Passive Transport: Movement Without Energy

Lesson Plan - Get It!

Audio:

What Is Passive Transport?

Passive transport is the movement of substances across the cell membrane without using energy.

That’s the key idea: no energy required.

Particles move naturally from areas where there are more of them to areas where there are fewer of them. This movement continues until things are more evenly spread out.

This process is driven by something called a concentration gradient.

High concentration = crowded with particles

Low concentration = fewer particles

Particles naturally move from crowded areas to less crowded areas until a balance is reached.

The cell membrane acts like a filter during this process. It does not push particles or decide their direction—it simply allows or blocks movement based on structure. The direction is always from high to low concentration.

Diffusion: Spreading Out Naturally

Diffusion is the basic form of passive transport.

It happens when particles spread out evenly over time.

Examples you see every day:

• Food coloring slowly mixing in water.
• A drop of dye spreading across a paper towel.
• Smell traveling through the air.

In cells, diffusion allows small molecules such as oxygen and carbon dioxide to move directly across the membrane.

These molecules are small enough and compatible with the membrane structure to pass straight through the phospholipid bilayer.

Osmosis: When Water Moves

Osmosis is a special type of diffusion that only involves water.

Water moves across the cell membrane to balance concentrations on both sides.

If one side of the membrane has more dissolved substances (like salt or sugar), water moves toward that side to balance the concentration.

This movement helps cells:

• Maintain shape.
• Prevent shrinking or bursting.
• Keep internal conditions stable.

The membrane is selectively permeable, meaning water can move through it, but not all substances can.

Facilitated Diffusion: Getting a Little Help

Not all particles can slip through the membrane easily.

Larger or charged particles—like glucose or ions (sodium, potassium, calcium)—need assistance.

That’s where facilitated diffusion comes in.

In this process:

• Proteins in the membrane act like doors or tunnels.
• Some proteins form channels that let specific particles pass.
• Others act like carriers that change shape to move particles across.

These proteins are highly selective. Each one only allows certain substances to pass through.

Even though proteins are involved, no energy is used. Movement still follows the concentration gradient—from high to low.

What Affects How Fast Things Move?

Not all diffusion happens at the same speed.

The rate of movement depends on:

• Concentration difference (bigger difference = faster movement)
• Size of the particles (smaller = faster)
• Whether the substance can dissolve in lipids
• Availability of protein channels

This is why some substances move quickly across membranes, while others take longer or need help.

Three Types, One Big Idea

All forms of passive transport share one rule:

No energy. Movement happens naturally.

The three main types are:

• Diffusion (small particles move directly)
• Osmosis (water moves to balance concentration)
• Facilitated diffusion (proteins help larger or charged particles)

Together, these processes keep cells balanced and functioning properly.

From Learning to Doing

You’ve seen how substances move into and out of cells without any energy at all.

Now it’s time to test that understanding by predicting how particles move and explaining what’s happening step by step.