Cell Biology — Structure, Organelles & Cellular Processes

Cell Theory & Types

Cell theory (Schleiden, Schwann, 1838–1839; Virchow, 1855): all living organisms are composed of cells; the cell is the basic unit of life; all cells arise from pre-existing cells. Prokaryotic cells (bacteria, archaea): no membrane-bound nucleus, circular DNA, 70S ribosomes, cell wall (peptidoglycan in bacteria), some have flagella. Size: 0.5–5 μm. Eukaryotic cells (animals, plants, fungi, protists): membrane-bound nucleus, linear chromosomes, 80S ribosomes, complex organelles. Size: 10–100 μm. Plant cells additionally have: cell wall (cellulose), chloroplasts, and large central vacuole.

Organelles & Their Functions

Nucleus : contains DNA, site of transcription, bounded by nuclear envelope with nuclear pores. Mitochondria : "powerhouse" — ATP synthesis via oxidative phosphorylation, own DNA (16,569 bp), double membrane. Endoplasmic reticulum : rough ER (ribosomes, protein synthesis/folding) and smooth ER (lipid synthesis, detoxification). Golgi apparatus : modifies, sorts, and packages proteins (cis → trans). Lysosomes : acidic vesicles (pH ~5) with hydrolytic enzymes for intracellular digestion. Peroxisomes : oxidize fatty acids, neutralize H₂O₂. Cytoskeleton : microfilaments (actin, 7 nm), intermediate filaments (10 nm), microtubules (tubulin, 25 nm) — shape, movement, intracellular transport. Ribosomes : protein synthesis (translation).

Cell Cycle & Division

The cell cycle: Interphase (G₁: growth, S: DNA synthesis, G₂: preparation) and M phase (mitosis + cytokinesis). Mitosis stages: prophase (chromatin condenses), prometaphase (nuclear envelope breaks), metaphase (chromosomes align at metaphase plate), anaphase (sister chromatids separate), telophase (nuclear envelopes reform). Cytokinesis divides the cytoplasm. Cell cycle regulation: CDKs (cyclin-dependent kinases) + cyclins, checkpoints (G₁/S, G₂/M, spindle assembly). Tumor suppressors (p53, Rb) and proto-oncogenes control proliferation. Dysregulation → cancer.

Meiosis & Sexual Reproduction

Meiosis produces haploid gametes (n) from diploid cells (2n) via two consecutive divisions. Meiosis I (reductional): homologous chromosomes pair (synapsis), crossing over exchanges genetic material, then homologs separate. Meiosis II (equational): sister chromatids separate, like mitosis. Results: 4 genetically unique haploid cells. Genetic variation arises from: crossing over (recombination), independent assortment (2²³ = 8.4 million chromosome combinations in humans), and random fertilization. Errors: nondisjunction causes aneuploidy (e.g., trisomy 21 = Down syndrome, monosomy X = Turner syndrome).

Membrane Transport & Signaling

The plasma membrane (fluid mosaic model, Singer-Nicolson 1972): phospholipid bilayer with embedded proteins, cholesterol, and glycoproteins. Transport: passive — diffusion, osmosis, facilitated diffusion (channel/carrier proteins, no ATP). Active — Na⁺/K⁺-ATPase (3 Na⁺ out, 2 K⁺ in, 1 ATP), H⁺ pump, ABC transporters. Vesicular — endocytosis (phagocytosis, pinocytosis, receptor-mediated) and exocytosis. Cell signaling: autocrine, paracrine, endocrine. Signal transduction pathways: ligand binds receptor → second messengers (cAMP, Ca²⁺, IP₃) → kinase cascades (MAPK, PI3K/Akt) → cellular response (gene expression, metabolism, motility).

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❓ Frequently Asked Questions

What is the difference between mitosis and meiosis?

Mitosis: 1 division, 2 identical diploid (2n) daughter cells, for growth/repair. Meiosis: 2 divisions, 4 genetically unique haploid (n) cells (gametes), includes crossing over and independent assortment for genetic diversity. Mitosis maintains chromosome number; meiosis halves it.

Why are mitochondria called the powerhouse of the cell?

Mitochondria produce ~90% of cellular ATP through oxidative phosphorylation. Glucose metabolism (glycolysis → Krebs cycle → electron transport chain) yields ~30–32 ATP per glucose molecule, most generated in mitochondria. They have their own DNA, supporting the endosymbiotic theory (evolved from ancient bacteria).

What is osmosis?

Osmosis is the passive movement of water across a semipermeable membrane from a region of lower solute concentration (hypotonic) to higher (hypertonic). Water moves to equalize solute concentrations. Osmotic pressure (π = iMRT) drives this process. Cells in hypotonic solution swell (lysis); in hypertonic solution they shrink (crenation/plasmolysis).

How does the cell cycle relate to cancer?

Cancer results from uncontrolled cell division due to mutations in cell cycle regulators. Proto-oncogenes (growth promoters like RAS) become oncogenes when mutated. Tumor suppressors (p53, Rb, BRCA1) normally halt the cycle or trigger apoptosis. When both systems fail, cells proliferate unchecked, forming tumors.

What is the endosymbiotic theory?

Proposed by Lynn Margulis (1967): mitochondria and chloroplasts were once free-living bacteria engulfed by ancestral eukaryotic cells. Evidence: double membranes, own circular DNA, 70S ribosomes, binary fission, size similar to bacteria. This symbiosis provided a huge energetic advantage, enabling complex multicellular life.

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