Premed · Premed · Cell Biology

Lecture 28: Course Review

Cell Biology

Learning Objectives

By the end of this lecture, students will be able to:

1. Integrate concepts across all major topics covered in this course
2. Identify connections between cellular structures, signaling pathways, and disease mechanisms
3. Apply cell biology principles to solve multi-step problems
4. Prepare effectively for the final examination by understanding key themes and high-yield topics

Lecture Content

I. Cell Structure and Compartmentalization (Lectures 1-3)

Cells are the fundamental unit of life; all cells share common features (plasma membrane, ribosomes, DNA genome) Prokaryotes vs. eukaryotes: eukaryotes have membrane-bound organelles that compartmentalize functions. Endomembrane system: ER, Golgi, lysosomes, endosomes, vesicles — interconnected by vesicular transport. Macromolecules: proteins, nucleic acids, carbohydrates, lipids — structure determines function. Key concept: compartmentalization allows simultaneous, incompatible biochemical reactions and increases efficiency.

II. Membranes and Transport (Lectures 4-6)

Fluid mosaic model: lipid bilayer with embedded and peripheral proteins. Membrane fluidity determined by: fatty acid saturation, chain length, cholesterol content, temperature. Asymmetry: different lipid and protein composition on each leaflet. Transport: Passive: simple diffusion, facilitated diffusion (channels, carriers) — down the electrochemical gradient; no energy required. Active: primary (ATP-driven pumps: Na+/K+-ATPase, Ca2+-ATPase, H+/K+-ATPase) and secondary (co-transporters using ion gradients, e.g., SGLT1) Na+/K+-ATPase: 3 Na+ out, 2 K+ in per ATP; establishes the Na+ and K+ gradients essential for membrane potential, secondary active transport, and cell volume regulation. Ion channels: selective, gated pores. Voltage-gated (Na+, K+, Ca2+ channels): action potentials. Ligand-gated: neurotransmitter receptors (nAChR) Mechanosensitive: Piezo1, Piezo2. Key diseases: cystic fibrosis (CFTR Cl- channel), channelopathies (long QT syndrome, epilepsy).

III. Cell Signaling (Lectures 7-9)

General principles: signal -> receptor -> intracellular transduction -> cellular response. Amplification, specificity, integration, adaptation/desensitization. GPCRs (7-TM receptors): largest receptor family. G-alpha-s -> adenylyl cyclase -> cAMP -> PKA. G-alpha-q -> PLC -> IP3 (Ca2+ release) + DAG (PKC activation) G-alpha-i -> inhibits adenylyl cyclase. Desensitization: GRK phosphorylation -> beta-arrestin binding -> internalization. RTKs: dimerization -> autophosphorylation -> recruitment of SH2-domain proteins. Ras-MAPK pathway (proliferation): RTK -> Grb2 -> SOS -> Ras-GTP -> Raf -> MEK -> ERK. PI3K-Akt pathway (survival): RTK -> PI3K -> PIP3 -> Akt (inhibited by PTEN) Both pathways are major oncogenic drivers when constitutively activated. Intracellular receptors: steroid/thyroid hormones cross the membrane; bind nuclear receptors -> transcription factor activation. Integration theme: the same pathways appear in multiple contexts (development, physiology, cancer).

<image>Integrated signaling overview. Panel A: Side-by-side comparison of the three major receptor types — GPCR (7-TM, heterotrimeric G protein, cAMP/Ca2+ second messengers), RTK (dimerization, autophosphorylation, Ras-MAPK and PI3K-Akt cascades), and nuclear receptor (ligand crosses membrane, binds intracellular receptor, receptor-ligand complex acts as transcription factor binding DNA response elements). Panel B: Convergence and crosstalk — arrows showing how GPCR signaling (via PKA/PKC) can modulate MAPK pathway activity, how Akt cross-talks with the cell cycle (inhibits p27, activates cyclin D/MDM2), and how these pathways converge on transcription factors (CREB, Myc, Fos/Jun) that regulate proliferation, survival, and differentiation.</image>

IV. Endomembrane System and Protein Trafficking (Lectures 10-15)

ER: protein synthesis (rough ER), lipid synthesis and detoxification (smooth ER) Signal hypothesis: signal peptide -> SRP -> SRP receptor -> translocon -> co-translational translocation. Protein folding: BiP, calnexin/calreticulin, PDI. Unfolded protein response (UPR): IRE1, PERK, ATF6 sensors. ER quality control: ERAD (misfolded proteins retrotranslocated and degraded by proteasome) Golgi: cis -> medial -> trans processing; glycosylation (N-linked modification, O-linked addition) Sorting at the TGN: signals direct proteins to lysosomes (M6P), plasma membrane (default), or secretory granules. Vesicular transport: COPII (ER to Golgi), COPI (Golgi to ER retrograde), clathrin (TGN to endosomes, PM to endosomes) Coat proteins, Rab GTPases (tethering), SNAREs (membrane fusion) Endocytosis: clathrin-mediated, caveolae, receptor-mediated (LDL receptor pathway) Lysosomes: acid hydrolases (pH ~4.5-5.0), autophagy (macroautophagy: phagophore -> autophagosome -> autolysosome) Lysosomal storage diseases: Tay-Sachs, Gaucher, I-cell disease. Mitochondria: oxidative phosphorylation, TCA cycle; double membrane; own DNA; maternal inheritance. Electron transport chain: complexes I-IV -> proton gradient -> ATP synthase (complex V) Peroxisomes: beta-oxidation of very long chain fatty acids; catalase (H2O2 detoxification) Zellweger syndrome: peroxisome biogenesis disorder.

V. The Cytoskeleton and Cell Motility (Lectures 16-19)

Three cytoskeletal systems:

Motor proteins: myosins (actin), kinesins (MT, mostly plus-end), dyneins (MT, minus-end) Muscle contraction: sarcomere, sliding filament model, Ca2+-troponin-tropomyosin regulation. Cell migration: protrusion (actin polymerization) -> adhesion (integrins) -> traction (myosin II contraction) -> retraction.

VI. Cell Cycle, Division, and Death (Lectures 20-22)

Cell cycle: G1 -> S -> G2 -> M; driven by cyclin-CDK complexes. Cyclin D-CDK4/6 (G1) -> Cyclin E-CDK2 (G1/S) -> Cyclin A-CDK2 (S) -> Cyclin B-CDK1 (G2/M) Restriction point: Rb phosphorylation releases E2F -> commitment to S phase. Checkpoints: G1/S (p53/p21), G2/M (Cdc25/Wee1), SAC (Mad2-APC/C) Mitosis: prophase -> prometaphase -> metaphase -> anaphase -> telophase. APC/C-Cdc20: ubiquitinates securin (freeing separase -> cohesin cleavage) and cyclin B (CDK1 inactivation) SAC prevents premature anaphase until all kinetochores are bi-oriented. Cytokinesis: RhoA -> contractile ring (actin + myosin II) -> furrow ingression -> ESCRT-III-mediated abscission. Apoptosis: Intrinsic: stress -> BH3-only proteins -> Bax/Bak -> MOMP -> cytochrome c -> apoptosome -> caspase-9 -> caspase-3. Extrinsic: death ligand -> death receptor -> DISC -> caspase-8 -> caspase-3. Regulated by Bcl-2 family (anti- vs. pro-apoptotic balance) and IAPs/Smac.

<image>Cell cycle and apoptosis integration. Panel A: Cell fate decision diagram — a cell receiving growth factor signals progresses through the cell cycle (G1->S->G2->M, with cyclin-CDK complexes driving each transition); DNA damage activates p53, which can either arrest the cycle (via p21 inhibiting CDKs) or induce apoptosis (via PUMA/NOXA activating the intrinsic mitochondrial pathway). Loss of growth factor signals can lead to apoptosis (Bad activation) or quiescence (G0). Panel B: Connections to cancer — oncogenic mutations that override each control point are indicated: cyclin D amplification or Rb loss (bypasses G1 restriction), p53 mutation (bypasses DNA damage arrest and apoptosis), Bcl-2 overexpression (blocks intrinsic apoptosis), SAC defects (causes aneuploidy). Panel C: Overview of the apoptosis pathways — extrinsic (Fas/FADD/caspase-8) and intrinsic (Bax/cytochrome c/apoptosome/caspase-9) converging on executioner caspases (caspase-3) leading to cell dismantling and phagocytic clearance.</image>

VII. Cell Adhesion, ECM, and Tissue Organization (Lectures 23-24)

Cell-cell junctions: tight junctions (barrier/fence), adherens junctions (E-cadherin-actin), desmosomes (desmosomal cadherins-IFs), gap junctions (connexins, direct communication) Cell-ECM adhesion: integrins (bidirectional signaling, focal adhesions) ECM components: collagen (tensile strength), proteoglycans/GAGs (compression resistance, hydration), laminin and fibronectin (adhesion, signaling) Basement membrane: type IV collagen + laminin + nidogen + perlecan. Mechanotransduction: cells sense ECM stiffness and respond (YAP/TAZ, FAK/Src) Clinical connections: epidermolysis bullosa (keratin/integrin mutations), pemphigus (anti-desmoglein antibodies), Alport syndrome (collagen IV), fibrosis (excessive ECM), cancer invasion (MMPs, EMT).

VIII. Stem Cells, Cancer, and Modern Methods (Lectures 25-27)

Stem cells: self-renewal + differentiation; niche regulates behavior. ESCs, iPSCs (Yamanaka factors: Oct4, Sox2, Klf4, c-Myc), adult stem cells. Wnt, Notch, BMP, Hedgehog: key niche signaling pathways. Cancer: accumulation of genetic/epigenetic alterations; hallmarks of cancer framework. Oncogenes (gain-of-function): Ras, Myc, HER2, BCR-ABL, cyclin D. Tumor suppressors (loss-of-function): p53, Rb, APC, BRCA1/2, PTEN. Metastatic cascade: invasion -> intravasation -> circulation -> extravasation -> colonization. Targeted therapy: imatinib, trastuzumab, PARP inhibitors, checkpoint inhibitors. CRISPR: Cas9 + sgRNA -> DSB -> NHEJ (knockout) or HDR (knockin) Advanced tools: base editing, prime editing, CRISPRi/a, genome-wide screens. Therapeutic: exa-cel for sickle cell disease (first approved CRISPR therapy) Single-cell technologies: scRNA-seq reveals cellular heterogeneity; spatial transcriptomics preserves tissue context. Perturb-seq: CRISPR + scRNA-seq for functional genomics.

IX. Exam Preparation: Key Themes and Integration

Recurring principles to master: Structure-function relationships at every level (protein, organelle, cell, tissue) Compartmentalization and sorting signals. GTPase switches (Ras, Rho, Rab, Ran, heterotrimeric G proteins): common regulatory logic. Phosphorylation cascades as signal amplifiers and integration points. Ubiquitin-proteasome system: targeted protein degradation drives irreversibility (cell cycle, signaling) Cytoskeletal dynamics: assembly/disassembly regulated by nucleotide hydrolysis and accessory proteins. From cell biology to disease: nearly every cellular pathway has a disease associated with its malfunction. High-yield clinical connections: cystic fibrosis, lysosomal storage diseases, Kartagener syndrome, epidermolysis bullosa, laminopathies, cancer (multiple oncogenes and tumor suppressors), immunodeficiencies, neurodegenerative diseases. Practice strategy: focus on understanding mechanisms (not just memorizing facts); draw pathway diagrams from memory; explain how a mutation in a specific gene would affect downstream events.

Why this matters

This comprehensive review is your roadmap for integrating the vast material covered in cell biology. On the MCAT, you'll encounter complex scenarios that require connecting seemingly disparate concepts—for instance, understanding how a mutation affecting a cytoskeletal protein impacts cell migration and contributes to metastatic cancer. In the clinic, understanding the cellular and molecular basis of diseases, from genetic disorders like cystic fibrosis to multifactorial conditions like cancer, is fundamental for diagnosis, prognosis, and developing targeted therapies.

A picture in words

Imagine your brain is a massive library. Each lecture is a new book on a specific topic. This review lecture isn't just a reading list; it's the head librarian walking you through how all the books are interconnected. They show you where the cell signaling books are next to the cancer books, and how understanding membrane transport helps explain drug delivery, illustrating the flow of information and function across different sections.

Checkpoint

How does the interconnectedness of cellular pathways, as highlighted in this review, become particularly relevant when considering drug side effects? Understanding pathway interconnectedness is crucial because drugs rarely act with absolute specificity; inhibiting one pathway can inadvertently impact others that share common components or crosstalk, leading to off-target effects and adverse reactions. This necessitates a holistic view of cellular biology to anticipate potential drug interactions and side effects.

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