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AP Biology · Unit 2 · Enzymes and Cell Structure

Enzymes and Cells: How the Exam Actually Asks It

This unit is where the data questions start in earnest. Enzyme rate curves, potato cores, agar cubes, a paramecium pumping water. The biology is a handful of ideas. The questions dress them up in lab equipment.

What This Unit Looks Like on the Exam

This unit opens with enzymes (the College Board files them under Unit 3, Cellular Energetics, but they are the bridge from protein structure to everything cells do) and then covers what the College Board calls Unit 2: organelles and compartments, surface area to volume, the membrane and what crosses it, and water (osmosis, tonicity, water potential). Together that is roughly 15% of the exam. The free response section loves this material because enzyme rate graphs and water potential both come with data, and water potential has a formula on the equation sheet.

The same three question types show up, with a Unit 2 flavor:

Spot the false clause. Four descriptions of an enzyme, a transport type, or a membrane component. One wrong word each. "Against" instead of "down." "Raises" activation energy instead of lowers. "Requires ATP" where it does not.
The scenario with no data. An enzyme in a new environment. A cell in a new solution. A drug that blocks one organelle. Translate the story into the concept, then run the chain.
The data dump. A lab you have probably done: catalase and peroxide, cores in sucrose, cubes in dye. A table, a graph, and a question that is usually about one idea: what limits the rate, or which way does water go.

This page gives you the 15 ideas you need, the five moves you make on every question, and one fully dissected example of each question type.

Zoom out

The 15 Core Concepts of Unit 2

Every Unit 2 question is secretly about one or two of these. When a question feels new, your first job is to figure out which number it really is.

What an enzyme isA protein catalyst. It lowers the activation energy of a reaction by binding substrate at its active site and holding it in a shape that makes the bond change easier. It is not used up, it does not change the products or the overall energy change, it only makes the reaction faster.

Asked as: an energy diagram with and without enzyme, or the trap that says the enzyme "adds energy" or "changes the products."

Enzymes and their environmentEach enzyme has an optimum temperature and pH. Move away from it and the R group interactions holding the active site shift; move far enough and the enzyme denatures. Human enzymes peak near 37°C, pepsin near pH 2, trypsin near pH 8. This is Unit 1 protein structure wearing a lab coat.

Asked as: a bell shaped rate curve, or "what happens to lactase at 70°C." Answer with the mechanism: shape change at the active site.

What limits the rateAdd substrate and the rate rises until every active site is busy, then it plateaus (saturation). Add enzyme and the plateau rises. A competitive inhibitor fights for the active site and can be outcompeted by more substrate. A noncompetitive inhibitor binds elsewhere, changes the shape, and more substrate does not help.

Asked as: rate vs substrate concentration curves, with and without an inhibitor. Which one flattens at the same maximum (competitive) and which one flattens lower (noncompetitive).

What every cell has, and what compartments addAll cells: plasma membrane, cytosol, ribosomes, DNA. Prokaryotes stop there (plus wall, maybe capsule, pili, flagella, plasmids). Eukaryotes add internal membranes that keep different conditions in different places (the lysosome is acidic, the cytosol is not) and pack more membrane surface into the same volume.

Asked as: "Which structure is found in all cells?" and "What advantage do internal membranes provide?" Answer the second with both halves.

The endomembrane pathA secreted or membrane protein is made on a ribosome bound to the rough ER, moves by vesicle to the Golgi for modification and sorting, then by vesicle to the plasma membrane (exocytosis) or to a lysosome. Free ribosomes make cytosolic proteins.

Asked as: "A drug blocks X. Where does the protein accumulate?" Trace the path and stop at the block.

Mitochondria and chloroplastsBoth have folded inner membranes (cristae, thylakoids stacked into grana) because the reactions happen on membrane surfaces. Both have their own DNA and ribosomes, which is the evidence for endosymbiosis.

Asked as: why the inner membrane is folded, or what evidence supports endosymbiotic origin.

Lysosomes, peroxisomes, vacuolesLysosomes hold hydrolytic enzymes at low pH and digest worn parts and engulfed material. Peroxisomes break down fats and detoxify. The plant central vacuole stores water and pushes outward (turgor pressure). Contractile vacuoles in freshwater protists pump water out.

Asked as: a cell that cannot break something down, or a plant that wilts.

Surface area to volumeAs a cell grows, volume rises faster than surface area, so the ratio falls and the membrane cannot keep up with the inside. Cells stay small, go flat, grow microvilli, or fold their internal membranes. Formulas are on the equation sheet.

Asked as: agar cubes in dye, a cell that doubles in size, why intestinal cells have microvilli. Compute the ratio and say what it means for exchange.

Fluid mosaic membraneAmphipathic phospholipids form a bilayer, tails in, heads out. Proteins (integral, peripheral) sit in and on it. Cholesterol buffers fluidity in animal cells. Carbohydrates face outward on glycoproteins and glycolipids for recognition. Everything drifts sideways.

Asked as: which component does what, and the trap that puts the carbohydrates on the inside.

Selective permeabilitySmall nonpolar molecules (O₂, CO₂) cross the bilayer freely. Small polar molecules like water cross slowly. Ions and large polar molecules (glucose, Na⁺) need a protein. The hydrophobic core is the gatekeeper.

Asked as: "Which molecule can cross without a protein?" Sort by size and polarity, not by what sounds important.

Passive transportDown the gradient, no ATP. Simple diffusion through the bilayer. Facilitated diffusion through a channel or carrier protein. Still passive even though a protein is involved. Rate depends on gradient steepness, temperature, size, and charge.

Asked as: the trap that says facilitated diffusion needs energy because it needs a protein.

Active and bulk transportActive transport moves against the gradient, costs ATP, always through a pump, and builds the gradients the cell uses later (sodium potassium pump, proton pumps). Endocytosis and exocytosis move big cargo in vesicles and recycle membrane. Both cost energy.

Asked as: what happens when ATP runs out, or how a secreted protein leaves the cell.

Osmosis and aquaporinsWater diffuses across a selectively permeable membrane toward the side with more solute. It squeezes through the bilayer slowly or passes through aquaporin channels fast. No aquaporins means slower, not zero.

Asked as: oocytes with and without aquaporins, kidney cells, any question where water moves and you must say which way.

TonicityIsotonic: no net water movement. Hypotonic outside: water enters, animal cells swell and lyse, plant cells go turgid because the wall pushes back. Hypertonic outside: water leaves, animal cells shrivel, plant cells plasmolyze. Tonicity describes the solution relative to the cell.

Asked as: red blood cell and plant cell in the same beaker, different outcomes. The cell wall is the difference.

Water potential and the osmosis labψ = ψ_P + ψ_S. Water moves from higher ψ to lower ψ. Pure water is zero. Solutes make ψ_S negative (ψ_S = −iCRT). Pressure makes ψ_P positive. In a potato core or dialysis bag lab, mass change is positive when water enters, negative when it leaves, and zero at the isotonic point, which tells you the tissue's solute concentration.

Asked as: calculate ψ_S from a molarity, compare two compartments, or find the zero crossing on a graph. Sign errors are the whole game.

The formulas that ride along From the AP equation sheet: surface area and volume of a sphere, cube, and cylinder; ψ = ψ_P + ψ_S; and ψ_S = −iCRT with R = 0.0831 liter bars per mole K and T in kelvin. You do not memorize them. You do have to know what to plug in and what the sign means. Enzyme questions have no formula, but they have a graph shape you should be able to sketch from memory: rate rises, then flattens.

The strategy

The Five Moves You Make on Every Question

Same five moves as Unit 1. Same five moves as the SAT. The content changed. The reading did not.

In this order, every time

Close read the stem. Find the verb. "Describe" wants a fact. "Explain" wants a mechanism. "Predict" wants a causal chain. "Calculate" wants a number with a sign and a unit.
Dissect. Split every sentence into clauses. Each clause is its own claim and gets judged on its own.
Hunt for right and wrong. Shade each clause green (true) or red (false). You are looking for the only answer with zero red.
Scrutinize the survivors. If two choices have no red, one of them is answering a different question. Reread the verb.
Zoom in, zoom out. Zoom in on the one word that flips everything: "down" vs "against," "into" vs "out of," "higher" vs "lower." Zoom out and name which of the 15 concepts this question is really about.

The Unit 2 version of the rule

An answer choice is 100% right or 100% wrong. In this unit the single false clause is usually a direction word. Water into the cell or out of it. Molecules down the gradient or against it. Higher water potential or lower. Every other word in the choice can be perfect, and one backward direction makes the whole thing false. Read the direction words twice.

When and how

The Three Question Types and What Each One Wants

Type	What it looks like in Unit 2	What actually wins
1. Spot the false clause	Four descriptions of a transport type, a membrane part, or an organelle.	Clause by clause. Check every "down," "against," "requires," "without." Zero red wins.
2. Scenario, no data	A cell changes environments, loses an organelle, or changes size. "Which best explains" or "predict."	Sort class words from scenario words. Decide which way water or molecules move. Build the chain, then shade.
3. Data dump	An osmosis or diffusion lab with a table and a graph. Often a calculation at the end.	Stem, text, table, figure. Find the zero crossing or the trend. State it in one sentence before the answers.

Quick filter for all three Ask: "Which way does water (or the molecule) actually move here, and does this choice say the same?" If the direction is wrong, stop reading. It is out.

Worked example 1: Spot the false clause

Facilitated Diffusion, Four Ways

Lowest level question type, highest number of careless misses. Three of the four choices are mostly right. Read the question plain first. Decide. Then turn on the X ray.

Question 1

Which of the following best describes facilitated diffusion across a cell membrane?

AMolecules move through transport proteins down their concentration gradient, and the carrier protein uses ATP to change shape and release the molecule.

BSmall nonpolar molecules such as O₂ pass directly through the phospholipid bilayer down their concentration gradient without the help of a protein.

CIons or large polar molecules move through channel or carrier proteins down their concentration gradient, with no input of metabolic energy.

DIons move through channel proteins against their concentration gradient, powered by the gradient of a second molecule moving the other way.

Move 1: Close read the stem The verb is describes. Fact check. The term is facilitated diffusion. Before you read a choice, say the definition in your own words: "Through a protein. Down the gradient. No ATP." Three claims. Every choice gets checked against all three.

Move 5 (early): Zoom out This is Concept 11, with Concept 10 (who needs a protein) and Concept 12 (what active transport is) as the distractors. The exam builds wrong answers by borrowing one true clause from a neighboring concept.

Green: factually correct Red: factually wrong

A Molecules move through transport proteins down their concentration gradient, and the carrier protein uses ATP to change shape and release the molecule.

Wrong

Two of three claims are perfect. Then ATP shows up. Facilitated diffusion is passive; the carrier changes shape, but the gradient drives it, not ATP. Ties to Concept 11 and 12. Trap: borrowed from the neighbor. ATP belongs to active transport.

B Small nonpolar molecules such as O₂ pass directly through the phospholipid bilayer down their concentration gradient without the help of a protein.

Wrong

Every clause here is a true statement. It is just a true statement about simple diffusion. The red clause is red because the stem asked about facilitated diffusion, and "facilitated" means a protein is involved. Ties to Concept 10 and 11. Trap: right fact, wrong question. This is the one that catches students who know the material.

C Ions or large polar molecules move through channel or carrier proteins down their concentration gradient, with no input of metabolic energy.

Correct

Who needs it (ions, large polar), how (protein), which way (down), what it costs (nothing). All three claims from your definition, plus the correct cargo. Zero red.

D Ions move through channel proteins against their concentration gradient, powered by the gradient of a second molecule moving the other way.

Wrong

This describes secondary active transport (a cotransporter), which is real and which you may have seen. But "against" is the opposite of diffusion. One direction word kills it. Ties to Concept 12. Trap: flipped direction.

Look at what just happened Choice B had no false biology in it. It was still wrong, because it answered a question that was not asked. Choice D was wrong on one word. Check the direction word in every choice before you check anything else.

Worked example 2: Scenario, no data

The Paramecium Slows Down

The exam hides the biology inside a story about one organism. Nothing in the story is a vocabulary word. Read the question plain first. Try to answer it before you scroll.

Question 2

Paramecium is a single celled organism that lives in freshwater ponds. It has a contractile vacuole, a structure that collects water from the cytoplasm and periodically squeezes it out of the cell. In pond water, the contractile vacuole of a particular Paramecium contracts about once every 10 seconds. A student transfers the organism to a solution that contains more dissolved solute than pond water but still less than the cytoplasm of the cell. The contractile vacuole now contracts about once every 40 seconds.

Which of the following best explains the change in contraction rate?

AThe new solution is hypertonic to the cell, so water leaves the cell by osmosis and the vacuole has nothing to pump.

BThe new solution has a water potential closer to that of the cytoplasm, so water enters the cell by osmosis more slowly and the vacuole fills less often.

CThe added solute enters the cell by active transport and is stored in the contractile vacuole, which slows its contraction.

DThe higher solute concentration reduces the ATP available to the cell, so the vacuole, which contracts by facilitated diffusion, cannot work as fast.

Step 1: Close read the stem before anything else

Find the verb Explains. Mechanism question. The right answer must connect "more solute outside" to "slower contraction" with cause and effect. A choice that is true but does not make that connection is answering the wrong question.

Step 2: Dissect the scenario into two piles

Blue: you learned this in class Amber: specific to this question

Paramecium is a single celled organism that lives in freshwater ponds. It has a contractile vacuole, a structure that collects water from the cytoplasm and periodically squeezes it out of the cell. In pond water, the contractile vacuole contracts about once every 10 seconds. A student transfers the organism to a solution that contains more dissolved solute than pond water but still less than the cytoplasm of the cell. The contractile vacuole now contracts about once every 40 seconds.

Step 3: Translate the scenario into your notes

The question says	Your notes call it	Concept
"lives in freshwater ponds"	Pond water is hypotonic to the cell. Water constantly enters by osmosis.	13, 14
"collects water and squeezes it out"	Osmoregulation. The vacuole's job is to undo the water that leaks in. Its rate is a readout of how fast water is entering.	7, 15
"more solute than pond water, still less than the cytoplasm"	Still hypotonic, just less so. The water potential gap between outside and inside shrank, but water still goes in.	15
"once every 40 seconds"	Slower. Less water is coming in per second.	15

Zoom in: the clause that decides the question

"But still less than the cytoplasm of the cell." That clause exists to kill one specific wrong answer. If the solution is still less concentrated than the inside, it is still hypotonic, water still goes in, and any choice that says water leaves the cell is contradicting the passage. The exam volunteered that detail. Use it.

Step 4: Run the causal chain

External solute goes up, but stays below the cytoplasm'stherefore
External water potential drops, but stays above the cell's. The gap shrinks.therefore
Net osmosis into the cell slows down (still inward, just slower)therefore
The contractile vacuole takes longer to filltherefore
It contracts less often. 10 seconds becomes 40.

Step 5: Now shade the answers

A The new solution is hypertonic to the cell, so water leaves the cell by osmosis and the vacuole has nothing to pump.

Wrong

The passage said the solution is still less concentrated than the cytoplasm. That is hypotonic, not hypertonic, and water still goes in. The last clause would be true if the first two were, which is why it feels right. Trap: contradicts the passage, flipped direction.

B The new solution has a water potential closer to that of the cytoplasm, so water enters the cell by osmosis more slowly and the vacuole fills less often.

Correct

Smaller water potential gap, slower inward osmosis, slower filling, slower contraction. That is the chain, link for link, with the direction right (still entering) and the magnitude right (more slowly). Concepts 13 and 15.

C The added solute enters the cell by active transport and is stored in the contractile vacuole, which slows its contraction.

Wrong

Nothing in the passage says solute enters, and the contractile vacuole handles water, not solute. The passage told you what it does. Trap: outside the scenario, invents a mechanism.

D The higher solute concentration reduces the ATP available to the cell, so the vacuole, which contracts by facilitated diffusion, cannot work as fast.

Wrong

No link between external solute and ATP is given or true. And a vacuole contracting is not facilitated diffusion; facilitated diffusion is a molecule crossing a membrane through a protein. Trap: right vocabulary, wrong use (Concept 11).

Why the chain matters If you only remembered "freshwater, water comes in, vacuole pumps it out," you might have picked A because it mentions osmosis and a vacuole. The chain forces you to decide the direction and the rate separately. Direction: still in. Rate: slower. Only B says both.

Worked example 3: The data dump

Potato Cores in Sucrose

This is the Unit 2 data question. You have probably done this lab. The exam knows that, and it is counting on you to read the table from memory instead of from the page. Read it from the page, in the right order.

The reading order for every data question

Stem first. Scroll to the actual question. Find the verb.
Text second. Sort class words from scenario words. Underline any definition or constraint the passage hands you.
Table third. Title, headers, units. Find the trend. Find where it changes sign.
Figure last. Labels, axes, the point where the line crosses zero.
State the trend and the mechanism in one line before you read a single answer choice.

Question 3

Students cut cylinders of potato tissue, recorded the initial mass of each, and placed one cylinder in each of six sucrose solutions for 24 hours at 22°C. The cylinders were then blotted and weighed again. Percent change in mass is shown in Table 1 and plotted in Figure 1. The potato cell membranes are permeable to water but not to sucrose.

Table 1. Percent change in mass of potato cylinders

Sucrose concentration (M)	Percent change in mass
0.0	+18
0.2	+8
0.4	−2
0.6	−9
0.8	−15
1.0	−20

Figure 1. Percent change in mass vs sucrose concentration

Dashed line marks zero percent change. The data line crosses it between 0.2 M and 0.4 M.

Which of the following best explains why the cylinders in the 0.0 M solution gained mass?

ASucrose diffused out of the potato cells into the distilled water, and the lost sucrose was replaced by an equal mass of water.

BThe distilled water had a higher water potential than the potato cells, so water moved into the cells by osmosis and the cylinders gained mass.

CThe potato cells actively transported water inward using ATP, because cells in a hypotonic solution must take up water to survive.

DThe distilled water had a lower water potential than the potato cells, so water moved into the cells down its water potential gradient.

Step 1: Stem first

Find the verb, then the target "Explains why the cylinders in the 0.0 M solution gained mass." Mechanism question about one row of the table. You do not need the whole graph for this one. You need to know what 0.0 M means and which way water went.

Step 2: Text second, sorted into piles

Blue: from class Amber: from this question only

Students cut cylinders of potato tissue, recorded the initial mass, and placed one in each of six sucrose solutions for 24 hours at 22°C. The cylinders were then blotted and weighed again. Percent change in mass is shown in Table 1 and plotted in Figure 1. The potato cell membranes are permeable to water but not to sucrose.

The passage handed you a gift "Permeable to water but not to sucrose." That one clause kills every answer in which sucrose moves. It also tells you this is an osmosis question and nothing else. Keep it loaded.

Step 3: Table third

Headers, units, trend, sign change Column 1 is sucrose in molar. Column 2 is percent change in mass. Read down: +18, +8, −2, −9, −15, −20. Positive means the cylinder gained water. Negative means it lost water. The sign flips between 0.2 and 0.4 M. One sentence: "Less sucrose outside, water goes in. More sucrose outside, water goes out. Somewhere near 0.35 M, nothing happens."

Step 4: Figure last

Axes, zero line, crossing Same data, drawn. The dashed line is zero. The blue line crosses it a little past 0.3 M. That crossing is the sucrose concentration that is isotonic to potato tissue, which means it is also the potato's own solute concentration. The figure does not add anything the table did not have, but the crossing point is easier to see here.

Step 5: One line before the answers

Say this, then read the choices

"0.0 M is pure water. Pure water has a water potential of zero, which is the highest it gets. The potato has solutes, so its water potential is negative. Water moves from higher to lower, so it moved into the potato. That is the gain." Concepts 13 and 15. Now every choice gets judged against that sentence.

Step 6: Shade

A Sucrose diffused out of the potato cells into the distilled water, and the lost sucrose was replaced by an equal mass of water.

Wrong

The passage says the membrane is not permeable to sucrose. First clause dead. And even if it were, an equal swap would give zero change, not +18. Trap: contradicts the passage.

B The distilled water had a higher water potential than the potato cells, so water moved into the cells by osmosis and the cylinders gained mass.

Correct

Pure water, ψ = 0, higher than the potato's negative ψ. Water goes from higher to lower, so inward. Mass goes up. This is your one line, in the exam's words.

C The potato cells actively transported water inward using ATP, because cells in a hypotonic solution must take up water to survive.

Wrong

Water is never actively transported; it moves by osmosis. And a plant cell in a hypotonic solution does not need water to survive, it just ends up turgid. Trap: borrowed from the neighbor (Concept 12), then a made up rule.

D The distilled water had a lower water potential than the potato cells, so water moved into the cells down its water potential gradient.

Wrong

The conclusion is right. The direction of the gradient is backward. Pure water is zero, the potato is negative, so the water outside is higher, not lower. The choice is internally inconsistent: water cannot move "down" a gradient from lower to higher. Trap: flipped direction, true destination. This is the most common Unit 2 miss. Read the sign.

Bonus: the calculation the exam usually attaches

Question 3b

Using the data, estimate the solute potential (ψ_S) of the potato tissue at 22°C. Assume sucrose does not ionize (i = 1) and R = 0.0831 liter bars per mole K.

A+8.8 bars

B−0.36 bars

C−8.8 bars

D−24.5 bars

Zoom out, then plug in The verb is estimate and the stem says using the data, so you need a number from the graph first. The isotonic point is where the line crosses zero: about 0.36 M. At that concentration the solution's ψ_S equals the potato's. So ψ_S = −iCRT = −(1)(0.36)(0.0831)(295 K) ≈ −8.8 bars. Choice C. Three traps: A forgot the negative sign, B stopped at the molarity, D used 22 instead of 295 and multiplied wrong. Convert to kelvin. Keep the sign.

What to watch for

The Six Traps Unit 2 Sets

✗Flipped direction. Into vs out of. Down vs against. Higher vs lower water potential. Hypo vs hyper. The single most common Unit 2 error. Read every direction word twice.

✗Borrowed from the neighbor. A true clause from the concept next door: ATP in a facilitated diffusion answer, a protein in a simple diffusion answer, active transport of water.

✗Right fact, wrong question. A perfect description of simple diffusion when the stem asked about facilitated. Every clause true, question unanswered.

✗Contradicts the passage. "Not permeable to sucrose" kills every choice where sucrose moves. "Still less than the cytoplasm" kills every choice where water leaves. The passage is the law.

✗Sign and unit errors. ψ_S is negative. T is in kelvin. A solute potential of +8.8 bars does not exist.

✗Right vocabulary, wrong use. "Facilitated diffusion" applied to a vacuole contracting. "Tonicity" applied to a single solution without a reference cell.

✓The survivor: every clause matches your notes, every direction word points the right way, nothing contradicts the passage, and the whole choice answers the verb.

The golden rule

An answer is 100% right or 100% wrong. One red clause makes the whole choice wrong, and in this unit the red clause is usually a direction. Decide which way water moves before you read a single choice. Then find the only choice that agrees.

Ready to Practice?

10 questions for this unit: 3 spot the false clause, 3 scenarios, 4 data. Mark up the question, write what you need, hunt for red, then pick. Three scores per question.

Start the Unit 2 practice quiz