Cell Membranes & Transport — Quick Review

Apoptosis is a natural, non-harmful part of development (e.g., removing webbing between fingers). Not all cell death is bad.
Cancer involves cells that bypass checkpoint controls and divide uncontrollably; a major health concern and research focus.

Organelles are either membrane-bound or non-membrane-bound.
- Membrane-bound examples include nucleus, ER, Golgi, mitochondria, lysosomes, vesicles, etc.
- Non-membrane-bound examples include ribosomes, cytoskeleton, centrioles/centrosomes, etc.
Primary role of membranes: compartmentalization and organization (like pockets in a backpack).
Membranes are largely phospholipid bilayers:
- Hydrophobic tails (inside) and hydrophilic heads (outside).
- Some proteins span both layers (integral/transmembrane proteins); others bind to one side (peripheral proteins).
- Decorations: glycoproteins, glycolipids; cholesterol provides stability.
Membrane proteins provide: channels, carriers, pumps, receptors, enzymes, anchors, and identity markers (e.g., blood type).

Membrane transport is governed by selective permeability and concentration gradients.
Two broad transport categories:
- Passive transport: no ATP required; molecules move down their concentration gradient.
- Active transport: requires ATP; molecules move against the gradient.
Vesicular transport: ATP-dependent, uses vesicles; not gradient-driven.
Concentration gradient: difference in solute concentration across a selectively permeable membrane.
Selectively permeable membrane: some substances cross, others do not.

Diffusion (simple): movement from high to low concentration until equilibrium; no ATP.
- Small, nonpolar molecules (e.g., O$2$, CO$2$) cross directly.
- Rate depends on gradient and temperature; larger gradient and higher temperature speed up diffusion.
- Mathematical note: ext{Rate} \propto \Delta C \cdot T where \Delta C = C{\text{high}} - C{\text{low}}
Facilitated diffusion uses membrane proteins:
- Channel-mediated diffusion: ions pass through channels (some channels are leaky; still down gradient).
- Carrier-mediated diffusion: carrier protein binds the molecule, flips it to the other side; can saturate at a transport maximum V_{\max}.
Osmosis: diffusion of water across a selectively permeable membrane, from higher to lower water concentration.
- Water moves toward higher solute concentration (lower water activity).
- Aquaporins are water-channel proteins that enable bulk water movement.
- Osmotic pressure: \pi \propto C_{\text{solute}} (higher solute concentration increases pressure driving water movement).

Osmotic pressure drives water movement to equalize solute concentrations.
Tonicity compares two solutions relative to the cell interior (two-solution scenario: inside vs outside).
Isotonic: inside and outside solute concentrations are equal; no net water movement.
Hypotonic: outside solution has lower solute concentration; water moves into cell; cell may swell and undergo lysis if excessive.
Hypertonic: outside solution has higher solute concentration; water moves out of cell; cell shrivels (crenation).
Key terms:
- Lysis: cell bursting due to excessive water influx.
- Crenation: cell shrinking due to water efflux.

Primary distinctions:
- Passive transport: no ATP; down gradient toward equilibrium.
- Active transport: requires ATP; moves against gradient; never reaches equilibrium.
Vesicular transport: ATP-dependent; includes endocytosis/exocytosis; not gradient-driven.
For diffusion, some solutes cross easily; others require facilitated diffusion (channel or carrier).
Important examples to remember:
- Diffusion: O$2$, CO$2$ (simple diffusion).
- Facilitated diffusion: ions via channels; larger polar solutes via carriers.
- Osmosis: water via aquaporins when needed.

The memory cues about protein decorations and membrane composition help explain permeability and transport behavior.
The discussion of apoptosis and cancer provides context for why cellular transport and membrane integrity matter for health.