---
title: "Turgor Pressure — AP Bio Definition & Exam Guide"
description: "Turgor pressure is the water pressure inside a plant cell's central vacuole that keeps it firm. Learn how it ties to vacuoles, tonicity, and AP Bio Unit 2."
canonical: "https://fiveable.me/ap-bio/key-terms/turgor-pressure"
type: "key-term"
subject: "AP Biology"
unit: "Unit 2"
---

# Turgor Pressure — AP Bio Definition & Exam Guide

## Definition

Turgor pressure is the outward pressure that water inside a plant cell's large central vacuole exerts against the cell wall, keeping the cell firm and maintaining the plant's shape and rigidity (CED Topic 2.1).

## What It Is

Turgor pressure is the force that water pushes outward against the [cell wall](/ap-bio/key-terms/cell-wall "fv-autolink") in a plant cell. Here's the setup: a plant cell has a big **central vacuole** that fills up with water and dissolved stuff. As it fills, it swells and presses the [cell membrane](/ap-bio/unit-2/cell-structure-function/study-guide/znjrRPCY6596o2nWt05n "fv-autolink") against the rigid cell wall. That push is turgor pressure, and it's what keeps a plant standing up instead of flopping over.

This connects directly to CED [Topic 2.1](/ap-bio/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9 "fv-autolink"), which is all about how subcellular components and organelles contribute to cell function. The central vacuole is part of the endomembrane system (EK 2.1.A.2), and one of its big jobs is storage. By holding water, it does mechanical work for the whole plant. When a plant wilts, it's literally losing turgor pressure as water leaves those vacuoles. So when you water a droopy plant and it perks back up, you're watching turgor pressure get restored in real time.

## Why It Matters

Turgor pressure lives in **[Unit 2](/ap-bio/unit-2 "fv-autolink"): Cells**, under Topic 2.1, and it supports learning objective **[AP Bio](/ap-bio "fv-autolink") 2.1.A**: explaining how the structure and function of organelles contribute to the function of cells. It's the clearest example of an organelle (the central vacuole) doing a structural job for the entire organism. On the exam, this term is a bridge between cell structure and the bigger theme of how cells maintain themselves. It only fully makes sense once you also understand water movement across membranes, which is why it keeps showing up in questions about tonicity and osmosis.

## Connections

### Central Vacuole and the Endomembrane System (Unit 2)

Turgor pressure exists because the large [central vacuole](/ap-bio/key-terms/central-vacuole "fv-autolink") stores water. The vacuole is part of the endomembrane system (EK 2.1.A.2), so this is your concrete example of an organelle's structure (a big water-filled sac) directly producing a function (cell rigidity).

### Osmosis and Tonicity (Unit 2)

Turgor pressure rises and falls based on which way water moves. In a hypotonic solution, water flows in and turgor pressure goes up; in a [hypertonic solution](/ap-bio/key-terms/hypertonic-solution "fv-autolink"), water leaves and the cell loses turgor and plasmolyzes. You can't explain turgor without explaining osmosis first.

### [Cell Lysis (Unit 2)](/ap-bio/key-terms/cell-lysis)

This is the animal-cell contrast that makes turgor pressure click. An animal cell in a [hypotonic](/ap-bio/unit-2/tonicity-osmoregulation/study-guide/i3qUckt9PGfT4pQlHq5B "fv-autolink") solution swells and bursts (lysis) because it has no cell wall. A plant cell doesn't burst because the rigid wall pushes back, and that pushback is turgor pressure doing its job.

## On the AP Exam

Expect this term in multiple-choice questions about plant cells in different solutions. A classic stem places plant cells in a hypertonic solution and asks what happens (answer: water leaves the vacuole, turgor pressure drops, and the cell plasmolyzes). Other questions ask you to name the pressure that keeps a plant cell firm, or to compare vacuole function across tissues like leaf cells versus root storage cells. No released FRQ uses 'turgor pressure' by name, but the underlying logic (organelle structure producing a function, water moving by osmosis) shows up in free-response prompts about cell structure and transport. What you need to DO: connect the central vacuole's water storage to whole-cell rigidity, and predict turgor changes from the tonicity of the surrounding solution.

## turgor pressure vs cell lysis

Both involve water rushing into a cell in a hypotonic solution, but the outcome is opposite. A plant cell gains turgor pressure and stays firm because its cell wall resists the swelling. An animal cell has no wall, so it just keeps swelling until it bursts, which is cell lysis. The cell wall is the whole difference.

## Key Takeaways

- Turgor pressure is water in the central vacuole pushing the membrane against the cell wall, which keeps a plant cell firm.
- It's a direct example of CED objective 2.1.A: an organelle's structure (a water-storing vacuole) producing a function (rigidity).
- In a hypotonic solution, water enters and turgor pressure increases; in a hypertonic solution, water leaves and the cell loses turgor and plasmolyzes.
- A wilting plant is a plant that has lost turgor pressure, and watering it restores that pressure.
- Plant cells gain turgor instead of bursting because the cell wall pushes back, unlike wall-less animal cells that undergo lysis.

## FAQs

### What is turgor pressure in AP Bio?

It's the outward pressure water exerts inside a plant cell's large central vacuole, pressing the membrane against the cell wall to keep the cell firm. It ties to CED Topic 2.1 as an example of how organelle structure drives cell function.

### Does turgor pressure cause plant cells to burst like animal cells do?

No. A plant cell in a hypotonic solution gains turgor pressure but doesn't burst, because its rigid cell wall pushes back against the swelling. An animal cell has no wall, so it keeps swelling and undergoes cell lysis.

### How is turgor pressure different from cell lysis?

Both start with water entering a cell in a hypotonic solution, but turgor pressure is the controlled firmness a plant cell gains because its wall resists swelling, while cell lysis is an animal cell bursting because it has no wall to stop the swelling.

### What happens to turgor pressure in a hypertonic solution?

It drops. Water leaves the vacuole by osmosis, the cell shrinks, and the membrane pulls away from the wall in a process called plasmolysis. This is the classic hypertonic-plant-cell question on the AP exam.

### Why does a plant wilt without water?

Without enough water, the central vacuoles lose water and turgor pressure falls, so the cells go limp and the plant droops. Watering it lets the vacuoles refill, turgor pressure returns, and the plant stands back up.

## Related Study Guides

- [2.1 Cell Structure and Function](/ap-bio/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9)

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