---
title: "Isotonic — AP Biology Definition & Exam Guide"
description: "Isotonic means equal solute concentration inside and outside a cell, so there's no net water movement. Key to osmosis, tonicity, and osmoregulation in AP Bio Unit 2."
canonical: "https://fiveable.me/ap-bio/key-terms/isotonic"
type: "key-term"
subject: "AP Biology"
unit: "Unit 2"
---

# Isotonic — AP Biology Definition & Exam Guide

## Definition

In AP Biology, isotonic describes a solution whose solute concentration equals that inside a cell, so water moves equally in both directions and there is no net movement across the membrane.

## What It Is

Isotonic is one of three words you use to compare two solutions: **isotonic, [hypotonic](/ap-bio/unit-2/tonicity-osmoregulation/study-guide/i3qUckt9PGfT4pQlHq5B "fv-autolink"), and [hypertonic](/ap-bio/key-terms/hypertonic "fv-autolink")**. "Iso" means equal, so an isotonic solution has the *same* solute concentration as the inside of the cell. Because the concentrations match, water still moves across the membrane in both directions, but the amounts cancel out. The result is **no net movement of water** and a cell that holds its shape.

Think of it as a tug-of-war where both sides pull equally hard, so the rope doesn't move. The cell isn't shrinking and it isn't swelling. Connect this to **[water potential](/ap-bio/key-terms/water-potential "fv-autolink") (ψ = ψₚ + ψₛ)**: in an isotonic situation the water potential inside the cell equals the water potential outside, so there's no gradient driving water one way or the other. This is the calm, balanced state, and it's the comparison point you measure hypotonic and hypertonic against.

## Why It Matters

Isotonic lives in **[Unit 2](/ap-bio/unit-2 "fv-autolink"): Cells**, specifically topic 2.7 on tonicity and osmoregulation. It directly supports **[AP Bio](/ap-bio "fv-autolink") 2.7.A**, which asks you to explain how concentration gradients drive movement across membranes, and **AP Bio 2.7.B**, which connects water balance to the survival of whole organisms. The big idea is that water moves by osmosis from high water potential (low solute) to low water potential (high solute). Isotonic is the special case where that gradient is zero. Mastering it lets you reason about every osmosis scenario, predict whether a cell swells, shrinks, or stays put, and tie cell-level physics to homeostasis in living things.

## Connections

### Hypertonic and Hypotonic (Unit 2)

Isotonic only makes sense next to its two neighbors. Hypotonic means lower outside [solute](/ap-bio/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ "fv-autolink"), so water rushes in; hypertonic means higher outside solute, so water leaves. Isotonic is the balance point between them where neither happens.

### [Osmolarity (Unit 2)](/ap-bio/key-terms/osmolarity)

[Osmolarity](/ap-bio/key-terms/osmolarity "fv-autolink") is the actual number you compare to decide tonicity. When the osmolarity inside the cell equals the osmolarity of the surrounding fluid, that solution is isotonic by definition.

### Contractile Vacuole and Osmoregulation (Unit 2)

Freshwater protists live in hypotonic surroundings, so water constantly leaks in. Their [contractile vacuole](/ap-bio/key-terms/contractile-vacuole "fv-autolink") pumps that excess water back out, doing the work to keep the cell from bursting since the environment isn't conveniently isotonic.

### Passive Transport and Concentration Gradient (Unit 2)

Osmosis is passive transport, meaning it needs no ATP. An isotonic solution shows what happens when the concentration gradient hits zero: passive movement still occurs both ways, but there's nothing pushing a net direction.

## On the AP Exam

Multiple-choice stems love to hand you two solute concentrations and ask you to label the relationship. If the inside and outside numbers are equal, the answer is isotonic, and you should be able to say the cell stays the same size with no net water movement. One common version asks what happens to a red blood cell or plant cell placed in an isotonic solution, and the correct response is that it neither swells nor shrinks. For plant cells specifically, an isotonic solution means the cell is flaccid, not turgid, because there's no inward water pressure pushing against the wall. No released FRQ has used "isotonic" verbatim, but the tonicity reasoning behind it shows up in osmoregulation and water-potential free-response work, where you may calculate ψ using ψ = ψₚ + ψₛ and explain why water does or doesn't move.

## isotonic vs hypertonic

Both compare a solution to the cell, but they're opposites in effect. Isotonic means equal solute, so no net water movement and the cell stays the same. Hypertonic means the surrounding solution has MORE solute, so water leaves the cell and it shrinks (in plants, the membrane pulls away from the wall, called plasmolysis).

## Key Takeaways

- Isotonic means the solute concentration outside a cell equals the concentration inside, so there is no net movement of water across the membrane.
- Water still crosses the membrane in both directions in an isotonic solution; the two flows just cancel out.
- A red blood cell in an isotonic solution keeps its normal shape, neither swelling nor shrinking.
- A plant cell in an isotonic solution becomes flaccid because there's no inward water pressure to make it turgid.
- Isotonic is the zero-gradient case for water potential: ψ inside equals ψ outside, so osmosis has no net direction.
- You judge tonicity by comparing osmolarity, and equal osmolarity on both sides is what makes a solution isotonic.

## FAQs

### What does isotonic mean in AP Biology?

Isotonic describes a solution with the same solute concentration as the inside of a cell. Because the concentrations match, there's no net movement of water across the membrane and the cell stays the same size.

### Does water stop moving in an isotonic solution?

No. Water keeps moving across the membrane in both directions, but equal amounts move in and out, so there is no *net* movement. That's a common trap on the exam, so say "no net movement," not "no movement."

### How is isotonic different from hypertonic and hypotonic?

Isotonic means equal solute, so no net water movement. Hypotonic means lower solute outside, so water enters and the cell swells. Hypertonic means higher solute outside, so water leaves and the cell shrinks.

### What happens to a plant cell in an isotonic solution?

The plant cell becomes flaccid. There's no net water entering, so there's no pressure pushing the membrane against the cell wall, which means the cell isn't turgid and isn't plasmolyzed either.

### How do you use water potential to explain isotonic conditions?

Use ψ = ψₚ + ψₛ. In an isotonic situation the water potential inside the cell equals the water potential of the surroundings, so there's no gradient and water has no net direction to flow.

## Related Study Guides

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

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