Membrane Transport

Passive transport: no energy, moves with the concentration gradient (high→low). Types: simple diffusion (small nonpolar molecules directly through bilayer, e.g. O2/CO2), facilitated diffusion (protein-mediated — channels or carriers — for large/polar molecules and ions), osmosis (water across a semipermeable membrane).
Tonicity: hypertonic (higher solute outside) → water leaves cell → shrinks. Hypotonic (lower solute outside) → water enters → swells/bursts. Isotonic (equal) → balanced, normal size.
Osmotic pressure π = iMRT (i = van't Hoff factor, M = molarity, R = gas constant, T = Kelvin) — a colligative property, depends only on particle number, not identity.
Active transport: requires energy, moves against the gradient (low→high). Primary active transport uses ATP directly via ATPases (e.g. sodium-potassium pump). Secondary active transport uses an existing electrochemical gradient — symport (same direction) or antiport (opposite directions), e.g. sodium-glucose symporter.
Bulk transport: endocytosis (membrane invaginates, engulfs material — pinocytosis = fluids, phagocytosis = large particles) and exocytosis (vesicles fuse with membrane, release contents) — both use vesicles.

Facilitated diffusion: Passive, protein-mediated transport (channels or carriers) — no energy required, still follows the gradient.
Osmotic pressure: Colligative property quantifying the force driving osmosis; depends on particle count, not identity.
Primary active transport: Uses ATP directly (via an ATPase) to move a solute against its gradient.
Secondary active transport: Uses energy stored in an existing electrochemical gradient (not ATP directly) to move a solute against its gradient.
Symport / Antiport: Secondary active transport moving two particles in the same direction / opposite directions.
Pinocytosis / Phagocytosis: Endocytosis of fluids and dissolved substances / large particles or solids.

Feature	Passive vs. Active
Energy required?	No / Yes (ATP, direct or indirect)
Direction relative to gradient	With the gradient (high→low) / Against the gradient (low→high)
Examples	Simple/facilitated diffusion, osmosis / Na⁺-K⁺ pump, Na⁺-glucose symporter

Solution	Water Movement → Cell Result
Hypertonic (higher solute outside)	Water leaves the cell → shrinks
Hypotonic (lower solute outside)	Water enters the cell → swells, may burst
Isotonic (equal)	Balanced in/out → normal size maintained

π = iMRT

Osmotic pressure is colligative — depends only on the number of dissolved particles, not their identity.
In cells, osmotic pressure acts against the cell membrane (not a fluid column), so solute balance must be tightly regulated to avoid excess water gain/loss.

Facilitated diffusion is still passive (no energy) even though it needs a protein — don't classify it as active transport just because a channel/carrier is involved.
Osmotic pressure depends on the NUMBER of particles (accounting for dissociation via i), not the type of solute — 1 M NaCl (i≈2) produces roughly double the osmotic pressure of 1 M glucose (i=1).
Secondary active transport doesn't use ATP directly, but it's still "active" because it moves a solute against its gradient, powered indirectly by a gradient primary active transport created.

What two protein types carry out facilitated diffusion?

What happens to a cell placed in a hypotonic solution?

What does the van't Hoff factor (i) represent in the osmotic pressure equation?

What powers secondary active transport if not ATP directly?

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