Passive transport

Selectively Permeable Membrane: Plasma membranes control which substances pass through to maintain cellular life.
Passive Transport: Natural movement of substances from an area of higher concentration to lower concentration without expending cellular energy (ATP).
Concentration Gradient: A physical space where a single substance exists in a range of concentrations.

Definition: A passive transport process where a single substance moves down its concentration gradient until evenly distributed.
Dynamic Equilibrium: A state reached when a substance is evenly distributed; molecules continue moving, but there is no net movement.
Concentration gradients store potential energy that dissipates as diffusion occurs.

Extent of Gradient: Larger concentration differences increase diffusion rate.
Molecular Mass: Lighter molecules move and diffuse faster than heavier ones.
Temperature: Higher temperatures increase thermal energy and molecule velocity, accelerating diffusion.
Solvent Density: Denser solvent media slow molecular movement (e.g., dehydration thickens cytoplasm, slowing diffusion and impairing cell/neuron function).
Solubility: Nonpolar (lipid-soluble) materials cross the membrane faster than polar materials.
Surface Area & Membrane Thickness: Increased surface area speeds up diffusion; thicker membranes reduce it.
Distance: Long distances slow diffusion, limiting maximum cell size.
Filtration: Substance movement down a concentration gradient through a membrane, often accelerated by fluid pressure (e.g., blood pressure forcing solutes into kidney tubules).

Definition: Movement of polar or charged substances across the hydrophobic plasma membrane via integral transport proteins without spending energy.
Channel Proteins:
- Transmembrane proteins with hydrophilic cores providing hydrated pathways.
- Transport substances rapidly (tens of millions of molecules per second).
- Can be continuously open or gated (opened/closed by specific signals or ions).
- Aquaporins: Specialized channel proteins for high-speed water movement.
Carrier Proteins:
- Bind specific molecules, triggering a conformational (shape) change to transfer them across.
- Slower transport rate (1,000 to 1,000,000 molecules per second).
- Have finite transport capacities and can become saturated (e.g., excess glucose spilling into urine in diabetes).

Definition: The movement of free water molecules across a selectively permeable membrane down water's concentration gradient.
Solutes unable to cross the membrane drive water movement from low solute concentration (high free water) to high solute concentration (low free water).

Definition: The ability of an extracellular solution to alter a cell's volume by affecting osmosis.
Osmolarity: Total solute concentration of a solution.
Types of Solutions:
- Hypotonic: Extracellular fluid has lower osmolarity (lower solute, higher water concentration) than cell cytoplasm. Water enters the cell, causing it to swell or burst (lysis).
- Hypertonic: Extracellular fluid has higher osmolarity (higher solute, lower water concentration) than cell cytoplasm. Water leaves the cell, causing it to shrivel.
- Isotonic: Extracellular fluid and cytoplasm have equal osmolarity. No net water movement occurs.