The Cell
The cell is the fundamental unit of life. Unit 2 covers everything from organelle structure and function, to how membranes control what enters and exits cells, to the evolutionary origin of the eukaryotic cell. Membrane transport — especially osmosis — is the highest-tested topic in this unit.
Cell Structure and Function
All cells share four features: (1) a plasma membrane, (2) cytoplasm, (3) DNA as genetic material, and (4) ribosomes. Beyond these fundamentals, cells are broadly classified into two cell types: prokaryotic (no membrane-bound nucleus) and eukaryotic (membrane-bound nucleus and organelles). Note: "domain" is a taxonomic rank — the three domains of life are Bacteria, Archaea, and Eukarya, not prokaryote/eukaryote.
Prokaryotes vs. Eukaryotes
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Nucleus | Absent; DNA in nucleoid region (not membrane-bound) | Present; DNA enclosed in nuclear envelope (double membrane) |
| DNA structure | Single circular chromosome; often has plasmids | Multiple linear chromosomes; associated with histone proteins |
| Membrane-bound organelles | Absent | Present (mitochondria, ER, Golgi, etc.) |
| Ribosomes | 70S (smaller) | 80S in cytoplasm; 70S in mitochondria/chloroplasts |
| Cell wall | Usually present (peptidoglycan in bacteria) | Plants: cellulose; Fungi: chitin; Animals: absent |
| Size | 1–10 μm | 10–100 μm |
| Examples | Bacteria, Archaea | Plants, Animals, Fungi, Protists |
Key Eukaryotic Organelles — Structure Dictates Function
Contains DNA; site of transcription (DNA → mRNA). Enclosed by nuclear envelope (double membrane with pores). Nuclear pores control movement of mRNA, proteins, and ribosomal subunits.
Dense region within the nucleus; site of rRNA synthesis and ribosome subunit assembly. Not membrane-bound. Larger and more active in cells that produce many proteins.
Site of translation (mRNA → protein). Free ribosomes in cytoplasm make proteins for the cell interior. Ribosomes on the rough ER make proteins for secretion, the membrane, or lysosomes.
Studded with ribosomes. Receives newly synthesized proteins; performs initial protein folding and glycosylation. Manufactures membrane lipids. Continuous with nuclear envelope.
No ribosomes. Functions: lipid synthesis (phospholipids, steroids), detoxification of drugs/toxins (abundant in liver cells), calcium storage (in muscle cells — triggers contraction).
"Post office" of the cell. Receives vesicles from ER (cis face), modifies and packages proteins (glycosylation, phosphorylation), then ships to destinations via vesicles (trans face) — lysosomes, plasma membrane, or secretion.
Contain hydrolytic enzymes (hydrolases) at pH ~5. Digest worn organelles (autophagy), cellular debris, food particles, pathogens. Found only in animal cells. If lysosomes rupture, the cell self-digests.
Site of cellular respiration (produces most ATP). Double membrane: outer membrane + highly folded inner membrane (cristae) which maximizes surface area for ATP synthase. Matrix contains circular DNA, ribosomes (70S) — evidence for endosymbiosis.
Site of photosynthesis. Double membrane + internal membrane system of thylakoids (stacked grana). Stroma surrounds thylakoids. Contains circular DNA, 70S ribosomes — also evidence for endosymbiosis. Only in plants and some protists.
Large central vacuole in plant cells: maintains turgor pressure, stores water/ions/pigments. In animal cells: smaller, used for storage or waste. In protists: contractile vacuoles expel excess water (osmoregulation).
Network of protein filaments: microfilaments (actin — cell shape, amoeboid movement), microtubules (tubulin — chromosome separation in mitosis, cilia/flagella structure, vesicle transport), intermediate filaments (structural support, anchor organelles).
Made of cellulose microfibrils. Provides rigid support, prevents excessive water uptake (limits cell swelling). Freely permeable to water and small molecules — does NOT restrict diffusion the way the plasma membrane does.
Secretory pathway: Ribosomes (on rough ER) → ER lumen → transport vesicle → Golgi (cis face) → modified → vesicle → plasma membrane (exocytosis) or lysosome. Know this sequence cold — it appears in FRQs about how a secreted protein gets from DNA to outside the cell.
Mitochondria and chloroplasts have 70S ribosomes and circular DNA — just like bacteria. This is the evidence for the endosymbiotic theory. The exam will ask you to "support or refute" endosymbiosis using this evidence.
Structure → function: Cells with high secretory activity (e.g., pancreatic cells, antibody-producing B cells) have abundant rough ER and Golgi. Muscle cells have abundant smooth ER (calcium store). Liver cells have abundant smooth ER (detoxification). Always connect organelle abundance to cell function.
A scientist studying a newly discovered unicellular organism finds it has circular DNA, 70S ribosomes, no nuclear envelope, and no membrane-bound organelles. Describe whether this organism is prokaryotic or eukaryotic, and justify your answer using TWO pieces of evidence.
Evidence 1 — No nuclear envelope: In prokaryotes, DNA is not enclosed within a membrane-bound nucleus. The organism's DNA is in a nucleoid region open to the cytoplasm. Eukaryotes always have a double-membrane nuclear envelope surrounding their chromosomes.
Evidence 2 — Circular DNA + 70S ribosomes: Prokaryotes have a single, circular chromosome (no linear chromosomes with histones). Their ribosomes are 70S, smaller than eukaryotic cytoplasmic ribosomes (80S). Both features match the prokaryotic cell type.
Cell Size
Why can't cells simply keep growing larger? The answer lies in the surface area-to-volume ratio (SA:V). As a cell grows, its volume increases much faster than its surface area. A cell's plasma membrane (surface area) is its interface for exchanging nutrients, gases, and wastes with the environment. If volume becomes too large relative to surface area, the cell can no longer sustain the metabolic demands of its interior.
The Math of SA:V
For a cube of side length s: Surface Area = 6s², Volume = s³, SA:V = 6/s. As s increases, SA:V decreases. This means larger cells are less efficient at exchange.
| Cube Side (μm) | Surface Area (μm²) | Volume (μm³) | SA:V Ratio |
|---|---|---|---|
| 1 | 6 | 1 | 6.0 |
| 2 | 24 | 8 | 3.0 |
| 4 | 96 | 64 | 1.5 |
| 8 | 384 | 512 | 0.75 |
Cells maintain high SA:V by staying small, or by adopting shapes that maximize surface area: microvilli (intestinal epithelium), highly folded cristae (mitochondria), and flattened thylakoid membranes (chloroplasts) are all adaptations that increase membrane surface area relative to volume.
SA:V calculation: The AP exam frequently provides a data table or asks you to calculate SA:V for cells of different sizes. Know the formulas for cubes AND spheres (SA = 4πr², V = ⁴⁄₃πr³). Always show your work and include units.
Applying SA:V to biology: Small intestine villi/microvilli increase SA for absorption. Alveoli in lungs are small and spherical — high SA for gas exchange. Neurons are elongated — connects to function, not SA:V per se. Flat red blood cells maximize SA for oxygen loading.
Prediction questions: "Which cell will reach equilibrium with a dye faster — a 1mm cube or a 3mm cube?" → The smaller cube (higher SA:V). Slower equilibration = larger cube.
A spherical cell has a radius of 2 μm. If the cell doubles its radius to 4 μm, what happens to its surface area-to-volume ratio?
- (A) It doubles, because the surface area doubles.
- (B) It decreases by half, because volume increases proportionally faster than surface area.
- (C) It stays the same, because both surface area and volume increase proportionally.
- (D) It increases by half, because the surface area increases faster than volume.
Plasma Membrane
The plasma membrane is described by the Fluid Mosaic Model (Singer & Nicolson, 1972). "Fluid" because phospholipids and proteins can move laterally within the bilayer — the membrane is not a rigid structure. "Mosaic" because proteins of various types are embedded in or attached to the lipid bilayer, creating a mosaic appearance.
Membrane Composition
The structural foundation. Hydrophilic phosphate heads face outward (toward water); hydrophobic fatty acid tails face inward. Spontaneously forms bilayer in water. Selectively permeable — allows small nonpolar molecules to pass freely; blocks large polar/charged molecules.
Embedded within or spanning the entire bilayer (transmembrane proteins). Functions: channel proteins (pores for ions), carrier proteins (transport), receptor proteins, enzymes. The hydrophobic portion is embedded in the lipid tails; hydrophilic portions face water.
Attached to the surface of the membrane (inner or outer face) — not embedded in the lipid bilayer. Functions: cell signaling, structural support (connect to cytoskeleton), enzymes. Easily removed by changing salt concentration.
Intercalated between phospholipid molecules. Fluidity buffer: at high temperatures, reduces fluidity (prevents membrane from becoming too fluid); at low temperatures, prevents membrane from solidifying. Amount varies by organism and temperature adaptation.
Carbohydrate chains attached to proteins or lipids on the outer face of the membrane. Form the glycocalyx — involved in cell-cell recognition, immune response (self vs. non-self), cell adhesion, and signal reception. ABO blood type antigens are glycoproteins/glycolipids.
Factors Affecting Membrane Fluidity
| Factor | Effect on Fluidity | Biological Relevance |
|---|---|---|
| ↑ Temperature | ↑ Fluidity (more kinetic energy → lipids move more) | Fever, fever-reducing drugs affect membrane function |
| ↑ Unsaturated fatty acids | ↑ Fluidity (kinks prevent tight packing) | Cold-water fish, deep-sea organisms have more unsaturated lipids |
| ↑ Cholesterol (warm temps) | ↓ Fluidity (fills gaps between lipids) | Stabilizes membrane, prevents excessive movement |
| ↑ Cholesterol (cold temps) | ↑ Fluidity (disrupts crystal-packing of saturated lipids) | Prevents membrane from becoming too rigid at low temp |
| Shorter fatty acid tails | ↑ Fluidity (less surface area for van der Waals forces) | Bacteria regulate fatty acid chain length for homeoviscous adaptation |
❌ The membrane is not static. Individual phospholipids and most proteins can move laterally (flip-flop between layers is rare and slow). This lateral movement is key to understanding protein clustering and signal transduction.
❌ Glycoproteins face OUTWARD only. Carbohydrate chains of glycoproteins/glycolipids are always on the extracellular face — they are synthesized that way in the ER/Golgi. They are never on the cytoplasmic face.
Membrane Permeability
The plasma membrane is selectively permeable — it allows some substances to cross freely while restricting others. This selectivity is determined by the size, charge, and polarity of the molecule attempting to cross.
Rules for Membrane Crossing
| Molecule Type | Example | Can Cross Freely? | Reason |
|---|---|---|---|
| Small nonpolar | O₂, CO₂, N₂ | ✅ Yes (simple diffusion) | Dissolve easily in hydrophobic lipid tails; small enough to slip through |
| Small uncharged polar | H₂O, ethanol, urea | ⚠️ Slowly (or via aquaporins for water) | Small size helps, but polarity slows crossing through hydrophobic core |
| Ions (any charge) | Na⁺, K⁺, Cl⁻, Ca²⁺ | ❌ No (need channels) | Charge makes them strongly attracted to water (hydration shell) — cannot enter nonpolar core |
| Large polar molecules | Glucose, amino acids | ❌ No (need carriers) | Too large and/or polar to diffuse through lipid bilayer |
| Macromolecules | Proteins, polysaccharides, DNA | ❌ No (need vesicles) | Far too large; require endo/exocytosis |
Aquaporins — Water Channels
Although water is small and uncharged, its high polarity makes diffusion across the lipid bilayer slow. Aquaporins are integral membrane proteins that form water-specific channels, dramatically accelerating water movement. They are critical in the kidneys (water reabsorption), plant roots (water uptake), and red blood cells. Aquaporin discovery won the 2003 Nobel Prize in Chemistry.
"Why can't glucose cross the membrane by simple diffusion?" — Glucose is a large, polar molecule. Even though it has no net charge, its many –OH groups make it hydrophilic, and its size prevents it from fitting between phospholipid molecules.
Steroid hormones cross membranes; protein hormones do not. This distinction is tested in Unit 4 (Cell Communication). Steroids are nonpolar lipids → they freely diffuse across the membrane and bind intracellular receptors. Protein hormones are large polar → they bind surface receptors and initiate signal transduction cascades.
Membrane Transport — Passive Transport
Passive transport moves substances across a membrane down their concentration gradient (from high to low concentration). It requires no energy (ATP) — the movement is driven by the concentration gradient itself (a thermodynamically favorable process). The two types are simple diffusion and osmosis.
Simple Diffusion
Direct movement of small nonpolar molecules (O₂, CO₂, lipids, steroids) through the phospholipid bilayer without any protein assistance. Rate of diffusion is determined by:
- Concentration gradient magnitude — steeper gradient → faster diffusion
- Temperature — higher temperature → faster molecular movement → faster diffusion
- Membrane permeability — more fluid membranes → faster diffusion
- Surface area — larger membrane area → more diffusion events simultaneously
- Distance — shorter diffusion path → faster equilibration (Fick's Law)
Osmosis — Diffusion of Water
Osmosis is the diffusion of water across a selectively permeable membrane, moving from a region of higher water potential (lower solute concentration / hypotonic) to a region of lower water potential (higher solute concentration / hypertonic). Water moves to equalize solute concentrations on both sides.
Key concept: Water moves toward the side with more solute — or equivalently, toward the side with lower water concentration. Think of it as water "following" solutes.
Water potential (Ψ) determines the direction of water movement. Ψ = Ψs + Ψp, where Ψs = solute potential (always negative — solutes lower water potential) and Ψp = pressure potential (turgor pressure in plant cells, usually positive).
Water always flows from higher Ψ to lower Ψ. Pure water has Ψ = 0 (reference point). Adding solutes makes Ψ more negative. Applying pressure makes Ψ more positive.
In plant cells: As water enters by osmosis, turgor pressure builds (Ψp increases), raising Ψ. Equilibrium is reached when Ψinside = Ψoutside.
A cell with a solute potential of −8 bars and a pressure potential of +3 bars is placed in a solution with a water potential of −4 bars. In which direction will water move?
- (A) Water will move out of the cell, because the cell's water potential is lower than the solution's.
- (B) Water will move into the cell, because the solution has a higher water potential than the cell.
- (C) No net water movement, because the solute potentials are equal.
- (D) Water will move out of the cell, because the pressure potential is positive.
A plant cell has Ψs = −10 bars and Ψp = +4 bars. Cell Ψ = −10 + 4 = −6 bars. It is placed in a solution with Ψ = −3 bars.
Since solution Ψ (−3) > cell Ψ (−6), water moves from the solution into the cell (the solution is hypotonic = less concentrated than the cell). As water enters, turgor pressure (Ψp) increases, raising cell Ψ until equilibrium (Ψcell = Ψsolution).
Facilitated Diffusion
Facilitated diffusion is passive transport that uses membrane proteins (channels or carriers) to move large polar molecules or ions down their concentration gradient. Like simple diffusion, it requires no ATP — transport is still driven by the concentration gradient. The proteins simply provide a pathway through the hydrophobic core.
Form a water-filled pore through the membrane. Most are gated — they open and close in response to signals (voltage, ligands, mechanical stretch). Ion channels are highly selective (size and charge of pore matches specific ion). Aquaporins are an example (uncharged channel, highly specific for water).
Bind to a specific substrate, undergo a conformational change, and release the substrate on the other side. More selective than channels. Slower than channel-mediated transport (must bind and change shape for each molecule). Example: GLUT transporters for glucose into red blood cells.
Facilitated diffusion is still PASSIVE. Even though it uses proteins, no ATP is consumed. The movement is still down the concentration gradient. This is a very common exam trap — do not equate "uses a protein" with "requires energy."
Saturation kinetics: Unlike simple diffusion, facilitated diffusion can be saturated — when all carrier/channel proteins are occupied, increasing concentration further will not increase transport rate. This creates a plateau in a rate vs. concentration graph. This is analogous to enzyme saturation (Michaelis-Menten kinetics).
A researcher increases the concentration of glucose outside a red blood cell. Initially, glucose transport into the cell increases proportionally, but eventually the transport rate levels off even as glucose concentration continues to rise. Which of the following best explains this observation?
- (A) The cell has reached its maximum ATP production capacity.
- (B) The concentration gradient no longer exists at high glucose concentrations.
- (C) All available GLUT carrier proteins are saturated, so the transport rate cannot increase further.
- (D) The cell membrane becomes impermeable to glucose at high concentrations.
Tonicity and Osmoregulation
Tonicity describes the relative concentration of solutes in a solution compared to the interior of a cell, and determines the net direction of osmosis. Three terms describe relative tonicity:
| Tonicity | Definition | Effect on Animal Cell | Effect on Plant Cell |
|---|---|---|---|
| Hypotonic | Solution has LOWER solute concentration than cell interior | Water enters → cell swells → may lyse (osmotic lysis / cytolysis). RBCs in pure water → hemolysis. | Water enters → turgid (firm). Turgor pressure pushes against cell wall. Normal, desirable state for plant cells. |
| Isotonic | Equal solute concentration — no net osmosis | Normal shape maintained. No net water movement. | Cell is flaccid (no turgor pressure). Wilted appearance despite no water deficit at cellular level. |
| Hypertonic | Solution has HIGHER solute concentration than cell interior | Water exits → cell shrinks / crenates. RBCs become spiky "crenated" cells. | Water exits → plasmolysis. Plasma membrane pulls away from cell wall. Cell cannot function. |
Why Plant Cells Differ from Animal Cells
Plant cells have a rigid cell wall. When water enters a hypotonic solution, the cell wall resists expansion — building up turgor pressure (Ψp). Turgor pressure keeps plants upright. Wilting occurs when turgor pressure decreases (plant cells become flaccid in isotonic or hypertonic conditions) — not because cells lyse. Animal cells lack cell walls and will lyse in hypotonic solutions.
Plasmolysis vs. Crenation: Plasmolysis = plant cell in hypertonic solution (membrane pulls from wall). Crenation = animal cell in hypertonic solution (cell shrinks/spikes). Lysis = animal cell in hypotonic (bursts). Turgid = plant cell in hypotonic (ideal). These four terms appear constantly in osmosis questions.
Contractile vacuoles in freshwater protists (like Paramecium) pump out excess water entering by osmosis from the hypotonic environment. This is an energy-requiring (active) process of osmoregulation.
"Potato / egg / dialysis tubing" lab questions: If a potato slice placed in salt water loses mass → hypertonic solution (water left the potato). If it gains mass → hypotonic solution (water entered potato). Mass change direction = direction of net water movement.
A student cuts four equal-sized pieces of potato and places each piece in a solution of different NaCl concentration: 0.0 M, 0.2 M, 0.4 M, and 0.8 M. After 30 minutes, she measures the mass change (%). She observes that the 0.0 M piece gained 8% mass, the 0.2 M piece gained 3%, the 0.4 M piece showed 0% change, and the 0.8 M piece lost 12%. What is the approximate solute concentration of the potato cells?
The piece with 0% mass change is in an isotonic solution — meaning the solution's solute concentration equals the potato cell's internal solute concentration. No net osmosis occurs. Therefore, the potato cells have a solute concentration approximately equal to 0.4 M NaCl. This type of calculation (finding the isotonic point from a mass change vs. concentration graph) is a classic AP Biology lab question.
Mechanisms of Transport — Active Transport & Bulk Transport
Active transport moves substances against their concentration gradient (from low to high concentration). This is thermodynamically unfavorable and requires an energy source. Primary active transport is directly powered by ATP hydrolysis (e.g., Na⁺/K⁺-ATPase). Secondary active transport is driven indirectly — it uses the electrochemical gradient established by a primary pump (e.g., glucose entering intestinal cells co-transported with Na⁺ flowing down its gradient). Both types require carrier proteins.
The Sodium-Potassium Pump (Na⁺/K⁺-ATPase)
The canonical example of primary active transport. For each ATP hydrolyzed, the pump moves 3 Na⁺ out of the cell and 2 K⁺ in — against both concentration and electrical gradients for each ion. This maintains the resting membrane potential (negative inside) critical for nerve and muscle function.
The pump cycle: (1) Na⁺ binds inside the cell → (2) ATP phosphorylates the pump → (3) conformational change opens pump to outside → (4) Na⁺ released outside, K⁺ binds → (5) dephosphorylation → (6) conformational change → K⁺ released inside. Cycle repeats.
Secondary Active Transport (Co-transport)
Uses the electrochemical gradient established by a pump (e.g., Na⁺/K⁺-ATPase creates a Na⁺ gradient) to drive another molecule against ITS gradient. The Na⁺ gradient is a form of stored potential energy. Example: Sodium-glucose cotransporter (SGLT) in intestinal cells uses Na⁺ flowing down its gradient to drag glucose in against the glucose gradient — even when glucose is already concentrated inside. Indirectly requires ATP (to maintain the Na⁺ gradient).
Bulk Transport — Moving Large Materials
Cell takes material IN by engulfing it with the plasma membrane, forming a vesicle. Phagocytosis ("cell eating") — engulfs large particles (bacteria, food particles). Pinocytosis ("cell drinking") — nonspecific uptake of extracellular fluid. Receptor-mediated endocytosis — specific; ligand binds to receptor → clathrin-coated pit forms → vesicle pinches off (e.g., LDL cholesterol uptake).
Cell secretes material OUT by fusing a vesicle with the plasma membrane. Used for: secreting hormones (insulin from pancreatic β cells), neurotransmitters (into synapse), digestive enzymes, extracellular matrix components. The vesicle membrane becomes part of the plasma membrane.
Passive vs. Active transport comparison: The AP exam frequently asks you to compare them. Key distinction: passive transport moves with the gradient and requires no ATP; active transport moves against the gradient and requires an energy source — either directly from ATP (primary active transport) or indirectly from an ion gradient established by an ATP-driven pump (secondary active transport). Both active and passive transport may use membrane proteins.
Na⁺/K⁺-ATPase creates the membrane potential used in nerve signal transmission (Unit 4). Understanding this pump is foundational for neuroscience questions.
Receptor-mediated endocytosis + LDL: Familial hypercholesterolemia is caused by a defective LDL receptor — LDL cannot be endocytosed → accumulates in blood → cardiovascular disease. This connects biochemistry to disease in a testable scenario.
A cell is treated with a drug that inhibits ATP synthase, preventing ATP production. Which of the following transport processes would be MOST directly affected?
- (A) Diffusion of O₂ across the plasma membrane
- (B) Osmosis of water through aquaporins
- (C) Facilitated diffusion of glucose via GLUT transporters
- (D) Active transport of Na⁺ against its concentration gradient via Na⁺/K⁺-ATPase
Cell Compartmentalization
Eukaryotic cells are divided into compartments by internal membranes. This compartmentalization is a major advantage — it allows incompatible reactions to occur simultaneously, concentrates enzymes and substrates, and creates specialized microenvironments.
Why Compartmentalization Matters
DNA synthesis occurs in the nucleus while protein synthesis occurs in the cytoplasm — these processes are spatially separated. Lysosomal hydrolases work at pH 5 inside lysosomes while the cytoplasm maintains pH 7.2 — without compartmentalization, hydrolases would digest the entire cell.
Enzymes of a metabolic pathway are co-localized in a specific organelle (e.g., Krebs cycle enzymes in the mitochondrial matrix). Concentrating reactants in a small space dramatically increases reaction rates — rather than diluting molecules throughout the cytoplasm.
Inner mitochondrial membrane creates a proton (H⁺) gradient used to drive ATP synthesis (chemiosmosis). The thylakoid membrane of chloroplasts does the same. These gradients can only be maintained because membranes create compartments that separate high and low H⁺ concentrations.
Lysosomes isolate hydrolytic enzymes from the rest of the cell. Peroxisomes contain catalase to neutralize H₂O₂ before it damages other cellular components. Compartmentalization prevents cellular self-destruction.
FRQ scenario: "Explain why a cell with a non-functional lysosomal membrane would be harmed." → Hydrolytic enzymes at pH 5 would be released into the cytoplasm (pH 7.2) — the pH would still partially inhibit them, but enough activity remains to digest cytoplasmic proteins, DNA, and organelles → cell death. This is related to lysosomal storage diseases.
Chemiosmosis requires compartmentalization: The proton gradient across the inner mitochondrial membrane (or thylakoid membrane) can only exist because the membrane creates two separate compartments with different H⁺ concentrations. Disrupting the membrane dissipates the gradient and stops ATP synthesis.
Origins of Cell Compartmentalization — Endosymbiotic Theory
The Endosymbiotic Theory (Lynn Margulis, 1967) proposes that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by a larger host cell, and instead of being digested, established a mutually beneficial (endosymbiotic) relationship. Over time, the endosymbiont became permanently integrated as an organelle.
Evidence Supporting Endosymbiosis
| Evidence | Mitochondria | Chloroplasts | Why It Supports Endosymbiosis |
|---|---|---|---|
| Circular DNA | ✅ Circular chromosome | ✅ Circular chromosome | Like bacteria — not linear like eukaryotic nuclear DNA |
| Ribosome size | ✅ 70S ribosomes | ✅ 70S ribosomes | Same size as bacterial ribosomes; 80S in eukaryotic cytoplasm |
| Double membrane | ✅ Inner + outer membrane | ✅ Inner + outer membrane | Outer membrane came from the host cell's phagocytic vesicle; inner membrane is the original prokaryote's plasma membrane |
| Binary fission | ✅ Divide by binary fission | ✅ Divide by binary fission | Replicate independently like bacteria — not by the cell cycle |
| Antibiotic sensitivity | ✅ Inhibited by bacterial antibiotics | ✅ Inhibited by bacterial antibiotics | 70S ribosomes are targets of many antibiotics (e.g., streptomycin); confirms prokaryotic origin |
| Ancestral relationship | Most similar to α-proteobacteria | Most similar to cyanobacteria | Sequence analysis confirms evolutionary relationship to free-living bacteria |
1. Large anaerobic prokaryote (ancestor of eukaryote) engulfs a smaller aerobic α-proteobacterium by phagocytosis.
2. Instead of being digested, the endosymbiont survives inside the host and begins supplying the host with ATP from aerobic respiration (mutually beneficial).
3. Over evolutionary time, genes transfer from the endosymbiont's genome to the host nucleus (gene transfer). The endosymbiont becomes permanently dependent — it is now the mitochondrion.
4. A separate event: a eukaryote engulfs a photosynthetic cyanobacterium → becomes the chloroplast. This happened in the ancestor of plants and algae.
A student claims that mitochondria originated as free-living bacteria that were engulfed by a eukaryotic ancestor. Provide THREE distinct pieces of evidence that support this claim.
Evidence 2 — 70S ribosomes: Mitochondria contain 70S ribosomes, which are the same size and type as bacterial ribosomes. Eukaryotic cytoplasmic ribosomes are 80S. Mitochondrial ribosomes are also sensitive to antibiotics (e.g., chloramphenicol) that inhibit bacterial 70S ribosomes but not eukaryotic 80S ribosomes — reflecting their prokaryotic origin.
Evidence 3 — Double membrane and binary fission: Mitochondria are surrounded by a double membrane — the outer membrane is derived from the host's phagocytic vesicle, while the inner membrane is the original bacterial plasma membrane. Furthermore, mitochondria reproduce by binary fission (like bacteria), dividing independently of the cell cycle — not by mitosis as would be expected for a purely eukaryotic structure.
Mixed Practice Questions
A scientist treats cells with cytochalasin D, a drug that disrupts actin microfilaments. Which of the following processes would most likely be affected?
- (A) Chromosome separation during mitosis
- (B) Phagocytosis of bacteria by a macrophage
- (C) Movement of vesicles from ER to Golgi
- (D) Synthesis of ATP in the mitochondria
Describe the complete pathway of a secretory protein, from the gene encoding it to its release outside the cell. Name each cellular structure involved and describe what happens at each step.
Step 2 — Rough ER: Ribosomes on the rough ER translate the mRNA. The protein is threaded into the ER lumen as it is synthesized. Initial folding and glycosylation occur in the ER.
Step 3 — Transport vesicle: The protein is packaged into a transport vesicle that buds from the ER membrane.
Step 4 — Golgi apparatus (cis face → trans face): The vesicle fuses with the cis face of the Golgi. As the protein moves through Golgi cisternae, it is further modified (additional glycosylation, phosphorylation, cleavage). At the trans face, the protein is sorted and packaged into a secretory vesicle.
Step 5 — Secretory vesicle → Plasma membrane: The secretory vesicle moves to the plasma membrane. The vesicle membrane fuses with the plasma membrane (exocytosis), releasing the protein into the extracellular space.
High-Frequency Errors to Avoid
- 🔵Equating "uses a protein" with "requires ATP"Facilitated diffusion uses channel and carrier proteins but requires NO ATP. Only active transport (against the gradient) requires energy. The energy source is the concentration gradient for all passive transport.
- 🌿Saying plant cells lyse in hypotonic solutionsPlant cells do NOT lyse in hypotonic solutions — the cell wall prevents lysis. They become turgid (swell under turgor pressure), which is the normal, healthy state. Animal cells lack cell walls and WILL lyse (crenate in hypertonic; lyse in hypotonic).
- 🔬Saying the nucleolus is a separate organelle from the nucleusThe nucleolus is a dense region WITHIN the nucleus — it is not a membrane-bound organelle. It is the site of rRNA synthesis and ribosome subunit assembly, inside the nuclear envelope.
- 📏Thinking larger cells are more efficientLarger cells have LOWER SA:V ratios — they are LESS efficient at exchanging materials with the environment. Small cells are more efficient. Cells divide when they get too large, restoring a high SA:V ratio.
- 🦠Saying prokaryotes have no ribosomesProkaryotes DO have ribosomes — they are just 70S (smaller than eukaryotic 80S cytoplasmic ribosomes). Ribosomes are present in ALL living cells. Without ribosomes, no protein synthesis is possible.
- 🧬Confusing osmosis directionWater moves from LOW solute (hypotonic) to HIGH solute (hypertonic) — equivalently from HIGH water potential to LOW water potential. Do not say "water follows solutes" — it moves toward higher solute concentration, but water molecules themselves move, not solutes.
- 🍃Thinking chloroplasts are only found in leavesChloroplasts are found in all green parts of plants (stems, unripe fruit, guard cells). They contain their own DNA, divide by binary fission, and have 70S ribosomes — just like mitochondria. All of this is endosymbiotic evidence.
Unit 2 — Key Takeaways
Prokaryotes: no nucleus, circular DNA, 70S ribosomes. Eukaryotes: membrane-bound organelles. Know the secretory pathway (RER → Golgi → vesicle → outside).
SA:V decreases as cell grows. Cells must stay small to efficiently exchange materials. SA = 6s² (cube); SA:V = 6/s. Microvilli, cristae, alveoli all increase SA.
Fluid Mosaic Model: phospholipid bilayer with integral/peripheral proteins, cholesterol, glycoproteins. Fluidity: ↑ unsaturation → ↑ fluidity; cholesterol buffers fluidity.
Small nonpolar → freely crosses. Ions/large polar → need proteins. Aquaporins speed water movement. Steroid hormones cross; protein hormones cannot.
Simple diffusion: small nonpolar, no protein, no ATP. Osmosis: water down Ψ gradient. Facilitated diffusion: ions/glucose via channels/carriers, no ATP, saturatable.
Hypotonic → water in (turgid plant / lysed animal). Isotonic → no change (flaccid plant). Hypertonic → water out (plasmolysis plant / crenation animal).
Against gradient, requires ATP. Na⁺/K⁺-ATPase: 3 Na⁺ out, 2 K⁺ in. Co-transport: uses Na⁺ gradient for glucose uptake. Endo/exocytosis for bulk material.
Allows simultaneous incompatible reactions; maintains H⁺ gradients for ATP synthesis. Endosymbiosis: mitochondria + chloroplasts from bacteria (circular DNA, 70S ribosomes, binary fission, double membrane).
Unit 2 = 10–13% of the AP Biology Exam. The highest-yield topics are: membrane transport (passive vs. active), tonicity and osmosis calculations (including water potential math), the secretory pathway, endosymbiotic theory evidence, and SA:V ratio calculations. Osmosis and tonicity questions appear in almost every exam. Know the difference between turgid/flaccid/plasmolysis (plant) and normal/crenated/lysed (animal) clearly.