In AP Bio, a hypertonic solution is one where the solute concentration outside the cell is higher than inside, so water moves out of the cell by osmosis, causing it to shrink (crenation in animal cells, plasmolysis in plant cells).
A hypertonic solution has more dissolved stuff (solute) on the outside of the cell than on the inside. Water always moves toward the higher solute concentration during osmosis, so in a hypertonic environment water leaves the cell. The cell shrivels.
This ties directly to EK 2.5.A.1 and 2.5.A.2: membranes are selectively permeable, which lets concentration gradients build up across them, and water moves down its own gradient by passive transport (no ATP needed). "Hypertonic" is just a comparison word. It only makes sense relative to the inside of the cell. Flip it: if the inside had more solute, the outside would be hypotonic and water would rush in. An isotonic solution means the concentrations match and there's no net water movement. Remember that water follows solute, so it heads toward the saltier side.
This lives in Unit 2: Cells, Topic 2.5 Membrane Transport, and supports learning objective AP Bio 2.5.A (how organisms maintain solute and water balance). The whole point of the membrane transport topic is that a selectively permeable membrane plus a concentration gradient drives water movement, and hypertonic/hypotonic/isotonic is how you label which way water flows. It connects to the big idea of homeostasis: cells and organisms constantly work to keep their internal water and solute levels stable, and a hypertonic environment is the classic threat to that balance.
Keep studying AP Biology Unit 2
Crenation and Plasmolysis (Unit 2)
These are the visible results of a hypertonic solution. In animal cells the membrane crinkles inward (crenation); in plant cells the membrane pulls away from the cell wall as the vacuole shrinks (plasmolysis). Same cause, different look because plants have a wall.
Passive Transport and Osmosis (Unit 2)
Water leaving a cell in a hypertonic solution is passive transport (EK 2.5.A.2), so it needs no ATP. Water just slides down its own concentration gradient. Hypertonic conditions are basically osmosis with a label telling you which direction it goes.
Active Transport and the Contractile Vacuole (Unit 2)
A freshwater Paramecium normally pumps incoming water out using a contractile vacuole, which takes energy (EK 2.5.A.3). Drop it in a hypertonic solution and water stops flooding in, so the pumping rate plummets. This shows how active transport defends water balance when osmosis turns against the cell.
Expect this in MCQs that hand you a scenario and ask which way water moves or which cells survive best. One classic stem asks which cells are LEAST affected by a hypertonic solution because of their cell wall (plant and bacterial cells resist shrinking). Another gives a Paramecium whose contractile vacuole slows down in hypertonic saline and asks you to connect that evidence to reduced water influx. You may also see a plant cell diagram with the membrane pulled off the wall, asking you to identify the water-potential principle at work. The skill is always the same: figure out where solute is highest, then predict that water moves there. No released FRQ uses 'hypertonic' word-for-word, but it supports the kind of osmosis-and-homeostasis reasoning free-response questions reward.
They're opposites and easy to flip. Hypertonic means MORE solute outside, so water leaves and the cell shrinks. Hypotonic means LESS solute outside, so water enters and the cell swells (and can burst, or lyse). Trick: 'hyper' sounds big, but it makes the cell small because the solute is on the outside pulling water out.
A hypertonic solution has a higher solute concentration outside the cell than inside, so water leaves the cell by osmosis.
Water always moves toward the side with more solute, which is why the cell shrinks in a hypertonic environment.
Animal cells crenate (shrivel) and plant cells plasmolyze (membrane pulls off the wall) in hypertonic solutions.
This is passive transport, so no ATP is needed for the water to move out (EK 2.5.A.2).
Cells with rigid walls (plants, bacteria) resist hypertonic damage better than wall-less animal cells.
'Hypertonic' is always a comparison to the inside of the cell, so always identify which side has more solute first.
It's a solution with a higher solute concentration outside the cell than inside, which causes water to move out of the cell by osmosis and the cell to shrink. It maps to Topic 2.5 Membrane Transport and learning objective AP Bio 2.5.A.
It shrinks. Because there's more solute outside, water leaves the cell to move toward the higher solute concentration. Animal cells crenate and plant cells undergo plasmolysis.
They're opposites. Hypertonic means more solute outside, so water leaves and the cell shrinks; hypotonic means less solute outside, so water enters and the cell swells or bursts. An isotonic solution has equal concentrations and no net water movement.
Their rigid cell walls limit how much the cell can collapse. Even though water still leaves and the membrane plasmolyzes, the wall keeps the overall structure intact, which is exactly what a common AP MCQ tests.
No. Water leaving the cell in a hypertonic solution is osmosis, a form of passive transport (EK 2.5.A.2), so it happens down the gradient without any energy input. The energy cost shows up only when a cell fights osmosis, like a contractile vacuole pumping water against the gradient.