Human Anatomy U1

Discovery & Definition of Cells

• Mid-1600s: Robert Hooke uses the light microscope to examine cork.- Observes hundreds of tiny chambers; reminiscent of monks’ rooms (Latin: “cellula”).

  • Coins the term “cells,” establishing the basic unit of life.

• Modern perspective- Trillions of cells in the human body; $50{-}100 \times 10^{12}$ total.

  • Over $200$ distinct cell types, each specialized.

• Coaching analogy (softball/baseball)- Nine field positions → each with a precise task, yet united by a common goal (winning).

  • Likewise, organelles perform discrete jobs that collectively keep the cell alive.

External Cell Structures

• Universal feature: cell (plasma) membrane — thin, flexible, semipermeable.

• Additional layer in plants, many fungi, most bacteria: cell wall.- Thicker, rigid; provides shape & mechanical support.

  • Absent in animal cells.

• Size range- Bacteria \approx $1\,\mu\text{m}$ in diameter.

  • Human oocyte (egg) \approx $1\,\text{mm}$ (visible unaided).

  • Relation: $1\,\text{mm} = 1000\,\mu\text{m}$ (\approx fingernail thickness).

Cell Membrane: Composition & Analogy

• Phospholipid bilayer- Double sheet of water-insoluble phospholipids.

  • Hydrophilic phosphate heads, hydrophobic fatty-acid tails.

• Embedded proteins- Act as selective “doors” for import/export of molecules.

  • Pump ions (e.g., $\text{Na}^+, \text{K}^+$) using energy.

• Carbohydrate chains on outer leaflet- Cellular “ID tags.”

  • Enable immune system to distinguish self vs. foreign → eliminates unrecognized cells.

• House-wall metaphor- Keeps unwanted outsiders (wild animals, strangers, rain) out while retaining valuables (heat, furniture, children). Similarly, membrane maintains intracellular conditions.

Cytoplasm & Basic Organelles

• Cytoplasm- $\approx90\%$ water; gelatinous matrix where all metabolic reactions occur.

• Organelle overview (memorize names ↔ jobs)

  • Cell membrane: The thin, flexible, semipermeable outer boundary of a cell that regulates the passage of substances and helps maintain cell shape.

  • Nucleus: The command center of the cell, containing DNA, storing genetic information, and initiating ribosome production.

  • Nucleolus: A dense structure within the nucleus responsible for assembling ribosomal subunits.

  • Chromosomes: Condensed structures of DNA that become visible during cell division, carrying genetic information.

  • Ribosomes: Cellular "protein factories" responsible for synthesizing proteins, found freely in the cytoplasm or bound to the ER.

  • Endoplasmic reticulum (ER)

    • Rough ER (with ribosomes): An extensive network of membranes studded with ribosomes, responsible for synthesizing and transporting proteins to the Golgi apparatus.

    • Smooth ER: A network of membranes without ribosomes, involved in lipid synthesis and detoxification of drugs.

  • Golgi apparatus: An organelle that modifies, packages, and ships biomolecules, acting as the cell's "post office."

  • Lysosome: An acidic sac containing digestive enzymes that break down and recycle macromolecules and cellular waste.

  • Mitochondrion: The "powerhouse" of the cell, where cellular respiration occurs to convert food into ATP (adenosine triphosphate), the cell's main energy currency.

Levels of Biological Organization

• Five classical levels (expanded figure shows 7):

  1. Cells (basic units).

  2. Tissues (similar cells, shared function).

  3. Organs (multiple tissue types, complex tasks).

  4. Organ systems (coordinated organs, e.g., digestive, nervous).

  5. Organism (complete living entity).

• Construction metaphor (nails → walls → heating/electrical/plumbing systems → building).

Tissue Types

• Epithelial- Barrier, absorption, filtration/excretion, sensory reception.

  • Two forms

  • Covering & lining: single or multiple layers; various shapes (squamous, cuboidal, columnar).

  • Glandular: tubular or sac-like clusters secreting hormones, enzymes, sweat, oil, mucous.

• Muscle- High mitochondria content; cause movement.

  • Skeletal: striated, multinucleate, voluntary; attached to bones (biceps, triceps).

  • Cardiac: striated, branched, single nucleus; involuntary; heart walls; pumps blood.

  • Smooth: spindle-shaped, non-striated; involuntary; walls of hollow organs (esophagus, intestine, uterus).

• Connective (most abundant)- Common design: matrix + fibers + cells.

  • Loose (areolar/fat): cushions organs, stores energy.

  • Dense (tendons, ligaments): strong collagen; resists tensile stress.

  • Cartilage (joints, nose, ear): flexible support; shock absorber.

  • Bone: calcified collagen; structural framework; mineral storage; marrow.

  • Blood: cells in plasma; transport & immunity.

• Nervous- Neurons (dendrites + axon) conduct electrical impulses up to $\approx1\,\text{m}$.

  • Neuroglia support, insulate, protect neurons.

Organs & Systems

• Organ = functional grouping of at least two tissue types (e.g., skeletal muscle contains muscle, connective, nervous, blood).

• Organ system = coordinated organs (11 major human systems).

• Organism = sum total; shows emergent properties.

Central Dogma & Historical Experiments

• Goal: crack the hereditary “code.”

• Griffith (1928) — Transformation- Heat-killed virulent pneumonia bacteria + live harmless strain → mice die.

  • Inferred “transforming factor” (later identified as gene).

• Avery (1944)- Systematically destroyed proteins, carbs, lipids—only nucleic acids retained ability to transform → genes must be DNA/RNA.

• Hershey & Chase (1952)- Radio-labelled bacteriophage proteins with $^{35}\text{S}$, DNA with $^{32}\text{P}$.

  • Only $^{32}\text{P}$ entered bacteria → DNA carries genetic info.

• Chargaff (1950)- Base-pair rule: $[A]=[T],\;[G]=[C]$ regardless of species.

• Franklin (1952)- X-ray diffraction → X-shaped pattern → double-stranded helix.

• Watson & Crick (1953)- Integrate prior data → publish double helix model; earn Nobel Prize.

DNA Structure

• Components: phosphate, deoxyribose sugar, nitrogenous base.

• Four bases: adenine (A), thymine (T), guanine (G), cytosine (C).

• Complementarity: $A\leftrightarrow T$, $G\leftrightarrow C$.

• Double helix resembles spiral staircase; base pairs form “steps.”

DNA Replication

• Occurs prior to cell division; ensures genetic continuity.

• Steps

  1. Helix unwinds.

  2. Hydrogen bonds break; strands unzip.

  3. DNA polymerase reads template, adds complementary nucleotides.

• Semi-conservative: each daughter molecule = 1 old strand + 1 new.

• Example exercise: template AGTCGCTA → complement $TCAGCGAT$.

RNA vs. DNA (4 key differences)

• Strands: DNA double; RNA single.

• Sugar: deoxyribose vs. ribose.

• Bases: RNA substitutes uracil (U) for thymine.

• Location: DNA confined to nucleus; RNA travels to cytoplasm.

Protein Synthesis

• Construction company analogy- Nucleus = main office; DNA = boss; mRNA = messenger boy; ribosome = construction site; amino acids = nails/wood; tRNA = workers; protein = finished house.

Transcription

• DNA template → complementary mRNA inside nucleus.

• Rules: $A\rightarrow U,\;T\rightarrow A,\;G\leftrightarrow C$.

• Example: DNA AGTGCA → mRNA $UCACGU$.

Translation

• mRNA exits nucleus, binds ribosome.

• Ribosome reads codons (3 bases each).

• Each codon specifies an amino acid; tRNA with anticodon delivers.

• Chain elongates → polypeptide; folds → functional protein.

• Diagram (Fig. 1.19) couples both steps.

Protein Types & Roles

• Structural: keratin (hair, nails), collagen (bones, tendons, ligaments).

• Movement: actin & myosin (muscle contraction).

• Transport: hemoglobin (carries $O_2$), membrane channels.

• Defense: antibodies.

• pH regulation: albumin buffers blood.

• Metabolism & catalysis: enzymes (digestion), hormones (insulin, growth hormone).

Mutations

• Definition: permanent change in DNA sequence.

• Gene mutations- Point mutation: single-base substitution.

  • Frameshift: insertion/deletion shifts reading frame; usually severe.

• Chromosomal mutations- Deletion, duplication, inversion, translocation; affect many genes.

• Causes (mutagens)- Cigarette smoke, UV/ionizing radiation, certain pesticides/food chemicals, industrial agents, alcohol during pregnancy.

• Consequences: cancer, birth defects.

• Biotechnology responses- Human Genome Project: mapping gene loci.

  • Genetic engineering: design & insert healthy genes.

  • Stem cell therapy as future delivery system.

Membrane Transport Mechanisms

• Passive (no energy)

  • Diffusion: The passive movement of solutes from an area of higher concentration to an area of lower concentration until equilibrium is reached.

  • Example: $CO<em>2, O</em>2$ exchange.

  • Facilitated diffusion: A type of passive transport where large or charged molecules (e.g., glucose, ions) cross the cell membrane with the help of specific protein channels or carriers.

  • Osmosis: The passive movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration.

  • Generates osmotic pressure.

  • Solution categories

    • Hypotonic solution: A solution with a lower solute concentration than the cell's cytoplasm, causing water to move into the cell, leading to swelling or lysis (bursting).

    • Hypertonic solution: A solution with a higher solute concentration than the cell's cytoplasm, causing water to move out of the cell, leading to shrinking (crenation).

    • Isotonic solution: A solution with a solute concentration equal to that of the cell's cytoplasm, resulting in no net water movement and maintaining cell volume.

  • Adaptations

    • Plant cell wall resists bursting.

    • Protists use contractile vacuoles (Fig. 1.22).

• Active (energy-dependent)

  • Active transport: An energy-dependent process where protein pumps move substances across the membrane against their concentration gradient.

  • Endocytosis: An active process where the cell membrane engulfs material from outside the cell, forming a vesicle to bring it inside. Phagocytosis is a specific type of endocytosis involving the ingestion of large particles, such as by white blood cells.

  • Exocytosis: An active process where a vesicle inside the cell fuses with the cell membrane, expelling its contents to the outside.

Homeostasis & Metabolism

• Homeostasis: maintaining stable internal conditions (temperature, pH, fluid volume, blood pressure).- Scout camp story: bonfire to offset $-15^{\circ}\text{F}$ storm; body similarly counters deviations (shivering, sweating).

• Metabolism: sum of all chemical reactions → millions per second; engine that sustains homeostasis.