transport, and contrast passive vs active transport with examples.

🧠 Membrane Proteins & Their Roles

Membranes contain many proteins that alter structure and function. Integral (transmembrane) proteins span the bilayer and have hydrophobic residues facing the tails. Peripheral proteins associate without crossing the membrane. Proteins can be amphipathic and fold to create functional domains.

🚪 Channels — Fast, Selective Pathways

Channel proteins form hydrophilic pores enabling passive transport (facilitated diffusion) of ions or small polar molecules. They are selective (e.g., Na⁺ channels) and often gated — opening/closing in response to signals (ligand binding, voltage, ATP). Example: CFTR is an ATP‑gated Cl⁻ channel; mutations cause cystic fibrosis due to defective chloride transport and thick mucus.

🚰 Aquaporins

Aquaporins are specialized water channels that increase water permeability ~10× compared with lipid bilayers alone. They support rapid water movement where needed (e.g., intestinal absorption) and can be gated in some cases.

🔁 Carrier Proteins — Conformational Transporters

Carrier proteins bind solutes and undergo conformational changes to move them across the membrane. They can mediate facilitated diffusion (passive) or active transport. Types: uniport (one solute), symport (two solutes same direction), antiport (two solutes opposite directions). Example uniporter: GLUT‑1 for glucose; example symporter: SGLT (Na⁺/glucose cotransporter).

⚡ Pumps — Active Transporters

Pumps use energy (often ATP) to move ions/molecules against their gradients. The sodium–potassium pump (Na⁺/K⁺ ATPase) exports 3 Na⁺ and imports 2 K⁺ per ATP, establishing concentration and electrical gradients crucial for cell function and secondary transport.

🔗 Coupled Transport (Secondary Active Transport)

Pumps create electrochemical gradients that store potential energy. Coupled transporters (symporters/antiporters) use these gradients to move other solutes without direct ATP hydrolysis. Example: SGLT uses the Na⁺ gradient to import glucose uphill.

🧩 Electrochemical Gradients & Physiological Impact

Ionic gradients combine concentration and charge differences to form electrochemical gradients. These gradients drive processes like nutrient uptake, nerve signaling, and maintain cell volume. Defects in membrane transport proteins can cause disease (e.g., CFTR in cystic fibrosis).

📝 Quick Comparison: Channels vs Carriers vs Pumps

• Channels: passive, fast, selective, gated.
• Carriers: bind and transport via conformational change; can be passive or active; suitable for larger solutes (e.g., glucose).
• Pumps: active, ATP‑dependent, move solutes against gradients.

🔎 Study Tips

Focus on: membrane structure → types of transport (diffusion, osmosis, facilitated, active) → examples (GLUT, SGLT, CFTR, Na⁺/K⁺ pump) → how gradients power coupled transport. Use the learning objectives as checkpoints to test understanding.