Membrane Transport

Passive transport moves substances downhill — from where they are more concentrated to where they are less — using the substances' own kinetic energy, so the cell spends no ATP.

• Simple diffusion: small nonpolar molecules (O₂, CO₂) drift directly through the lipid bilayer from high to low concentration until evenly spread. This is how oxygen enters cells and carbon dioxide leaves.
• Facilitated diffusion: ions and polar molecules (like glucose) can't cross the oily core, so they pass through channel or carrier proteins — still moving down their gradient, still no ATP.
• Osmosis: the diffusion of water across the membrane, moving toward the side with more solute (less free water). Because cells are mostly water, osmosis governs cell volume.
• Filtration: here movement is driven by pressure, not concentration, pushing water and small solutes through a membrane. In the kidneys, blood pressure filters plasma into the nephron — the first step of urine formation.

Active transport moves substances uphill — against their gradient — and therefore needs energy.

• Primary active transport uses ATP directly. The classic example is the sodium–potassium pump (Na⁺/K⁺-ATPase), which pushes 3 Na⁺ out and 2 K⁺ in per ATP, against both gradients. This maintains the high-K⁺/low-Na⁺ interior essential for nerve and muscle excitability, and it runs constantly in every cell.
• Secondary active transport uses the energy stored in a gradient (usually the Na⁺ gradient built by the pump) to drag another substance along. Glucose absorption in the intestine, for instance, rides the inward flow of sodium.

Vesicular transport moves large particles or bulk material in membrane sacs (vesicles), using ATP:

• Endocytosis brings material in (phagocytosis, "cell eating," engulfs large particles like bacteria; pinocytosis, "cell drinking," takes in fluid).
• Exocytosis sends material out — how cells secrete hormones, neurotransmitters, and mucus.

Tonicity: predicting what happens to a cell. Tonicity describes how a surrounding solution affects cell water via osmosis:

• Isotonic: equal solute concentration inside and out; no net water movement; the cell stays normal. (Normal saline, 0.9% NaCl, is isotonic to blood.)
• Hypotonic: the solution has less solute than the cell, so water moves in; the cell swells and may lyse (burst).
• Hypertonic: the solution has more solute than the cell, so water moves out; the cell shrinks (crenates).

Remember that water follows solute — it moves toward the saltier side.

How it works

Classifying a transport event:

1. Down the gradient, no ATP? → passive (diffusion/osmosis/facilitated/filtration).
2. Against the gradient, uses ATP? → active (pumps).
3. Bulk material in a vesicle? → endocytosis (in) or exocytosis (out).
4. Cell in a solution? → compare solute: isotonic (stable), hypotonic (swells), hypertonic (shrinks).

Comparisons

Common confusions

• Diffusion vs osmosis. Osmosis is specifically the diffusion of water.
• Facilitated diffusion is still passive. Using a protein does not mean using ATP; it's downhill movement.
• Hypotonic vs hypertonic effect. Water moves toward higher solute — hypertonic surroundings shrink the cell; hypotonic swell it.
• Active transport direction. Active transport moves things against the gradient (that's why it costs energy).

Memory aids

• "Passive = downhill (free); Active = uphill (costs ATP)."
• Osmosis: "water walks toward salt."
• HypO = O for "swells like a balloon" (water in); HypER = "shrivels."
• Phago = "eat" (Pac-Man); Pino = "drink."

Quick review

• Passive transport (no ATP, down the gradient): simple diffusion, facilitated diffusion, osmosis (water), and filtration (pressure).
• Active transport (uses ATP, against the gradient): the Na⁺/K⁺ pump (primary) and gradient-powered secondary transport; endo/exocytosis move bulk material.
• Tonicity: isotonic (cell normal), hypotonic (water in → swell/burst), hypertonic (water out → shrink) — water follows solute.