A&P 3 AM Survival Guide: Cells, Bones & Muscles
Written like we're cramming in the break room with cold coffee and 14 minutes left. Three units, quick-hit tables, dumb-but-sticky mnemonics, the traps professors love, and practice questions you tap to check. Intro-level college A&P, built from OpenStax and NCBI StatPearls (sources at the bottom).
🔬 Companion: A&P 1 Lab Practical guide (skin, bones & muscles) ↗
How to use it: skim the Quick Take, study the tables, say the mnemonics out loud (yes, out loud), then do the questions before you peek.
Unit 1 · The Cell: The Night Crew Nobody Thanks
↑ Back to indexQuick Take: The cell is the smallest unit of life. Every cell has a plasma membrane, cytoplasm (cytosol + organelles), and (except mature red blood cells) a nucleus. Structure predicts function: lots of mitochondria = high energy demand; lots of rough ER = protein factory.
The plasma membrane
↑ Back to index- Phospholipid bilayer: hydrophilic (water-loving) phosphate heads face out, hydrophobic fatty-acid tails face in. Cholesterol tucked in adjusts fluidity.
- Proteins: integral (span the membrane: channels, carriers, receptors) and peripheral (stuck to one surface). Glycoproteins/glycolipids form the glycocalyx, the cell's ID badge.
- Selectively permeable: small nonpolar stuff (O₂, CO₂, lipids) slides through; ions and big polar molecules need proteins.
Moving stuff across the membrane
↑ Back to index| Process | Energy (ATP)? | Direction | Example |
|---|---|---|---|
| Simple diffusion | No (passive) | High → low concentration | O₂ into cells, CO₂ out |
| Facilitated diffusion | No | High → low, through a channel or carrier | Glucose via GLUT carriers; ions via channels |
| Osmosis | No | Water moves toward higher solute concentration | Water through aquaporins |
| Active transport (primary) | Yes, directly | Low → high (against gradient) | Na⁺/K⁺ pump: 3 Na⁺ out, 2 K⁺ in per ATP |
| Secondary active transport | Indirectly (uses a gradient the pump built) | One substance rides another's gradient | Na⁺–glucose symport |
| Endocytosis / exocytosis | Yes (vesicles) | Bulk in / bulk out | Phagocytosis ("cell eating"), releasing neurotransmitter |
Tonicity (what happens to a red blood cell)
↑ Back to index| Solution | Solute outside vs inside | Water moves | Cell |
|---|---|---|---|
| Isotonic | Equal | No net movement | Stays normal |
| Hypotonic | Less outside | Into the cell | Swells, may burst (lysis) |
| Hypertonic | More outside | Out of the cell | Shrinks (crenation) |
🧠 Memory hook: hYPO = the cell gets fat like a hipPO. hYPER = hyper cell sweats all its water out and shrivels.
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Organelles: who does what on the unit
↑ Back to index| Organelle | Job | Night-shift translation |
|---|---|---|
| Nucleus | Holds DNA; controls the cell; site of transcription (DNA → mRNA) | The charge nurse with the master plan |
| Nucleolus | Makes ribosomal subunits (rRNA) | Supply closet restocker |
| Ribosomes | Translation: build proteins from mRNA (free = cytosol proteins; bound = export/membrane proteins) | The hands doing the actual work |
| Rough ER | Ribosome-studded; makes and folds proteins for export or membranes | Med room prepping orders |
| Smooth ER | Lipid and steroid synthesis, detox, stores Ca²⁺ (called sarcoplasmic reticulum in muscle) | Pharmacy + calcium vault |
| Golgi apparatus | Modifies, sorts, packages proteins into vesicles | Tube station / mailroom |
| Mitochondria | Aerobic cellular respiration → most ATP; has its own DNA | The coffee machine. Everything stops without it. |
| Lysosomes | Digestive enzymes break down worn-out parts and debris | Housekeeping |
| Peroxisomes | Break down fatty acids, detoxify; neutralize hydrogen peroxide | Hazmat team |
| Cytoskeleton | Shape, support, movement (microfilaments, intermediate filaments, microtubules) | The building's framing |
| Centrosome/centrioles | Organize microtubules and the mitotic spindle | Organizes the move when the unit splits |
| Cilia / flagella | Cilia move stuff across a surface; a flagellum moves the cell (sperm) | Transport team |
Charge nurse tip: If an exam describes a cell that "secretes lots of protein," think rough ER + Golgi. "Makes steroid hormones" or "detoxes"? Smooth ER. "Huge energy needs" (muscle, sperm)? Packed with mitochondria.
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Protein synthesis in two moves
↑ Back to index- Transcription (in the nucleus): DNA is copied into mRNA.
- Translation (at the ribosome): mRNA codons are read three bases at a time; tRNA brings matching amino acids; the ribosome links them into a polypeptide.
Cell cycle & mitosis
↑ Back to indexInterphase (G₁ grow → S copy DNA → G₂ prep) takes up most of the cycle. Then mitosis (nuclear division) + cytokinesis (cytoplasm splits) → two identical diploid daughter cells.
| Phase | What happens |
|---|---|
| Prophase | Chromosomes condense; spindle forms; nuclear envelope breaks down |
| Metaphase | Chromosomes line up at the Middle (metaphase plate) |
| Anaphase | Sister chromatids pulled Apart to opposite poles |
| Telophase | Two new nuclei form; chromosomes decondense (cytokinesis usually overlaps) |
🧠 Mnemonic: "I Pee More At Twilight" → Interphase, Prophase, Metaphase, Anaphase, Telophase. (Relatable at 0300.) Bonus: Metaphase = Middle, Anaphase = Apart.
⚠️ Exam traps
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- Interphase is not a phase of mitosis; it's the part of the cell cycle between divisions. DNA replicates in the S phase, not during mitosis.
- Mitosis = 2 identical diploid cells (body cells). Meiosis = 4 genetically different haploid gametes.
- Osmosis is about water moving, and water moves toward the higher solute concentration.
- The Na⁺/K⁺ pump is 3 Na⁺ out, 2 K⁺ in. Flipping that is a classic wrong answer.
- Mature red blood cells have no nucleus and no mitochondria.
Unit 1 practice questions
↑ Back to index ↑ Back to index ↑ Back to indexUnit 2 · The Skeletal System: The Building Is Always Under Renovation
↑ Back to indexQuick Take: Bone is living, constantly remodeled tissue. Functions: support, protection, movement (levers for muscles), mineral storage (calcium, phosphate), blood cell formation (hematopoiesis in red marrow), and fat storage (yellow marrow). The adult skeleton has 206 bones: axial (80) + appendicular (126).
🧠 Mnemonic for functions: "Some People Must Stay Bloody Funny" → Support, Protection, Movement, Storage (minerals), Blood cell formation, Fat storage.
Bone classification by shape
↑ Back to index| Type | Description | Examples |
|---|---|---|
| Long | Longer than wide; shaft + two ends; act as levers | Femur, humerus, phalanges |
| Short | Cube-like; stability with some motion | Carpals, tarsals |
| Flat | Thin, often curved; protect organs, muscle attachment | Skull bones, sternum, ribs, scapula |
| Irregular | Complex shapes | Vertebrae, facial bones |
| Sesamoid | Small, round, formed in tendons; protect tendons from stress | Patella |
Anatomy of a long bone
↑ Back to index- Diaphysis: the shaft; compact bone around the medullary cavity (yellow marrow in adults).
- Epiphyses: the ends; spongy bone inside (red marrow), covered by articular cartilage (hyaline).
- Metaphysis & epiphyseal plate: the metaphysis is the region between the diaphysis and epiphysis; it holds the epiphyseal (growth) plate of hyaline cartilage in kids, which becomes the epiphyseal line when growth stops.
- Periosteum: outer membrane with blood vessels, nerves, and bone cells; tendons and ligaments anchor here. Endosteum lines the medullary cavity.
Compact vs spongy bone
↑ Back to index| Compact (cortical) | Spongy (cancellous) | |
|---|---|---|
| Structure | Osteons (Haversian systems): concentric lamellae around a central canal | Trabeculae (lattice of struts), no osteons |
| Where | Outer layer of all bones; diaphysis | Epiphyses, inside flat bones |
| Job | Strength and support under stress | Lightweight; houses red marrow |
Osteon parts: central (Haversian) canal (vessels, nerves) running lengthwise; perforating (Volkmann's) canals running crosswise; lacunae (little lakes) holding osteocytes; canaliculi (tiny channels) connecting them.
↑ Back to indexBone cells
↑ Back to index| Cell | Job | Memory |
|---|---|---|
| Osteogenic (osteoprogenitor) cell | Stem cell; divides and becomes an osteoblast | The new grad |
| Osteoblast | Builds bone; secretes osteoid (collagen matrix) that then mineralizes | Blast = Build |
| Osteocyte | Mature bone cell trapped in a lacuna; maintains matrix, senses stress | Cyte = sits in its "Cell" and monitors |
| Osteoclast | Large multinucleated cell (from monocyte/macrophage line) that resorbs (breaks down) bone | Clast = Chew/Carve |
🧠 "Blasts Build, Clasts Cut." OsteoBlasts Build, osteoClasts Cut (and they Chew up Calcium into the blood).
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Bone matrix
↑ Back to indexAbout one-third organic (mostly collagen → flexibility, tensile strength) and two-thirds inorganic (hydroxyapatite, calcium phosphate crystals → hardness). Lose collagen = brittle. Lose mineral = bendy.
↑ Back to indexBone formation (ossification)
↑ Back to index| Type | Starts from | Makes |
|---|---|---|
| Intramembranous | Mesenchyme (embryonic connective tissue) directly | Flat bones of skull, mandible, clavicle |
| Endochondral | A hyaline cartilage model that is replaced by bone | Most bones, including long bones |
Growth in length happens at the epiphyseal plate (cartilage grows, then ossifies). Growth in width = appositional growth (osteoblasts add bone under the periosteum while osteoclasts widen the medullary cavity).
↑ Back to indexRemodeling & calcium balance
↑ Back to indexRemodeling is ongoing resorption by osteoclasts and deposition by osteoblasts. It responds to mechanical stress (Wolff's law: bone adapts to the loads placed on it) and to hormones.
| Hormone | Released when | Effect on bone and blood Ca²⁺ |
|---|---|---|
| Parathyroid hormone (PTH) | Blood Ca²⁺ low | Stimulates osteoclast activity (indirectly), increases kidney Ca²⁺ reabsorption and calcitriol production → raises blood Ca²⁺ |
| Calcitonin (thyroid) | Blood Ca²⁺ high | Inhibits osteoclasts → lowers blood Ca²⁺ |
| Calcitriol (active vitamin D) | Stimulated by PTH | Increases Ca²⁺ absorption from the gut |
| Growth hormone, sex hormones (estrogen, testosterone) | Growth/puberty | Promote growth; sex hormones eventually close the epiphyseal plates; estrogen helps preserve bone mass |
🧠 "Calci-TONin TONES DOWN calcium. PTH PULLS calcium (out of bone, into the blood)."
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Fracture repair (4 steps)
↑ Back to index- Hematoma forms (torn vessels bleed and clot).
- Fibrocartilaginous (soft) callus bridges the break.
- Bony (hard) callus: spongy bone replaces the soft callus.
- Remodeling: compact bone restores the original shape.
🧠 "Hot Sauce Burns Raw" → Hematoma, Soft callus, Bony callus, Remodeling.
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Joints (articulations) quick hit
↑ Back to index| Structural | Functional | Examples |
|---|---|---|
| Fibrous | Mostly synarthrosis (immovable) | Skull sutures |
| Cartilaginous | Amphiarthrosis (slightly movable) | Pubic symphysis, intervertebral discs |
| Synovial | Diarthrosis (freely movable) | Knee, shoulder, hip, elbow |
Charge nurse tip: Ligaments connect bone to bone; tendons connect muscle to bone. "Ligament = Link bones."
⚠️ Exam traps
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- Red marrow makes blood cells; in adults it's mostly in spongy bone of flat bones, vertebrae, and the ends of long bones like the femur and humerus. Yellow marrow is mostly fat.
- Osteocytes do not divide or build new bone; they maintain it. Osteoblasts build.
- PTH raises blood calcium; calcitonin lowers it. Opposites, don't swap them.
- The patella is a sesamoid bone, not a short bone. Vertebrae are irregular.
- Long bones grow in length at the epiphyseal plate, not the periosteum.
Unit 2 practice questions
↑ Back to index ↑ Back to index ↑ Back to indexUnit 3 · The Muscular System: Code Team Mechanics
↑ Back to indexQuick Take: Muscle tissue is excitable, contractile, extensible, and elastic. Three types: skeletal, cardiac, smooth. Contraction = myosin pulling actin filaments toward the center of the sarcomere using ATP, switched on by calcium. Muscles only pull; they never push.
Three muscle tissue types
↑ Back to index| Feature | Skeletal | Cardiac | Smooth |
|---|---|---|---|
| Location | Attached to bones (and skin in the face) | Heart wall | Walls of hollow organs, vessels, airways |
| Striated? | Yes | Yes | No |
| Nuclei | Many, at periphery (multinucleated) | Usually one (sometimes two), central | One, central |
| Cell shape | Long cylindrical fibers | Branched, joined by intercalated discs (gap junctions + desmosomes) | Spindle-shaped |
| Control | Voluntary (somatic nervous system) | Involuntary; autorhythmic (pacemaker cells) | Involuntary (autonomic, hormones) |
| Speed | Fast | Moderate, rhythmic | Slow, sustained |
🧠 "Skeletal is the Soldier (voluntary, takes orders). Cardiac is the Clock (beats on its own). Smooth is the Sloth (slow and steady)."
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Skeletal muscle structure, biggest to smallest
↑ Back to indexMuscle (wrapped by epimysium) → fascicle (bundle, wrapped by perimysium) → muscle fiber/cell (wrapped by endomysium) → myofibrils → sarcomeres → myofilaments (actin = thin, myosin = thick).
🧠 Epi = on top (outermost). Peri = around the bundles. Endo = inside, around each fiber.
Fiber terms: sarcolemma = plasma membrane; sarcoplasm = cytoplasm; sarcoplasmic reticulum (SR) = smooth ER that stores Ca²⁺; T-tubules = sarcolemma tunnels that carry the action potential deep into the fiber.
↑ Back to indexThe sarcomere: the functional unit
↑ Back to index| Part | What it is | During contraction |
|---|---|---|
| Z disc (Z line) | Boundary of a sarcomere; anchors thin filaments | Z discs move closer together |
| A band | Full length of thick (myosin) filaments; dArk | Stays the same length |
| I band | Thin filaments only; lIght | Shortens |
| H zone | Center of A band, thick filaments only | Shortens / disappears |
| M line | Middle; anchors thick filaments | Stays central |
🧠 "HI, I'm shrinking, but the A band is Always the same." H zone and I band shorten; A band stays constant.
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Neuromuscular junction → contraction (excitation-contraction coupling)
↑ Back to index- A motor neuron action potential reaches the axon terminal; voltage-gated Ca²⁺ channels open.
- The neuron releases acetylcholine (ACh) into the synaptic cleft.
- ACh binds receptors on the motor end plate; Na⁺ rushes in → the sarcolemma depolarizes → a muscle action potential spreads.
- The action potential travels down the T-tubules.
- The SR releases Ca²⁺ into the sarcoplasm.
- Ca²⁺ binds troponin → troponin shifts tropomyosin off the myosin-binding sites on actin.
- Cross-bridge cycling begins (below). Meanwhile, acetylcholinesterase breaks down ACh in the cleft.
Cross-bridge cycle (sliding filament model)
↑ Back to index- Cocking: myosin head hydrolyzes ATP → ADP + Pi, and the head swings to its high-energy "cocked" position.
- Attach: the cocked head binds an exposed site on actin, forming a cross-bridge.
- Power stroke: Pi releases, the head pivots and pulls actin toward the M line; ADP releases.
- Detach: a new ATP binds myosin and the head lets go of actin. Repeat as long as Ca²⁺ and ATP are present.
Relaxation: ACh is cleared, Ca²⁺ is actively pumped back into the SR (uses ATP), tropomyosin re-covers binding sites.
🧠 "Cock, Attach, Pull, Let go." And remember: ATP is needed to let GO. No ATP after death = myosin stays stuck = rigor mortis.
Charge nurse tip: Calcium is the ON switch, ATP is the fuel and the release. If a question asks "what directly causes detachment," the answer is ATP binding myosin, not calcium.
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Energy for contraction
↑ Back to index| Source | Speed | How long it lasts | Notes |
|---|---|---|---|
| Stored ATP + creatine phosphate | Fastest | About 15 seconds of max effort | Creatine phosphate donates phosphate to ADP |
| Anaerobic glycolysis | Fast | About a minute | 2 ATP per glucose; makes lactate (lactic acid) |
| Aerobic respiration | Slow | Long duration | In mitochondria; about 30+ ATP per glucose; needs O₂ |
Fiber types & motor units
↑ Back to index- Slow oxidative (type I): lots of mitochondria and myoglobin (red), fatigue-resistant: posture, marathon muscles.
- Fast glycolytic (type IIx): pale, powerful, fatigue fast: sprinting, lifting.
- Fast oxidative (type IIa): in between.
- Motor unit = one motor neuron + all the fibers it controls. Small units = fine control (eyes, fingers); large units = power (thigh). More force = recruit more units.
- Muscle tone = constant low-level contraction at rest. Isotonic contraction changes length (concentric shortens, eccentric lengthens); isometric builds tension without changing length.
Charge nurse tip: Prime mover (agonist) does the main job; antagonist does the opposite; synergists help; fixators stabilize the origin. Origin = the fixed attachment, insertion = the attachment that moves.
⚠️ Exam traps
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- The A band does not shorten during contraction; the filaments slide, they don't shrink.
- Calcium binds troponin, not tropomyosin and not myosin.
- Calcium for skeletal muscle contraction comes mainly from the sarcoplasmic reticulum.
- Cardiac muscle is striated but involuntary; smooth muscle is not striated.
- ATP is needed both to power the stroke and to detach myosin, and to pump Ca²⁺ back into the SR.
Unit 3 practice questions
↑ Back to index ↑ Back to index ↑ Back to indexSources
↑ Back to indexFacts checked against these references (accessed October 2026). Your professor's textbook and lecture always win if they differ.
- OpenStax, Anatomy and Physiology 2e (Betts et al.), Ch. 3 "The Cellular Level of Organization": openstax.org/books/anatomy-and-physiology-2e/pages/3-introduction
- OpenStax, Anatomy and Physiology 2e, Ch. 6 "Bone Tissue and the Skeletal System": openstax.org/books/anatomy-and-physiology-2e/pages/6-introduction
- OpenStax, Anatomy and Physiology 2e, Ch. 9 "Joints": openstax.org/books/anatomy-and-physiology-2e/pages/9-introduction
- OpenStax, Anatomy and Physiology 2e, Ch. 10 "Muscle Tissue": openstax.org/books/anatomy-and-physiology-2e/pages/10-introduction
- NCBI StatPearls, "Physiology, Bone Remodeling": ncbi.nlm.nih.gov/books/NBK499863
- NCBI StatPearls, "Physiology, Skeletal Muscle": ncbi.nlm.nih.gov/books/NBK537139
- NCBI StatPearls, "Physiology, Osmosis": ncbi.nlm.nih.gov/books/NBK557609