AP Biology · Unit 2 · 10–13% of Exam ⚡ SPRINT MODE
The Cell
The most calculation-heavy unit in the course. Osmosis, water potential, and SA:V appear on almost every AP Bio exam. Know all three transport types cold, and master the tonicity grid — these are pure points.
Prokaryote vs. Eukaryote
SA:V Ratio
Fluid Mosaic Model
Osmosis
Water Potential
Tonicity
Na⁺/K⁺ Pump
Endosymbiosis
⚡ Quick Glance — All Topics at a Glance
| Topic | Priority | Exam Format | Key Trap |
|---|---|---|---|
| 2.1 Cell Structure | ★★★ | MCQFRQ | Prokaryotes DO have ribosomes (70S); nucleolus is INSIDE nucleus |
| 2.2 Cell Size / SA:V | ★★★ | MCQCalc | Larger cell = LOWER SA:V = LESS efficient (not more) |
| 2.3 Plasma Membrane | ★★★ | MCQFRQ | Cholesterol is a BUFFER — increases fluidity in cold, decreases in heat |
| 2.4 Permeability | ★★ | MCQ | Water crosses slowly (via aquaporins); ions CANNOT cross without channels |
| 2.5–6 Passive Transport | ★★★ | MCQFRQ | Facilitated diffusion uses proteins but needs NO ATP — still passive! |
| 2.7 Tonicity / Osmosis | ★★★ | MCQCalcData | Plant cells: hypo → turgid (NOT lysis); animal cells: hypo → lysis |
| 2.8 Active Transport | ★★★ | MCQFRQ | Na⁺/K⁺ pump: 3 Na⁺ OUT, 2 K⁺ IN — net charge moves out → electrochemical gradient |
| 2.9–10 Compartments + Endosymbiosis | ★★★ | MCQFRQ | Must cite 3+ pieces of evidence for endosymbiosis; mitochondria in ALL eukaryotes |
MCQ StandaloneFRQ ComponentData AnalysisCalculation
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Topics 2.1
Cell Structure & Function
Prokaryote vs. Eukaryote — Must-Know Comparison
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Nucleus | ❌ None — DNA in nucleoid region (no membrane) | ✅ Membrane-bound nucleus (double membrane) |
| DNA shape | Single circular chromosome + plasmids | Multiple linear chromosomes + histones |
| Membrane-bound organelles | ❌ None | ✅ ER, Golgi, mitochondria, etc. |
| Ribosomes | ✅ 70S (smaller) — ALL cells have ribosomes! | 80S cytoplasm; 70S in mitochondria/chloroplasts |
| Cell wall | Peptidoglycan (bacteria) | Cellulose (plant); chitin (fungi); none (animal) |
| Size | 1–10 μm | 10–100 μm |
| Examples | Bacteria, Archaea | Animals, Plants, Fungi, Protists |
Key Organelles — Function = Exam Answer
Secretory Pathway — High Frequency
🏭 Protein Secretion Route
- Ribosome (on RER) → Rough ER (synthesis + initial folding) → transport vesicle → Golgi apparatus (modification, sorting, packaging) → secretory vesicle → plasma membrane (exocytosis)
- Golgi is the "post office" — receives, modifies, and ships proteins
- Lysosomes bud from Golgi → contain hydrolytic enzymes for intracellular digestion
Energy Organelles
⚡ Mitochondria & Chloroplasts
- Mitochondria: double membrane; inner membrane highly folded (cristae = ↑ surface area for ATP synthesis); site of cellular respiration; matrix contains enzymes for Krebs cycle
- Chloroplasts: double membrane + thylakoid membranes stacked into grana; site of photosynthesis; stroma = liquid between thylakoids
- Both have 70S ribosomes + circular DNA → endosymbiotic evidence (2.10)
Quick Reference
🔬 Other Key Organelles
- Nucleus: site of transcription; nuclear pores control traffic; nucleolus = rRNA synthesis (inside nucleus, NOT separate organelle)
- Smooth ER: lipid synthesis, detoxification (liver), calcium storage (muscle)
- Vacuole: central vacuole in plant cells → turgor pressure; contractile vacuole in protists (osmoregulation)
- Cytoskeleton: microfilaments (actin, cell shape/movement), microtubules (cilia/flagella, spindle fibers), intermediate filaments (structural)
🎯 Exam Sniper
- FRQ (classic): "A cell produces a protein that is secreted. Trace its path from synthesis to exit." → Ribosome on RER → RER lumen → transport vesicle → Golgi → secretory vesicle → exocytosis. Must include ALL steps for full credit
- MCQ: "A drug inhibits Golgi function. Which process is most directly affected?" → Secretion of modified proteins (glycoproteins)
- FRQ: "Provide evidence that mitochondria originated from a prokaryotic ancestor" → Must cite: double membrane, 70S ribosomes, circular DNA, binary fission — need at least 2–3 for credit
- MCQ: "Which organelle would be most abundant in a cell that produces large amounts of lipids?" → Smooth ER
💣 Trap Alert
- ❌ Prokaryotes DO have ribosomes — just 70S, not 80S. No ribosomes = no protein synthesis = dead cell
- ❌ The nucleolus is NOT a separate organelle — it is a dense region INSIDE the nucleus where rRNA is made
- ❌ Archaea are prokaryotes but are more closely related to eukaryotes than to bacteria — don't group them casually as "just bacteria"
Topic 2.2
Cell Size & Surface Area:Volume Ratio
The Core Idea
📐 Why SA:V Matters
- Surface area (membrane) = interface for exchanging nutrients, O₂, CO₂, waste
- Volume = metabolic demands of the interior
- As cell grows, volume increases faster than surface area
- Large cell = low SA:V = insufficient exchange = cell division triggered
- Small cells are MORE efficient at exchange
Formulas — Memorize
🧮 SA:V Calculations
- Cube (side = s): SA = 6s²; V = s³; SA:V = 6/s
- Sphere (radius = r): SA = 4πr²; V = ⁴⁄₃πr³
- Rule: Double the radius → SA ×4, V ×8 → SA:V halves
- Always show units; SA:V has units of 1/length (e.g., μm⁻¹)
Cube s=1: SA=6, V=1, SA:V=6
Cube s=2: SA=24, V=8, SA:V=3
Cube s=4: SA=96, V=64, SA:V=1.5
Cube s=2: SA=24, V=8, SA:V=3
Cube s=4: SA=96, V=64, SA:V=1.5
Biological Applications
🔬 High SA:V Adaptations
- Microvilli in intestine → ↑ SA for nutrient absorption
- Alveoli in lungs → small spheres, high SA for gas exchange
- Cristae in mitochondria → ↑ inner membrane SA for ATP synthesis
- Root hair cells → elongated projections ↑ SA for water/ion uptake
- Red blood cells → flattened biconcave disc → maximizes SA relative to V
🎯 Exam Sniper
- Calculation MCQ: Given two cells (e.g., a 2 μm cube and a 4 μm cube), calculate both SA:V values → smaller cell has higher SA:V → explain that it exchanges materials more efficiently
- Data Analysis: Graph shows dye penetration into agar cubes of different sizes over time — smaller cubes equilibrate faster → higher SA:V
- MCQ (reasoning): "Why do cells divide rather than continue growing?" → As V grows faster than SA, SA:V falls below the threshold needed to sustain metabolic demands of the interior
Two cuboidal cells are compared: Cell A has sides of 1 μm and Cell B has sides of 3 μm. Which cell is more efficient at exchanging materials with its environment, and what is the SA:V ratio for each?
- (A) Cell B is more efficient; Cell A SA:V = 2, Cell B SA:V = 6
- (B) Cell A is more efficient; Cell A SA:V = 6, Cell B SA:V = 2
- (C) Both are equally efficient; SA:V = 1 for both
- (D) Cell B is more efficient; larger cells have more surface area
Answer: (B) — Cell A (s=1): SA = 6(1²) = 6; V = 1³ = 1; SA:V = 6. Cell B (s=3): SA = 6(9) = 54; V = 27; SA:V = 54/27 = 2. Cell A has a SA:V of 6 versus Cell B's 2. The smaller cell (A) is more efficient because it has proportionally more surface membrane per unit of metabolic volume. This is why cells stay small and divide rather than grow indefinitely.
Topic 2.3
Plasma Membrane — Fluid Mosaic Model
Model Name
🫧 Fluid Mosaic Model
- "Fluid": phospholipids and proteins can move laterally within each layer — not rigid
- "Mosaic": proteins of many types embedded throughout, like tiles in a mosaic
- Proposed by Singer & Nicolson, 1972
- Selectively permeable: controls what enters/exits
- Phospholipid bilayer: hydrophilic heads face water (outer); hydrophobic tails face inward
Components
🔩 Membrane Proteins
- Integral proteins: span the bilayer (transmembrane); channel proteins (pores for ions/water), carrier proteins (transport), receptor proteins — cannot be removed without detergent
- Peripheral proteins: attached to membrane surface; involved in signaling and structural support; can be removed by changing salt concentration
- Glycoproteins/glycolipids: carbohydrate chains on outer face only; cell-cell recognition, immune response (ABO blood type), cell adhesion
Fluidity Control — High Exam Yield
🌡 Factors Affecting Fluidity
- ↑ Temperature → ↑ kinetic energy → ↑ fluidity
- ↑ Unsaturated fatty acids (kinked tails) → can't pack tightly → ↑ fluidity
- Cholesterol = fluidity buffer: at high temp → restrains movement → ↓ fluidity; at low temp → prevents freezing → ↑ fluidity
- Shorter fatty acid chains → less van der Waals force → ↑ fluidity
- Cold-water organisms: more unsaturated lipids → membrane stays fluid in cold
🎯 Exam Sniper
- MCQ (classic): "A fish is moved from warm to cold water. How would its membrane composition change to maintain normal fluidity?" → Increase proportion of unsaturated fatty acids (kinked tails prevent tight packing → maintain fluidity in cold)
- MCQ: "What is the role of cholesterol in the cell membrane at body temperature (37°C)?" → Reduces fluidity / stabilizes the membrane by restricting phospholipid movement
- FRQ: "Explain why glycoproteins are found only on the outer face of the plasma membrane." → Synthesized in the ER/Golgi with carbohydrate chains added in the lumen; when vesicles fuse to plasma membrane they remain facing outward
- MCQ: "Which component of the membrane is responsible for cell-cell recognition?" → Glycoproteins (carbohydrate portion on extracellular face)
💣 Trap Alert
- ❌ Cholesterol does NOT simply "increase fluidity" or "decrease fluidity" — it's a buffer. The answer depends on temperature
- ❌ Glycoproteins are on the outer face only — they are never found on the cytoplasmic face
- ❌ Phospholipids move laterally (side-to-side) easily; flip-flop between leaflets is rare and requires enzymes (flippases)
Topic 2.4
Membrane Permeability
| Molecule Type | Example | Crosses Freely? | Why? |
|---|---|---|---|
| Small nonpolar | O₂, CO₂, N₂, steroids | ✅ Yes — simple diffusion | Dissolve in hydrophobic core; small enough to pass |
| Small uncharged polar | H₂O, urea, ethanol | ⚠️ Slowly (aquaporins speed up water) | Small helps, but polarity slows crossing |
| Ions (any charge) | Na⁺, K⁺, Cl⁻, Ca²⁺ | ❌ No — need ion channels | Hydration shells too large to enter hydrophobic core |
| Large polar molecules | Glucose, amino acids | ❌ No — need carrier proteins | Too large and polar to diffuse through lipid layer |
| Macromolecules | Proteins, RNA, polysaccharides | ❌ No — need vesicle transport | Far too large; require endo/exocytosis |
🎯 Exam Sniper
- MCQ: "Why can't glucose cross the membrane by simple diffusion?" → Too large AND polar — many –OH groups make it hydrophilic; cannot dissolve in the hydrophobic core
- Unit 1 connection: Steroid hormones (lipid, nonpolar) cross freely → bind intracellular receptor. Protein hormones (large, polar) cannot cross → bind surface receptor → signal transduction cascade (Unit 4)
- MCQ: "Aquaporins increase the rate of water movement across membranes. What does this tell you about water's ability to cross without aquaporins?" → Water crosses slowly/inefficiently without them (it can cross but is slow because it's polar)
Topics 2.5–2.6
Passive Transport — Diffusion & Facilitated Diffusion
All Passive Transport = No ATP, Down Concentration Gradient
| Type | What Moves | Protein Required? | ATP Required? | Direction |
|---|---|---|---|---|
| Simple Diffusion | Small nonpolar gases (O₂, CO₂), steroids | ❌ No | ❌ No | High → Low concentration |
| Osmosis | Water (H₂O) | Via aquaporins (speeds up) | ❌ No | Low solute → High solute (High Ψ → Low Ψ) |
| Facilitated Diffusion | Ions, glucose, amino acids | ✅ Yes (channel or carrier) | ❌ No | High → Low concentration |
Facilitated Diffusion — Key Details
🚪 Channel vs. Carrier Proteins
- Channel proteins: form a pore; allow ions to flow through; can be gated (open/close in response to signals — voltage, ligand, mechanical)
- Carrier proteins: bind specific molecule, change shape, release on other side; slower; exhibits saturation
- Both: specific (one type of molecule), no ATP, down gradient
- Saturation: when all carriers occupied, rate maxes out even if concentration gradient increases → unlike simple diffusion (no maximum)
Diffusion Rate Factors
📈 What Affects Diffusion Speed
- ↑ Concentration gradient → ↑ diffusion rate
- ↑ Temperature → ↑ kinetic energy → ↑ rate
- ↑ Surface area → ↑ rate (more membrane = more crossings)
- ↓ Distance → ↑ rate (Fick's Law: rate ∝ SA×ΔC / distance)
- Membrane fluidity: more fluid → faster diffusion
- Molecule size: smaller → crosses faster
🎯 Exam Sniper
- MCQ (top trap): "Glucose enters a cell using a carrier protein with no ATP used. What type of transport is this?" → Facilitated diffusion (NOT active transport — no energy used, moving down gradient)
- MCQ: "A graph shows that transport rate plateaus even as concentration gradient increases. What does this suggest?" → Carrier-mediated transport (saturation of carrier proteins) — would NOT happen with simple diffusion
- FRQ: "Explain how alveoli are adapted for efficient gas exchange." → Must mention small size (high SA:V), short diffusion distance, large total surface area, and thin membrane — connects SA:V (2.2) to diffusion rate (2.5)
💣 Trap Alert
- ❌ Facilitated diffusion uses proteins but requires NO ATP — "uses a protein" ≠ "active transport." ATP is needed only to move things AGAINST the gradient
- ❌ Simple diffusion has NO maximum rate; facilitated diffusion via carriers DOES have a maximum (saturation) — this graph distinction is a common data question
Topic 2.7
Tonicity, Osmosis & Water Potential
Osmosis = diffusion of water across a selectively permeable membrane, from low solute → high solute (equivalently: from high water potential → low water potential). Water moves to equalize solute concentrations.
Hypotonic Solution
LOWER solute than cell
- Water moves INTO cell
- Animal cell → swells → LYSES (bursts)
- Plant cell → swells → TURGID ✓ (cell wall holds; normal healthy state)
- RBC placed in distilled water → lysis
Isotonic Solution
EQUAL solute to cell
- No net water movement
- Animal cell → normal ✓
- Plant cell → flaccid (limp — not ideal for plants)
- 0.9% NaCl = isotonic to human blood
Hypertonic Solution
HIGHER solute than cell
- Water moves OUT of cell
- Animal cell → CRENATES (shrivels)
- Plant cell → PLASMOLYSIS (membrane pulls away from cell wall)
- Saltwater fish, salted meat preservation
Water Potential (Ψ) — Calculation Target
Ψ = Ψs + Ψp
Ψs = solute potential = −iCRT (always ≤ 0; solutes lower water potential)
Ψp = pressure potential (turgor pressure; usually 0 in open containers)
Pure water: Ψ = 0 (reference point, highest possible)
Water flows from higher Ψ → lower Ψ (more negative = lower)
Ψs = solute potential = −iCRT (always ≤ 0; solutes lower water potential)
Ψp = pressure potential (turgor pressure; usually 0 in open containers)
Pure water: Ψ = 0 (reference point, highest possible)
Water flows from higher Ψ → lower Ψ (more negative = lower)
Ψs Calculation
🧮 Solute Potential Formula
- Ψs = −iCRT
- i = ionization constant (1 for non-electrolytes like glucose/sucrose; 2 for NaCl; 3 for CaCl₂)
- C = molar concentration (mol/L)
- R = 0.0831 L·bar/mol·K
- T = temperature in Kelvin (°C + 273)
- Result is always negative (solutes lower Ψ)
Plant Cell Water Potential
🌿 Turgor Pressure in Plants
- Ψp = 0 in open systems (like a beaker); positive in turgid plant cells
- At equilibrium: Ψ of cell = Ψ of solution (no net water movement)
- Turgor pressure prevents wilting; drives stomatal opening/closing
- Guard cells: gain water → become turgid → stoma opens
- Wilted plant: low turgor pressure → Ψp ≈ 0 → low Ψ → gains water when watered
🎯 Exam Sniper
- Calculation FRQ (appears almost every year): Given concentration and temperature, calculate Ψs; determine which direction water will flow between two cells or cell and solution
- MCQ: "A plant cell has Ψ = −4 bar. The surrounding solution has Ψ = −2 bar. Which direction does water move?" → Water moves from −2 (higher Ψ) to −4 (lower Ψ) → INTO the cell
- Data Analysis: Graph of cell mass vs. sucrose concentration — the concentration at which mass doesn't change = isotonic point. Cells lose mass in hypertonic (water exits) and gain in hypotonic (water enters)
- MCQ (critical distinction): "What happens to a plant cell in a hypotonic solution?" → Turgid, NOT lysed. Cell wall provides structural support — this is the most common tonicity error!
💣 Trap Alert
- ❌ Plant cells in hypotonic solution become TURGID, NOT lysed — the cell wall prevents lysis
- ❌ Water moves from low solute (hypotonic) to high solute (hypertonic) — NOT the other way. Water moves toward MORE solute, not LESS
- ❌ In water potential: more negative Ψ = lower Ψ = water moves there. Don't confuse the negative sign — −8 bar is LOWER potential than −3 bar
- ❌ "Isotonic" doesn't mean "no water movement" — it means no NET movement. Individual water molecules still cross in both directions
A student places red blood cells and plant cells into three solutions: distilled water, 0.9% NaCl, and 3% NaCl. In which solution would the plant cell become turgid AND the red blood cell lyse?
- (A) 3% NaCl — it has the highest solute concentration
- (B) Distilled water — it is hypotonic to both cell types
- (C) 0.9% NaCl — it is isotonic to human cells
- (D) Both 3% NaCl and distilled water cause turgidity and lysis
Answer: (B) — Distilled water is hypotonic (lower solute than the cytoplasm). Water enters BOTH cell types by osmosis. The plant cell gains water and becomes turgid — its cell wall generates wall pressure that prevents lysis. The red blood cell (animal cell — no cell wall) has nothing to oppose the influx of water, so it swells and bursts (lyses). This is the single most-tested tonicity concept on the AP Biology exam.
Topic 2.8
Active Transport, Bulk Transport & Co-transport
Definition
⚡ Active Transport
- Moves substances against concentration gradient (low → high)
- Requires ATP (energy-driven)
- Uses pump proteins (carrier proteins that use ATP)
- Maintains concentration gradients essential for nerve impulses, nutrient uptake, pH regulation
Most-Tested Pump
🔄 Na⁺/K⁺-ATPase Pump
- Pumps 3 Na⁺ OUT and 2 K⁺ IN per ATP hydrolyzed
- Both move against their gradients (Na⁺ higher outside; K⁺ higher inside)
- Net result: net positive charge moves out → inside of cell is more negative → resting membrane potential (−70 mV)
- Critical for: nerve impulses, muscle contraction, secondary active transport
- Memory: NaKe 3-2 rule: 3 Na out, 2 K in
Indirect Active Transport
🔗 Co-transport (Secondary Active)
- Uses an existing ion gradient (usually Na⁺) to drive transport of another molecule — no direct ATP use
- Example: glucose-Na⁺ symport in intestinal epithelium → Na⁺ gradient (created by Na⁺/K⁺ pump) pulls glucose in against its gradient
- The Na⁺ gradient = indirect energy source (ATP was used to establish it)
- Symport: both molecules same direction; Antiport: opposite directions
Bulk Transport
📦 Endo & Exocytosis
- Endocytosis: cell engulfs material using membrane vesicles → requires ATP; includes phagocytosis (solid particles), pinocytosis (fluid/small molecules), receptor-mediated endocytosis (specific ligands)
- Exocytosis: vesicles fuse with plasma membrane → release contents outside → secretory pathway ends here
- Both require ATP and involve membrane fusion
🎯 Exam Sniper
- MCQ: "A cell is treated with a drug that blocks ATP production. Which transport processes would be most directly affected?" → Active transport (Na⁺/K⁺ pump), co-transport (indirectly), and endocytosis/exocytosis. Passive transport continues
- FRQ: "Explain how the Na⁺/K⁺ pump contributes to establishing the resting membrane potential." → Pump exports 3 Na⁺ for every 2 K⁺ imported → net outflow of positive charge → inside becomes negative relative to outside → resting potential of ~−70 mV
- MCQ: "Glucose absorption in the intestine uses a Na⁺/glucose co-transporter. Which form of transport is this?" → Secondary active transport (co-transport) — glucose moves against its gradient, driven by the Na⁺ gradient established by the Na⁺/K⁺ pump
- MCQ: "Which transport mechanism requires a vesicle?" → Endocytosis and exocytosis (bulk transport)
💣 Trap Alert
- ❌ Na⁺/K⁺ pump: 3 Na⁺ OUT, 2 K⁺ IN — NOT 2 Na⁺ out and 3 K⁺ in. The asymmetry (more Na⁺ out) creates the membrane potential
- ❌ Co-transport is NOT simple passive transport — it uses the energy of an ionic gradient, which was created by ATP (indirect energy use)
- ❌ Phagocytosis (cell eating) and pinocytosis (cell drinking) are both forms of endocytosis — both require ATP and form vesicles
Topics 2.9–2.10
Cell Compartmentalization & Endosymbiotic Theory
Why Compartmentalize?
🏗 Advantages of Membrane Compartments
- Allows simultaneously incompatible reactions to occur (e.g., fatty acid synthesis and oxidation; DNA transcription isolated from translation)
- Concentrates reactants → ↑ reaction efficiency
- Creates H⁺ gradients (proton motive force) for ATP synthesis in mitochondria and chloroplasts
- Isolates digestive enzymes (lysosomes) to prevent self-digestion
- Nucleus separates transcription from translation (post-transcriptional modification possible)
FRQ Must-Know
🔬 Endosymbiotic Theory — Evidence
- Double membrane: inner membrane = ancestral prokaryote membrane; outer = original host's phagocytic vesicle
- 70S ribosomes: same as prokaryotes (not 80S like eukaryotic cytoplasm)
- Circular DNA: no histones, like prokaryotic chromosomes
- Reproduce by binary fission: divide independently of the cell cycle
- Size similar to bacteria: ~1–10 μm
- Antibiotic sensitivity: some antibiotics that target prokaryotes also affect mitochondria
Theory Details
🧬 Who Was Engulfed?
- Proposed by Lynn Margulis
- Mitochondria: descended from aerobic α-proteobacteria; found in ALL eukaryotes
- Chloroplasts: descended from cyanobacteria (photosynthetic); found only in photosynthetic eukaryotes (plants, algae)
- Host cell was an anaerobic archaean-like cell that phagocytosed but did not digest these bacteria
- Over time, most bacterial genes transferred to host nucleus → reduced organelle genome
🎯 Exam Sniper
- FRQ (very common): "Provide evidence that mitochondria originated from a prokaryotic ancestor." → Must list multiple pieces: 70S ribosomes, circular DNA without histones, double membrane, reproduce by binary fission. Minimum 2–3 pieces needed for full credit
- MCQ: "A new antibiotic is found to inhibit 70S ribosomes. What eukaryotic organelle might also be affected?" → Mitochondria (and chloroplasts) — they have 70S ribosomes from their prokaryotic ancestors
- MCQ: "Why does compartmentalization benefit eukaryotic cells?" → Allows simultaneous incompatible reactions; creates specialized microenvironments; isolates potentially harmful enzymes
- MCQ: "Which organelle is found in plant cells but NOT animal cells?" → Chloroplast (and central vacuole, cell wall) — NOT mitochondria (all eukaryotes have those)
Practice
Sprint Practice — Mixed Questions
A researcher treats intestinal epithelial cells with ouabain, a drug that specifically inhibits the Na⁺/K⁺-ATPase pump. Which of the following would be the MOST DIRECT effect on glucose absorption in these cells?
- (A) Glucose diffusion would increase because the concentration gradient would steepen
- (B) Glucose uptake via the Na⁺/glucose co-transporter would decrease because the Na⁺ gradient would dissipate
- (C) Glucose would be exported from the cell by exocytosis instead
- (D) Passive diffusion of glucose would compensate for the blocked pump
Answer: (B) — The Na⁺/glucose co-transporter uses the Na⁺ concentration gradient (high outside, low inside) to drive glucose uptake against its own concentration gradient. This Na⁺ gradient is maintained by the Na⁺/K⁺-ATPase pump. When ouabain blocks the pump, Na⁺ builds up inside the cell → the Na⁺ gradient collapses → the driving force for glucose co-transport is lost → glucose absorption decreases. This is a perfect chain of reasoning: ATP → Na⁺ gradient → co-transport → glucose uptake.
A student places identical pieces of potato tissue (each 5 grams) into sucrose solutions of different concentrations and records mass after 2 hours. The results are: 0.0 M → 5.6 g; 0.2 M → 5.2 g; 0.4 M → 4.9 g; 0.6 M → 4.8 g; 0.8 M → 4.5 g. What is the approximate solute concentration of the potato cells?
- (A) 0.0 M — cells gain the most water here
- (B) 0.8 M — cells lose the most water here
- (C) Approximately 0.3–0.4 M — where mass change is closest to zero (isotonic point)
- (D) 0.6 M — because the mass plateaus here
Answer: (C) — At 0.4 M, the cells lose the least mass (4.9 g from 5.0 g). At 0.2 M they gain mass (5.2 g). The isotonic point — where there is no net water movement — is somewhere between 0.2 M and 0.4 M (approximately 0.3 M). At this concentration, the solute concentration inside the potato cells equals the sucrose solution outside. Below this concentration, the solution is hypotonic → cells gain water. Above, hypertonic → cells lose water. This experiment design appears repeatedly on AP Biology exams.
A scientist discovers a new eukaryotic organism with organelles resembling mitochondria. She claims these organelles evolved via endosymbiosis. Describe THREE independent pieces of evidence she could present to support this claim.
- (A) The organelles have 80S ribosomes, double membranes, and produce ATP
- (B) The organelles have 70S ribosomes, circular DNA without histones, and reproduce by binary fission
- (C) The organelles are enclosed in a single membrane, contain ATP synthase, and are inherited from the mother
- (D) The organelles are found in all eukaryotes, use oxygen, and contain cristae
Answer: (B) — The three key pieces of endosymbiotic evidence are: (1) 70S ribosomes — same size as prokaryotic ribosomes, NOT the 80S found in the eukaryotic cytoplasm; (2) Circular DNA without histone proteins — like prokaryotic chromosomes, not eukaryotic linear chromosomes; (3) Reproduce by binary fission — divide independently using the same mechanism as bacteria, not through the eukaryotic cell cycle. (A) is wrong because 80S ribosomes are eukaryotic. (C) is wrong — double membrane (not single) is the correct feature. (D) describes functions but not evidence of prokaryotic ancestry.
⚠ Trap Alert
Unit 2 High-Frequency Exam Traps
- ⚡Facilitated diffusion uses proteins but requires NO ATPThis is Unit 2's #1 trap. "Uses a protein carrier" does NOT mean active transport. Active transport = against gradient + ATP. Facilitated diffusion = with gradient + protein + NO ATP. The energy source is the concentration gradient itself.
- 🌿Plant cells in hypotonic solution become TURGID, NOT lysedThe cell wall prevents lysis in plants. Turgidity is actually the normal, healthy state for plant cells. Only animal cells (no cell wall) lyse in hypotonic solutions. Conversely, in hypertonic solutions: animals crenate; plants undergo plasmolysis (membrane pulls away from wall).
- 💧Water moves from LOW solute (hypotonic) to HIGH solute (hypertonic)Water moves toward the more concentrated solution. Equivalently: from high water potential (Ψ) to low water potential. In water potential terms: −2 bar is higher than −8 bar, so water moves from −2 to −8. A negative sign doesn't mean "more negative = more movement toward".
- 📐Larger cells have LOWER SA:V → LESS efficient exchangeLarger cells have proportionally more volume relative to surface area. They are worse, not better, at exchanging materials. Small cells are more efficient. This is why cells divide when they get too big.
- 🦠Prokaryotes DO have ribosomes — just 70S, not 80SAll living cells have ribosomes (needed for protein synthesis). Prokaryotes have 70S ribosomes; eukaryotic cytoplasm has 80S. BOTH mitochondria and chloroplasts have 70S ribosomes — endosymbiotic evidence. This means some antibiotics that target 70S ribosomes (like erythromycin) can also affect mitochondria.
- 🌡Cholesterol is a FLUIDITY BUFFER — answer depends on temperatureAt high temperatures: cholesterol reduces fluidity (fills spaces, restricts movement). At low temperatures: cholesterol prevents freezing/rigidity (disrupts tight packing). It's never just "increases" or "decreases" — always specify the temperature context.
- 🔄Na⁺/K⁺ pump: 3 Na⁺ OUT, 2 K⁺ IN (not the reverse)Three sodium ions are exported per two potassium ions imported per ATP. The asymmetry means net positive charge leaves the cell → inside becomes negative → resting membrane potential ~−70 mV. This gradient powers co-transport (secondary active transport) of glucose and amino acids.
- 🔬The nucleolus is INSIDE the nucleus — NOT a separate organelleThe nucleolus is a dense region within the nucleus where rRNA is synthesized and ribosomal subunits are assembled. It is NOT membrane-bound and NOT a separate compartment from the nucleus.
✓ Last-Min Checklist
Pre-Exam 10-Minute Checklist
Click each item to check off. Any unchecked = review before exam.
Cell Structure (2.1)
- Prokaryotes: no nucleus, circular DNA, 70S ribosomes, peptidoglycan wall (bacteria)
- Secretory pathway: ribosome on RER → RER → vesicle → Golgi → vesicle → plasma membrane/lysosome
- Nucleolus is INSIDE the nucleus — not a separate organelle
- Smooth ER: lipid synthesis + detox; Rough ER: protein synthesis (ribosomes attached)
Cell Size & Membrane (2.2–2.3)
- SA:V for a cube = 6s²/s³ = 6/s — larger cell = lower SA:V = less efficient
- Can calculate SA:V for cube and sphere given dimensions
- Fluid Mosaic Model: fluid (lateral movement of lipids/proteins) + mosaic (proteins embedded throughout)
- Cholesterol = fluidity buffer: reduces fluidity in heat; prevents rigidity in cold
- Glycoproteins on outer face ONLY — cell-cell recognition, immune response (ABO blood type)
- More unsaturated fatty acids → more fluid membrane (cold-water organisms, organisms adapting to cold)
Transport (2.4–2.8)
- Simple diffusion: small nonpolar only (O₂, CO₂, steroids) — no protein, no ATP
- Facilitated diffusion: ions + glucose + amino acids — protein required, NO ATP
- Osmosis: water from low solute (hypotonic) to high solute (hypertonic) = high Ψ to low Ψ
- Tonicity: Hypotonic → plant turgid / animal lyse; Isotonic → plant flaccid / animal normal; Hypertonic → plant plasmolysis / animal crenate
- Water potential: Ψ = Ψs + Ψp; water moves from high Ψ to low Ψ; pure water Ψ = 0
- Na⁺/K⁺ pump: 3 Na⁺ OUT, 2 K⁺ IN, uses ATP, against gradient — establishes resting potential
- Co-transport (secondary active): uses Na⁺ gradient to move glucose in — no direct ATP
Compartmentalization & Endosymbiosis (2.9–2.10)
- Compartmentalization allows incompatible reactions simultaneously; creates proton gradients
- Endosymbiotic evidence: 70S ribosomes, circular DNA (no histones), double membrane, binary fission
- Mitochondria: ALL eukaryotes; Chloroplasts: photosynthetic eukaryotes only
⚡ Final Sprint Strategy for Unit 2
- Biggest point-earners: osmosis direction + tonicity outcomes (plant vs. animal), water potential calculation, SA:V calculation, Na⁺/K⁺ pump (3 out/2 in), endosymbiosis evidence list
- FRQ danger zones: When explaining transport, always state: (1) direction of movement, (2) name of mechanism, (3) whether ATP is required, (4) protein involved if any. Partial answers lose partial credit
- Data questions: In osmosis experiments (potato/cell mass vs. sucrose concentration), find the isotonic point = where mass doesn't change. Points above = hypertonic → mass loss; below = hypotonic → mass gain
- Connection to other units: Membrane fluidity → Unit 1 (lipid structure); Na⁺ gradient → Unit 4 (signal transduction, nerve impulses); Chloroplast/mitochondria structure → Unit 3 (photosynthesis/respiration); Endosymbiosis → Unit 7 (evolution evidence)