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AP Biology Practice Quiz: Tonicity and Osmoregulation

Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026

Test your understanding with short quizzes. This quiz has 14 questions to check your progress.

Question 1 of 14

According to the provided text, what fundamental principle governs the movement of water via osmosis?

All Questions (14)

According to the provided text, what fundamental principle governs the movement of water via osmosis?

A) Water moves from areas of low water potential to areas of high water potential.

B) Water moves from areas of high water potential to areas of low water potential.

C) Water movement is driven by active transport, requiring energy.

D) Water moves to equalize solute concentrations, regardless of water potential.

Correct Answer: B

The provided content explicitly states, 'Water moves by osmosis from areas of high water potential to areas of low water potential.'

What is the primary factor that affects the movement of molecules across membranes?

A) The size of the cell

B) The temperature of the external environment

C) The presence of concentration gradients

D) The age of the organism

Correct Answer: C

The content specifies that concentration gradients affect the movement of molecules across membranes.

A plant cell is placed in a solution, and water is observed to move into the cell, causing it to become turgid. How would the external environment be classified relative to the cell?

A) Isotonic

B) Hypertonic

C) Hypotonic

D) Saturated

Correct Answer: C

The external environment is hypotonic. Water moves by osmosis from an area of high water potential (the hypotonic solution) to an area of low water potential (the cell), causing the cell to swell.

If a cell is placed in a hypertonic solution, what is the expected net movement of water?

A) Water will move out of the cell.

B) Water will move into the cell.

C) There will be no net movement of water.

D) The direction of water movement cannot be predicted.

Correct Answer: A

A hypertonic solution has a lower water potential than the cell. Therefore, water will move by osmosis from the area of higher water potential (inside the cell) to the area of lower water potential (outside the cell).

The constant movement of molecules across membranes is identified as essential for which of the following biological processes?

A) Reproduction and genetic variation

B) Growth and homeostasis

C) Photosynthesis and cellular respiration

D) Nervous and endocrine signaling

Correct Answer: B

The provided text directly states, 'The constant movement of molecules across membranes is essential for maintaining growth and homeostasis.'

What is the primary function of osmoregulation in an organism?

A) To generate energy through the breakdown of solutes.

B) To make the internal environment identical to the external environment.

C) To control internal solute concentration and water potential.

D) To stop the movement of water across all cell membranes.

Correct Answer: C

The content states that 'Osmoregulation maintains water balance and allows organisms to control their internal solute concentration and water potential.'

When a cell is in an isotonic external environment, what is the net movement of water across its membrane?

A) Water moves rapidly out of the cell.

B) Water moves rapidly into the cell.

C) There is no net movement of water.

D) The membrane becomes impermeable to water.

Correct Answer: C

In an isotonic environment, the water potential inside and outside the cell is equal. This results in no net movement of water across the membrane, as water moves in and out at equal rates.

Why are osmoregulatory mechanisms fundamentally important for living things?

A) They are the only way cells can communicate with each other.

B) They provide structural support to the entire organism.

C) They are the primary source of metabolic energy.

D) They contribute to the health and survival of organisms.

Correct Answer: D

The provided content explicitly states that 'osmoregulatory mechanisms contribute to the health and survival of organisms.'

A U-tube is divided by a selectively permeable membrane. Side A contains a solution with a high water potential, and Side B contains a solution with a low water potential. What will be the net direction of water movement?

A) From Side B to Side A.

B) From Side A to Side B.

C) There will be no net movement of water.

D) Solutes will move from Side B to Side A.

Correct Answer: B

This is a direct application of the principle stated in the text: 'Water moves by osmosis from areas of high water potential to areas of low water potential.' Therefore, water will move from Side A to Side B.

An organism's ability to maintain a stable internal water balance despite changes in its external environment is a direct result of which process?

A) Osmosis

B) Osmoregulation

C) Homeostasis

D) Concentration gradients

Correct Answer: B

The content defines osmoregulation as the process that 'maintains water balance and allows organisms to control their internal solute concentration and water potential,' which is crucial for dealing with external environmental changes.

If an organism's osmoregulatory mechanisms fail while it is in a hypotonic environment, what is the most likely consequence for its cells?

A) The cells will lose water and shrink due to a lack of solute control.

B) The cells will take on excess water, potentially leading to lysis (bursting).

C) The cells will become isotonic with the environment, achieving a stable state.

D) The cells will stop all transport across their membranes to prevent damage.

Correct Answer: B

A hypotonic environment has a higher water potential than the cell's interior. Without osmoregulatory mechanisms to counteract this, water will continuously move into the cells from the high water potential environment, causing them to swell and possibly burst. This demonstrates the importance of osmoregulation for survival.

Which statement best describes the relationship between osmosis and osmoregulation?

A) They are unrelated processes, one physical and one chemical.

B) Osmosis is the active, energy-requiring form of osmoregulation.

C) Osmoregulation involves mechanisms that control the passive process of osmosis to maintain water balance.

D) Osmoregulation is the movement of water, and osmosis is the control of that movement.

Correct Answer: C

Osmosis is the physical movement of water down its water potential gradient. Osmoregulation is the biological process by which an organism actively controls its internal water and solute levels, thereby managing the effects of osmosis to ensure health and survival.

How does the establishment of concentration gradients relate to an organism's ability to maintain homeostasis through osmoregulation?

A) Concentration gradients are eliminated by osmoregulation to achieve a static internal state.

B) Osmoregulation creates water potential differences (a type of concentration gradient) that drive the necessary movement of water to maintain internal stability.

C) Homeostasis is achieved when concentration gradients across all membranes are equal to zero.

D) Concentration gradients only affect solute movement, while osmoregulation only affects water movement, so they are independent.

Correct Answer: B

Osmoregulation allows an organism to control its internal solute concentration, which in turn establishes a specific water potential. This creates a gradient relative to the external environment, and managing the resulting water movement (osmosis) is key to maintaining homeostasis and growth.

A cell that neither gains nor loses a net amount of water when placed in a solution is in an environment that is _________ relative to its cytoplasm.

A) hypertonic

B) hypotonic

C) isotonic

D) at a lower water potential

Correct Answer: C

An isotonic environment has the same water potential as the cell's cytoplasm. This means that water moves across the membrane in both directions at an equal rate, resulting in no net gain or loss of water.