Cell Biology

Cell Biology for the MCAT

Part 1 of 7 — Cell Structure & Organelles

Prokaryotes vs. Eukaryotes

Key Organelles

The Endomembrane System — Protein Trafficking

The MCAT frequently tests the path of a secretory protein:

$\text{Ribosome on RER} \to \text{ER lumen} \to \text{transport vesicle} \to \text{cis-Golgi} \to \text{trans-Golgi} \to \text{secretory vesicle} \to \text{plasma membrane}$

• Signal peptide: N-terminal sequence that directs the ribosome to the RER
• Signal recognition particle (SRP): Binds signal peptide and docks ribosome on RER
• Glycosylation begins in the ER (N-linked) and is modified in the Golgi (O-linked added)
• Mannose-6-phosphate tag: targets proteins to lysosomes

Endosymbiotic Theory — Evidence Checklist

Why mitochondria (and chloroplasts) were once free-living bacteria:

• Own circular DNA (like bacteria)
• 70S ribosomes (not 80S like the rest of the eukaryotic cell)
• Double membrane (inner = original bacterial membrane; outer = host's endocytic vesicle)
• Reproduce by binary fission
• Maternal inheritance (mitochondria come from the egg)

Cytoskeleton Overview

• Cilia: 9+2 microtubule arrangement (motile) or 9+0 (primary/sensory)
• Dynein: motor protein that moves cargo toward minus end (toward cell center)
• Kinesin: motor protein that moves cargo toward plus end (toward periphery)

Cell Structure & Organelles 🎯

Passage-Style Thinking: Organelle Dysfunction

MCAT passages often describe a disease and ask you to identify the organelle involved. Key pattern recognitions:

Free vs. Bound Ribosomes

• Free ribosomes: Make proteins that stay in the cytoplasm (enzymes, structural proteins)
• Bound ribosomes (on RER): Make secretory proteins, membrane proteins, and lysosomal enzymes
• The ribosome itself is identical — the signal peptide determines where it goes
• This is a common MCAT distractor: the ribosome does not "know" where it needs to be in advance

Deeper Concepts 🎯

Key Takeaways — Part 1

• Know every organelle's function and the diseases that result from dysfunction
• Protein trafficking path: RER → transport vesicle → cis-Golgi → trans-Golgi → destination
• Mannose-6-phosphate = lysosome targeting signal; defects cause I-cell disease
• Endosymbiotic theory evidence: circular DNA, 70S ribosomes, double membrane, binary fission
• Cytoskeleton: microfilaments (actin, 7nm), intermediate filaments (10nm), microtubules (tubulin, 25nm)
• Motor proteins: dynein (minus-end), kinesin (plus-end); dynein arms also drive cilia
• Free ribosomes → cytoplasmic proteins; bound ribosomes → secretory/membrane/lysosomal proteins

Cell Biology for the MCAT

Part 2 of 7 — Cell Membrane & Transport

Membrane Structure (Fluid Mosaic Model)

• Phospholipid bilayer: Hydrophilic heads out, hydrophobic tails in
• Cholesterol: Regulates fluidity — prevents crystallization at low temp, prevents excess fluidity at high temp (acts as a "fluidity buffer")
• Integral proteins: Span the membrane (channels, receptors, transporters)
• Peripheral proteins: Loosely attached to surface (often via electrostatic interactions)
• Glycoproteins/Glycolipids: Carbohydrate chains on extracellular face only — cell recognition, immune identity

Transport Mechanisms

Na$^+$/K$^+$ ATPase (ULTRA HIGH YIELD)

Per ATP hydrolyzed: 3 Na$^+$ out, 2 K$^+$ in

• Creates electrochemical gradient for both ions
• Maintains resting membrane potential (~$-70$ mV)
• Electrogenic: net positive charge moved out (3+ out vs 2+ in)
• Powers secondary active transport (Na$^+$ gradient drives glucose uptake in intestine)

Osmosis and Tonicity

Key distinction: Osmolarity = total solute concentration. Tonicity = the effect on cell volume (only non-penetrating solutes matter). Urea is an osmole but freely crosses membranes, so it does not affect tonicity.

Membrane Transport 🎯

Membrane Selectivity: What Crosses and What Cannot

This is a fundamental MCAT reasoning skill — predicting what can cross a lipid bilayer:

Crosses freely (simple diffusion):

• Small, nonpolar molecules: O$_2$, CO$_2$, N$_2$
• Small, uncharged polar: H$_2$O (slowly), ethanol, urea
• Hydrophobic molecules: steroid hormones, fatty acids

Cannot cross without help:

• Ions: Na$^+$, K$^+$, Ca$^{2+}$, Cl$^-$ (charged = repelled by hydrophobic core)
• Large polar molecules: glucose, amino acids
• Macromolecules: proteins, nucleic acids

Receptor-Mediated Endocytosis

• Ligand binds receptor → clathrin-coated pit forms → vesicle internalized
• Example: LDL cholesterol uptake via LDL receptors
• Familial hypercholesterolemia: defective LDL receptors → LDL stays in blood → atherosclerosis
• This is a favorite MCAT passage topic linking cell biology to disease

Membrane Potential — Nernst Equation

For a single ion, the equilibrium potential is:

$E_{ion} = \frac{61}{z} \log \frac{[ion]_{outside}}{[ion]_{inside}} \text{ (at 37°C, in mV)}$

• $E_K \approx -90$ mV (K$^+$ higher inside)
• $E_{Na} \approx +60$ mV (Na$^+$ higher outside)
• Resting potential (~$-70$ mV) is closer to $E_K$ because the membrane is more permeable to K$^+$ at rest

Advanced Transport 🎯

Key Takeaways — Part 2

• Fluid mosaic model: phospholipids + cholesterol (fluidity buffer) + integral/peripheral proteins + glycocalyx
• Crossing rules: small nonpolar = free diffusion; charged/large polar = need channels or transporters
• Na$^+$/K$^+$ ATPase: 3 Na$^+$ out, 2 K$^+$ in — electrogenic, powers secondary active transport
• Osmolarity ≠ tonicity: only non-penetrating solutes affect cell volume (urea penetrates → does not contribute to tonicity)
• Receptor-mediated endocytosis: clathrin-coated pits (LDL uptake → familial hypercholesterolemia link)
• Resting membrane potential (~$-$70 mV) determined mostly by K$^+$ leak channels, with small contribution from Na$^+$/K$^+$ ATPase

Cell Biology for the MCAT

Part 3 of 7 — Cell Cycle & Mitosis

The Cell Cycle

Interphase = G$_1$ + S + G$_2$ (where the cell spends ~95% of its time)

DNA Content Through the Cell Cycle

Key insight: After S phase, the chromosome number does NOT double — sisters are still joined at the centromere. The DNA content doubles (2C → 4C) but chromosome count stays at 2n until anaphase of meiosis I.

Mitosis Stages (PMAT)

1. Prophase: Chromatin condenses → chromosomes visible. Nuclear envelope breaks down. Centrosomes migrate to poles, spindle begins forming.
2. Prometaphase: Kinetochore microtubules attach to centromeres. Chromosomes move to center.
3. Metaphase: Chromosomes align at metaphase plate. Spindle assembly checkpoint ensures all kinetochores are attached.
4. Anaphase: Cohesin proteins cleaved → sister chromatids separate and are pulled to opposite poles by shortening kinetochore microtubules.
5. Telophase: Nuclear envelopes reform around each chromosome set. Chromosomes decondense. Cytokinesis begins.

Cytokinesis

• Animal cells: Cleavage furrow (contractile ring of actin and myosin pinches the cell)
• Plant cells: Cell plate forms from Golgi-derived vesicles (no cleavage furrow — rigid cell wall)

Cell Cycle Regulation — Cyclins and CDKs

• CDKs (cyclin-dependent kinases) are always present but inactive without their cyclin partner
• CDK inhibitors (p21, p27) act as brakes — upregulated by p53

Cell Cycle Checkpoints

Cell Cycle & Mitosis 🎯

Cancer Biology — Oncogenes vs. Tumor Suppressors

This is one of the most tested MCAT topics in cell biology. Understand the analogy:

• Proto-oncogenes → mutated → Oncogenes: "Gas pedal stuck ON"

  • Gain-of-function mutation (only need ONE allele mutated = dominant)
  • Examples: Ras (GTPase stuck in active state), Myc (transcription factor overexpressed), HER2 (receptor always active)

• Tumor suppressors: "Brakes removed"

  • Loss-of-function mutation (need BOTH alleles lost = recessive at cellular level)
  • Two-hit hypothesis (Knudson): both copies must be inactivated
  • Examples: p53 (G$_1$/S checkpoint), Rb (retinoblastoma protein binds E2F), APC (colon cancer), BRCA1/2 (DNA repair)

Rb Pathway — How It Works

1. Rb normally binds and inhibits E2F (a transcription factor for S-phase genes)
2. Growth factor signals → Cyclin D/CDK4 phosphorylates Rb → releases E2F
3. Free E2F activates genes needed for DNA replication
4. If Rb is mutated: E2F is always free → uncontrolled entry into S phase

The APC/C (Anaphase-Promoting Complex)

• Ubiquitin ligase activated at the metaphase-to-anaphase transition
• Targets securin for degradation → separase released → cleaves cohesin → sister chromatids separate
• Also targets cyclin B for degradation → MPF inactivated → cell exits mitosis

Cancer & Regulation 🎯

Key Takeaways — Part 3

• Cell cycle: G$_1$ → S (DNA doubles) → G$_2$ → M (mitosis + cytokinesis). Interphase = G$_1$+S+G$_2$
• After S phase: 46 chromosomes (unchanged), 92 chromatids, 4C DNA content
• Cyclins fluctuate; CDKs are constitutive. MPF (Cyclin B + CDK1) drives M-phase entry
• p53 → p21 → CDK inhibition at G$_1$/S checkpoint. p53 loss = cancer hallmark
• Oncogenes: gain-of-function, dominant (Ras, Myc, HER2). Tumor suppressors: loss-of-function, both alleles (p53, Rb, BRCA)
• Rb normally sequesters E2F; phosphorylation by CDK releases E2F for S-phase gene activation
• APC/C ubiquitinates securin and cyclin B → triggers anaphase and mitotic exit

Cell Biology for the MCAT

Part 4 of 7 — Meiosis & Genetic Diversity

Meiosis Overview

$\text{Diploid (2n, 4C)} \xrightarrow{\text{Meiosis I}} \text{Haploid (1n, 2C)} \xrightarrow{\text{Meiosis II}} \text{4 haploid gametes (1n, 1C)}$

Meiosis I vs. Meiosis II

Prophase I — The Key Stage

Prophase I is the longest and most complex phase:

1. Synapsis: Homologous chromosomes pair up (form a bivalent/tetrad)
2. Synaptonemal complex: Protein structure holds homologs together
3. Crossing over: Non-sister chromatids exchange DNA segments at chiasmata
4. This produces recombinant chromosomes with novel allele combinations

Sources of Genetic Diversity

Comparing Mitosis and Meiosis

Meiosis Fundamentals 🎯

Nondisjunction — When Chromosome Separation Fails

Common aneuploidies from nondisjunction:

• Trisomy 21 (Down syndrome): 3 copies of chromosome 21
• Turner syndrome (45, XO): only one X chromosome in females
• Klinefelter syndrome (47, XXY): extra X in males
• Trisomy 18 (Edwards syndrome), Trisomy 13 (Patau syndrome)

Oogenesis vs. Spermatogenesis

Why this matters for MCAT: Older maternal age → higher nondisjunction risk because oocytes arrested in prophase I for decades, cohesin proteins degrade over time.

Ploidy vs. DNA Content — Master This Distinction

The MCAT loves to test this:

• Ploidy (n): Number of unique chromosomes (haploid = n, diploid = 2n)
• DNA content (C): Amount of DNA (doubles after S phase)
• A cell can be 1n but 2C (after meiosis I, before meiosis II)
• Always track both independently through the cell cycle

Nondisjunction & Gametogenesis 🎯

Key Takeaways — Part 4

• Meiosis I: homologs separate (2n → 1n, reduction division). Meiosis II: sisters separate (like mitosis)
• Prophase I is unique: synapsis, crossing over at chiasmata, recombinant chromosomes
• Genetic diversity: crossing over + independent assortment ($2^{23}$) + random fertilization
• Nondisjunction in meiosis I → all 4 gametes abnormal; in meiosis II → 2 of 4 abnormal
• Spermatogenesis → 4 functional sperm; Oogenesis → 1 egg + 3 polar bodies
• Oocyte arrest at prophase I for decades → cohesin degradation → maternal age-related aneuploidy
• Always track ploidy (n) and DNA content (C) independently

Cell Biology for the MCAT

Part 5 of 7 — Cell Signaling

Signal Transduction — The Universal Framework

$\text{Signal (ligand)} \to \text{Receptor} \to \text{Transduction (amplification)} \to \text{Cellular Response}$

Signal amplification is critical: one hormone molecule can activate millions of downstream effectors through enzyme cascades. Each step multiplies the signal.

Types of Signaling

Major Receptor Types

GPCR Signaling — The Most Tested Pathway

1. Ligand binds GPCR (7-transmembrane domain receptor)
2. Conformational change → G$\alpha$ subunit exchanges GDP for GTP (activation)
3. G$\alpha$-GTP activates effector enzyme:
   • G$_s$ → activates adenylyl cyclase → cAMP ↑ → PKA activated
   • G$_i$ → inhibits adenylyl cyclase → cAMP ↓
   • G$_q$ → activates phospholipase C → IP$_3$ + DAG
4. G$\alpha$ has intrinsic GTPase activity → hydrolyzes GTP → returns to inactive state

Second Messengers

Cell Signaling 🎯

Signal Amplification — Why One Molecule Matters

MCAT passages test quantitative reasoning about amplification:

$\text{1 epinephrine} \to \text{1 GPCR} \to \text{many G-proteins} \to \text{many adenylyl cyclase} \to \text{many cAMP} \to \text{many PKA} \to \text{millions of products}$

Each enzyme activates multiple substrates, creating an exponential amplification cascade. This is why hormones work at nanomolar concentrations.

Key Pathway Connections for MCAT

Receptor Desensitization

Cells can turn down signaling when overstimulated:

• Receptor phosphorylation: Kinases phosphorylate the receptor → arrestin binds → blocks G-protein coupling
• Receptor internalization: Endocytosis removes receptors from the surface
• Downregulation: Decreased receptor gene expression
• This explains drug tolerance and why chronic stimulation leads to diminished response

Nitric Oxide (NO) Signaling — Unique Pathway

• NO is a gas that diffuses freely through membranes (no receptor needed at surface)
• Activates soluble guanylyl cyclase → cGMP ↑ → PKG → smooth muscle relaxation → vasodilation
• Very short-lived (seconds)
• NO synthase uses arginine + O$_2$ → citrulline + NO
• Clinical: nitroglycerin releases NO → relieves angina

Advanced Signaling 🎯

Key Takeaways — Part 5

• Signal transduction: ligand → receptor → transduction (amplification) → response
• GPCRs: G$_s$ activates adenylyl cyclase (cAMP ↑), G$_i$ inhibits it, G$_q$ activates PLC (IP$_3$ + DAG)
• RTKs: dimerize and autophosphorylate → Ras → MAPK cascade (growth signals)
• Steroid hormones: cross membrane, bind intracellular receptors, act as transcription factors (slow but lasting)
• Second messengers: cAMP, IP$_3$, DAG, Ca$^{2+}$, cGMP — know what produces each and what each activates
• Cholera = G$_s$ locked ON; Pertussis = G$_i$ locked OFF; both raise cAMP
• Signal amplification: each cascade step multiplies the signal exponentially
• NO: gaseous signal → guanylyl cyclase → cGMP → vasodilation

Cell Biology for the MCAT

Part 6 of 7 — Apoptosis & Cellular Processes

Apoptosis (Programmed Cell Death)

Apoptosis is an orderly, energy-requiring process — fundamentally different from necrosis:

• Cell shrinks, chromatin condenses, DNA fragments into 180 bp ladder
• Membrane blebs form (but does NOT rupture — no inflammation)
• "Eat me" signals (phosphatidylserine on outer leaflet) attract phagocytes
• Regulated by caspases — a protease cascade

Apoptosis Pathways

Key Regulators of Apoptosis

Apoptosis vs. Necrosis

Apoptosis 🎯

Autophagy — Self-Eating for Survival

Autophagy is distinct from apoptosis — it is a survival mechanism, not a death pathway:

• Cell digests its own damaged organelles or misfolded proteins
• Double-membrane vesicle (autophagosome) engulfs target → fuses with lysosome → contents degraded
• Activated by nutrient deprivation, stress, mTOR inhibition
• Provides amino acids and energy during starvation

MCAT distinction: Apoptosis = programmed cell DEATH. Autophagy = programmed cell SURVIVAL under stress.

Necroptosis — Programmed Necrosis

• A regulated form of necrosis (combines features of both)
• Triggered by death receptors (like extrinsic apoptosis) but when caspase-8 is inhibited
• RIPK1 → RIPK3 → MLKL → membrane rupture
• Results in inflammation (like necrosis) but is genetically programmed (like apoptosis)

Clinical Connections — MCAT Favorites

During Development — Apoptosis Is Essential

• Removes cells between developing fingers and toes
• Eliminates self-reactive T cells in the thymus (negative selection)
• Shapes the nervous system by removing neurons without proper connections
• The tadpole tail resorbs during metamorphosis via apoptosis

Cell Death Mechanisms 🎯

Key Takeaways — Part 6

• Apoptosis: orderly, ATP-requiring, no inflammation. Necrosis: chaotic, membrane rupture, inflammation.
• Intrinsic: stress → Bax/Bak pores → cytochrome c released → caspase-9. Extrinsic: death ligand → caspase-8. Both → caspase-3.
• Bcl-2 = anti-apoptotic (cancer when overexpressed); p53 and Bax = pro-apoptotic
• DNA ladder (~180 bp) = apoptosis. DNA smear = necrosis.
• PS exposure on outer membrane = "eat me" signal for phagocytes
• Autophagy = survival mechanism (self-digestion under stress), NOT death
• Apoptosis essential in development (digit separation, thymic negative selection, neural pruning)
• Too little apoptosis → cancer; too much → neurodegeneration, immunodeficiency

Cell Biology for the MCAT

Part 7 of 7 — Specialized Cell Types & Tissues

The Four Tissue Types

Epithelial Classifications

Rule: Simple = one layer (diffusion/absorption). Stratified = multiple layers (protection).

Muscle Types — Comparison

Connective Tissue Components

• Collagen: Most abundant protein in the body; provides tensile strength (Type I in bone/tendon, Type II in cartilage, Type IV in basement membranes)
• Elastin: Allows stretch and recoil (lungs, arteries, skin)
• Fibroblasts: Produce collagen and ECM components
• Ground substance: Gel-like matrix of proteoglycans and glycosaminoglycans (GAGs)

Tissues & Cell Types 🎯

Stem Cells — Potency Hierarchy

Cell Junctions — Holding Tissues Together

Extracellular Matrix (ECM) Signaling

• Integrins: Transmembrane receptors linking ECM to cytoskeleton
• Bidirectional signaling: "outside-in" (ECM signals affect cell behavior) and "inside-out" (cell regulates integrin adhesion)
• ECM composition influences cell fate: stiff ECM → bone differentiation; soft ECM → neuronal differentiation
• Matrix metalloproteinases (MMPs): Enzymes that degrade ECM (important in wound healing, but exploited by cancer cells for invasion/metastasis)

Germ Layer Origins — What Comes From Where

Advanced Topics 🎯

Cell Biology — Complete! ✅

Key Takeaways — Part 7

• Four tissue types: epithelial (cover), connective (support), muscle (contract), nervous (signal)
• Epithelial: simple = 1 layer (exchange); stratified = multiple layers (protection). Shape: squamous, cuboidal, columnar
• Muscle: skeletal (voluntary, striated), cardiac (involuntary, striated, intercalated discs), smooth (involuntary, non-striated)
• Collagen: most abundant protein; vitamin C required for hydroxylation (scurvy connection)
• Stem cell potency: totipotent → pluripotent → multipotent → oligopotent → unipotent
• Cell junctions: tight (barrier), adherens/desmosomes (adhesion), gap (communication), hemidesmosomes (to basement membrane)
• Integrins: link ECM to cytoskeleton; bidirectional signaling
• Germ layers: ectoderm (nerves, skin), mesoderm (muscle, bone, blood), endoderm (GI lining, liver, lungs)