Cellular Level of Organization (Ch. 3)

Biological Membranes & Transport

The cell membrane is a dynamic structure that regulates the movement of substances into and out of the cell, maintaining homeostasis.

• Fluid Mosaic Model: The membrane consists of a fluid phospholipid bilayer with embedded proteins that move laterally, allowing flexibility and selective permeability.

• Permeability: Small, uncharged, non-polar molecules (e.g., O2, CO2, N2) diffuse freely. Large, polar, or charged molecules (e.g., Na+, K+, glucose) require transport proteins.

• Passive Transport: Does not require energy.

  • Simple Diffusion: Movement down the concentration gradient without proteins.

  • Facilitated Diffusion: Uses transport proteins to move substances down their gradient.

  • Osmosis: Diffusion of water toward higher solute concentrations.

    • Hypotonic: Water enters cell; cell swells/lyses.

    • Isotonic: No net water movement; preferred by animal cells.

    • Hypertonic: Water exits cell; cell shrivels.

• Active Transport: Requires ATP.

  • Primary Active Transport: Direct ATP use (e.g., Na+/K+ pump).

  • Secondary Active Transport: Uses gradients established by primary pumps (e.g., sodium-glucose cotransporter).

• Bulk Transport:

  • Endocytosis: Cell engulfs material (phagocytosis, pinocytosis, receptor-mediated).

  • Exocytosis: Vesicles expel substances (e.g., hormones, neurotransmitters).

Example: The Na+/K+ pump maintains cellular ion gradients essential for nerve impulse transmission.

Equation:

per ATP hydrolyzed

Eukaryotic Organelles & Endomembrane System

Eukaryotic cells contain specialized organelles that compartmentalize functions for efficient cellular operation.

• Nucleus: Stores DNA; nuclear envelope with pores; nucleolus assembles ribosomes.

• Endoplasmic Reticulum (ER):

  • Rough ER: Protein synthesis and folding.

  • Smooth ER: Lipid synthesis and detoxification.

• Golgi Apparatus: Processes, sorts, and packages proteins/lipids for transport.

• Lysosomes & Peroxisomes: Digestive and detoxification vesicles; peroxisomes handle H2O2.

• Mitochondria: Site of ATP production; double membrane; own DNA and ribosomes.

• Cytoskeleton: Microfilaments, intermediate filaments, and microtubules provide structure and movement.

Example: Mitochondria generate ATP via cellular respiration, crucial for muscle contraction.

Central Dogma, Transcription, and Translation

The central dogma describes the flow of genetic information from DNA to RNA to protein.

• DNA Organization: DNA wraps around histones forming nucleosomes, chromatin, and chromosomes.

• Transcription: DNA is transcribed to RNA in the nucleus by RNA polymerase.

  • Direction: 5' to 3'

• Translation: RNA is translated to protein at ribosomes using mRNA and tRNA.

  • Ribosomal Sites: A-site (amino-acyl tRNA), P-site (polypeptide chain), E-site (exit).

  • Steps: Initiation (AUG start codon), elongation, termination (stop codon).

Equation:

The Cell Cycle and Mitosis

The cell cycle governs cell growth and division, ensuring genetic continuity.

• Interphase: G1 (growth), S (DNA replication), G2 (preparation); G0 is a resting state.

• Mitosis: Division of somatic cells into two identical diploid cells.

  • Prophase: Chromosomes condense, spindle forms.

  • Prometaphase: Nuclear envelope breaks, spindle attaches.

  • Metaphase: Chromosomes align at equator.

  • Anaphase: Sister chromatids separate.

  • Telophase: Nuclear envelopes reform.

• Cytokinesis: Division of cytoplasm; cleavage furrow in animals, cell plate in plants.

• Regulation & Cancer: Controlled by checkpoints (G1, S, G2, M) and genes; mutations can cause tumors.

Example: Cancer results from loss of cell cycle control, leading to uncontrolled proliferation.

Tissue Level of Organization (Ch. 4)

Overview of Primary Tissues

Tissues are groups of cells with similar structure and function, studied in histology.

• Four Types: Epithelial, Connective, Muscle, Nervous.

• Extracellular Matrix (ECM): Non-cellular component providing structural support.

Epithelial Tissue

Epithelial tissue covers surfaces, lines cavities, and forms glands.

• Characteristics: Polar, tightly packed, avascular, highly regenerative.

• Basement Membrane: Basal lamina + reticular lamina.

• Classification:

  • Simple Squamous: Rapid diffusion (lungs, capillaries).

  • Simple Cuboidal: Secretion/absorption (kidney tubules).

  • Simple Columnar: Absorption/secretion (digestive tract).

  • Pseudostratified Columnar: Appears stratified; often ciliated (respiratory tract).

  • Stratified Squamous: Protection (skin, mouth).

  • Transitional: Changes shape (urinary bladder).

• Glandular Epithelium:

  • Endocrine: Ductless; secrete hormones into blood.

  • Exocrine: Secrete onto surfaces/ducts; merocrine, holocrine, apocrine modes.

Example: Skin's epidermis is stratified squamous epithelium for protection.

Connective Tissue

Connective tissue supports, binds, and protects other tissues and organs.

• Characteristics: Abundant, derived from mesenchyme, prominent ECM.

• Protein Fibers: Collagen (strength), reticular (scaffolding), elastic (stretch).

• Cell Types: -blast (immature, build ECM), -cyte (mature, maintain ECM).

• Classes:

  • Loose: Areolar, reticular, adipose.

  • Dense: Regular, irregular, elastic.

  • Specialized: Cartilage (hyaline, fibrocartilage, elastic), bone, blood.

Example: Tendons are dense regular connective tissue for muscle-bone attachment.

Muscle and Nervous Tissue

Muscle tissue enables movement; nervous tissue transmits electrical signals.

• Muscle Tissue:

  • Skeletal: Voluntary, striated, multinucleated.

  • Cardiac: Involuntary, striated, branched, intercalated discs.

  • Smooth: Involuntary, non-striated, spindle-shaped.

• Nervous Tissue:

  • Neurons: Dendrites, soma, axon.

  • Neuroglia: Support and protect neurons.

Example: Cardiac muscle contracts rhythmically to pump blood.

The Integumentary System (Ch. 5)

Layers of the Skin

The skin is composed of three main layers, each with distinct functions.

• Epidermis: Stratified squamous epithelium; layers (deep to superficial):

  1. Stratum Basale: Stem cells, melanocytes, tactile cells.

  2. Stratum Spinosum: Keratinocytes, desmosomes.

  3. Stratum Granulosum: Keratin granules.

  4. Stratum Lucidum: Clear, dead cells (thick skin only).

  5. Stratum Corneum: Dead, keratinized cells.

• Dermis: Connective tissue; papillary (areolar, dermal papillae) and reticular (dense irregular, collagen, glands, hair follicles).

• Hypodermis: Areolar and adipose tissue; anchors skin, site for injections.

Example: The dermis contains blood vessels and sensory receptors for touch and pressure.

Appendages and Glands

Skin appendages include hair, nails, and glands, each serving specialized functions.

• Hair & Follicles: Keratinized hairs; arrector pili muscles cause goosebumps; sebaceous glands secrete oil.

• Sweat Glands:

  • Apocrine: Axillary/groin; sticky, odorous secretion.

  • Merocrine (Eccrine): Widely distributed; watery sweat for thermoregulation.

Example: Eccrine sweat glands help cool the body during exercise.

Clinical Applications

Skin injuries and diseases can affect function and require medical intervention.

• Burns:

  • First-Degree: Epidermis only; minor pain.

  • Second-Degree: Epidermis and upper dermis; blisters.

  • Third-Degree: Full-thickness; requires grafts, high infection risk.

• Rule of Nines: Estimates burn area for adults.

  Body Region	% of Surface Area
  Head	9%
  Each Upper Limb	9%
  Trunk	36%
  Each Lower Limb	18%
  Genitalia	1%

• Gangrene: Tissue death from poor blood supply.

Example: Third-degree burns may require skin grafts to restore function.

Bone Tissue & Skeletal System (Ch. 6)

Functions of the Skeletal System

The skeletal system provides structural support, protection, and facilitates movement.

• Support: Framework for the body.

• Mineral Storage: Calcium and lipids.

• Blood Cell Production: Red marrow produces blood cells.

• Protection: Shields vital organs.

• Leverage: Muscles act on bones for movement.

Bone Cells

Bone tissue contains specialized cells for growth, maintenance, and remodeling.

• Osteoprogenitor Cells: Stem cells in endosteum/periosteum; differentiate into osteoblasts.

• Osteoblasts: Build bone matrix (osteoid).

• Osteocytes: Mature cells in lacunae; maintain matrix.

• Osteoclasts: Resorb bone; derived from macrophages.

Example: Osteoclast activity increases during bone remodeling and calcium release.

Bone Structure & Growth

Bones grow and develop through distinct processes, resulting in compact and spongy regions.

• Compact Bone: Dense, strong outer layer.

• Spongy Bone: Trabecular network; houses marrow.

• Endochondral Ossification: Replaces cartilage with bone; forms most bones.

• Intramembranous Ossification: Forms flat bones from mesenchyme.

• Appositional Growth: Increases bone width.

Example: Long bones grow in length at the epiphyseal plate via endochondral ossification.

Calcium Homeostasis & Hormonal Control

Blood calcium levels are tightly regulated by hormones acting on bone, kidneys, and intestines.

• Parathyroid Hormone (PTH): Increases blood Ca2+ by stimulating osteoclasts, increasing absorption, decreasing excretion.

• Calcitonin: Lowers blood Ca2+ by inhibiting osteoclasts, increasing excretion.

• Other Hormones: Calcitriol, growth hormone, thyroxine, sex hormones.

Equation:

Fracture Repair & Aging

Bones repair themselves through a multi-step process; aging affects bone mass and strength.

• Fracture Repair:

  1. Hematoma formation

  2. Callus formation

  3. Spongy bone replacement

  4. Remodeling

• Aging: Osteopenia (bone mass loss) begins at 30–40 years; osteoporosis is severe loss, common post-menopause.

Example: Elderly individuals are at higher risk for fractures due to osteoporosis.

Additional info: Academic context and examples were added to clarify and expand brief points for exam preparation.