Cell Biology Ultimate Cheat Sheet
A comprehensive 1000+ word study module. Master the high-yield concepts, vesicular transport pathways, cell signaling, and apoptosis mechanisms guaranteed to appear in the DBT BET JRF 2026 Exam.
1. Plasma Membrane & Membrane Transport
The plasma membrane is the foundational topic of Cell Biology. In the DBT BET exam, examiners frequently test your understanding of membrane fluidity, lipid composition, and specific transport mechanisms.
Membrane Dynamics & Lipid Rafts
According to the Fluid Mosaic Model, the membrane is a dynamic structure. Membrane fluidity is assessed experimentally using FRAP (Fluorescence Recovery After Photobleaching). Faster recovery in FRAP indicates higher lateral mobility of membrane proteins. Lipid Rafts are specialized microdomains rich in cholesterol and sphingolipids. They are thicker than the rest of the membrane and serve as signaling platforms and entry points for viruses (like HIV and Influenza).
Types of Membrane Transporters
- P-type ATPases: Undergo phosphorylation during the transport cycle. Example: Na⁺/K⁺ ATPase (pumps 3 Na⁺ out, 2 K⁺ in) and Ca²⁺ ATPase in the sarcoplasmic reticulum.
- V-type ATPases: 'V' stands for Vacuolar. They use ATP to pump protons (H⁺) into organelles to acidify them. Found heavily in the membranes of Lysosomes and plant vacuoles.
- F-type ATPases: The ATP Synthase in mitochondria and chloroplasts. They usually run in reverse, using a proton gradient to synthesize ATP.
- ABC Transporters: ATP-Binding Cassette transporters. Clinically highly relevant due to MDR1 (Multidrug Resistance Protein), which pumps chemotherapy drugs out of cancer cells.
2. Intracellular Compartments & Vesicular Transport
How does a cell know where to send a newly synthesized protein? Protein sorting and vesicular trafficking are favorite topics of DBT examiners, especially the different coated vesicles.
Vesicular Coats (Strictly Memorize)
- COP II vesicles: Mediate Anterograde transport from the Rough ER to the cis-Golgi network. Activated by the Sar1 GTPase.
- COP I vesicles: Mediate Retrograde transport from the Golgi back to the ER. Used to retrieve escaped ER resident proteins (which contain the KDEL or KKXX sorting signals). Activated by ARF GTPase.
- Clathrin-coated vesicles: Mediate transport from the trans-Golgi network to Lysosomes, and also mediate receptor-mediated endocytosis at the plasma membrane.
| Organelle | Marker Enzyme | Primary Exam Focus / Function |
|---|---|---|
| Plasma Membrane | Na⁺/K⁺ ATPase, 5'-Nucleotidase | Cell boundary, Signal transduction, Transport |
| Mitochondria (Inner) | Cytochrome C Oxidase, ATP Synthase | Electron Transport Chain (ETC), Apoptosis trigger |
| Mitochondria (Matrix) | Citrate Synthase, Pyruvate Dehydrogenase | TCA Cycle, Beta-oxidation of short fatty acids |
| Lysosome | Acid Phosphatase, Cathepsins | Degradation of macromolecules at pH 5.0 |
| Golgi Apparatus | Galactosyl Transferase | O-linked glycosylation, Protein sorting |
| Rough ER | Glucose-6-phosphatase | Protein folding, N-linked glycosylation core synthesis |
| Peroxisome | Catalase, Urate Oxidase | VLCFA (Very Long Chain Fatty Acid) oxidation, ROS detox |
3. The Cell Cycle, Checkpoints & Apoptosis
Cell division must be tightly regulated. Cancer is essentially a failure of cell cycle checkpoints and apoptotic mechanisms. This makes it a high-yield concept for molecular biology exams.
Tumor Suppressors and Checkpoints
The G1/S Checkpoint (Restriction Point) is crucial. Here, the protein p53 (Guardian of the Genome) checks for DNA damage. If damage is found, p53 induces the transcription of p21 (a CDK inhibitor), which halts the cell cycle to allow for repair. Another key player is the Retinoblastoma (Rb) protein. In its unphosphorylated state, Rb binds and inhibits the E2F transcription factor. Cyclin D/CDK4 phosphorylates Rb, releasing E2F to push the cell into S phase.
| Phase | Active Cyclin | Active CDK | Critical Function in Cycle |
|---|---|---|---|
| G1 Phase | Cyclin D | CDK 4, 6 | Hyperphosphorylates Rb protein, releasing E2F |
| Late G1 / S | Cyclin E | CDK 2 | Commits cell to DNA replication |
| S Phase | Cyclin A | CDK 2 | Initiates DNA replication & fires replication origins once |
| M Phase | Cyclin B | CDK 1 (cdc2) | Forms Maturation Promoting Factor (MPF); drives mitosis |
Apoptosis (Programmed Cell Death)
Apoptosis is distinct from necrosis (which causes inflammation). Apoptosis is characterized by chromatin condensation, DNA fragmentation (laddering pattern on gels), and membrane blebbing. It relies on the Bcl-2 protein family:
- Pro-apoptotic factors: Bax, Bak, Bad, Bid. (They poke holes in the mitochondria).
- Anti-apoptotic factors: Bcl-2, Bcl-xL. (They protect the mitochondria).
4. Cell Signaling Pathways
Understanding how external signals change gene expression is non-negotiable for DBT JRF. Here are the two most tested pathways.
G-Protein Coupled Receptors (GPCRs)
GPCRs have 7 transmembrane domains. Upon ligand binding, the alpha subunit of the G-protein exchanges GDP for GTP and becomes active. It can activate two major effector enzymes:
- Adenylyl Cyclase: Converts ATP to cyclic AMP (cAMP), which then activates Protein Kinase A (PKA).
- Phospholipase C (PLC): Cleaves PIP2 in the membrane to form IP3 and DAG. IP3 travels to the smooth ER to release Calcium (Ca²⁺), while DAG stays in the membrane to activate Protein Kinase C (PKC).
Receptor Tyrosine Kinases (RTKs)
Used primarily by growth factors (like EGF, Insulin). Ligand binding causes receptor dimerization and cross-autophosphorylation of tyrosine residues. This recruits adapter proteins like Grb2 and SOS, ultimately activating the Ras-MAPK cascade or the PI3K-Akt pathway (critical for cell survival and growth).
5. The Cytoskeleton & Motors
The cytoskeleton provides structural support and the "highways" for intracellular transport.
| Filament Type | Monomer Subunit | Key Properties & Associated Motors |
|---|---|---|
| Microtubules (25 nm) | α/β Tubulin dimers (binds GTP) | Exhibits dynamic instability. Motors: Kinesin (+ end), Dynein (- end). Target of Taxol & Colchicine. |
| Microfilaments (7 nm) | G-Actin (binds ATP) | Exhibits treadmilling. Crucial for cytokinesis (cleavage furrow). Motor: Myosin. |
| Intermediate Filaments | Keratin, Vimentin, Lamins | No polarity. No motor proteins. Provides pure mechanical tensile strength. |
Guaranteed Exam Hits
- M6P Sorting Signal: The attachment of Mannose-6-Phosphate (M6P) in the cis-Golgi is the universal signal that targets acid hydrolase enzymes directly to the Lysosomes. Defect causes I-cell disease.
- Apoptotic Caspases: The Intrinsic pathway activates Caspase 9 via the apoptosome. The Extrinsic pathway activates Caspase 8. Both converge to activate executioner Caspase 3.
- ER Retention Motif: The amino acid sequence KDEL at the C-terminus acts as a retrieval signal, ensuring ER proteins are sent back from the Golgi via COP I vesicles.
- Nuclear Import via Ran-GTPase: Transport of cargo into the nucleus requires Nuclear Localization Signals (NLS). Driven by the Ran GTPase. High Ran-GTP in the nucleus promotes cargo release.
- SNARE Hypothesis: Vesicle fusion is mediated by v-SNAREs (vesicle) and t-SNAREs (target). Botulinum toxin cleaves SNAREs, preventing neurotransmitter release.
- Cell Junctions: Cadherins mediate calcium-dependent cell-cell adhesion. Integrins mediate cell-ECM adhesion. Gap junctions allow transfer of ions between adjacent cells.
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