Cell Theory & Overview

Cell Theory

The cell is the fundamental structural and functional unit of all living organisms. Cell theory is a foundational concept in biology that describes the properties of cells.

• Definition: A cell is the basic structural and functional unit of life.

• Principles of Cell Theory:

  1. All living organisms are composed of one or more cells.

  2. The cell is the smallest unit of life.

  3. All cells arise only from pre-existing cells by cell division.

Three Main Parts of a Cell

• Plasma membrane: A selectively permeable outer barrier that controls the movement of substances in and out of the cell.

• Cytoplasm: The intracellular fluid containing organelles and cytosol.

• Nucleus: The control center of the cell, containing genetic material (DNA).

Extracellular Materials

• Extracellular fluids: Include interstitial fluid, plasma, and cerebrospinal fluid, which bathe and surround cells.

• Cellular secretions: Substances such as digestive and lubricating fluids produced by cells.

• Extracellular matrix: A network of proteins and polysaccharides that provides structural and biochemical support to surrounding cells.

Plasma Membrane

Structure

The plasma membrane is a dynamic structure that separates the internal environment of the cell from the external environment.

• Phospholipid bilayer: Composed of hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

Membrane Proteins

• Integral proteins: Span the membrane and function as transporters or channels for molecules.

• Peripheral proteins: Attached to integral proteins or membrane lipids; function as enzymes or provide structural support.

Cell Junctions

• Tight junctions: Form impermeable barriers between cells.

• Desmosomes: Anchoring junctions that prevent cells from being pulled apart.

• Gap junctions: Allow communication between cells through the passage of ions and small molecules.

Glycocalyx

• A carbohydrate-rich area on the cell surface important for cell recognition and interaction.

Membrane Transport

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradient.

• Primary active transport: Direct use of ATP to transport molecules (e.g., Na+/K+ pump).

• Secondary active transport: Uses energy stored in gradients created by primary active transport.

Passive Transport

Passive transport does not require energy and moves substances down their concentration gradient.

• Simple diffusion: Movement of small or lipid-soluble molecules directly through the membrane.

• Facilitated diffusion: Movement of larger or charged molecules via carrier proteins or channels (e.g., glucose, amino acids, ions).

• Osmosis: Diffusion of water through aquaporins; affected by tonicity (isotonic, hypertonic, hypotonic solutions).

Vesicular Transport

• Endocytosis: Uptake of materials into the cell via vesicles.

• Phagocytosis: "Cell eating"; engulfment of large particles.

• Pinocytosis: "Cell drinking"; uptake of extracellular fluid.

• Receptor-mediated endocytosis: Specific uptake of molecules after binding to receptors.

• Exocytosis: Secretion of materials out of the cell via vesicles.

Membrane Potential & Cell Signaling

Resting Membrane Potential

The resting membrane potential is the voltage difference across the plasma membrane when the cell is at rest, typically around -70 mV.

• Primarily established by the diffusion of K+ ions and maintained by the Na+/K+ ATPase pump.

Cell Adhesion Molecules (CAMs)

• Proteins that anchor cells to each other and to the extracellular matrix; also involved in signaling.

Membrane Receptors

• Contact signaling: Direct cell-to-cell contact for communication.

• Chemical signaling: Ligands (such as hormones or neurotransmitters) bind to receptors, triggering intracellular signaling pathways (e.g., G protein-coupled receptors and second messengers).

Cell Organelles

Major Organelles and Their Functions

• Mitochondria: Site of ATP (energy) production.

• Ribosomes: Sites of protein synthesis; can be free in cytoplasm or attached to rough ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Involved in lipid metabolism and detoxification.

• Golgi Apparatus: Modifies, packages, and ships proteins and lipids; forms lysosomes.

• Lysosomes: Contain digestive enzymes to break down ingested material and worn-out organelles.

• Peroxisomes: Detoxify harmful substances.

• Cytoskeleton: Provides structural support and facilitates cell movement.

  • Microtubules: Support and transport.

  • Microfilaments: Cell shape and motility.

  • Intermediate filaments: Stability.

• Centrosome/Centrioles: Organize spindle fibers during cell division; base for cilia and flagella.

• Cilia: Move substances across cell surfaces.

• Flagella: Propel cells (e.g., sperm).

• Microvilli: Increase surface area for absorption.

Nucleus

• Nuclear envelope: Double membrane with nuclear pores.

• Nucleolus: Site of ribosome assembly.

• Chromatin: DNA and histone proteins; condenses into chromosomes during cell division.

Tissues

Four Basic Types of Tissues

• Epithelial tissue

• Connective tissue

• Muscle tissue

• Nervous tissue

Epithelial Tissue

Functions include protection, absorption, filtration, excretion, secretion, and sensory reception. Epithelial tissue is characterized by polarity, specialized contacts, support by connective tissue, avascularity but innervation, and high regenerative capacity.

• Cell layers: Simple (one layer) vs. stratified (multiple layers).

• Cell shapes: Squamous (flat), cuboidal (cube-shaped), columnar (tall).

• Examples:

  • Simple squamous: Filtration/exchange (e.g., alveoli).

  • Simple cuboidal: Small ducts, kidneys.

  • Simple columnar: Digestive tract.

  • Pseudostratified: Respiratory tract.

  • Stratified squamous: Skin.

  • Stratified cuboidal: Large ducts.

  • Stratified columnar: Pharynx, male urethra.

  • Transitional: Urinary bladder.

Glandular Epithelium

• Endocrine glands: Ductless, secrete hormones into blood.

• Exocrine glands: Secrete onto body surfaces or into ducts.

• Unicellular: Mucus-secreting cells.

• Multicellular: Simple (unbranched) vs. compound (branched); merocrine (exocytosis) vs. holocrine (cell rupture).

Connective Tissue

Connective tissue supports, protects, and binds other tissues. It is characterized by an abundant extracellular matrix, mesenchymal origin, and variable vascularity.

• Components: Ground substance, fibers (collagen, elastic, reticular), and cells (blast/cyte forms, adipocytes, macrophages, mast cells).

• Types:

  • Loose (areolar, adipose, reticular)

  • Dense (regular, irregular, elastic)

  • Cartilage (hyaline, elastic, fibrocartilage)

  • Bone (osteoblasts, osteocytes, osteons)

  • Blood (plasma, cells)

Muscle Tissue

• Skeletal muscle: Striated, voluntary, multinucleate.

• Cardiac muscle: Striated, involuntary, branched, intercalated discs.

• Smooth muscle: Non-striated, involuntary, spindle-shaped.

Nervous Tissue

• Neurons: Conduct electrical impulses; consist of dendrites (input) and axons (output).

• Supporting cells (glial cells): Support, insulate, and protect neurons.

Membranes & Tissue Repair

Types of Membranes

• Cutaneous membrane: Skin.

• Mucous membranes: Line open body cavities.

• Serous membranes: Line closed body cavities.

Tissue Repair

• Occurs in three steps: inflammation, organization (restoration of blood supply), and regeneration/fibrosis (replacement of tissue).

Integumentary System (Skin)

Structure of the Skin

• Epidermis: Superficial epithelial layer.

• Dermis: Deeper connective tissue layer.

• Hypodermis: Subcutaneous adipose tissue; anchors skin and provides insulation.

Cells of the Epidermis

• Keratinocytes: Produce keratin protein for protection.

• Melanocytes: Produce melanin pigment for UV protection.

• Dendritic (Langerhans) cells: Immune defense.

• Tactile (Merkel) cells: Sensory receptors.

Layers of the Epidermis (Deep to Superficial)

• Stratum basale (mitosis)

• Stratum spinosum (keratinocytes, dendritic cells)

• Stratum granulosum (keratinization)

• Stratum lucidum (thick skin only)

• Stratum corneum (dead cells, barrier)

Mnemonic: Come Let's Get Sun Burned (Corneum, Lucidum, Granulosum, Spinosum, Basale)

Dermis

• Papillary layer: Areolar connective tissue, dermal papillae with capillaries and receptors.

• Reticular layer: Dense irregular connective tissue, provides strength and elasticity.

Skin Color

• Melanin: Brown-black pigment, UV protection.

• Carotene: Yellow-orange pigment from diet.

• Hemoglobin: Red hue, especially in fair skin.

Functions of the Skin

• Protection

• Body temperature regulation

• Cutaneous sensation

• Metabolic functions (e.g., vitamin D synthesis)

• Blood reservoir

• Excretion

Hair

• Structure: Shaft (above skin), root (within skin).

• Layers: Medulla (inner), cortex (middle), cuticle (outer).

• Follicle: Extends into dermis/hypodermis; bulb, root hair plexus (sensation), matrix (growth).

• Arrector pili muscle: Causes "goosebumps" by contracting.

• Types: Vellus (fine), terminal (coarse).

• Growth: Influenced by nutrition and hormones.

• Baldness: Alopecia, male pattern baldness.

Glands

• Eccrine (merocrine) sweat glands: Watery sweat for thermoregulation.

• Apocrine sweat glands: Axillary/genital regions; protein/lipid-rich secretion.

• Ceruminous glands: Produce earwax.

• Mammary glands: Produce milk.

• Sebaceous (oil) glands: Secrete sebum; lubricates and is bactericidal.

Skin Disorders

• Skin cancers:

  • Basal cell carcinoma: Most common, least malignant.

  • Squamous cell carcinoma: Faster-growing, may metastasize.

  • Melanoma: Most dangerous; use ABCD rule (Asymmetry, Border, Color, Diameter).

• Burns:

  • Risks: Dehydration (immediate), infection.

  • Degrees:

    • 1st degree: Epidermis only, redness, pain.

    • 2nd degree: Epidermis + dermis, blisters.

    • 3rd degree: Full-thickness, no pain (nerve damage), requires grafting.

  • Rule of nines: Method to estimate body surface area affected by burns.

Sample Table: Types of Tissues and Key Properties

Key Equations

• Resting Membrane Potential (Nernst Equation):

• Osmosis (Water Potential):

Additional info: Equations provided for academic completeness; not all may have been explicitly mentioned in the original materials.