---
title: "Hypertonic — AP Bio Definition & Exam Guide"
description: "Hypertonic means the outside solution has more solute than the cell, so water leaves the cell. Key for osmosis, water potential, and Unit 2 tonicity questions."
canonical: "https://fiveable.me/ap-bio/key-terms/hypertonic"
type: "key-term"
subject: "AP Biology"
unit: "Unit 2"
---

# Hypertonic — AP Bio Definition & Exam Guide

## Definition

In AP Bio, a hypertonic solution has a higher solute concentration than the inside of the cell, so water moves out of the cell by osmosis (from high water potential to low water potential), often causing the cell to shrink or lose turgor pressure.

## What It Is

Hypertonic describes the external environment when it has *more* dissolved stuff ([solute](/ap-bio/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ "fv-autolink")) than the inside of the cell. "Hyper" means more, so think "more solute outside." Because water always moves toward the saltier side, water flows *out* of the cell into the [hypertonic solution](/ap-bio/key-terms/hypertonic-solution "fv-autolink"). The cell shrinks.

Here's the cleaner way the CED wants you to say it: water moves by osmosis from regions of **high water potential** to regions of **low water potential**, which is the same as moving from low solute ([hypotonic](/ap-bio/unit-2/tonicity-osmoregulation/study-guide/i3qUckt9PGfT4pQlHq5B "fv-autolink")) toward high solute (hypertonic). More solute means lower water potential, and water chases the lower water potential. So in a hypertonic environment, the outside has the lower water potential, and water leaves. If you ever get tangled, remember the equation ψ = ψₚ + ψₛ. Adding solute makes the solute potential (ψₛ) more negative, which drags total water potential down.

## Why It Matters

This term lives in **[Unit 2](/ap-bio/unit-2 "fv-autolink"): Cells**, specifically topic **2.7 Tonicity and Osmoregulation**. It directly supports learning objective **[AP Bio](/ap-bio "fv-autolink") 2.7.A** (how concentration gradients drive molecule movement across membranes) and connects to **AP Bio 2.7.B** (how osmoregulation keeps organisms alive). Tonicity is one of those concepts the exam loves because it ties together membranes, passive transport, water potential math, and homeostasis all at once. If you can correctly label a setup as hypertonic and predict which way water flows, you've shown you understand osmosis, not just memorized a vocab word.

## Connections

### Hypotonic and Isotonic (Unit 2)

These three are a matched set. Hypertonic = more solute outside (water leaves), hypotonic = less solute outside (water enters), [isotonic](/ap-bio/key-terms/isotonic "fv-autolink") = equal (no net movement). You can't really know one without the other two, because tonicity is always a comparison.

### [Water Potential (Unit 2)](/ap-bio/key-terms/water-potential)

Hypertonic is the words version of the math. A hypertonic solution has the more negative (lower) [water potential](/ap-bio/key-terms/water-potential "fv-autolink"), and ψ = ψₚ + ψₛ lets you prove it. Adding solute makes ψₛ more negative, so water flows toward it.

### Contractile Vacuole and Osmoregulation (Unit 2)

Freshwater protists live in a hypotonic world, so water constantly floods in. Their [contractile vacuole](/ap-bio/key-terms/contractile-vacuole "fv-autolink") pumps that water back out. It's the body's answer to never being in a comfortable isotonic balance, which is exactly what 2.7.B is testing.

### [Passive Transport (Unit 2)](/ap-bio/key-terms/passive-transport)

Osmosis is passive transport, no ATP needed. Water moving out of a cell into a hypertonic solution happens for free, driven only by the water potential gradient. That's why tonicity questions sit next to diffusion and facilitated diffusion.

## On the AP Exam

Multiple-choice questions hand you two solute concentrations and ask you to name the relationship. If the outside is 0.3% and the inside is 0.9%, the outside is hypotonic and the inside is hypertonic, so water enters the cell. Flip the numbers and you flip the answer. A common plant-cell twist asks what drops when a cell sits in a hypertonic solution: the answer is **turgor pressure (pressure potential, ψₚ)**, because water leaving the cell pulls away from the cell wall. FRQs rarely use the bare word "hypertonic," but the 2019 Long FRQ on aquatic protists leans on osmoregulation, the exact survival problem tonicity creates. Your job is to predict water direction, justify it with high-to-low water potential, and connect it to whether the cell shrinks, swells, or stays put.

## hypertonic vs hypotonic

These trip up everyone. Hypertonic = MORE solute outside the cell, so water leaves and the cell shrinks. Hypotonic = LESS solute outside, so water enters and the cell swells. The trick is that both words describe the OUTSIDE solution relative to the cell, not the cell itself. When in doubt, find the side with more solute, then send the water there.

## Key Takeaways

- Hypertonic means the solution outside the cell has a higher solute concentration than the inside, so water moves out and the cell shrinks.
- Water always moves from high water potential to low water potential, which is the same as moving from hypotonic toward hypertonic.
- Adding solute makes solute potential (ψₛ) more negative and lowers total water potential, so the hypertonic side has the lower water potential.
- In a plant cell placed in a hypertonic solution, turgor pressure (ψₚ) drops as water leaves the cell.
- Osmosis is passive transport, so water leaving a cell in a hypertonic environment needs no ATP, only a gradient.

## FAQs

### What does hypertonic mean in AP Bio?

A hypertonic solution has a higher solute concentration than the inside of the cell. Because water moves toward the saltier side, water leaves the cell by osmosis and the cell shrinks.

### Does water move into or out of a cell in a hypertonic solution?

Out. The hypertonic solution outside has more solute and lower water potential, so water flows from the cell (higher water potential) into the surrounding solution (lower water potential).

### How is hypertonic different from hypotonic?

Hypertonic means more solute OUTSIDE the cell, so water leaves and the cell shrinks. Hypotonic means less solute outside, so water enters and the cell swells. Both terms describe the outside solution relative to the cell.

### Is a cell in a hypertonic solution going to burst?

No, that's backwards. Cells burst (lyse) in HYPOTONIC solutions when too much water rushes in. In a hypertonic solution, water leaves, so the cell shrivels (crenates) instead.

### Which part of water potential decreases when a plant cell is in a hypertonic solution?

Pressure potential (ψₚ), also called turgor pressure. As water leaves the cell, the cell membrane pulls away from the wall and ψₚ drops.

## Related Study Guides

- [2.7 Tonicity and Osmoregulation](/ap-bio/unit-2/tonicity-osmoregulation/study-guide/i3qUckt9PGfT4pQlHq5B)

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