Membrane Bound Transport

Head = polar; Unequal charge
Tails = nonpolar; Equally distributed charge
New terms:
- Head = hydrophilic (Loves water)
- Tail = hydrophobic (Avoids water)
Amphipathic: having both hydrophilic and hydrophobic parts
Figure 3.2 (contextual)

Bilayer forms to satisfy both amphipathic parts
Arrangement:
- Nonpolar/hydrophobic tails come together in the interior
- Polar/hydrophilic heads face water on both sides
Bilayer acts as a good barrier to polar molecules
Polar/hydrophilic molecules tend to avoid the center
All membranes are very flexible
Inside/outside terminology:
- Intracellular fluid → inside the cell
- Extracellular fluid → outside the cell
- Interstitial fluid → extracellular fluid not in blood vessels
Figure 3.3 (contextual)

Other components: proteins
Functions: help move information across the barrier
Membrane protein classes:
- Peripheral: associated on one side of the membrane
- Integral: cross the entire membrane
- Channel protein: pass through to allow ions and other substances
- Receptor: receive external signals and cause internal changes
External signal = ligand
Figure 3.4 (Receptor-Ligand context)

Cell membranes are selectively permeable
Transport types:
- Passive transport: energy required = none
- Active transport: energy required
Key concepts:
- Concentration gradient: difference in substance concentration across space
- Movement down the gradient: from high to low concentration (downhill)
Conceptual analogies:
- Number of people in a room (diffusion)
- Dye diffusing in a pool (diffusion)

Gases (e.g., O₂) diffuse across the membrane (high → low)
Liquid molecules often need help: facilitated diffusion
Facilitated diffusion via:
- Channel proteins or transporter (integral proteins)
Example: glucose uses a transporter for facilitated diffusion
Figure 3.6 and Figure 3.5 (contextual)

Osmosis: diffusion of water through a semipermeable membrane
Water-specific integral protein channels (aquaporins) allow water movement; other molecules cannot freely pass
Isotonic: dissolved molecule concentration is the same inside and outside the cell
- Water moves in and out to balance
Hypertonic: dissolved molecule concentration higher outside
- Water moves out; intracellular concentration effectively increases; red blood cells may shrivel
Hypotonic: dissolved molecule concentration lower outside
- Water moves in; intracellular concentration decreases; cells may swell and burst

Active transport uses energy to move substances against their concentration gradient
Most common and important example: sodium-potassium pump
Na⁺/K⁺ pump details:
- Uses ATP
- 3 Na⁺ ions out of the cell
- 2 K⁺ ions into the cell
- Both moves are against their gradients and create an electrical gradient
- Resulting negative charge inside the cell is important for nervous system function
Figure 3.9 (contextual)

Large sodium gradient created by Na⁺/K⁺ pump enables secondary transport
Does not directly consume energy but relies on the Na⁺ gradient
Types:
- Symport: Na⁺ down its gradient coupled with another molecule (e.g., glucose) moving into the cell; both substances are transported inward together
- Antiport: Na⁺ down its gradient enters the cell while another molecule (e.g., H⁺) moves outward
Note: requires Na⁺/K⁺ pump activity to maintain gradient