Biology for AP Courses · Structure and Function of Plasma Membranes
Passive Transport
On this page 9 sections
In 30 seconds
Molecules are in constant, random motion — and that motion moves substances across membranes for free. Passive transport Movement across a membrane down the gradient, using no cellular energy Full entry → is movement across a membrane down the Concentration gradient Difference in concentration between two regions Full entry → (high to low) with no energy input from the cell. This topic covers the three forms: Simple diffusion Movement directly through the lipid bilayer Full entry → (through the lipid bilayer), Facilitated diffusion Down-gradient movement through a channel or carrier Full entry → (through protein channels or carriers), and Osmosis Diffusion of water toward higher solute concentration Full entry → (the Diffusion Net movement of molecules from high to low concentration Full entry → of water). Because it needs no ATP, passive transport is the cell's "free ride" — bringing in oxygen and glucose, letting out carbon dioxide, and moving water throughout the body. Gradients, equilibrium, and Tonicity Effect of a solution on cell water movement Full entry → are essential for active transport (Topic 3) and plant physiology later.
Why this matters
Passive transport is the workhorse of everyday physiology. Oxygen diffuses from the lungs into the blood and then into tissues; carbon dioxide diffuses back out. Glucose enters cells by facilitated diffusion down its gradient, and plants absorb water from the soil by osmosis. In healthcare, isotonic, hypotonic, and hypertonic solutions govern IV fluid therapy and explain what happens to red blood cells in fluid imbalance. On the AP exam, expect to classify transport as passive or active, predict net movement from a gradient, and explain why certain molecules need channels or carriers.
The college version
Core Concepts
Diffusion and concentration gradients
Diffusion is the net movement of molecules from higher to lower concentration, driven by the molecules' own kinetic energy. When a concentration gradient exists — a difference in concentration between two regions — random motion produces a net movement from high to low. Movement continues until the molecules are evenly distributed (equilibrium), at which point net movement is zero. The steeper the gradient, the faster diffusion proceeds; warmth, larger surface area, and smaller molecules also speed it up.
Simple diffusion: through the lipid bilayer
Simple diffusion is the direct passage of a substance through the phospholipid bilayer, with no protein involved. Only molecules that can dissolve in the hydrophobic core get through this way: small nonpolar molecules such as O₂, CO₂, N₂ and lipid-soluble molecules such as steroid hormones and ethanol. Water also crosses directly, though slowly. It is always down the gradient and never costs energy.
Facilitated diffusion: through proteins, still downhill
Glucose, amino acids, and ions cannot cross the hydrophobic core because they are too large, too polar, or charged. They cross by facilitated diffusion: down the concentration gradient through a transport protein, with no ATP required. There are two kinds of transport proteins:
- Channel proteins form water-filled pores. Some are always open; others are gated, opening in response to a chemical, voltage change, or mechanical stretch. Ion channels are selective for specific ions; aquaporins let water move quickly.
- Carrier proteins bind the molecule on one side, change shape, and release it on the other. Because carriers must bind, they show specificity (each carries certain molecules) and saturation (at high concentrations all carriers are busy, so the rate stops increasing). Glucose enters most cells this way, down its gradient.
Facilitated diffusion is still passive: down the gradient, no cellular energy — it just needs a protein.
Osmosis: the diffusion of water
Osmosis is the diffusion of water across a selectively permeable membrane toward the side with higher solute concentration — water moves down its own gradient, toward the region where water is less concentrated because more solute is dissolved there. Aquaporins speed osmosis but are not required. The pressure that would stop osmosis is the Osmotic pressure Pressure needed to stop osmosis; rises with solute concentration Full entry →; more solute means higher osmotic pressure.
Tonicity: what solutions do to cells
Tonicity describes how a solution affects water movement into or out of a cell:
- Isotonic — same solute concentration as the cell interior; no net water movement; cells keep their shape.
- Hypotonic — more dilute; water enters; animal cells swell and can burst (hemolysis); plant cells become turgid as the central vacuole fills.
- Hypertonic — more concentrated; water leaves; animal cells shrink (crenation); plant cells plasmolyze as the membrane pulls away from the wall.
Plants rely on Turgor pressure Outward water pressure inside a plant cell against the wall Full entry → (the outward push of water against the cell wall) to stand upright — a wilted plant is one whose cells have lost water. Tonicity terms are also the vocabulary of IV fluids.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Facilitated diffusion | Active transport | Both use carrier proteins, but facilitated diffusion is down the gradient with no energy; active transport goes against the gradient and requires ATP |
| Osmosis | Diffusion of solutes | Osmosis is specifically the movement of water; diffusion can move any molecule |
| Hypotonic | Hypertonic | Hypotonic = more dilute outside → water enters, cell swells; hypertonic = more concentrated outside → water leaves, cell shrinks |
| Channel protein | Carrier protein | A channel is an open (often gated) pore; a carrier binds the molecule and changes shape |
| Equilibrium | No movement | At equilibrium molecules keep moving randomly; only net movement is zero |
| Osmotic pressure | Turgor pressure | Osmotic pressure is a property of a solution; turgor pressure is the actual outward pressure inside a plant cell |
| Water moves toward higher water concentration | Water moves toward higher solute concentration | Water is less concentrated where solute is high, so water moves toward the solute-rich side |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine a crowded room with an open door to an empty room: more people wander into the empty room than back because one side is crowded — that is diffusion, and it needs no effort. Now imagine a big box that will not fit through the door: a helper opens a special hatch for it (facilitated diffusion — still no energy, just a door). Water plays by its own rules: if one side of a wall is very salty, water moves toward the salty side — that is osmosis. A balloon shrinks in salt water and puffs up in fresh water.
Worked example
A classic thought experiment (and common AP free-response setup) places identical red blood cells in three beakers. Beaker 1 holds 0.9% salt — roughly isotonic; water moves in and out equally, so cells keep their disc shape. Beaker 2 holds distilled water — hypotonic; water diffuses in, cells swell and, if the gradient is steep enough, burst (hemolysis). Beaker 3 holds very salty solution — hypertonic; water leaves and cells shrivel (crenate). The same logic explains why plant cells become turgid in fresh water and plasmolyze in salt water — and why clinicians choose isotonic IV fluids for routine rehydration (actual choices depend on the patient's status and current guidelines).
Key takeaways
- Passive transport = no ATP, always down the gradient (high → low concentration).
- Simple diffusion: small nonpolar molecules (O₂, CO₂, steroids) straight through the bilayer.
- Facilitated diffusion: polar molecules and ions via channel proteins or carrier proteins; still down the gradient, no energy.
- Carriers show specificity and saturation; channels show ion selectivity.
- Osmosis = water moves toward higher solute concentration; aquaporins speed it up.
- Isotonic: no net water movement. Hypotonic: water enters → animal cell swells/lyses, plant cell turgid. Hypertonic: water leaves → animal cell shrinks (crenation), plant cell plasmolysis.
- Equilibrium ≠ no movement — molecules keep moving; only net movement is zero.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What single feature defines all passive transport?
Show answer
Passive transport moves substances down their concentration gradient and requires no cellular energy (no ATP).
Why can oxygen cross the membrane directly, while glucose cannot?
Show answer
Oxygen is small and nonpolar, so it dissolves through the hydrophobic core of the bilayer. Glucose is a large polar molecule that cannot cross the lipid core; it needs a carrier protein (facilitated diffusion).
How is facilitated diffusion different from simple diffusion — and what do they share?
Show answer
Simple diffusion moves small nonpolar molecules directly through the bilayer; facilitated diffusion moves them through channel or carrier proteins. Both are passive: down the gradient, no energy.
A cell is placed in a hypertonic solution. Which way does water move, and what happens to the cell?
Show answer
The solution outside is more concentrated, so water leaves the cell by osmosis; the cell shrinks (crenates in a red blood cell; plasmolyzes in a plant cell).
What is an Aquaporin Water channel protein Full entry →, and why is it useful?
Show answer
An aquaporin is a water channel that lets water cross the membrane much faster than through the bilayer alone.
What does "saturation" mean for a Carrier protein Protein that binds a molecule and changes shape Full entry →?
Show answer
Saturation means that at high concentrations all carrier proteins are busy, so the transport rate reaches a maximum and stops increasing.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Passive transport
- Movement across a membrane down the gradient, using no cellular energy
- Concentration gradient
- Difference in concentration between two regions
- Diffusion
- Net movement of molecules from high to low concentration
- Simple diffusion
- Movement directly through the lipid bilayer
- Facilitated diffusion
- Down-gradient movement through a channel or carrier
- Channel protein
- Protein pore through the membrane, often selective or gated
- Carrier protein
- Protein that binds a molecule and changes shape
- Aquaporin
- Water channel protein
- Osmosis
- Diffusion of water toward higher solute concentration
- Osmotic pressure
- Pressure needed to stop osmosis; rises with solute concentration
- Tonicity
- Effect of a solution on cell water movement
- Turgor pressure
- Outward water pressure inside a plant cell against the wall
- Plasmolysis
- Shrinking of plant cell cytoplasm away from the wall
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.