Chapter 13: High-Yield Cheat Sheet

This is the MCAT biology cheat sheet you wish you had the week before test day. Every high-yield fact from all 12 chapters, compressed into one page. Hormones, germ layers, enzymes, nephron segments, neurotransmitters, immune cells, inheritance patterns, the sarcomere - all the tables you need to memorize, none of the prose you do not. If you can recall 80% of what is on this page, you will recognize most of what the MCAT throws at you in Bio/Biochem and Psych/Soc biology passages.

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1. The Cell and Organelles

Eukaryote = membrane-bound nucleus and organelles, 80S ribosomes, linear DNA with histones. Prokaryote = no nucleus, 70S ribosomes, one circular chromosome in a nucleoid, no membrane-bound organelles.

Organelle	Function	Must-Know Detail
Nucleus	Stores DNA, site of transcription	Nuclear pores gate mRNA export
Nucleolus	rRNA synthesis, ribosome assembly	Dense region inside nucleus
Rough ER	Synthesizes secreted/membrane proteins	Ribosomes on cytosolic face
Smooth ER	Lipid synthesis, detox, Ca²⁺ storage	Abundant in liver, gonads
Golgi	Modifies, sorts, ships proteins	cis = receiving, trans = shipping
Lysosome	Intracellular digestion	Acidic (pH ~4.5), hydrolytic enzymes
Peroxisome	β-oxidation of very-long-chain FAs, H₂O₂ detox	Uses catalase
Mitochondrion	ATP synthesis (ETC + ox phos)	Own 70S ribosomes + circular DNA; maternal inheritance
Ribosome	Protein synthesis	80S eukaryotic, 70S prokaryotic/mitochondrial
Centrosome/centrioles	Organize mitotic spindle	Absent in plants

Cytoskeleton:

• Microfilaments (actin, 7 nm): cell shape, cytokinesis, muscle contraction
• Intermediate filaments (keratin/vimentin/lamin, 10 nm): tensile strength, anchor nucleus
• Microtubules (α+β-tubulin dimers, 25 nm): vesicle transport (kinesin = +end, dynein = -end), mitotic spindle, cilia/flagella (9+2)

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2. Cell Membrane and Transport

Fluid mosaic model: phospholipid bilayer with embedded proteins, cholesterol (fluidity buffer), and glycolipids/glycoproteins (cell ID).

Transport Type	ATP?	Direction	Example
Simple diffusion	No	Down gradient	O₂, CO₂, steroids
Facilitated diffusion	No	Down gradient	Glucose via GLUT, ion channels
Osmosis	No	Water, down water potential	Across aquaporins
Primary active	Yes	Against gradient	Na⁺/K⁺ pump (3 out, 2 in)
Secondary active	Indirect	Couples to gradient	SGLT (Na⁺/glucose cotransport)
Endocytosis	Yes	In (bulk)	Phago-, pino-, receptor-mediated
Exocytosis	Yes	Out (bulk)	Neurotransmitter release

Cell junctions: tight (water-tight seal, claudin/occludin), desmosome (spot weld, keratin + cadherin), gap (cytoplasmic channel, connexin - heart, smooth muscle).

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3. Cell Cycle, Mitosis, Meiosis

Cell cycle: G₁ → S → G₂ → M. Restriction point (R) late in G₁ is the commit-to-divide checkpoint. Cyclins rise and fall; CDKs are always present but only active when bound to cyclin. p53 halts the cycle for DNA damage repair.

Feature	Mitosis	Meiosis
Cells produced	2 diploid	4 haploid
DNA replications	1	1
Divisions	1	2
Crossing over	No	Yes (prophase I)
Daughter cells	Genetically identical	Genetically unique
Synapsis/tetrads	No	Yes (prophase I)
Purpose	Growth, repair	Gamete production

PMAT (mitosis): Prophase → Metaphase → Anaphase → Telophase (then cytokinesis).

Meiosis I = reductional (homologs separate). Meiosis II = equational (sister chromatids separate, like mitosis of a haploid cell).

Nondisjunction in meiosis I = all gametes abnormal. In meiosis II = half gametes abnormal. Trisomy 21 (Down), 18 (Edwards), 13 (Patau); XXY (Klinefelter), XO (Turner).

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4. DNA, RNA, Protein Synthesis

DNA: double helix, antiparallel, A–T (2 H-bonds), G–C (3 H-bonds). Replication is semiconservative, bidirectional from origins, 5’ → 3’ only. Leading strand continuous; lagging strand = Okazaki fragments joined by ligase. Telomerase extends telomeres in stem/germ/cancer cells.

Enzyme	Job
Helicase	Unwinds DNA at replication fork
Topoisomerase (gyrase)	Relieves supercoiling
Primase	Lays down RNA primer
DNA polymerase III (prokaryotic)	Adds nucleotides 5’→3’, 3’→5’ proofreading
DNA polymerase I (prokaryotic)	Removes primer, fills gap
DNA ligase	Seals nicks
RNA polymerase II	Transcribes mRNA in eukaryotes
Reverse transcriptase	RNA → DNA (retroviruses)

Transcription: promoter (TATA box ~-25), 5’ cap + 3’ poly-A tail + splicing (spliceosome removes introns) in the nucleus.

Translation: mRNA codon (3 nt) + tRNA anticodon, ribosome A→P→E sites. Start codon AUG (methionine); stop codons UAA, UAG, UGA. Genetic code is degenerate (multiple codons per amino acid) and universal.

Gene regulation (prokaryote): lac operon inducible, on when lactose present (allolactose displaces repressor); CAP+cAMP boosts transcription when glucose is low. trp operon repressible, off when tryptophan abundant.

Mutations: silent (no AA change), missense (different AA), nonsense (→ stop), frameshift (indel not multiple of 3 - usually catastrophic).

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5. Genetics and Inheritance

Mendel: segregation (alleles separate in gametes) and independent assortment (genes on different chromosomes sort independently).

Pattern	Clues
Autosomal dominant	Every affected child has ≥1 affected parent; no skipping
Autosomal recessive	Can skip generations; parents are carriers
X-linked recessive	Males far more affected; no father-to-son
X-linked dominant	No father-to-son; all daughters of affected father affected
Mitochondrial	Maternal only; all children of affected mother affected

Hardy-Weinberg equilibrium (no evolution): assumes (1) no mutation, (2) no selection, (3) random mating, (4) no gene flow, (5) large population.

Chi-square test of observed vs expected ratios:
$\chi^2 = \sum \frac{(O - E)^2}{E}$

Linkage: genes close together co-segregate; recombination frequency ≈ map distance (1 cM = 1% recombination).

Non-Mendelian: incomplete dominance (blended), codominance (both expressed, e.g., AB blood), pleiotropy (one gene → many traits), polygenic (many genes → one trait), epistasis (one gene masks another), genomic imprinting (parent-of-origin).

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6. Embryogenesis: The Germ Layer Table (very high-yield)

Fertilization (ampulla of fallopian tube) → zygote → cleavage → morula (~16 cells) → blastocyst (inner cell mass + trophoblast) → implants day ~7 → gastrulation forms three germ layers → neurulation (notochord induces neural tube).

Germ Layer	Derivatives
Ectoderm (“attract-o-derm”, outer)	Epidermis, hair, nails, nervous system (brain, spinal cord, peripheral nerves), adrenal medulla, lens/cornea, inner ear
Mesoderm (“meso-means-middle”)	Muscle (all three types), bone/cartilage, dermis, circulatory system, kidneys, gonads, adrenal cortex, connective tissue
Endoderm (“endo-into-gut”)	Epithelial lining of GI tract and respiratory tract, liver, pancreas, thyroid, bladder, inner ear epithelium

Extra-embryonic membranes: amnion (cushion), yolk sac (first blood cells), allantois (→ umbilical vessels), chorion (→ placenta). Apgar score at 1 and 5 min: Appearance, Pulse, Grimace, Activity, Respiration (0-2 each, max 10).

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7. Nervous System

Neuron parts: dendrites (receive) → soma → axon hillock (trigger zone) → axon → synaptic terminals. Myelin (oligodendrocytes in CNS, Schwann cells in PNS) speeds conduction via saltatory conduction at nodes of Ranvier.

Resting membrane potential: ~-70 mV. Driven by Na⁺/K⁺ pump (3 Na⁺ out, 2 K⁺ in per ATP) and K⁺ leak.

Action potential phases:

1. Threshold (~-55 mV): voltage-gated Na⁺ channels open
2. Depolarization: Na⁺ rushes in, membrane → +35 mV
3. Repolarization: Na⁺ channels inactivate, voltage-gated K⁺ channels open, K⁺ leaves
4. Hyperpolarization (undershoot): K⁺ channels slow to close
5. Return to rest: Na⁺/K⁺ pump restores gradients

Refractory periods: absolute (Na⁺ channels inactivated, cannot fire) → relative (requires stronger stimulus). Ensures one-way propagation.

Neurotransmitter	Main Role	Key Fact
Acetylcholine (ACh)	NMJ, parasympathetic, CNS	Blocked by curare; ↓ in Alzheimer’s
Dopamine	Reward, movement	↓ in Parkinson’s, ↑ in schizophrenia
Norepinephrine	Sympathetic, arousal	Fight-or-flight
Serotonin (5-HT)	Mood, sleep	SSRIs block reuptake
GABA	Main inhibitory (CNS)	Benzos, alcohol potentiate
Glutamate	Main excitatory (CNS)	NMDA receptor = memory (LTP)
Glycine	Inhibitory (spinal cord)
Endorphins	Natural opioids	Pain modulation

CNS divisions: cerebrum (higher function), cerebellum (coordination), brainstem (vitals). PNS: somatic (voluntary) + autonomic (sympathetic “fight or flight” + parasympathetic “rest and digest”).

Reflex arc: sensory neuron → interneuron (in spinal cord) → motor neuron. Knee-jerk is monosynaptic (no interneuron).

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8. The Endocrine System (Hormone Master Table)

This is the most-tested table on the MCAT. Memorize source, target, action, and feedback direction.

Hormone	Source	Target / Action
GH	Anterior pituitary	Growth of bone/soft tissue; antagonizes insulin
Prolactin	Anterior pituitary	Milk production
TSH	Anterior pituitary	Thyroid → T₃/T₄
ACTH	Anterior pituitary	Adrenal cortex → cortisol
FSH	Anterior pituitary	Follicle maturation (F) / spermatogenesis (M)
LH	Anterior pituitary	Ovulation / testosterone (Leydig cells)
ADH (vasopressin)	Post. pituitary (made in hypothalamus)	Kidney collecting duct: ↑ aquaporins → H₂O reabsorption
Oxytocin	Posterior pituitary	Uterine contraction, milk letdown
T₃/T₄	Thyroid (follicles)	↑ basal metabolic rate
Calcitonin	Thyroid (C cells)	↓ blood Ca²⁺ (“tones down” calcium)
PTH	Parathyroid	↑ blood Ca²⁺ (bone resorption, kidney reabsorption, activates vitamin D)
Aldosterone	Adrenal cortex (zona glomerulosa)	DCT/collecting duct: ↑ Na⁺ and H₂O retention, ↑ K⁺ excretion
Cortisol	Adrenal cortex (zona fasciculata)	↑ blood glucose, immunosuppression, anti-inflammatory
Androgens	Adrenal cortex (zona reticularis)	Sex steroids
Epinephrine/NE	Adrenal medulla	Fight-or-flight; ↑ HR, ↑ BP, ↑ glucose
Insulin	Pancreas β-cells	↓ blood glucose (into muscle/fat); anabolic
Glucagon	Pancreas α-cells	↑ blood glucose (glycogenolysis, gluconeogenesis)
Somatostatin	Pancreas δ-cells, hypothalamus	Inhibits insulin, glucagon, GH
Testosterone	Testes (Leydig)	Male 2° sex traits, spermatogenesis
Estrogen	Ovaries (follicle), placenta	Female 2° sex traits, uterine lining proliferation
Progesterone	Corpus luteum, placenta	Maintains endometrium, maintains pregnancy
hCG	Syncytiotrophoblast	Maintains corpus luteum in 1st trimester (pregnancy test target)
Erythropoietin (EPO)	Kidney	↑ RBC production in bone marrow
ANP	Cardiac atria	↓ Na⁺ reabsorption → ↓ blood volume
Leptin	Adipose	Satiety signal
Ghrelin	Stomach	Hunger signal
Melatonin	Pineal	Circadian rhythm

Hormone classes:

• Peptide (insulin, GH, ADH): water-soluble, surface receptors, fast, 2nd messengers (cAMP, IP₃, DAG)
• Steroid (cortisol, aldosterone, sex hormones): lipid-soluble, intracellular receptors, slow, directly alter transcription
• Amino-acid-derived: thyroid hormones (act like steroids), catecholamines (act like peptides)

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9. Cardiovascular System

Blood flow (memorize): SVC/IVC → RA → tricuspid → RV → pulmonary semilunar → pulmonary arteries → lungs → pulmonary veins → LA → bicuspid (mitral) → LV → aortic semilunar → aorta → body.

Electrical path: SA node (pacemaker, 60-100 bpm) → AV node (delay) → Bundle of His → Purkinje fibers.

ECG Wave	Means
P wave	Atrial depolarization
QRS	Ventricular depolarization (hides atrial repolarization)
T wave	Ventricular repolarization

Blood vessels: tunica intima (endothelium), tunica media (smooth muscle + elastic), tunica externa (connective tissue). Arterioles = main resistance vessels. Capillaries are one cell thick - site of exchange.

Starling forces: hydrostatic pressure pushes fluid out at arteriolar end; oncotic (colloid osmotic) pressure pulls fluid back at venular end. Lymphatics return excess interstitial fluid.

Blood composition: 55% plasma (water, proteins, ions), 45% formed elements. Hematocrit = RBC%. RBCs (anucleate, 120 day lifespan, hemoglobin), WBCs (neutrophils, lymphocytes, monocytes, eosinophils, basophils = “Never Let Monkeys Eat Bananas”), platelets (clotting).

O₂-Hb dissociation curve:

• Right shift (releases more O₂ to tissues): ↑CO₂, ↑H⁺ (↓pH), ↑temp, ↑2,3-BPG - “exercising muscle”
• Left shift (holds O₂ tighter): opposite conditions; fetal Hb (γ instead of β) is left-shifted to pull O₂ from maternal blood

Bohr effect = CO₂/H⁺ shift curve right. Haldane effect = deoxygenated Hb carries more CO₂.

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10. Respiratory System

Path: nose → pharynx → larynx → trachea → bronchi → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli (gas exchange).

Muscles of inspiration: diaphragm (contracts/flattens), external intercostals. Expiration at rest = passive (elastic recoil). Forced expiration: internal intercostals, abdominals.

Volume/Capacity	Definition	Typical (mL)
Tidal volume (TV)	Normal breath	500
IRV	Above tidal inhalation	3000
ERV	Below tidal exhalation	1100
Residual volume (RV)	Air left after max exhale	1200
Vital capacity	TV + IRV + ERV	4600
Total lung capacity	VC + RV	5800

Surfactant (produced by type II pneumocytes starting ~week 24) reduces surface tension, prevents alveolar collapse. Deficient in preemies → neonatal respiratory distress syndrome.

Gas transport: O₂ bound to Hb (~98%); CO₂ mostly as HCO₃⁻ (~70%), bound to Hb as carbamino (~23%), dissolved (~7%). CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (carbonic anhydrase, especially in RBCs).

Chemoreceptors: central (medulla) respond to CSF H⁺ (indirectly CO₂); peripheral (carotid, aortic) respond to O₂, CO₂, pH.

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11. Digestive System

Path: mouth → pharynx → esophagus → stomach → small intestine (duodenum → jejunum → ileum) → large intestine (cecum → colon → rectum → anus). Accessory: salivary glands, liver, gallbladder, pancreas.

Enzyme	Source	Substrate	Product
Salivary/pancreatic amylase	Saliva, pancreas	Starch	Disaccharides
Pepsin	Stomach (chief cells)	Proteins	Peptides (active only at low pH)
Trypsin, chymotrypsin, elastase	Pancreas	Proteins	Small peptides
Carboxypeptidase	Pancreas	C-terminus cleavage	AAs
Aminopeptidase, dipeptidase	Brush border	Peptides	AAs
Pancreatic lipase	Pancreas	TAGs	FAs + monoglycerides
Lactase, sucrase, maltase	Brush border	Disaccharides	Monosaccharides
Enterokinase	Duodenum	Trypsinogen	Trypsin (activates cascade)
Bile (not enzyme)	Liver, stored in gallbladder	Fats	Emulsifies (micelles)

Key GI hormones:

• Gastrin (G cells of stomach): ↑ HCl secretion
• Secretin (duodenum): ↑ pancreatic HCO₃⁻ (neutralizes chyme)
• CCK (duodenum): ↑ bile release, ↑ pancreatic enzymes, satiety
• GIP/GLP-1 (incretins): ↑ insulin release in response to food

Absorption sites: stomach (alcohol, small amounts), duodenum (Fe, Ca), jejunum (most nutrients), ileum (B₁₂ + bile salts), colon (H₂O, electrolytes, vitamin K from bacteria).

Liver: detox, gluconeogenesis, urea cycle, bile production, plasma protein synthesis (albumin, clotting factors).

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12. Renal System / Homeostasis

Nephron path: Bowman’s capsule (glomerular filtration) → proximal convoluted tubule → descending loop of Henle → ascending loop of Henle → distal convoluted tubule → collecting duct → renal pelvis → ureter.

Nephron Segment	Permeability / Action
Glomerulus/Bowman’s	Filters by size/charge; ~20% of plasma; no cells or large proteins
PCT	Bulk reabsorption: glucose, AAs, 65% Na⁺, 65% H₂O, HCO₃⁻
Descending loop	H₂O permeable only (passive concentration)
Ascending loop (thick)	Na⁺/K⁺/2Cl⁻ pump; impermeable to H₂O (dilutes filtrate)
DCT	Na⁺ reabsorption (aldosterone-sensitive); Ca²⁺ (PTH)
Collecting duct	Variable H₂O reabsorption via aquaporins (ADH-controlled)

Countercurrent multiplier in loop of Henle creates medullary osmotic gradient (~300 mOsm cortex → ~1200 mOsm inner medulla) that drives water reabsorption.

Key hormones:

• ADH: collecting duct → ↑ aquaporins → concentrated urine
• Aldosterone: DCT/collecting duct → ↑ Na⁺ and H₂O retention, ↑ K⁺ excretion
• ANP: ↓ Na⁺ reabsorption → natriuresis
• Renin (from juxtaglomerular cells when BP low): angiotensinogen → angiotensin I → ACE → angiotensin II (vasoconstrictor, stimulates aldosterone)

Acid-base: kidney excretes H⁺ and reabsorbs HCO₃⁻ (slow, days); lungs exhale CO₂ (fast, minutes). Henderson-Hasselbalch:

$$\mathrm{pH} = 6.1 + \log \frac{[\mathrm{HCO_3^-}]}{0.03 \times \mathrm{PaCO_2}}$$

Normal pH 7.35-7.45. Acidosis <7.35, alkalosis >7.45. Metabolic = HCO₃⁻ change; respiratory = CO₂ change.

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13. Immune System

Innate (fast, nonspecific): barriers, macrophages, neutrophils, NK cells, complement, inflammation, fever.

Adaptive (slow, specific, memory): B cells (humoral, antibodies), T cells (cellular).

Immune Cell	Role
Neutrophil	Most abundant WBC; phagocytose bacteria
Macrophage (from monocytes)	Phagocytose + present antigen on MHC II
Dendritic cell	Professional APC
NK cell	Kill virus-infected & tumor cells (no MHC I needed)
B cell	Mature in bone marrow; make antibodies; present on MHC II
Plasma cell	Activated B cell; antibody factory
$T_{H}$ (CD4)	Activate B cells and Tc via cytokines; recognize MHC II
$T_{C}$ (CD8, cytotoxic)	Kill infected cells; recognize MHC I
$T_{\text{reg}}$	Suppress autoreactive responses
Mast cell / Basophil	Release histamine (allergy)
Eosinophil	Parasites, allergy

MHC I = on all nucleated cells; presents endogenous antigen to CD8. MHC II = on APCs; presents exogenous antigen to CD4.

Antibody classes:

• IgG most abundant in serum; crosses placenta; secondary response
• IgM first in primary response; pentamer
• IgA mucosal secretions (saliva, tears, breast milk); dimer
• IgE allergy and parasites; binds mast cells
• IgD B-cell receptor

Primary response slow, IgM dominant. Secondary response fast, large, IgG dominant (memory).

Lymphoid organs: primary (bone marrow, thymus) = where lymphocytes mature. Secondary (lymph nodes, spleen, tonsils, MALT) = where they meet antigen.

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14. Musculoskeletal System

Muscle types:

Type	Control	Striated?	Nuclei	Key
Skeletal	Voluntary (somatic)	Yes	Multi	Attached to bone
Cardiac	Involuntary (autonomic + intrinsic)	Yes	1-2	Intercalated discs, gap junctions
Smooth	Involuntary	No	1	Organ walls, vessels

Sarcomere (Z line → Z line):

• A band = myosin (thick) length - stays constant
• I band = actin only - shortens
• H zone = myosin only - shortens
• M line = center

Sliding filament (contraction):

1. AP travels down motor neuron → ACh at NMJ → muscle depolarizes
2. Depolarization travels down T-tubules → SR releases Ca²⁺
3. Ca²⁺ binds troponin → tropomyosin shifts → exposes actin binding sites
4. Myosin head (cocked with ATP hydrolyzed → ADP + Pi) binds actin
5. Power stroke: myosin pulls actin toward M line; releases ADP + Pi
6. New ATP binds myosin → detaches from actin
7. ATP hydrolysis re-cocks head; cycle repeats while Ca²⁺ present

Rigor mortis: no ATP → myosin cannot detach from actin.

Bone: compact (outer, osteons with central Haversian canals) + spongy (trabeculae, marrow). Cells: osteoblasts build, osteoclasts resorb, osteocytes maintain. Ca²⁺ regulation: PTH ↑, calcitonin ↓, vitamin D ↑.

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15. Reproductive System

Male: seminiferous tubules (spermatogenesis, Sertoli cells nurse sperm + produce inhibin) → epididymis (mature, store) → vas deferens → ejaculatory duct → urethra. Leydig cells make testosterone (LH-stimulated). Spermatogenesis is continuous from puberty.

Female: ovary (oogonia → arrested primary oocyte at prophase I until puberty → completes meiosis I at ovulation → meiosis II only if fertilized). Path: ovary → fallopian tube (fertilization in ampulla) → uterus (implantation) → cervix → vagina.

Menstrual cycle (~28 days):

1. Follicular phase (days 1-14): FSH → follicle matures → estrogen rises
2. Ovulation (~day 14): estrogen peak → LH surge → release of secondary oocyte
3. Luteal phase (days 14-28): corpus luteum → progesterone (maintains endometrium). If no pregnancy → CL degenerates → progesterone drops → menstruation.
4. If pregnancy: hCG from syncytiotrophoblast rescues CL for 1st trimester; placenta takes over after.

Menopause = ovarian follicles depleted → estrogen ↓, LH/FSH ↑ (loss of negative feedback).

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16. Viruses and Bacteria

Virus: nucleic acid (DNA or RNA, ss or ds) + capsid ± envelope. Obligate intracellular parasite.

• Lytic cycle: replicate → lyse host
• Lysogenic cycle: integrate as prophage → dormant → can go lytic
• Retrovirus (e.g., HIV): RNA → DNA (reverse transcriptase) → integrated by integrase

Bacteria: single circular chromosome, plasmids (can carry antibiotic resistance), no membrane-bound organelles, 70S ribosomes, peptidoglycan cell wall.

• Gram-positive: thick peptidoglycan, stains purple
• Gram-negative: thin peptidoglycan + outer LPS membrane, stains pink
• Shapes: cocci (round), bacilli (rod), spirilli (spiral)
• Reproduction: binary fission (asexual)
• Genetic transfer: transformation (free DNA uptake), transduction (via phage), conjugation (sex pilus, F plasmid)
• Growth curve: lag → log (exponential) → stationary → death

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17. Evolution and Population Genetics

Mechanisms of evolution:

• Natural selection (directional, stabilizing, disruptive)
• Sexual selection
• Genetic drift (bottleneck, founder effect) - stronger in small populations
• Gene flow (migration)
• Mutation (ultimate source of variation)

Fitness = relative reproductive success. Inclusive fitness = personal + relatives’ reproduction × relatedness (altruism toward kin).

Speciation:

• Allopatric = geographic isolation → divergence
• Sympatric = same location, different niches/polyploidy
• Prezygotic barriers: habitat, temporal, behavioral, mechanical, gametic
• Postzygotic barriers: hybrid inviability, sterility, breakdown

Evidence: fossil record, comparative anatomy (homologous = common ancestry; analogous = convergent), molecular (DNA/protein similarity), biogeography, embryology.

Convergent = similar trait, different ancestor (bat wing vs bird wing). Divergent = different traits, same ancestor (Darwin’s finches).

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18. Test-Day Memory Aids

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19. The 25 Facts Most Likely to Show Up on Test Day

1. Na⁺/K⁺ pump: 3 Na⁺ out, 2 K⁺ in per ATP.
2. Resting membrane potential ~-70 mV, threshold ~-55 mV, peak +35 mV.
3. Prokaryotic ribosomes 70S (50S + 30S); eukaryotic 80S (60S + 40S).
4. DNA polymerase synthesizes 5’ → 3’ only.
5. Ectoderm = nervous system + skin; Mesoderm = muscle + bone + blood + kidney; Endoderm = gut + lungs + liver + pancreas.
6. hCG maintains corpus luteum until placenta takes over (~week 10).
7. ADH → aquaporins in collecting duct; aldosterone → Na⁺ reabsorption in DCT.
8. SA node fires at 60-100 bpm; cardiac muscle has gap junctions at intercalated discs.
9. Fetal Hb (HbF, γ subunits) binds O₂ tighter than adult HbA.
10. Bohr effect: ↑CO₂/↑H⁺/↑temp/↑2,3-BPG → right shift → O₂ unloads to tissue.
11. Surfactant from type II pneumocytes; produced by ~week 24-28; prevents alveolar collapse.
12. Gastrin ↑ acid; Secretin ↑ pancreatic HCO₃⁻; CCK ↑ bile + pancreatic enzymes.
13. PTH ↑ blood Ca²⁺; Calcitonin ↓ blood Ca²⁺; vitamin D ↑ Ca²⁺ (gut absorption).
14. Insulin = anabolic, ↓ glucose (β-cells); Glucagon = catabolic, ↑ glucose (α-cells).
15. MHC I on all nucleated cells → CD8⁺. MHC II on APCs → CD4⁺.
16. IgM first; IgG most abundant and crosses placenta; IgA mucosal; IgE allergy/parasite.
17. Sarcomere shortens: I band and H zone shrink; A band stays the same.
18. Rigor mortis = no ATP → myosin cannot detach from actin.
19. Countercurrent multiplier in loop of Henle concentrates the medulla.
20. Hardy-Weinberg: p² + 2pq + q² = 1.
21. Start codon = AUG (Met); stop codons = UAA, UAG, UGA.
22. Mendel’s laws: segregation + independent assortment. Linked genes violate independent assortment.
23. Meiosis I = reductional (homologs separate); Meiosis II = equational (sisters separate).
24. Crossing over happens in prophase I (synapsis + tetrads at chiasmata).
25. Neural crest (from ectoderm) → adrenal medulla, Schwann cells, melanocytes, facial bones, most peripheral nerves.

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Next Steps

If something on this page feels unfamiliar, open the corresponding chapter and study the underlying concept. This cheat sheet gets you 80% recall on test day; the full chapters get you the reasoning that lets you solve novel passages. Bookmark this page and return to it the night before every practice test.