Cell biology Study Guide

📖 Core Concepts Cell biology – study of cell structure, function, and behavior; the foundation of all biology. Cell theory – (1) all living things are made of cells, (2) cells are the basic functional units, (3) all cells arise from pre‑existing cells. Prokaryotic vs. eukaryotic cells – prokaryotes lack a nucleus and membrane‑bound organelles; eukaryotes have both. Research techniques – culture (grow cells), microscopy (visualize), cytometry (measure/ sort), fractionation (isolate organelles). Cytopathology – diagnosis of disease by examining individual cells or tiny tissue fragments. --- 📌 Must Remember Schleiden & Schwann (1838) → cells = basic units of life. Virchow (1857) → “Omnis cellula e cellula” – every cell comes from another cell. Fluorescent proteins (e.g., GFP) are used to tag and visualize specific cellular components. Confocal microscopy → optical sectioning → 3‑D reconstruction of fluorescently labeled samples. Transmission EM → electron beams + heavy‑metal stains → nanometer‑scale images of internal structures. Cell fractionation → break cells → centrifuge → separate organelles for individual study. Key discoveries: Aquaporins (Agre) – water channels. Protein targeting to organelles (Blobel). Lysosomes (de Duve). Programmed cell death pathways (Horvitz). Chemiosmotic theory (Mitchell). Cell‑cycle regulators (Nurse). Autophagy mechanisms (Ohsumi). --- 🔄 Key Processes Cell Culture Workflow Sterile media preparation → inoculate cells → incubate (37 °C, 5 % CO₂) → monitor growth → passage or harvest. Fluorescence Microscopy Tag protein → excite fluorophore with specific wavelength → emitted light collected → image formation. Confocal Imaging Scan specimen point‑by‑point with a focused laser → reject out‑of‑focus light with a pinhole → stack images → reconstruct 3‑D model. Transmission EM Preparation Fix sample → stain with heavy metals → dehydrate → embed in resin → thin‑section → electron beam passes → image detector records contrast. Cytometry (Flow) Hydrodynamically focus cells → laser interrogation → detect forward scatter (size), side scatter (granularity), fluorescence (markers) → data analysis or sorting. Cell Fractionation Lyse cells (heat/sonication) → differential centrifugation (low speed → nuclei; higher speeds → mitochondria, lysosomes, etc.) → collect pellets for downstream assays. --- 🔍 Key Comparisons Phase‑contrast vs. Fluorescence microscopy Phase‑contrast: no stains; visualizes density differences; good for live, unlabeled cells. Fluorescence: requires fluorophores; specific labeling of structures; higher contrast for targeted components. Confocal vs. Transmission EM Confocal: optical sectioning, live‑cell possible, resolution 200 nm, 3‑D images. TEM: electron‑scale resolution (1 nm), requires fixed, thin sections, provides ultrastructure. Cell culture vs. Clonogenic assay Culture: bulk expansion for experiments, drug testing, protein production. Clonogenic assay: single‑cell plating to assess ability to form colonies; measures reproductive viability. --- ⚠️ Common Misunderstandings “Viruses are cells.” – Incorrect; they lack cellular machinery, studied in virology, not cell biology. “All microscopy shows the same detail.” – Phase‑contrast is low‑resolution; fluorescence adds specificity; confocal adds depth; TEM gives ultrastructural detail. “Cell fractionation destroys organelles.” – Properly controlled lysis and gentle centrifugation preserve organelle integrity for functional assays. --- 🧠 Mental Models / Intuition Cell as a “factory” – Nucleus = control room; organelles = specialized workstations; cytoskeleton = conveyor belts; membranes = security gates. Microscopy hierarchy – Think of lenses as “magnifying glasses”: phase‑contrast (wide‑angle view), fluorescence (color‑coded tags), confocal (layer‑by‑layer slices), TEM (electron‑microscope “X‑ray vision”). --- 🚩 Exceptions & Edge Cases Prokaryotes with internal membranes (e.g., Planctomycetes) blur the classic nucleus‑less definition. Fluorescent protein bleed‑through – overlapping emission spectra can cause false colocalization; need proper filter sets or spectral unmixing. Centrifugation overlap – some organelles have similar densities; gradient centrifugation (e.g., sucrose) may be required for clean separation. --- 📍 When to Use Which Choose microscopy Live, unstained cells → Phase‑contrast. Specific protein localization → Fluorescence. 3‑D reconstruction of labeled structures → Confocal. Sub‑nanometer detail of membranes or ribosomes → TEM. Choose cell analysis Bulk biochemical assays → Standard culture lysates. Single‑cell heterogeneity → Flow cytometry. Clonogenic potential → Clonogenic assay. Choose fractionation method Rough organelle separation → Differential centrifugation. Precise organelle purity → Density‑gradient centrifugation. --- 👀 Patterns to Recognize “GFP‑tag → green signal” in fluorescence images often indicates the protein of interest; look for co‑localization with organelle markers. Phase‑contrast halos around cells signal proper alignment of the phase ring; misaligned rings give uneven contrast. Centrifuge speed‑density relationship – low speed = large/heavy components; high speed = small/light components. Flow cytometry plots – forward scatter vs. side scatter clusters correspond to size/granularity; fluorescence shifts indicate marker expression. --- 🗂️ Exam Traps Distractor: “Viruses are studied in cell biology because they infect cells.” – Wrong; virology is separate because viruses lack cellular structure. Near‑miss: “Phase‑contrast microscopy uses fluorescence.” – Incorrect; it relies on optical phase differences, not fluorophores. Confusing statement: “TEM provides 3‑D images.” – TEM produces 2‑D projections; 3‑D reconstructions require tomography, not routine TEM. Misleading choice: “Cell fractionation always yields pure organelles.” – Not always; overlapping densities can cause cross‑contamination without gradient steps. ---