Memory tricks for the cell — the basic unit of life
Cell structure, organelles, membrane transport, the cell cycle, mitosis, and meiosis — cell biology is the foundation of all physiology. Understand the cell and you understand how every organ system works at its most fundamental level.
Proven Mnemonics & Acronyms — fast to learn, hard to forget.
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Cell Theory
Three tenets — All cells from cells · Cells are basic units · All living things are cells
Cell theory: the foundational framework of all biology
The three principles of cell theory — and why they matter for A&P
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.
Prokaryotic
No membrane-bound nucleus. Bacteria and archaea. No membrane organelles.
Eukaryotic
Membrane-bound nucleus. All human cells, plants, fungi, protists.
37 trillion
Estimated cell count in human body. More bacteria than human cells on/in us.
Cell Membrane Structure
Fluid Mosaic Model — phospholipid bilayer with floating proteins
Hydrophilic heads face out · Hydrophobic tails face in · Proteins float throughout
The plasma membrane — structure and the fluid mosaic model
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).
The five most important organelles — structure and function
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.
Mitochondria
ATP factory. Double membrane. Own DNA. Most in cardiac and skeletal muscle cells.
Rough ER
Ribosomes on surface — makes proteins for secretion or membrane insertion.
Smooth ER
Lipid synthesis, drug detox (liver), Ca2+ storage (muscle SR).
Golgi
Cis face receives (from ER), trans face ships. Glycosylation, sorting, vesicles.
The control center of the cell — houses DNA and controls protein synthesis
The nucleus — four structural features and why each matters
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.
23 pairs in somatic cells. 23 (haploid) in gametes. 22 pairs autosomes + 1 pair sex chromosomes.
Nucleolus
Makes rRNA → ribosome assembly. Disappears during cell division.
Histones
Proteins that DNA wraps around (nucleosome). Regulate gene access.
Membrane Transport
PACE — Passive · Active · Co-transport · Endocytosis/Exocytosis
No energy vs energy required — direction of concentration gradient determines type
How substances cross the cell membrane — four transport categories
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.
Simple diffusion
O2, CO2, lipid-soluble molecules — no protein needed, high to low concentration.
Facilitated
Glucose (GLUT), ions (channels) — protein assists, still down gradient.
Na+/K+ pump
3 Na+ out / 2 K+ in per ATP. Resting membrane potential. ~30% of resting ATP.
Osmosis
Water moves toward higher solute concentration. Aquaporins speed it up.
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.
Mitosis
2n → 2n. One division. Identical cells. Growth, repair, replacement.
3-base code on mRNA. 64 total — 61 code for amino acids, 3 are stop codons.
🎓 Common Exam Questions
Q: Describe the fluid mosaic model of the cell membrane.
A: Phospholipid bilayer: hydrophilic heads face outward (toward water), hydrophobic tails face inward. Cholesterol: between phospholipids, maintains fluidity — prevents too rigid at cold, too fluid at hot. Proteins: Integral (transmembrane) — span the bilayer, include channels, carriers, receptors, enzymes, cell identity markers. Peripheral — attached to surface, often enzymes or anchors for cytoskeleton. Glycoproteins and glycolipids: carbohydrate chains on outer surface — form glycocalyx — cell recognition, immune identity (ABO blood type), cell adhesion. 'Fluid' = lipids can move laterally; 'mosaic' = embedded proteins. Selectively permeable: allows some substances through, blocks others.
Q: What are the key organelles and their functions?
A: Nucleus: contains DNA, site of transcription. Nuclear envelope with pores. Nucleolus: rRNA synthesis, ribosome assembly. Ribosomes: protein synthesis — free (cytoplasmic proteins) or bound to RER (secretory proteins). RER: protein synthesis + folding + modification. SER: lipid synthesis, detoxification, Ca2+ storage (muscle). Golgi: 'post office' — sorts, modifies, packages proteins → secretory vesicles, lysosomes. Mitochondria: ATP production (aerobic respiration), have own DNA — endosymbiotic origin. Lysosomes: intracellular digestion — hydrolytic enzymes, pH 5. Peroxisomes: detoxify H2O2. Centrosomes/centrioles: organize mitotic spindle. Cytoskeleton: microfilaments (actin, cell shape), microtubules (transport, cilia, spindle), intermediate filaments (structural support).
Q: What are the types of membrane transport and which require energy?
A: Passive (no ATP): Simple diffusion — small nonpolar molecules (O2, CO2, lipids) cross directly down concentration gradient. Facilitated diffusion — polar/charged molecules use protein channels or carriers, still down gradient (glucose into cells, ions through channels). Osmosis — water moves through aquaporins down water potential gradient. Active transport (requires ATP): Primary active — Na+/K+ ATPase (3 Na+ out, 2 K+ in — maintains resting potential, cell volume). Secondary active — uses Na+ gradient created by primary active transport to move other molecules against gradient (Na+/glucose symport in intestine). Vesicular: Endocytosis (phagocytosis, pinocytosis, receptor-mediated), Exocytosis. Tonicity: Isotonic (no net water movement), Hypotonic (water enters → cell swells/lyses), Hypertonic (water exits → cell shrinks/crenates).
Q: Describe the cell cycle and what controls it.
A: Interphase (90% of cycle): G1 (growth, protein synthesis, checkpoint — cell size, nutrients, DNA damage), S (DNA replication — each chromosome duplicated into sister chromatids), G2 (preparation for division, checkpoint — DNA replication complete, damage check), G0 (quiescence — neurons, cardiac muscle). Mitotic phase: Mitosis (PMAT) + Cytokinesis. Checkpoints controlled by cyclins and CDKs (cyclin-dependent kinases): G1/S checkpoint (restriction point) — most important, controlled by Rb protein and cyclin D/CDK4. G2/M checkpoint — MPF (maturation promoting factor) = cyclin B/CDK1. Spindle checkpoint (M phase) — all chromosomes must attach to spindle. Cancer = loss of checkpoint control: tumor suppressors (p53, Rb) lost or oncogenes (Ras, Myc) overactivated.
Q: Describe protein synthesis from DNA to functional protein.
A: Transcription (nucleus): RNA polymerase unwinds DNA → reads template strand 3'→5' → synthesizes mRNA 5'→3'. Pre-mRNA processing: 5' cap added, poly-A tail added, introns spliced out (spliceosomes), exons joined → mature mRNA exits nucleus. Translation (ribosomes): mRNA codon (3 bases) read by ribosome → tRNA anticodon brings amino acid → peptide bond formed by peptidyl transferase activity of rRNA (ribozyme). Three sites: A (aminoacyl — new tRNA), P (peptidyl — growing chain), E (exit). Start codon AUG (methionine), stop codons UAA, UAG, UGA (no tRNA). Post-translational modifications: signal peptide → RER → Golgi → glycosylation, phosphorylation, cleavage, folding (chaperones). Protein folding diseases: Alzheimer (amyloid), prion diseases (misfolded prion protein).