Transmembrane Transport of Ions and Small Molecules 11.1-11.2

Plasma Membrane Function:
- Forms the barrier separating the cytoplasm from the external environment.
- Defines physical and chemical boundaries within the cell.
- Maintains differences in composition between cytosol and extracellular fluid.
- Concentrations of ions:
- Sodium ion ($[Na^+]_{cytosol}$) ~ 15 mM (inside cell) vs. 150 mM (extracellular fluid).
- Potassium ion ($[K^+]_{cytosol}$) is ~ 150 mM (inside cell) vs. lower outside.
- Critical for establishing the membrane potential.
Organelle Membranes:
- Separate cytosol from organelles.
- Example of differences in proton concentration: Lysosome interior has a pH of 5, much higher proton concentration than cytosol.
Membrane Composition:
- All cellular membranes are composed of a bilayer of phospholipids embedded with proteins.
- Phospholipid bilayers alone are impermeable to ions and large polar molecules, slightly permeable to water only.
Need for Selective Transport:
- Membranes serve as conduits, selectively transporting essential molecules (like glucose) and exporting waste.

Definition of Facilitated Transport:
- Mechanism smartly designed for transporting glucose and other solutes across the plasma membrane more efficiently than simple diffusion.
Uniport Transport Characteristics:
- Transport of a single molecule across a membrane via a uniporter is faster than simple diffusion.
- Distinct properties of uniport:
1. Faster than pure diffusion across pure phospholipid bilayer.
2. No interaction with hydrophobic core of membrane.
3. Limited number of uniporters establishes maximum transport rate ($V_{max}$).
4. Transport direction depends on concentration gradient.
5. Specificity for one type of molecule.
- Example: GLUT1 transporter in mammalian cells, especially in erythrocytes.

Transport Mechanisms:
- Cells require precise balance in import/export of molecules and ions.
- Energy from ATP hydrolysis powers transport against concentration gradients.
- Understand the alternating access model:
- Highlights conformational change mechanism in transporter proteins.
- Pumps, channels, and transporters vary in speed; Pumps are slower than channels due to single substrate movement.
Transport Types: (Based on energy usage and directionality)
- Channels: High transport rate; allow ions/molecules to move down gradients.
- Transporters: Medium transport rate; facilitate molecules moving predominantly down their gradients.
- ATP-powered pumps: Slowest; use ATP to transport ions/molecules against gradients.

Ionic Concentration:
- Importance of maintaining gradients of ions across the membrane.
- Examples of ionic gradients:
- Sodium and potassium ions are unequal across membranes; potassium is abundant inside cells, while sodium is more outside.

Cotransport Mechanisms:
- Symporters: Move two molecules in the same direction; one against its gradient, another down its gradient (e.g., Glucose-Sodium symporter).
- Antiporters: Move different molecules in opposite directions using similar gradient coupling.

Importance of selective transport in cells for cellular function and adaptation to environments. Understanding membrane transport mechanisms is essential for comprehending disease-causing mutations and therapeutic developments.
Transmembrane Transport and Health:
- Mutations in transport proteins can lead to diseases such as cystic fibrosis and long QT syndrome.
Recent advances in microscopy like cryo-EM are promising for future drug developments targeting transport proteins.