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πŸ”¬ Biology Β· Cell Biology

Memory tricks for DNA, heredity & mutations

From the nucleus to the plasma membrane β€” the cell is life's basic unit. These memory tricks lock in organelle functions, mitosis stages, and membrane transport so you can answer any cell biology question on your exam.

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πŸ”¬ Cell Biology

Memory Tricks

Proven mnemonics — fast to learn, hard to forget.

Cell Theory
All cells come from cells β€” "Omnis cellula e cellula"
3 tenets: all life = cells Β· cell = basic unit Β· cells from pre-existing cells
Cell Theory has three core tenets: (1) All living organisms are composed of one or more cells. (2) The cell is the basic unit of life. (3) All cells arise from pre-existing cells (Virchow, 1855). This disproved spontaneous generation. Prokaryotic cells (no nucleus, no membrane-bound organelles) vs Eukaryotic cells (nucleus + organelles).
Difficulty: Beginner
Prokaryote vs Eukaryote
Prokaryotes: no nucleus, no membrane-bound organelles, 70S ribosomes, usually smaller (1-10ΞΌm). Eukaryotes: true nucleus, membrane-bound organelles, 80S ribosomes, larger (10-100ΞΌm).
Scientists
Hooke (1665) β€” coined "cell" looking at cork. Leeuwenhoek β€” first to observe live cells. Schleiden (plants) + Schwann (animals) β€” all organisms made of cells. Virchow β€” cells from cells.
Cell size limits
Surface area to volume ratio limits cell size. As cells grow, volume increases faster than surface area β€” reduced ability to exchange materials. Cells divide before getting too large.
Smallest vs largest cells
Smallest: Mycoplasma (~0.2ΞΌm). Largest: ostrich egg yolk (single cell, ~10cm). Longest: motor neurons can be over 1 meter. Human cells average ~10ΞΌm.
Organelles
Mighty Men Can't Run Laps β€” Mitochondria, Membrane, Chloroplasts, Ribosomes, Lysosome
Key organelles and their functions
Major eukaryotic organelles: Nucleus (DNA storage, control center). Mitochondria (ATP production β€” "powerhouse"). Ribosomes (protein synthesis β€” rough ER has them). ER (protein/lipid processing). Golgi apparatus (packaging/shipping). Lysosomes (digestion). Chloroplasts (photosynthesis β€” plants only). Vacuole (storage β€” large in plants).
Difficulty: Beginner
Endomembrane system
Connected system: Nuclear envelope β†’ ER β†’ vesicles β†’ Golgi β†’ vesicles β†’ plasma membrane/lysosomes. Proteins made on rough ER, modified in Golgi, shipped to destination.
Mitochondria features
Double membrane (outer smooth, inner folded into cristae). Own DNA (circular, like bacteria). Own ribosomes (70S). Divide by binary fission. Evidence for endosymbiotic origin.
Rough vs Smooth ER
Rough ER: studded with ribosomes, makes membrane proteins and secretory proteins. Smooth ER: no ribosomes, makes lipids, detoxifies drugs, stores Ca²⁺ in muscle cells.
Golgi apparatus
Cis face (receives from ER) β†’ Trans face (ships to destination). Modifies, sorts, and packages proteins. Adds sugars to glycoproteins. "Post office of the cell."
Cell Membrane
Fluid Mosaic Model β€” phospholipid bilayer with floating proteins
Hydrophilic heads face out Β· hydrophobic tails face in Β· proteins float throughout
The plasma membrane is a fluid mosaic of phospholipids and proteins. Phospholipids: hydrophilic head (faces water) + hydrophobic tails (face inward β€” repel water). Proteins float in the bilayer (integral) or attach to surface (peripheral). Cholesterol stabilizes fluidity. Carbohydrates on outer surface form glycocalyx β€” cell recognition and communication.
Difficulty: Intermediate
Selective permeability
Small nonpolar molecules (Oβ‚‚, COβ‚‚, lipids) cross freely. Small polar molecules (water β€” via aquaporins) cross slowly. Ions and large molecules need protein channels or carriers.
Cholesterol role
Inserted between phospholipids. At high temp: reduces fluidity (prevents membrane from becoming too fluid). At low temp: prevents solidification. Acts as fluidity buffer.
Membrane proteins
Transport proteins (channels, carriers, pumps). Receptor proteins (signal transduction). Enzymes. Cell identity markers (MHC (Major Histocompatibility Complex) proteins). Cell adhesion molecules. Each protein has specific function.
Glycocalyx
Sugar coating on outer cell surface. Made of glycoproteins and glycolipids. Functions: cell-cell recognition, immune response, protection. ABO blood types determined by glycocalyx sugars.
Membrane Transport
PAID β€” Passive, Active, Inward (endocytosis), Discharge (exocytosis)
No energy needed for passive Β· ATP required for active Β· vesicles for bulk transport
Transport types: Passive (no ATP) β€” simple diffusion (down concentration gradient), facilitated diffusion (via channels/carriers), osmosis (water). Active transport (ATP required) β€” pumps move against gradient (Na⁺/K⁺ pump). Bulk transport: endocytosis (phagocytosis, pinocytosis, receptor-mediated) brings material IN; exocytosis releases material OUT.
Difficulty: Intermediate
Osmosis
Water moves from low solute (high water) to high solute (low water) via osmosis. Hypotonic solution β†’ cell swells (may lyse). Hypertonic β†’ cell shrinks (crenation in RBCs, plasmolysis in plants). Isotonic β†’ no net movement.
Na⁺/K⁺ pump
Uses 1 ATP to pump 3 Na⁺ OUT and 2 K⁺ IN. Maintains electrochemical gradient. Uses 30% of cell's ATP. Essential for nerve impulse transmission and muscle contraction.
Phagocytosis vs pinocytosis
Phagocytosis ("cell eating"): engulfs large particles (bacteria, dead cells). Pinocytosis ("cell drinking"): takes in extracellular fluid in small vesicles. Receptor-mediated: specific molecules bind receptors, then internalized.
Cotransport
One substance moved actively creates gradient that drives another substance passively. Na⁺/glucose cotransporter: Na⁺ gradient (from Na⁺/K⁺ pump) drives glucose into intestinal cells against its gradient. Secondary active transport.
Mitosis
PMAT β€” Prophase, Metaphase, Anaphase, Telophase
Prophase: condense Β· Metaphase: middle Β· Anaphase: apart Β· Telophase: two cells
Mitosis produces 2 identical diploid daughter cells for growth and repair. PMAT: Prophase β€” chromosomes condense, spindle forms, nuclear envelope breaks down. Metaphase β€” chromosomes align at cell equator (metaphase plate). Anaphase β€” sister chromatids pulled to opposite poles. Telophase β€” nuclear envelopes reform, chromosomes decondense. Cytokinesis divides the cytoplasm.
Difficulty: Intermediate
Interphase
G1 (cell growth, normal functions) β†’ S phase (DNA replication β€” each chromosome duplicated into 2 sister chromatids) β†’ G2 (preparation for division, protein synthesis). G0 = non-dividing state.
Spindle apparatus
Made of microtubules from centrosomes. Kinetochore microtubules attach to centromere of each chromosome. Motor proteins walk chromosomes along microtubules. Colchicine disrupts spindle β€” stops mitosis at metaphase.
Cytokinesis
Animal cells: cleavage furrow (actin-myosin ring pinches cell in two). Plant cells: cell plate forms from Golgi vesicles in the middle, builds new cell wall between daughter cells.
Cancer connection
Cancer = uncontrolled cell division. Tumor suppressor genes (p53, Rb) normally halt division. Proto-oncogenes (ras) promote division. Mutations converting them to oncogenes drive cancer. Checkpoints (G1, G2, M) normally catch errors.
Cellular Respiration
GLICK β€” Glycolysis, Link reaction, Intermediate, Citric acid, Krebs
Glucose β†’ Pyruvate β†’ Acetyl-CoA β†’ Krebs β†’ ETC (Electron Transport Chain) β†’ 36-38 ATP
Cellular respiration converts glucose to ATP in 3 stages. Glycolysis (cytoplasm): glucose β†’ 2 pyruvate, net 2 ATP, 2 NADH. Pyruvate oxidation (mitochondrial matrix): pyruvate β†’ acetyl-CoA + COβ‚‚. Krebs cycle (matrix): 2 ATP, 6 NADH, 2 FADHβ‚‚ per glucose. Electron Transport Chain (ETC) (inner mitochondrial membrane): 32-34 ATP via oxidative phosphorylation. Total: ~36-38 ATP.
Difficulty: Intermediate
Glycolysis details
10 enzyme-catalyzed steps. Glucose (6C) β†’ 2 pyruvate (3C). Invests 2 ATP, produces 4 ATP (net 2). Produces 2 NADH. Occurs in cytoplasm β€” no oxygen required.
Krebs cycle
Runs twice per glucose (one per pyruvate). Each turn: 3 NADH, 1 FADHβ‚‚, 1 ATP, 2 COβ‚‚. NADH and FADHβ‚‚ carry electrons to ETC. COβ‚‚ released β€” this is why you exhale COβ‚‚.
Electron Transport Chain
NADH and FADHβ‚‚ donate electrons to protein complexes I-IV in inner mitochondrial membrane. Electron flow pumps H⁺ into intermembrane space β†’ H⁺ flows back through ATP synthase β†’ makes ATP (chemiosmosis). Oβ‚‚ is final electron acceptor β†’ becomes Hβ‚‚O.
Anaerobic respiration
Without Oβ‚‚: glycolysis only (2 ATP). Pyruvate converted to lactate (animals/bacteria) or ethanol + COβ‚‚ (yeast β€” fermentation). Regenerates NAD⁺ to keep glycolysis running. Much less efficient than aerobic.
Photosynthesis
Light reactions make ATP + NADPH Β· Calvin cycle makes sugar
Light-dependent (thylakoid) + Light-independent/Calvin cycle (stroma)
Photosynthesis: 6COβ‚‚ + 6Hβ‚‚O + light energy β†’ C₆H₁₂O₆ + 6Oβ‚‚. Two stages: Light reactions (thylakoid membranes) β€” light splits water, produces ATP, NADPH, and Oβ‚‚. Calvin cycle (stroma) β€” uses ATP and NADPH to fix COβ‚‚ into G3P (used to make glucose). 3 COβ‚‚ fixed per turn; 3 turns needed per pyruvate equivalent.
Difficulty: Intermediate
Photosystems I and II
PSII (P680) absorbs light first β€” splits water (photolysis), releases Oβ‚‚, energizes electrons. PSI (P700) re-energizes electrons to make NADPH. "II before I" β€” PSII goes first despite the number.
Calvin cycle (C3)
3 stages: Carbon fixation (COβ‚‚ + RuBP β†’ 2 molecules of 3-PGA via RuBisCO), Reduction (ATP + NADPH reduce 3-PGA to G3P), Regeneration (ATP used to regenerate RuBP). 3 turns fix 3 COβ‚‚ and produce 1 G3P net.
C4 vs CAM plants
C3 (most plants): COβ‚‚ fixed directly by RuBisCO β€” problem in hot conditions (photorespiration). C4 (corn, sugarcane): pre-fix COβ‚‚ in mesophyll, concentrate it in bundle sheath. CAM (cacti): fix COβ‚‚ at night, release during day β€” minimizes water loss.
Chloroplast structure
Outer membrane β†’ inner membrane β†’ stroma (fluid) β†’ thylakoids (membrane sacs) β†’ grana (stacks of thylakoids). Light reactions on thylakoid membranes; Calvin cycle in stroma.
Cell Communication
Reception β†’ Transduction β†’ Response β€” "RTR: Receive the Signal, Transform it, React"
Signal molecule β†’ receptor β†’ cascade β†’ cellular response
Cell signaling has three stages: Reception (signal molecule binds receptor β€” often on cell surface). Transduction (signal converted to intracellular message β€” often via protein cascade and second messengers like cAMP). Response (gene expression change, enzyme activation, or cytoskeletal change). Allows cells to coordinate activities without direct contact.
Difficulty: Advanced
Types of signaling
Endocrine: hormones travel through blood (long range). Paracrine: signals to nearby cells (local). Autocrine: cell signals itself. Synaptic: neurotransmitters cross synapse. Direct contact: gap junctions, plasmodesmata.
Second messengers
cAMP: made by adenylyl cyclase when G-protein activated. Activates protein kinase A β†’ phosphorylation cascade. Ca²⁺: released from ER by IP₃ β†’ activates calmodulin and other proteins.
Receptor types
G-protein coupled receptors (GPCRs): most common, activate G-proteins. Receptor tyrosine kinases (RTKs): dimerize when ligand binds, phosphorylate each other β†’ RAS pathway. Ion channel receptors: ligand opens channel directly (nicotinic acetylcholine receptor).
Apoptosis
Programmed cell death β€” essential for development (finger separation, tadpole tail loss) and immune function. Triggered by internal/external signals. Caspases execute cell death. Failure = cancer; excess = neurodegenerative disease.
Cell Cycle Control
Checkpoints: G1, G2, M β€” "Good Grade Makes the Cut"
Three checkpoints verify cell is ready to proceed to next phase
Cell cycle checkpoints prevent damaged or incomplete cells from dividing. G1 checkpoint: is the cell large enough? Is DNA undamaged? Sufficient nutrients? G2 checkpoint: was DNA replicated correctly? M checkpoint (spindle assembly): are all chromosomes attached to spindle? Cyclin-CDK (cyclin-dependent kinase) complexes drive the cycle forward. p53 tumor suppressor halts cycle if DNA damaged β€” "guardian of the genome."
Difficulty: Advanced
Cyclins and CDKs
Cyclin levels rise and fall through the cell cycle. Cyclin binds CDK (cyclin-dependent kinase) β†’ active complex drives cycle forward. Different cyclins active at different phases. MPF (maturation promoting factor) = cyclin B + CDK1 β€” triggers mitosis.
p53 β€” guardian of genome
Most commonly mutated gene in human cancers. Activated by DNA damage β†’ stops cell cycle at G1 β†’ allows repair β†’ if unrepairable, triggers apoptosis. Loss of p53 = cell divides despite damage = cancer risk.
Proto-oncogenes vs tumor suppressors
Proto-oncogenes (ras, myc): normally promote growth β€” mutation β†’ oncogene β†’ uncontrolled growth (one mutated copy sufficient β€” dominant). Tumor suppressors (p53, Rb): normally inhibit growth β€” both copies must be mutated to lose function (recessive).
Contact inhibition
Normal cells stop dividing when touching neighboring cells β€” density-dependent inhibition. Cancer cells lose this β€” continue dividing, pile up, form tumors. Also lose anchorage dependence β€” can grow without attachment.
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Liver Functions
PUSH DG β€” Protein synthesis, Urea synthesis, Storage, Hormone synthesis, Detoxification, Glucose/fat metabolism
Six key liver functions in one acronym
The liver performs hundreds of functions, but these six are most tested: Protein synthesis (albumin, clotting factors). Urea synthesis (converts toxic ammonia β†’ urea for excretion). Storage (glycogen, vitamins A/D/B12, iron). Hormone synthesis (IGF-1 (insulin-like growth factor 1), thrombopoietin). Detoxification (drugs, alcohol, bilirubin). Glucose and fat metabolism (glycolysis, gluconeogenesis, lipid processing).
Difficulty: Intermediate
Detoxification detail
Liver converts harmful substances to water-soluble forms for excretion. Cytochrome P450 enzymes handle many drugs and toxins. Bilirubin (from broken-down hemoglobin) processed here β€” jaundice = liver failure to process bilirubin.
Glucose metabolism
After a meal: excess glucose β†’ glycogen (glycogenesis) or fat. Between meals: glycogen β†’ glucose (glycogenolysis). Fasting: amino acids/glycerol β†’ glucose (gluconeogenesis). Liver is the main blood glucose regulator.
Anterior Pituitary Hormones
FLAT PEG β€” FSH, LH, ACTH, TSH, Prolactin, Endorphins, Growth hormone
Seven hormones of the anterior pituitary gland
The anterior pituitary secretes 7 major hormones: FSH (follicle-stimulating hormone β€” egg/sperm development), LH (luteinizing hormone β€” ovulation/testosterone), ACTH (stimulates adrenal cortex), TSH (stimulates thyroid), Prolactin (milk production), Endorphins (pain relief), Growth hormone (GH β€” growth and metabolism).
Difficulty: Intermediate
Anterior vs Posterior pituitary
Anterior pituitary: glandular tissue, produces own hormones (FLAT PEG). Posterior pituitary: neural tissue, STORES and releases ADH and oxytocin (made in hypothalamus). Both controlled by hypothalamus via releasing/inhibiting hormones.
Tropic hormones
FSH, LH, ACTH, TSH are "tropic" β€” they target other endocrine glands. Negative feedback: target gland hormones inhibit pituitary and hypothalamus. Example: high cortisol β†’ inhibits ACTH release β†’ less cortisol made.
Liver Functions
PUSH DG β€” Protein synthesis, Urea synthesis, Storage, Hormone synthesis, Detoxification, Glucose/fat metabolism
Six key liver functions in one acronym
The liver performs hundreds of functions, but these six are most tested: Protein synthesis (albumin, clotting factors). Urea synthesis (converts toxic ammonia β†’ urea for excretion). Storage (glycogen, vitamins A/D/B12, iron). Hormone synthesis (IGF-1 (insulin-like growth factor 1), thrombopoietin). Detoxification (drugs, alcohol, bilirubin). Glucose and fat metabolism (glycolysis, gluconeogenesis, lipid processing).
Difficulty: Intermediate
Detoxification detail
Liver converts harmful substances to water-soluble forms for excretion. Cytochrome P450 enzymes handle many drugs and toxins. Bilirubin (from broken-down hemoglobin) processed here β€” jaundice = liver failure to process bilirubin.
Glucose metabolism
After a meal: excess glucose β†’ glycogen (glycogenesis) or fat. Between meals: glycogen β†’ glucose (glycogenolysis). Fasting: amino acids/glycerol β†’ glucose (gluconeogenesis). Liver is the main blood glucose regulator.
Anterior Pituitary Hormones
FLAT PEG β€” FSH, LH, ACTH, TSH, Prolactin, Endorphins, Growth hormone
Seven hormones of the anterior pituitary gland
The anterior pituitary secretes 7 major hormones: FSH (follicle-stimulating hormone β€” egg/sperm development), LH (luteinizing hormone β€” ovulation/testosterone), ACTH (stimulates adrenal cortex), TSH (stimulates thyroid), Prolactin (milk production), Endorphins (pain relief), Growth hormone (GH β€” growth and metabolism).
Difficulty: Intermediate
Anterior vs Posterior pituitary
Anterior pituitary: glandular tissue, produces own hormones (FLAT PEG). Posterior pituitary: neural tissue, STORES and releases ADH and oxytocin (made in hypothalamus). Both controlled by hypothalamus via releasing/inhibiting hormones.
Tropic hormones
FSH, LH, ACTH, TSH are "tropic" β€” they target other endocrine glands. Negative feedback: target gland hormones inhibit pituitary and hypothalamus. Example: high cortisol β†’ inhibits ACTH release β†’ less cortisol made.
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🎓 Common Exam Questions
Q: What are the three tenets of Cell Theory and who contributed to each?
A: The three tenets: (1) All living organisms are composed of one or more cells β€” Schleiden (plants) and Schwann (animals). (2) The cell is the basic unit of structure and function in life. (3) All cells arise from pre-existing cells β€” Virchow (1855), which disproved spontaneous generation. A fourth modern tenet is sometimes added: cells carry genetic information in DNA.
Q: Explain the PMAT stages of mitosis and what happens at each checkpoint.
A: PMAT: Prophase (chromosomes condense, spindle forms, nuclear envelope breaks down), Metaphase (chromosomes align at metaphase plate β€” checked by M/spindle assembly checkpoint), Anaphase (sister chromatids pulled to opposite poles), Telophase (nuclear envelopes reform, chromosomes decondense), then Cytokinesis. Three checkpoints: G1 (cell size, DNA integrity, nutrients), G2 (DNA replication complete, no damage), M (all chromosomes attached to spindle). p53 is the G1 checkpoint guardian β€” mutations in p53 are found in ~50% of cancers.
Q: What is the difference between active and passive transport? Give examples of each.
A: Passive transport: moves down concentration gradient, no ATP required. Examples: simple diffusion (O2, CO2, lipids cross freely), facilitated diffusion (glucose via GLUT transporters, ions via channels), osmosis (water via aquaporins). Active transport: moves against concentration gradient, requires ATP. Primary: Na+/K+ pump (3 Na+ out, 2 K+ in per ATP). Secondary: cotransport where Na+ gradient drives glucose uptake in intestinal cells. Bulk: endocytosis (phagocytosis, pinocytosis) and exocytosis move large materials via vesicles.
Q: Walk through cellular respiration from glucose to ATP β€” stages, locations, and yields.
A: Three stages: (1) Glycolysis (cytoplasm): glucose β†’ 2 pyruvate, net 2 ATP, 2 NADH β€” no oxygen needed. (2) Pyruvate oxidation + Krebs cycle (mitochondrial matrix): 2 ATP, 6 NADH, 2 FADH2, 6 CO2 per glucose. (3) Electron Transport Chain (ETC) (inner mitochondrial membrane): NADH and FADH2 donate electrons, H+ pumped into intermembrane space, flows back through ATP synthase β€” produces ~32-34 ATP. O2 is the final electron acceptor, forming water. Total: ~36-38 ATP per glucose. Without O2 (anaerobic): only glycolysis runs, pyruvate β†’ lactate (animals) or ethanol + CO2 (yeast).
Q: What does the FLAT PEG mnemonic cover, and what are the functions of each hormone?
A: FLAT PEG = the 7 anterior pituitary hormones: FSH (follicle-stimulating hormone β€” stimulates egg/sperm development), LH (luteinizing hormone β€” triggers ovulation in females, testosterone in males), ACTH (adrenocorticotropic hormone β€” stimulates adrenal cortex to produce cortisol), TSH (thyroid-stimulating hormone β€” stimulates thyroid hormone production), Prolactin (milk production), Endorphins (natural pain relief), GH (growth hormone β€” growth and metabolism). FSH, LH, ACTH, and TSH are 'tropic' hormones β€” they target other endocrine glands. All are regulated by hypothalamic releasing hormones and negative feedback.