Diffusion and Osmosis

Diffusion and Osmosis

Middle School

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Learning Objectives

• Define passive transport, diffusion, and osmosis and explain the differences between them.

• Explain how concentration gradients move substances across the cell membrane.

• Describe facilitated diffusion and the important role that transport proteins play.

• Predict how different solutions affect animal and plant cells.

Key Vocabulary

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Diffusion

The movement of a substance from an area of high concentration to an area of low concentration.

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Osmosis

The diffusion of water molecules across a selectively permeable membrane from high to low concentration.

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Homeostasis

The process of maintaining a stable internal environment inside a cell or an entire organism.

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Passive Transport

Substances cross the cell membrane without the cell needing to use any of its own energy.

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Concentration Gradient

The measurement of how the concentration of a substance changes from one area to another.

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Facilitated Diffusion

A type of diffusion that is assisted by transport proteins located in the cell membrane.

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Passive Transport and Diffusion

• Passive transport is movement across the cell membrane that requires no cell energy.

• Diffusion is a type of passive transport from high to low concentration.

• This is also known as moving down a concentration gradient.

• Movement stops at equilibrium, when molecules are distributed evenly on both sides.

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Multiple Choice

What is the primary driver of diffusion?

• Energy from the cell (ATP)

• The concentration gradient

• The size of the cell

• The temperature of the environment

Osmosis: The Diffusion of Water

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Hypertonic Solution

• A hypertonic solution has a higher solute concentration outside the cell.

• This higher concentration causes water to move out of the cell.

• As a result, the cell loses water and begins to shrivel.

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Hypotonic Solution

• A hypotonic solution has a lower solute concentration outside the cell.

• This lower concentration causes water to move into the cell.

• As a result, the cell takes in water and swells up.

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Isotonic Solution

• An isotonic solution has an equal solute concentration inside and outside.

• Water molecules move in and out of the cell at equal rates.

• There is no net water movement, so the cell remains stable.

Multiple Choice

What happens to a cell when it is placed in a hypotonic solution?

• It shrivels because water leaves the cell.

• It swells because water enters the cell.

• Its size does not change.

• It builds a cell wall for protection.

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Facilitated Diffusion

• This passive transport helps large or water-loving molecules cross the cell membrane.

• It uses transport proteins and does not require the cell to use energy.

• Channel proteins form a tunnel for small ions and water to pass through.

• Carrier proteins change shape to carry specific molecules across the membrane.

Multiple Choice

What is the role of transport proteins in facilitated diffusion?

• They use cellular energy to pump substances against the concentration gradient.

• They provide a pathway for large or hydrophilic molecules to cross the membrane passively.

• They break down large molecules into smaller ones.

• They are only used for active transport.

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Common Misconceptions

Misconception	Correction
Diffusion and osmosis are the same thing.	Osmosis is the diffusion of water across a membrane. Diffusion can refer to any substance.
Passive transport requires no cellular components.	Passive transport can require transport proteins, even though it does not use energy.
Molecules stop moving at equilibrium.	At equilibrium, molecules move equally in both directions, leading to no net change.

Multiple Choice

Why is facilitated diffusion classified as a form of passive transport?

• Because it moves substances against the concentration gradient.

• Because the proteins are not part of the cell.

• Because it does not require the cell to expend energy (ATP).

• Because it only moves water molecules.

Multiple Choice

If a plant cell's plasma membrane is observed pulling away from the cell wall, what kind of solution was it likely placed in and why?

• A hypotonic solution, because the cell gained too much water.

• An isotonic solution, because the cell volume remained stable.

• A hypertonic solution, because the cell lost water, causing it to shrink.

• A hypertonic solution, because the cell wall dissolved.

Multiple Choice

A freshwater amoeba, which is a single-celled organism, is placed in a saltwater aquarium. Based on the principles of osmosis, what is the most probable outcome for the amoeba?

• The amoeba will rapidly swell and likely burst as water rushes into it.

• The amoeba will shrink and shrivel as water moves out of it.

• The amoeba's size will remain stable as it is an adaptable organism.

• The amoeba will absorb salt from the water to achieve equilibrium.

Multiple Choice

A U-shaped tube is divided by a semipermeable membrane. Side A contains a 5% salt solution, and Side B contains a 20% salt solution. Predict the outcome after several hours.

• The water level on Side A will be higher.

• The water level on Side B will be higher.

• The salt will diffuse from Side B to Side A until both sides are 12.5%.

• The membrane will dissolve due to the salt.

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Summary

• Passive transport moves substances from high to low concentration without using energy.

• Osmosis is the specific diffusion of water across a semipermeable membrane.

• Facilitated diffusion uses transport proteins to move molecules across the membrane.

• Diffusion and osmosis are vital for maintaining cellular homeostasis.

Poll

On a scale of 1-4, how confident are you about the concepts of diffusion and osmosis?

• 1

• 2

• 3

• 4