Step 1
Find where concentration is higher.
Where Passive Transport Fits in Unit 2
The previous page, selective permeability, explained what the membrane allows through. This page explains what happens when substances move across the membrane without ATP.
After this page, you will study active transport, where cells use energy to move substances against gradients.
Current concept
Passive Transport and Diffusion
Substances move down gradients without ATP.
Learning Journey Checkpoint: Find the gradient, predict high-to-low movement, then check whether ATP is used.
- 1 Unit 2 Hub: Cell Structure and Function
- 2 Osmosis and Tonicity
- 3 Cell Structure and Function
- 4 Cell Organelles and Their Functions
- 5 Prokaryotic vs Eukaryotic Cells
- 6 Surface Area to Volume Ratio
- 7 Plasma Membrane Structure
- 8 Selective Permeability
- 9 Passive Transport and Diffusion You are here
- 10 Active Transport
- 11 Cell Compartmentalization
- 12 Unit 2 Practice Questions
What Is Passive Transport in AP Biology?
Passive transport AP Biology means movement across a membrane without ATP. Passive transport includes simple diffusion, facilitated diffusion, and osmosis, and substances move down concentration gradients from high concentration to low concentration.
On AP exams, name the gradient direction and ATP use before you label the transport type.
Say It Fast
- Passive = no ATP
- Transport = movement across membrane
- Gradient = concentration difference
- Diffusion moves high to low
- Facilitated diffusion uses proteins
- Osmosis is water movement
- Equilibrium means no net movement
AP Exam Clue: If the molecule moves down its gradient and no ATP is used, it is passive transport.
Transport Type Shortcut: Direction + ATP + Protein
Most AP Biology transport questions can be solved by checking three clues: direction, ATP, and protein use.
| Clue Pattern | Transport Type | Why |
|---|---|---|
| High to low + no ATP + no protein | Simple diffusion | Molecule crosses directly through the membrane |
| High to low + no ATP + protein | Facilitated diffusion | Protein helps movement down the gradient |
| Water movement + no ATP | Osmosis | Water moves passively across a selectively permeable membrane |
| Low to high + ATP | Active transport | Energy moves substances against the gradient |
| Equal concentration both sides | Equilibrium | Molecules still move, but no net movement occurs |
Tip: Scroll sideways to see the full table.
AP Exam Tip: Protein use alone does not prove active transport. Direction and ATP decide.
The Gradient Rule: High to Low
A concentration gradient is a difference in concentration between two areas. In passive transport, substances move down the gradient—from higher concentration to lower concentration—until particles are more evenly distributed.
Step 2
Find where concentration is lower.
Step 3
Predict movement from high to low.
Step 4
Check whether ATP is used.
Step 5
If no ATP and movement is high to low, it is passive transport.
AP Exam Tip: Do not memorize arrows. Find the gradient first.
Diffusion in AP Biology
Diffusion is the passive movement of particles from an area of higher concentration to an area of lower concentration. In cells, diffusion can happen across membranes when the molecule can cross the membrane.
Examples include oxygen diffusing into cells and carbon dioxide diffusing out. Small nonpolar molecules often cross the phospholipid bilayer directly—see selective permeability for which molecules cross easily.
| Feature | Diffusion |
|---|---|
| Requires ATP? | No |
| Direction | High to low concentration |
| Protein required? | Not for simple diffusion |
| Example | Oxygen crossing the membrane |
| AP clue | Down gradient, no ATP |
Tip: Scroll sideways to see the full table.
AP Exam Tip: Simple diffusion works best for small nonpolar molecules that can cross the phospholipid bilayer.
Facilitated Diffusion: Passive Transport With Proteins
Facilitated diffusion is passive transport that uses membrane proteins. It still moves substances down their concentration gradients and does not use ATP.
- Channel proteins allow specific substances through
- Carrier proteins change shape to move substances
- Ions and large polar molecules often need proteins
- Movement is still high to low
- No ATP is required
AP Exam Trap: Do not assume “protein used” means active transport. Facilitated diffusion uses proteins but does not require ATP.
Gradient Detective Lab: Predict the Direction
In AP Biology questions, the answer is often hidden in the concentration numbers. Circle the higher concentration, underline the lower concentration, then ask whether ATP is used.
0 of 8 gradient cases solved
Oxygen concentration is higher outside a cell than inside. Oxygen crosses the membrane without ATP.
Movement: Into the cell.Transport type: Simple diffusion.Clue used: High outside, low inside, no ATP, small nonpolar molecule.
Carbon dioxide concentration is higher inside a cell than outside.
Movement: Out of the cell.Transport type: Simple diffusion.Clue used: High inside, low outside, small nonpolar molecule.
Glucose concentration is higher outside the cell. Glucose enters through a carrier protein without ATP.
Movement: Into the cell.Transport type: Facilitated diffusion.Clue used: High outside, low inside, protein used, no ATP.
Sodium ions move through a channel from high concentration to low concentration.
Movement: Down the gradient through the channel.Transport type: Facilitated diffusion.Clue used: Ion uses protein, moves down gradient, no ATP.
Water moves through aquaporins toward the side with higher solute concentration.
Movement: Toward the side with higher solute concentration.Transport type: Osmosis.Clue used: Water movement through aquaporins, no ATP.
A substance moves from low concentration to high concentration using ATP.
Movement: Low to high (against the gradient).Transport type: Active transport.Clue used: Low to high and ATP used.
Particles are evenly distributed on both sides of a membrane, but molecules continue moving both ways.
Movement: Both directions with no net movement.Transport type: Equilibrium.Clue used: Equal concentration on both sides.
A large polar molecule moves through a protein down its concentration gradient.
Movement: Down the gradient through the protein.Transport type: Facilitated diffusion.Clue used: Protein used, down gradient, no ATP.
AP Exam Tip: Always identify direction and ATP use before naming the transport type.
Equilibrium Does Not Mean Movement Stops
At equilibrium, particles are evenly distributed. Molecules still move randomly in both directions, but there is no net movement because movement is balanced.
- Before equilibrium: net movement from high to low
- At equilibrium: movement continues both ways
- No net movement means no overall change in concentration
AP Exam Trap: Equilibrium does not mean molecules stop moving. It means there is no net movement.
Passive Transport vs Active Transport
Check ATP and direction. Passive transport moves down the gradient without ATP. Active transport moves against the gradient and usually requires energy.
| Feature | Passive Transport | Active Transport |
|---|---|---|
| ATP required? | No | Yes, usually |
| Direction | High to low | Low to high |
| Gradient | Down gradient | Against gradient |
| Proteins used? | Sometimes | Often |
| Examples | Diffusion, facilitated diffusion, osmosis | Pumps, sodium-potassium pump |
| AP clue | No ATP, down gradient | ATP, against gradient |
Tip: Scroll sideways to see the full table.
Continue with active transport for how cells move substances against gradients using ATP.
AP Exam Tip: Protein use alone does not prove active transport. Direction and ATP are the key clues.
Common Mistakes About Passive Transport and Diffusion
| Mistake | Better AP Biology Understanding |
|---|---|
| "Passive transport means nothing is happening" | Molecules are moving, but no ATP is used |
| "Facilitated diffusion is active transport" | It uses proteins but still moves down the gradient without ATP |
| "Equilibrium means molecules stop moving" | Molecules still move, but there is no net movement |
| "All diffusion needs proteins" | Simple diffusion does not require proteins |
| "All protein transport uses ATP" | Facilitated diffusion uses proteins without ATP |
| "Water movement is not passive" | Osmosis is passive water movement |
| "High to low means active" | High to low is passive if ATP is not used |
Tip: Scroll sideways to see the full table.
Passive Transport Practice Questions
Answer all ten questions. Choices shuffle on reload—justify each pick with gradient direction and ATP use.
Question 1 of 10
Start
Correct: 0
Answered: 0
Streak: 0
Accuracy: 0%
AP-Style FRQ Practice: Passive Transport and Diffusion
Open each card, draft your response, then compare to the rubric. In transport FRQs, link direction, gradient, ATP use, and whether a protein is involved.
0 of 2 FRQs opened
Prompt
- Describe what a concentration gradient is.
- Predict the direction of movement when oxygen concentration is higher outside a cell than inside.
- Explain why oxygen can cross the membrane by simple diffusion.
- Explain why this movement does not require ATP.
Scoring rubric (4 points)
- 1 pt — A concentration gradient is a difference in concentration between two areas.
- 1 pt — Oxygen will move into the cell.
- 1 pt — Oxygen is small and nonpolar, so it can pass through the phospholipid bilayer.
- 1 pt — Movement down a concentration gradient is passive and does not require ATP.
Sample response
A. A concentration gradient is a difference in concentration between two areas.
B. Oxygen will move into the cell.
C. Oxygen is small and nonpolar, so it can move through the hydrophobic interior of the bilayer.
D. The movement is down the gradient and passive, so no ATP is required.
Status: Draft your answer first—then open the rubric or sample.
Prompt
- Identify the type of transport used when glucose moves through a carrier protein down its concentration gradient.
- Explain why the process is still passive.
- Explain how facilitated diffusion differs from simple diffusion.
- Explain why a cell might need membrane proteins for passive transport.
Scoring rubric (4 points)
- 1 pt — Facilitated diffusion.
- 1 pt — It moves down the concentration gradient and does not use ATP.
- 1 pt — Facilitated diffusion uses proteins; simple diffusion does not.
- 1 pt — Proteins help ions or large polar molecules cross the membrane.
Sample response
A. Facilitated diffusion.
B. Glucose moves down its concentration gradient without ATP, so the process is passive.
C. Facilitated diffusion uses carrier proteins; simple diffusion crosses the bilayer without transport proteins.
D. Many ions and large polar molecules cannot cross the bilayer easily and need protein pathways.
Status: Draft your answer first—then open the rubric or sample.
FRQ Tip
In transport FRQs, always mention direction, gradient, ATP use, and whether a protein is involved.
AP FRQ Sentence Frame
Use this pattern when AP Biology asks you to explain passive transport.
Sentence frame
“Because [substance] is moving from [high concentration] to [low concentration] and [ATP is not used], the process is [passive transport/simple diffusion/facilitated diffusion]. If a membrane protein is involved, the process is facilitated diffusion.”
Examples
- Oxygen: Because oxygen moves from high concentration to low concentration without ATP, it moves by simple diffusion.
- Glucose: Because glucose moves down its concentration gradient through a carrier protein without ATP, it moves by facilitated diffusion.
- Sodium ion: Because sodium ions move through a channel protein down their concentration gradient without ATP, they move by facilitated diffusion.
FAQs About Passive Transport and Diffusion in AP Biology
What is passive transport in AP Biology?
Passive transport is movement across a membrane without ATP. Substances move down their concentration gradients from high concentration to low concentration.
Does passive transport require ATP?
No. Passive transport does not require ATP because substances move down their concentration gradients.
What is diffusion in AP Biology?
Diffusion is the passive movement of particles from an area of higher concentration to an area of lower concentration.
What is facilitated diffusion?
Facilitated diffusion is passive transport that uses membrane proteins to move substances down their concentration gradients without ATP.
Is facilitated diffusion active or passive?
Facilitated diffusion is passive because it does not use ATP and moves substances down their concentration gradients.
What is a concentration gradient?
A concentration gradient is a difference in concentration between two areas.
What happens at equilibrium?
At equilibrium, molecules still move randomly, but there is no net movement because concentrations are balanced.
How is passive transport different from active transport?
Passive transport moves substances down gradients without ATP, while active transport moves substances against gradients and usually requires energy.
Is osmosis passive transport?
Yes. Osmosis is passive movement of water across a selectively permeable membrane.
Before You Move On
- Can you define passive transport?
- Can you explain a concentration gradient?
- Can you predict high-to-low movement?
- Can you tell simple diffusion from facilitated diffusion?
- Can you explain why facilitated diffusion is still passive?
- Can you explain equilibrium as no net movement?
- Can you separate passive transport from active transport?
If yes, you are ready for Active Transport.
Passive Transport and Diffusion: Final Review
- Passive transport does not require ATP
- Substances move down concentration gradients
- Diffusion moves particles from high to low concentration
- Simple diffusion does not require transport proteins
- Facilitated diffusion uses proteins but does not use ATP
- Osmosis is passive water movement
- Equilibrium means no net movement
- Active transport moves against gradients using energy
- Strong AP answers mention direction, gradient, ATP, and protein use
You now know how substances move across membranes without ATP. Continue with Active Transport, or test yourself with Unit 2 practice questions.