AP Bio · Unit 2 Progress Check

AP Biology Unit 2 Progress Check Cell Structure and Function walkthrough.

AP Biology Unit 2 covers cell structure and function, and the Unit 2 Progress Check MCQ leans on membrane transport and water potential. This walkthrough explains what the questions test, the sign errors that cost points, and how to reason through them. Explanations only — no AP Classroom answer keys.

Updated August 2026Written by Mahmudul HasanFree · No signup

What Unit 2 covers

Unit 2 is Cell Structure and Function. Cell structure and function is roughly 10–13% of the AP Biology exam.

Organelles

Structure and function, and why compartmentalisation matters for efficiency.

Membrane structure

The fluid mosaic model, phospholipid bilayer, and embedded proteins.

Passive transport

Diffusion, osmosis, and facilitated diffusion — no ATP required.

Active transport

Pumps, endocytosis, and exocytosis — moving against the gradient using energy.

Water potential

Ψ = Ψ_s + Ψ_p, and predicting which way water moves.

Surface area to volume

Why cells stay small, and how the ratio constrains exchange.

The Progress Check MCQ: what each question type tests

These are the question patterns that recur on this Progress Check, and what each one is really asking.

Predict water movement

Water moves from higher to lower water potential. Compute both sides before deciding.

Tonicity questions

Hypertonic solution means the cell loses water. The prefix describes the solution, not the cell.

Transport identification

If it moves against the gradient or needs a protein plus ATP, it is active.

Organelle function

Often framed as “which organelle would be most abundant in…” — match function to cell type.

Surface area to volume reasoning

As a cell grows, volume outpaces surface area, limiting exchange.

The Progress Check FRQ

Unit 2 free response frequently asks for a water potential calculation followed by a prediction. Points come from using Ψ = Ψ_s + Ψ_p with correct signs, stating the direction of net water movement, and explaining the consequence for the cell.

Where students lose the most points

Sign errors in water potential

Solute potential is always negative or zero. Dropping the minus sign reverses your prediction.

Confusing hypertonic and hypotonic

Hypertonic describes the solution having more solute — the cell shrinks.

Saying water “seeks” solute

Water moves down its own potential gradient; describe it that way on the free response.

Calling facilitated diffusion active

It uses a protein but no ATP, so it is still passive.

Ignoring pressure potential

In plant cells Ψ_p is often non-zero and changes the answer.

How to work through this unit

Write both water potential values down before predicting anything. Water always moves from the less negative value to the more negative one — get the signs right and the prediction is automatic.

Once you have finished the Progress Check, put your raw score into our AP Biology Calculator to see roughly where that pace puts you on the 1–5 scale, and use the AP Biology Review for the full exam format and study plan.

Frequently asked questions

Quick answers — written by humans, not a chatbot.

What does the AP Biology Unit 2 Progress Check cover?

Cell structure and function: organelles, membrane structure, passive and active transport, water potential, and the surface area to volume ratio.

How do you calculate water potential?

Water potential equals solute potential plus pressure potential. Solute potential is negative, so keep the signs consistent, then compare the two sides to predict net water movement.

Which way does water move in a hypertonic solution?

Out of the cell. A hypertonic solution has more solute and therefore a lower water potential than the cell interior.

Do you publish AP Biology Unit 2 answer keys?

No. AP Classroom work is graded as your own, so we publish walkthroughs of the reasoning instead.