Memory tricks for the cell — the basic unit of life

All living organisms are composed of one or more cells. The cell is the basic structural and functional unit of life. All cells arise from pre-existing cells (cell division — no spontaneous generation). Human body contains ~37 trillion cells. Two major cell types: prokaryotic (no nucleus — bacteria) and eukaryotic (membrane-bound nucleus — all human cells). Human cells are eukaryotic. Cell size range: 2–200 micrometers. Red blood cells (7.5 μm) and muscle cells (up to 30 cm) illustrate the range. Cell shape reflects function — RBCs are biconcave for surface area, neurons have long axons for communication.

The plasma membrane is a phospholipid bilayer — two layers of phospholipids arranged tail-to-tail. Each phospholipid has a hydrophilic (water-loving) head facing outward toward water, and two hydrophobic (water-fearing) fatty acid tails facing inward away from water. Fluid mosaic model: the membrane is fluid (phospholipids move laterally) and mosaic (proteins embedded throughout). Membrane proteins: integral proteins span the membrane (transport, receptors), peripheral proteins attach to surface (cytoskeleton anchor). Cholesterol embedded between phospholipids — stabilizes membrane, prevents it from being too rigid or too fluid. Glycoproteins and glycolipids on outer surface — cell recognition and identification (ABO blood type).

Mitochondria: ATP production via oxidative phosphorylation. Double membrane. Own DNA — evidence of endosymbiosis. More mitochondria in high-energy cells (cardiac muscle). Endoplasmic reticulum: Rough ER (ribosomes attached → protein synthesis for export), Smooth ER (lipid synthesis, detox, Ca2+ storage in muscle). Ribosomes: protein synthesis — free ribosomes make cytoplasmic proteins, bound ribosomes make secretory proteins. Golgi apparatus: protein processing, sorting, packaging — "post office of the cell." Lysosomes: contain digestive enzymes at pH 4.5 — break down waste, cellular debris, and pathogens.

The nucleus is the control center of the cell — contains the genetic instructions for making every protein. Nuclear envelope: double membrane punctuated by nuclear pores — controls what enters and exits (mRNA exits, transcription factors enter). Connected to rough ER — continuous membrane system. DNA: organized into chromosomes (46 in human somatic cells, 23 pairs). Chromatin = DNA + histone proteins. Condensed = chromosomes (visible during division). Dispersed = chromatin (during gene expression). Nucleolus: dense region within nucleus — site of ribosomal RNA (rRNA) synthesis and ribosome assembly. Cells with high protein output have large nucleoli.

Passive transport: moves DOWN concentration gradient — no ATP. Simple diffusion (O2, CO2, lipids), facilitated diffusion (glucose via GLUT transporters, ions via channels), osmosis (water via aquaporins). Active transport: moves AGAINST gradient — requires ATP. Na+/K+ ATPase: 3 Na+ out, 2 K+ in per ATP — essential for nerve and muscle function. Secondary active transport (cotransport): uses Na+ gradient created by Na+/K+ pump to drive glucose/amino acid uptake. Endocytosis: phagocytosis (large particles), pinocytosis (fluid), receptor-mediated (specific molecules — LDL cholesterol). Exocytosis: secretion of proteins, hormones, neurotransmitters.

Interphase (90% of cell cycle): G1 — cell grows, makes proteins, prepares for DNA replication. S phase — DNA synthesis (replication), chromosomes duplicated. G2 — final growth, organelle duplication, prepares for division. Mitosis (PMAT): Prophase — chromatin condenses into visible chromosomes, spindle forms. Metaphase — chromosomes align at middle (metaphase plate). Anaphase — sister chromatids pulled to opposite poles. Telophase — nuclear envelopes reform, chromosomes decondense. Cytokinesis: cytoplasm divides → 2 identical daughter cells. Checkpoints: G1, G2, and M checkpoints ensure DNA is undamaged before proceeding. Cancer = checkpoint failure.

Mitosis: for growth and repair. One diploid cell (46 chromosomes) → two genetically identical diploid daughter cells (46 chromosomes each). One round of division. No crossing over. Meiosis: for sexual reproduction. One diploid cell → four genetically unique haploid cells (23 chromosomes each — sperm or eggs). Two rounds of division (Meiosis I and II). Crossing over in Prophase I creates genetic recombination — shuffles genes. Fertilization restores diploid number (23 + 23 = 46). Nondisjunction: failure of chromosomes to separate → aneuploidy. Trisomy 21 (Down syndrome) = extra chromosome 21.

The cytoskeleton is the cell's internal scaffolding — it maintains shape, enables movement, anchors organelles, and provides tracks for intracellular transport. Microfilaments (thinnest, 7 nm): actin filaments — muscle contraction, cell movement, cell division (contractile ring). Intermediate filaments (10 nm): keratin, vimentin, desmin — structural support, resist mechanical stress, anchor nucleus. Microtubules (thickest, 25 nm): tubulin — cell shape, mitotic spindle (chromosome movement), cilia and flagella (9+2 arrangement), motor protein tracks (kinesin and dynein move organelles). Centrioles: two perpendicular microtubule cylinders — form spindle during cell division.

Transcription (nucleus): DNA unwinds → RNA polymerase reads template strand → builds complementary mRNA strand (A-U, T-A, G-C, C-G). mRNA processed (introns removed, exons spliced together, 5' cap and poly-A tail added) → exits nucleus through nuclear pores. Translation (ribosome): ribosome reads mRNA codons (3 bases = 1 amino acid) → tRNA brings matching amino acid (anticodon matches codon) → peptide bonds form amino acid chain → polypeptide released. 64 codons, 20 amino acids, 3 stop codons (UAA, UAG, UGA). Mutations: substitution, insertion, deletion — can alter protein function.