Passive Transport

Passive Transport

High School

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Learning Objectives

• Define passive transport and identify its three main types.

• Explain how concentration gradient influences the movement of molecules.

• Differentiate between simple diffusion, facilitated diffusion, and osmosis.

• Describe the effects of different solutions on animal and plant cells.

Key Vocabulary

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Passive Transport

The movement of substances across a cell membrane without using cellular energy, following the concentration gradient.

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Concentration Gradient

The difference in the concentration of a substance between two areas, driving the movement of molecules.

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Equilibrium

The state where molecules are evenly distributed, resulting in no net change in concentration across the membrane.

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Facilitated Diffusion

The passive movement of molecules across the cell membrane through protein channels, requiring no cellular energy.

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Osmosis

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

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Hypertonic Solution

A solution that has a higher concentration of solutes compared to the inside of a cell.

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The Basics of Transport

• A cell’s semipermeable membrane regulates what enters and exits to maintain homeostasis.

• Passive transport is the movement of substances from high to low concentration.

• This process moves substances down the concentration gradient and requires no cellular energy.

• Dynamic equilibrium is when molecules are evenly spread but continue to move.

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Multiple Choice

What is the primary driving force for passive transport across the cell membrane?

• The concentration gradient

• The cell's nucleus

• Energy from ATP

• The hydrophobic tails of the lipid bilayer

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Types of Diffusion

Simple Diffusion

• Movement of substances directly across the cell's lipid bilayer without any assistance from proteins.

• This process occurs down the concentration gradient and does not require any metabolic energy.

• It is most effective for small, nonpolar molecules like oxygen (O₂) and carbon dioxide (CO₂).

Facilitated Diffusion

• Uses special transport proteins to move molecules that cannot cross the membrane directly.

• Moves specific particles like glucose and ions such as Cl^- and Na⁺ across the membrane.

• Although it uses proteins, it is a form of passive transport and requires no energy.

Multiple Choice

How does facilitated diffusion differ from simple diffusion?

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

• Facilitated diffusion is used for small, nonpolar molecules like oxygen.

• Facilitated diffusion requires cellular energy (ATP).

• Facilitated diffusion moves substances against the concentration gradient.

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Osmosis: The Diffusion of Water

• Osmosis is the diffusion of water across a selectively permeable membrane.

• ​Water moves from an area of high to low water concentration.

• The membrane's hydrophobic interior makes it difficult for water to pass through.

• Special channels called aquaporins allow water to move through the membrane.

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Multiple Choice

Which statement best describes the process of osmosis?

• It is the diffusion of water across a membrane through aquaporins.

• It is the movement of salt and sugar across the cell membrane.

• It is the direct movement of any small molecule through the lipid bilayer.

• It is a form of active transport requiring ATP.

Effects of Osmosis on Cells

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Isotonic Solution

• Solute concentration is the same inside and outside the cell.

• Water moves in and out of the cell at an equal rate.

• The cell’s size and shape remain stable and do not change.

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Hypotonic Solution

• Solute concentration is lower outside the cell than inside.

• Water moves into the cell, causing it to swell and expand.

• The cell can burst, but plant cells become rigid instead.

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Hypertonic Solution

• Solute concentration is higher outside the cell than inside.

• Water moves out of the cell, which causes it to shrink.

• This process causes wilting in plants due to water loss.

Multiple Choice

A cell placed in a beaker of solution immediately begins to shrink. What type of solution is in the beaker?

• A hypertonic solution

• A solution at equilibrium

• A hypotonic solution

• An isotonic solution

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Common Misconceptions about Passive Transport

Misconception	Correction
All movement stops once equilibrium is reached.	Molecules continue to move randomly, but the overall concentration remains balanced.
Osmosis is the movement of salt or sugar.	Osmosis is the movement of water towards an area with a higher solute concentration.
All passive transport is the same.	Passive transport includes simple diffusion, facilitated diffusion, and osmosis, which use different mechanisms.
Facilitated diffusion requires energy because it uses a protein.	Facilitated diffusion is passive and does not require energy as it follows the concentration gradient.

Multiple Choice

Based on their properties, why do oxygen molecules use simple diffusion while glucose molecules use facilitated diffusion to cross the cell membrane?

• Oxygen is larger than glucose and cannot fit through protein channels.

• Both molecules use simple diffusion, but glucose moves faster.

• Oxygen requires energy to cross, while glucose moves passively.

• Oxygen is small and nonpolar, while glucose is a larger molecule that needs a protein channel.

Multiple Choice

What is the primary role of the cell wall when a plant cell is placed in a hypotonic solution?

• The cell wall has no role in osmosis; only the cell membrane is involved.

• The cell wall prevents the cell from bursting by providing structural support against the incoming water pressure.

• The cell wall shrinks along with the cell membrane, causing the plant to wilt.

• The cell wall actively pumps excess water out of the cell.

Multiple Choice

A scientist observes that a certain substance moves into a cell from a high concentration to a low concentration, but the process stops when a chemical that blocks protein channels is added. What can the scientist conclude about the transport mechanism?

• The substance moves by osmosis, and the chemical blocked the aquaporins.

• The substance moves by simple diffusion, which is independent of proteins.

• The substance moves by facilitated diffusion, which requires specific protein channels.

• The substance moves by active transport, which was indirectly affected by the chemical.

Multiple Choice

Analyze the consequences for a marine fish (living in saltwater) if its cells suddenly lost the ability to regulate water movement via osmosis. What would be the immediate effect?

• Its cells would remain in an isotonic state with the surrounding water.

• Its cells would lose water to the hypertonic saltwater environment and shrink.

• Its cells would switch to active transport to manage salt levels.

• Its cells would rapidly absorb water from the saltwater and swell.

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Summary

• The cell membrane is semipermeable, controlling what enters and exits the cell.

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

• Simple diffusion, facilitated diffusion, and osmosis are the main types of passive transport.

• Osmosis is water diffusion; its effect on a cell depends on the solution.

Poll

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