Cell Transport

Cell Transport

Middle School

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Learning Objectives

• Describe the cell membrane and its role in cell transport.

• Compare passive transport, like diffusion and osmosis, with active transport.

• Explain how cells move large molecules using vesicle transport.

• Understand how the cell membrane helps maintain a stable internal environment.

Key Vocabulary

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Cell Membrane

A selectively permeable boundary that controls what enters and leaves a cell.

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Homeostasis

The essential process of an organism maintaining a stable internal environment.

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Passive Transport

The movement of substances across a cell membrane without using cellular energy.

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Active Transport

The movement of substances across a cell membrane that requires cellular energy.

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Osmosis

The specific diffusion of water molecules across a semipermeable cell membrane.

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Diffusion

The movement of substances from an area of high to low concentration.

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The Cell Membrane's Role

• Cell transport moves materials across the cell membrane, which acts as a boundary.

• The membrane controls what goes in and out, like a gatekeeper for the cell.

• It is selectively permeable, allowing only certain substances to pass through its gates.

• This maintains a stable internal environment, a process known as homeostasis.

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Multiple Choice

Why is the cell membrane described as 'semipermeable'?

• Because it blocks everything from passing through.

• Because it only allows certain substances to pass through.

• Because it lets everything pass through.

• Because it is a solid, rigid barrier.

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The Structure of the Cell Membrane

• The cell membrane is a phospholipid bilayer with proteins embedded in it.

• ​Each phospholipid has a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail.

• Heads face water on both sides; tails face inward, creating a barrier.

• The membrane is fluid, letting proteins move to transport substances.

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Multiple Choice

How are phospholipids arranged in the cell membrane?

• They are arranged in a single layer with tails facing out.

• They are mixed randomly with proteins.

• Hydrophobic tails face the water, and hydrophilic heads face inward.

• Hydrophilic heads face the water, and hydrophobic tails face inward.

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What Is Passive Transport?

• This is the movement of substances across a cell membrane without using energy.

• Substances move from a high to a low concentration area naturally.

• This is known as moving down the concentration gradient.

• Diffusion is the main type and continues until equilibrium is reached.

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Multiple Choice

What are the two key characteristics of passive transport?

• No energy required, moves from high to low concentration.

• Requires energy, moves from low to high concentration.

• Requires energy, moves from high to low concentration.

• No energy required, moves from low to high concentration.

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Types of Passive Transport

Osmosis

• ​This is a special type of diffusion that involves only water molecules.

• ​​Water moves across the cell membrane from a high to low water concentration.

• ​Like other forms of passive transport, this process does not require energy.

Facilitated Diffusion

• ​This process moves molecules that are too large to cross the membrane freely.

• ​​It is a passive process, so it does not require the cell to use energy.

• ​Special transport proteins, like channel and carrier proteins, help move the molecules.

Multiple Choice

What is the main difference between osmosis and facilitated diffusion?

• Osmosis is the diffusion of water, while facilitated diffusion uses proteins to help larger molecules move.

• Facilitated diffusion moves substances from low to high concentration.

• Osmosis moves any substance, while facilitated diffusion only moves water.

• Osmosis requires energy, while facilitated diffusion does not.

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Active Transport

• Active transport moves substances across a cell membrane by using cellular energy.

• ​This process moves substances against their concentration gradient, from low to high.

• It uses transport proteins embedded in the membrane to move the substances.

• The sodium-potassium pump is a key example for moving ions in and out.

Multiple Choice

Under what condition does a cell use active transport?

• To move substances from low to high concentration, which requires energy.

• To let molecules diffuse freely across the membrane.

• To move water across the membrane.

• To move substances from high to low concentration without energy.

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Vesicle Transport: Moving Big Cargo

Endocytosis

• This process moves very large substances into the cell using vesicles.

• The plasma membrane wraps around the substance and creates a pocket.

• This pocket pinches off, forming a vesicle that enters the cell.

Exocytosis

• This process moves very large substances out of the cell.

• A vesicle carrying the substance travels to the cell membrane.

• The vesicle fuses with the membrane and releases its contents outside.

Multiple Choice

A cell needs to bring a large food particle inside. Which process will it use?

• Endocytosis

• Diffusion

• Osmosis

• Exocytosis

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Common Misconceptions about the Cell Membrane

Misconception	Correction
The cell membrane is a solid, stiff wall.	The cell membrane is fluid and flexible, allowing it to move.
All types of cell transport require energy.	Passive transport, like diffusion, does not require any energy.
Osmosis and diffusion are exactly the same thing.	Osmosis is the specific diffusion of water across a membrane.
The cell membrane is completely impermeable.	The membrane is semipermeable, letting only certain molecules pass through.

Multiple Choice

If a cell is placed in a very salty solution, what will happen to the water inside the cell and why?

• Water will move out of the cell due to osmosis.

• Water will move into the cell due to osmosis.

• Nothing will happen.

• Salt will be actively pumped into the cell.

Multiple Choice

Which statement best contrasts the roles of passive and active transport in maintaining cellular function?

• Passive transport allows for balance without energy, while active transport concentrates substances where needed using energy.

• Active transport creates equilibrium, while passive transport uses ATP.

• Both use energy to move materials for cell balance.

• Passive transport moves waste out, while active transport moves nutrients in.

Multiple Choice

A cell needs to rapidly increase its internal concentration of a specific nutrient that is scarce outside the cell. Which transport mechanism is essential for this and why?

• Osmosis, because it will balance the concentration of water.

• Endocytosis, because the nutrient is very large.

• Facilitated diffusion, because it uses a protein to help.

• Active transport, because it can move substances against their concentration gradient using energy.

Multiple Choice

What would be the immediate impact on a cell if its ability to perform exocytosis was completely blocked?

• The cell could not get rid of large waste products, leading to a toxic buildup.

• The cell would be unable to move water, causing it to swell or shrink.

• The cell would be unable to generate ATP for energy.

• The cell could not take in necessary large nutrients.

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Summary

• The cell membrane controls what enters and leaves the cell to maintain balance.

• Passive transport moves substances without using any cell energy.

• Active transport requires energy to move substances against their natural flow.

• Vesicle transport moves very large molecules across the membrane.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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