Active Transport: AP Biology Membrane Transport Guide
Active transport AP Biology questions test how cells use energy to move substances across membranes against concentration gradients. Unlike passive transport, active transport can move materials from low concentration to high concentration when the cell needs to maintain specific internal conditions.
This guide helps you understand ATP use, membrane pumps, the sodium-potassium pump, active versus passive transport, bulk transport, and how active transport supports homeostasis.
Updated May 29, 2026 · Reviewed by APScore5 Editorial Team
Active transport uses energy to move substances against concentration gradients.
Learning journey
Where Active Transport Fits in Unit 2
The previous page, passive transport and diffusion, explained movement down gradients without ATP. This page explains active transport, where cells use energy to move substances against gradients or move large materials using vesicles.
After this page, you will study cell compartmentalization, which explains how eukaryotic cells organize processes in specialized spaces.
Active transport AP Biology means movement across a membrane that requires energy, often ATP. Active transport usually moves substances against their concentration gradients, from low concentration to high concentration, using membrane proteins such as pumps.
On AP exams, check direction and ATP use before you decide whether a process is active or passive.
Say It Fast
Active = energy required
ATP often powers movement
Movement can go low to high
Pumps move specific substances
Proteins can change shape
Bulk transport uses vesicles
Homeostasis depends on control
AP Exam Clue: If a substance moves against its gradient or ATP is used, the process is active transport.
Energy rule
The Energy Rule: Low to High Needs Power
A concentration gradient is like a hill. Passive transport moves downhill, from high concentration to low concentration. Active transport can move uphill, from low concentration to high concentration, but that requires energy.
Step 1
Find the high concentration side.
Step 2
Find the low concentration side.
Step 3
Check the movement direction.
Step 4
If movement is low to high, energy is needed.
Step 5
If ATP is used, it is active transport.
Active transport requires energy because substances move against their concentration gradients.
AP Exam Tip: Direction matters. A protein is not enough to prove active transport; ATP use or movement against the gradient is the key clue.
Membrane pumps
Membrane Pumps and Active Transport
Membrane pumps are transport proteins that use energy to move specific substances across the membrane. Many pumps change shape after ATP transfers energy to the protein.
Pumps are specific
Pumps often move ions
Pumps can move substances against gradients
ATP can cause shape changes in pump proteins
Pumps help cells maintain concentration differences
Feature
Membrane Pump
Requires ATP?
Often yes
Direction
Often low to high
Protein involved?
Yes
Specificity
Moves specific substances
Example
Sodium-potassium pump
AP clue
Against gradient or ATP used
Tip: Scroll sideways to see the full table.
Membrane pumps use energy to move specific substances across the plasma membrane.
AP Exam Tip: A pump is not just a hole. Pumps use energy and often change shape to move substances.
Sodium-potassium pump
Sodium-Potassium Pump: The Classic AP Biology Example
The sodium-potassium pump is a major example of active transport. It uses ATP to move sodium ions out of the cell and potassium ions into the cell.
3 sodium ions move out
2 potassium ions move in
ATP provides energy
Movement helps maintain ion gradients
Ion gradients support nerve signals, cell volume, and membrane potential
Memory box: 3 Na+ out · 2 K+ in · 1 ATP used
The sodium-potassium pump moves sodium out and potassium in using ATP.
AP Exam Tip: Do not just memorize “3 out, 2 in.” Explain that ATP helps maintain ion gradients across the membrane.
Active vs passive
Active Transport vs Passive Transport
The fastest way to tell active and passive transport apart is to check direction and ATP. Passive transport moves down gradients without ATP. Active transport moves against gradients using energy.
Passive transport moves down gradients without ATP, while active transport moves against gradients using energy.
AP Exam Trap: Facilitated diffusion uses proteins but is still passive because it moves down the gradient without ATP.
Pump power lab
Pump Power Lab: Is It Active Transport?
Use the Pump Power Method: direction first, ATP second, protein third. The strongest active transport clues are movement against the gradient and energy use.
0 of 8 transport cases solved
Pump Power Lab · Case 1
Calcium ions move from low concentration to high concentration using ATP.
Answer: Active transport.Clue used: Low to high movement and ATP use.
Pump Power Lab · Case 2
Glucose moves through a carrier protein from high concentration to low concentration without ATP.
Answer: Not active transport; facilitated diffusion.Clue used: Protein used, but movement is down the gradient and no ATP is used.
Pump Power Lab · Case 3
Sodium ions are pumped out of a cell while potassium ions are pumped in using ATP.
Answer: Active transport by the sodium-potassium pump.Clue used: ATP-powered ion movement.
Pump Power Lab · Case 4
Oxygen moves directly through the membrane from high concentration to low concentration.
Answer: Not active transport; simple diffusion.Clue used: Down gradient, no ATP.
Pump Power Lab · Case 5
A proton pump uses ATP to move hydrogen ions across a membrane.
Answer: Active transport.Clue used: ATP-powered pump.
Pump Power Lab · Case 6
Water moves across a membrane through aquaporins without ATP.
Answer: Not active transport; osmosis/facilitated water movement.Clue used: No ATP and passive water movement.
Pump Power Lab · Case 7
A cell engulfs a large particle by wrapping membrane around it.
Answer: Bulk transport by endocytosis.Clue used: Large material moved using vesicle formation.
Pump Power Lab · Case 8
A vesicle fuses with the membrane to release materials outside the cell.
Answer: Bulk transport by exocytosis.Clue used: Vesicle releases large materials.
The pump power method helps students classify transport by direction, ATP use, and protein involvement.
AP Exam Tip: Always identify direction, ATP use, and protein or vesicle involvement before naming the process.
Primary vs secondary
Primary vs Secondary Active Transport
Primary active transport directly uses ATP to move substances across a membrane. Secondary active transport uses energy stored in an existing gradient that was often created by primary active transport.
Type
Energy Source
Example Logic
Primary active transport
Direct ATP use
Pump uses ATP to move ions
Secondary active transport
Energy stored in gradient
One substance moves down gradient to help another move
AP level clue
Direct ATP mention
Coupled movement using an existing gradient
Tip: Scroll sideways to see the full table.
AP Exam Tip: For AP Biology, focus first on whether energy is used and whether movement goes against a gradient.
Bulk transport
Bulk Transport: Endocytosis and Exocytosis
Some materials are too large to pass through membrane proteins. Cells use vesicles to move large substances into or out of the cell. Vesicle movement and membrane rearrangement require cellular energy.
Endocytosis: materials enter the cell in vesicles
Exocytosis: materials exit when vesicles fuse with the membrane
Bulk transport uses vesicles to move large materials across the cell boundary.
AP Exam Tip: If a question describes vesicles, engulfing, or secretion of large materials, think bulk transport.
Homeostasis
How Active Transport Supports Homeostasis
Homeostasis means maintaining stable internal conditions. Active transport helps cells keep specific ion concentrations, nutrient levels, pH conditions, and electrochemical gradients.
Cell Need
Active Transport Role
Ion balance
Pumps maintain ion gradients
Cell volume
Ion gradients affect water movement
Nerve signaling
Sodium and potassium gradients matter
pH control
Proton pumps move hydrogen ions
Large material movement
Vesicles move materials in bulk
Tip: Scroll sideways to see the full table.
Connect ion gradients to osmosis and tonicity when water follows solute differences across the membrane.
Common mistakes
Common Mistakes About Active Transport
Mistake
Better AP Biology Understanding
"All protein transport is active"
Facilitated diffusion uses proteins but is passive
"Active transport always means movement into the cell"
Direction depends on the gradient and cell need
"ATP is never used in membrane transport"
ATP powers many pumps
"Low to high can happen passively"
Movement against a gradient requires energy
"Sodium-potassium pump only moves sodium"
It moves sodium out and potassium in
"Endocytosis is the same as diffusion"
Endocytosis uses vesicles to move large materials
"Active transport is unrelated to homeostasis"
Active transport helps maintain internal conditions
Tip: Scroll sideways to see the full table.
MCQ practice
Active Transport Practice Questions
Answer all ten questions. Choices shuffle on reload—justify each pick with direction, ATP use, and pump or vesicle clues.
Question 1 of 10Start
Correct: 0Answered: 0Streak: 0Accuracy: 0%
FRQ practice
AP-Style FRQ Practice: Active Transport
Open each card, draft your response, then compare to the rubric. In active transport FRQs, link energy, direction, gradient, and the pump or vesicle mechanism.
0 of 2 FRQs opened
Prompt
Describe what active transport means.
Explain why ATP may be required for active transport.
Identify one example of active transport.
Explain how active transport can help maintain homeostasis.
Scoring rubric (4 points)
1 pt — Active transport moves substances across membranes using energy, often against a concentration gradient.
1 pt — ATP provides energy needed to move substances from low to high concentration or power pump shape changes.
1 pt — Accept sodium-potassium pump, proton pump, calcium pump, endocytosis, or exocytosis when explained appropriately.
1 pt — Active transport maintains ion gradients, nutrient levels, pH, or internal conditions.
Sample response
A. Active transport moves substances across a membrane using energy, often against a concentration gradient.
B. ATP provides energy to move substances from low to high concentration and can change pump protein shape.
C. The sodium-potassium pump uses ATP to move sodium out and potassium into the cell.
D. Ion gradients maintained by pumps help control cell volume, signaling, and stable internal conditions.
Status: Draft your answer first—then open the rubric or sample.
Prompt
Identify whether glucose moving through a carrier protein from high to low concentration without ATP is active or passive.
Explain why protein use alone does not prove active transport.
Describe one way the sodium-potassium pump uses ATP.
Explain why sodium and potassium gradients are important to cells.
1 pt — Facilitated diffusion uses proteins but moves down the gradient without ATP.
1 pt — ATP powers the pump and helps change its shape to move ions.
1 pt — Ion gradients help maintain membrane potential, nerve signaling, cell volume, and homeostasis.
Sample response
A. Passive transport; specifically facilitated diffusion.
B. A carrier protein can move glucose down its gradient without ATP, so the process is still passive.
C. ATP binds to the sodium-potassium pump and powers conformational changes that move ions against their gradients.
D. Unequal sodium and potassium concentrations support membrane potential, signaling, and stable cell conditions.
Status: Draft your answer first—then open the rubric or sample.
FRQ Tip
In active transport FRQs, always mention energy, direction, gradient, and the transport protein or vesicle mechanism.
AP writing tool
AP FRQ Sentence Frame
Use this pattern when AP Biology asks you to explain active transport.
Sentence frame
“Because [substance] moves from [low concentration] to [high concentration], the cell must use [ATP/energy] through [pump/protein/vesicle transport] to move it against its concentration gradient.”
Examples
Calcium ion: Because calcium moves from low concentration to high concentration, the cell must use ATP through a pump to move it against its concentration gradient.
Sodium-potassium pump: Because sodium and potassium are moved to maintain unequal ion concentrations, ATP powers the sodium-potassium pump.
Endocytosis: Because a large particle cannot pass directly through the membrane, the cell uses vesicle formation to bring it inside.
