Is facilitated diffusion active transport?

Facilitated diffusion uses a protein and moves down an electrochemical gradient. The carrier or channel does not directly use ATP for that movement. Active transport requires an energy source to drive movement against a gradient.

Do I need water-potential calculations?

Yes. Use Ψ = Ψs + Ψp and keep the units consistent. Water moves toward the lower total water potential. A plant cell’s pressure term can change the prediction based on solute concentration.

Which calculator can I use on AP Biology in 2027?

Both sections allow a four-function calculator with square root or a nongraphing scientific calculator. Bluebook includes Desmos scientific. Handheld graphing calculators are not allowed in 2027.

AP Biology

AP Biology Unit 2 review: Cells

Unit 2 explains how cell structures, membranes and transport support cellular function. It is 10–13% of multiple choice. Expect to predict the movement of water or solutes, interpret cell-size data and explain how a specific organelle supports an observed process.

Try the practice questions Topic list

By Marteen Tang, founder of Ashore · Updated Nov 9, 2026 · Sources at the bottom of the page

Unit 2 on the May 2027 exam

AP Biology Unit 2 exam facts
	2027 exam
Weight	10–13% of the multiple-choice section
Exam date	Monday, May 3, 2027, afternoon session
Multiple choice	60 questions in 90 minutes, worth 50% of the exam score
Free response	6 questions in 90 minutes, worth 50%. Two long questions worth 9 points each and four short questions worth 4 points each.
Calculator	Both sections allow a four-function calculator with square root or a nongraphing scientific calculator. Bluebook includes Desmos scientific. Handheld graphing calculators are not allowed in 2027.
Class time	About 14–16 class periods in College Board's pacing guide
2026 results	15% of students scored a 5 on the whole exam, and about 71% scored 3 or higher

Sources: College Board's course and exam description, exam page, 2027 exam dates and score distributions.

What Unit 2 covers

College Board's course description splits the unit into 10 topics. The second column is what you should be able to do for each one.

AP Biology Unit 2 topics
Topic	What you should be able to do
2.1 Cell Structure and Function	Explain how organelle structures support their jobs, including protein synthesis, processing, transport and energy conversion.
2.2 Cell Size	Calculate surface area and volume. Use their ratio to explain constraints on exchange as a cell grows.
2.3 Plasma Membrane	Use the fluid mosaic model to locate phospholipids, proteins, carbohydrates and cholesterol.
2.4 Membrane Permeability	Predict which substances can cross a phospholipid bilayer from their size, polarity and charge.
2.5 Membrane Transport	Connect concentration gradients and selective permeability to passive and active transport.
2.6 Facilitated Diffusion	Explain transport through channels and carriers down a concentration or electrochemical gradient.
2.7 Tonicity and Osmoregulation	Predict water movement from water potential and tonicity, including pressure effects in plant cells.
2.8 Mechanisms of Transport	Explain ATP-powered pumps, cotransport, endocytosis and exocytosis using gradients and membrane movement.
2.9 Cell Compartmentalization	Explain how internal membranes separate reactions and maintain different local conditions.
2.10 Origins of Cell Compartmentalization	Evaluate evidence that mitochondria and chloroplasts arose from ancestral cells through endosymbiosis.

Choose a transport mechanism from the evidence

Check the substance, its direction of travel and the source of energy before naming the mechanism.

Choose a transport mechanism from the evidence
Mechanism	Direction and energy	Evidence to look for
Simple diffusion	Down a concentration gradient, no direct ATP use	Small nonpolar molecules pass through the bilayer.
Facilitated diffusion	Down an electrochemical gradient, no direct ATP use	A channel or carrier is required. Transport can be selective.
Osmosis	Water moves toward lower water potential	Compare solute potential and pressure potential on both sides.
Primary active transport	Against an electrochemical gradient using energy directly	A pump couples transport to ATP hydrolysis.
Secondary active transport	One substance moves downhill to drive another uphill	The driving gradient was established by another process.
Endocytosis or exocytosis	Energy-dependent movement using vesicles	Large cargo enters or leaves as membranes bud or fuse.
Cell-size constraint	For a cube, SA/V = 6/side length	Doubling the side halves SA/V, increasing exchange demands per unit surface.

Common mistakes

Each of these loses points on questions you otherwise understand.

Naming an organelle without explaining its function. Connect ribosomes to polypeptide synthesis or the Golgi to processing and sorting. The CED identifies this exam error on printed p. 45.
Dropping units or error-bar information when interpreting a transport experiment. Label variables and use the stated uncertainty measure in the conclusion. See CED p. 45.
Assuming every protein-mediated movement requires ATP. Facilitated diffusion can move a solute through a membrane protein down its gradient. See CED topic 2.6.
Predicting water movement from solute concentration alone when pressure differs. Add pressure potential to solute potential before comparing compartments. See CED topic 2.7.

6 practice questions with answers

Written by Ashore for this page, in the style of the multiple-choice section. Try each one before you open the answer.

Question 1 · 2.2
Two cube-shaped cells have side lengths of 2 μm and 6 μm. What is the ratio of the smaller cell’s surface-area-to-volume ratio to the larger cell’s?
- A1/3
- B1
- C3
- D9
Show the answer
C. For a cube, SA/V = 6/L. The two ratios are 3 μm⁻¹ and 1 μm⁻¹. Their ratio is 3. The smaller cell therefore has three times as much exchange surface per unit volume.
Question 2 · 2.4
Which substance is most likely to cross a pure phospholipid bilayer rapidly without a transport protein?
- AO₂
- BNa⁺
- CA large protein
- DGlucose
Show the answer
A. O₂ is small and nonpolar, so it can enter the hydrophobic interior of the bilayer. An ion has charge, while glucose is polar and a protein is large. These properties impede direct passage.
Question 3 · 2.7
A plant cell has solute potential −0.7 MPa and pressure potential +0.3 MPa. The surrounding solution has water potential −0.2 MPa. What is the initial net movement of water?
- AOutward; cell water potential is +0.4 MPa
- BNo net movement; both potentials are −0.2 MPa
- COutward; cell water potential is −0.7 MPa
- DInward; cell water potential is −0.4 MPa
Show the answer
D. The cell’s water potential is −0.7 + 0.3 = −0.4 MPa. Water moves from the higher potential outside, −0.2 MPa, to the lower potential inside. Both pressure and solute terms enter the comparison.
Question 4 · 2.6
An uncharged solute moves into a cell through a carrier from 12 mM outside to 3 mM inside. No ATP is used by the carrier. Which process fits these observations?
- AExocytosis
- BFacilitated diffusion
- CPrimary active transport
- DDNA replication
Show the answer
B. The solute travels down its concentration gradient through a membrane protein. Those observations support facilitated diffusion. Using a protein does not by itself establish active transport.
Question 5 · 2.1
A secreted enzyme is synthesized normally but accumulates in the rough ER. Which defect best explains the low amount reaching the cell exterior?
- AFailure of DNA base pairing
- BIncreased diffusion of oxygen
- CFailure of vesicle transport from ER to Golgi
- DLoss of starch from a vacuole
Show the answer
C. Secreted proteins enter the endomembrane pathway. Impaired ER-to-Golgi transport can trap a newly made protein before sorting and secretion. Normal synthesis helps localize the defect to a later step.
Question 6 · 2.10
Which finding best supports an endosymbiotic origin for mitochondria?
- ATheir DNA and ribosomes resemble bacterial versions
- BTheir chromosomes are identical to nuclear chromosomes
- CTheir membranes are made entirely of bacterial DNA
- DTheir proteins are all synthesized in the nucleus
Show the answer
A. Independent genetic material and bacterial-like translation machinery support descent from an ancestral engulfed cell. The explanation connects multiple inherited cellular features to common ancestry.

How to study Unit 2

Draw a membrane and mark the concentration on each side. For each molecule, show its net direction, route and energy source. Add a second drawing with unequal pressure to practice water-potential reasoning.

Use a secretion experiment to trace a protein through ribosome, ER, Golgi and vesicle. Predict where protein would accumulate after blocking each step. For cell-size questions, calculate both surface area and volume before interpreting the ratio.

Ashore's Biology course has a short lesson for every topic in this unit, each ending with a few questions on what it taught.

Questions

Still have a question? support@tryashore.com

The CED assigns 10–13% of the multiple-choice section to this unit. The whole multiple-choice section is half the exam score. Free-response questions can combine units. The suggested pacing is 14–16 class periods.

By Marteen Tang · Founder of Ashore

Marteen Tang founded Ashore and works on its SAT and AP lessons, practice and plans. Updated Nov 9, 2026.

Sources

Learn Biology one short lesson at a time

Free to start, no card. A short lesson for every topic in all 8 units.

Start the lessons free AP Biology overview

Starting check

SAT Math · 1 of 5

Calculator allowed

A jacket's price is increased by 20% and then the new price is decreased by 20%. The final price is what percent of the original price?

A $96\%$
B $100\%$
C $104\%$
D $80\%$