Cell Theory and Cellular Structure

Cell Theory

The cell is the fundamental structural and functional unit of all living organisms. The activities of an organism depend on the individual and collective activities of its cells. Structure and function are closely related, and all cells arise from preexisting cells.

• Cell: The smallest unit of life, capable of performing all vital physiological functions.

• Continuity of Life: Cells can only arise from other cells, ensuring the transmission of genetic information.

• Complementarity: The structure of a cell determines its function, and vice versa.

Generalized Cell Structure

Human cells exhibit diversity in size, shape, and organelle composition, which determines their specialized functions. Despite this diversity, all human cells share three basic components:

• Plasma Membrane: The outer boundary that regulates the movement of substances in and out of the cell.

• Cytoplasm: The intracellular fluid containing organelles.

• Nucleus: The control center containing DNA, responsible for regulating gene expression and cell activities.

Common Organelles: Mitochondria, rough and smooth endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes, ribosomes, cytoskeleton, centrosome, and centrioles.

Extracellular Environment

Extracellular Matrix and Fluids

The extracellular environment supports and interacts with cells, providing nutrients, removing waste, and facilitating communication.

• Extracellular Fluids (ECF):

  • Interstitial Fluid: Surrounds and bathes cells.

  • Blood Plasma: The fluid component of blood within vessels.

  • Cerebrospinal Fluid: Surrounds the brain and spinal cord.

• Cellular Secretions: Substances such as mucus and saliva that aid in digestion and lubrication.

• Extracellular Matrix: A network of proteins and polysaccharides that acts as a glue to hold cells together and provides structural support.

Plasma Membrane Structure and Function

Membrane Lipids

The plasma membrane is a selectively permeable barrier composed primarily of lipids and proteins, described by the fluid mosaic model.

• Phospholipids (75%):

  • Hydrophilic heads: Water-loving, face outward toward aqueous environments.

  • Hydrophobic tails: Water-fearing, face inward, away from water.

• Glycolipids (5%): Lipids with attached sugars, found on the outer surface, involved in cell recognition.

• Cholesterol (20%): Stabilizes membrane fluidity and integrity.

Membrane Proteins

• Integral Proteins: Firmly embedded in the membrane, often spanning the bilayer (transmembrane). Functions include transport, acting as enzymes, and serving as receptors.

• Peripheral Proteins: Loosely attached to integral proteins. Functions include acting as enzymes, motor proteins, and facilitating cell-to-cell connections.

Glycocalyx and Cell Recognition

• Glycocalyx: A carbohydrate-rich area on the cell surface, formed by glycoproteins and glycolipids. It provides a unique biological marker for cell recognition and helps the immune system distinguish self from non-self.

• Clinical Relevance: In cancer cells, the glycocalyx composition can change rapidly, aiding in immune evasion.

Cell Junctions

Types of Cell Junctions

• Desmosomes: Anchoring junctions with interlocking proteins that provide mechanical stability, especially in tissues under stress (e.g., skin, heart).

• Tight Junctions: Proteins fuse adjacent cells, forming an impermeable barrier to prevent leakage between cells (e.g., intestines).

• Gap Junctions: Transmembrane proteins form channels (connexons) allowing ions and small molecules to pass between cells, enabling electrical and metabolic coupling (e.g., cardiac and smooth muscle).

Membrane Transport

Passive Transport

• Simple Diffusion: Movement of molecules from high to low concentration without energy input.

• Facilitated Diffusion: Movement of larger or polar molecules via carrier or channel proteins (e.g., glucose, amino acids, ions).

• Osmosis: Movement of water across a selectively permeable membrane from low solute (high water) to high solute (low water) concentration.

Osmosis and Tonicity

• Isotonic: Same solute concentration inside and outside the cell; no net water movement.

• Hypertonic: Higher solute concentration outside; water moves out, cell shrinks (crenation).

• Hypotonic: Lower solute concentration outside; water moves in, cell swells and may burst (lysis).

Active Transport

• Primary Active Transport: Direct use of ATP to power transport proteins (e.g., sodium-potassium pump).

• Sodium-Potassium Pump: Pumps 3 Na+ out and 2 K+ into the cell, maintaining electrochemical gradients essential for nerve and muscle function. Equation:

• Secondary Active Transport: Uses energy stored in ion gradients created by primary transport to move multiple substances via cotransport proteins.

• Vesicular Transport: Uses vesicles to move large particles and fluids across membranes (e.g., endocytosis, exocytosis, phagocytosis, pinocytosis, receptor-mediated endocytosis, transcytosis).

Organelles and Cellular Functions

• Mitochondria: Site of ATP production via aerobic respiration; inner folds called cristae increase surface area.

• Rough Endoplasmic Reticulum (ER): Studded with ribosomes; synthesizes proteins and membranes.

• Smooth ER: Lipid metabolism, detoxification, and calcium storage.

• Golgi Apparatus: Modifies, sorts, and packages proteins for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes; break down bacteria, debris, and old organelles (autolysis).

• Peroxisomes: Detoxify harmful substances using oxidase and catalase; break down fatty acids and free radicals.

• Cytoskeleton: Network of rods for structural support and movement (microfilaments, intermediate filaments, microtubules).

• Cilia and Flagella: Cilia move substances across cell surfaces; flagella propel entire cells (e.g., sperm).

• Microvilli: Increase surface area for absorption (e.g., intestines).

• Nucleus: Largest organelle; contains DNA and nucleolus (site of ribosome production); surrounded by a double membrane with pores.

Tissues: Types and Functions

Overview of Tissues

Tissues are groups of cells similar in structure that perform a common or related function. The study of tissues is called histology.

• Epithelial Tissue: Covers surfaces and lines cavities; functions in protection, absorption, filtration, secretion, and sensory reception.

• Connective Tissue: Supports, binds, protects, insulates, stores fuel, and transports substances.

• Muscle Tissue: Responsible for movement.

• Nervous Tissue: Functions in control and communication.

Epithelial Tissue

• Characteristics: Polarity (apical and basal surfaces), specialized contacts (tight junctions, desmosomes), supported by connective tissue, avascular but innervated, high regeneration capacity.

• Classification by Layers:

  • Simple Epithelium: Single layer; absorption, secretion, filtration.

  • Stratified Epithelium: Multiple layers; protection.

  • Pseudostratified Epithelium: Appears layered but all cells touch the basement membrane.

  • Transitional Epithelium: Specialized for stretching (e.g., bladder).

• Classification by Shape:

  • Squamous: Flat and scale-like.

  • Cuboidal: Cube-shaped.

  • Columnar: Tall and column-like.

• Glandular Epithelium:

  • Unicellular Glands: Goblet cells produce mucin.

  • Multicellular Glands: Composed of a duct and secretory unit; classified as simple or compound, tubular, alveolar, or tubuloalveolar.

  • Modes of Secretion: Merocrine (exocytosis), holocrine (rupture), apocrine (apex ruptures; controversial in humans).

Connective Tissue

• Functions: Binding, support, protection, insulation, fuel storage, and transport.

• Main Classes: Connective tissue proper, cartilage, bone, blood.

• Components:

  • Cells: Blast cells (immature, secrete matrix), cyte cells (mature, maintain matrix), fat cells, white blood cells, mast cells, macrophages.

  • Matrix: Ground substance (interstitial fluid, cell adhesion proteins, proteoglycans) and fibers (collagen, elastic, reticular).

• Types of Fibers:

  • Collagen: Strongest, provides tensile strength.

  • Elastic: Stretch and recoil.

  • Reticular: Short, fine, branched; supportive mesh.

Connective Tissue Proper

• Loose Connective Tissue: Areolar, adipose, reticular.

• Dense Connective Tissue: Dense regular (tendons, ligaments), dense irregular (dermis, joint coverings), elastic (walls of large arteries).

Cartilage

• General Features: Matrix secreted by chondroblasts and maintained by chondrocytes; avascular, resilient, compressible.

• Types: Hyaline (glassy, ribs, nose), elastic (external ear), fibrocartilage (intervertebral discs).

Bone and Blood

• Bone (Osseous Tissue): Supports and protects, stores calcium, produces blood cells; highly vascularized.

• Blood: Atypical connective tissue; transports nutrients, wastes, and gases.

Muscle Tissue

• Skeletal Muscle: Voluntary, striated, multinucleated, long cylindrical cells.

• Cardiac Muscle: Involuntary, striated, one nucleus per cell, branched cells, intercalated discs.

• Smooth Muscle: Involuntary, non-striated, spindle-shaped cells, one nucleus per cell.

Nervous Tissue

• Main Component: Brain, spinal cord, nerves.

• Neurons: Specialized for generating and conducting nerve impulses.

• Glial Cells: Support, insulate, and protect neurons.

Membranes and Tissue Repair

• Regeneration: Replacement of destroyed tissue with the same kind of tissue; original function restored.

• Fibrosis: Replacement with scar tissue; function may be lost.

• Regenerative Capacity: High in epithelial, bone, areolar, dense irregular connective tissue, and blood; moderate in smooth muscle and dense regular connective tissue; poor/none in cardiac muscle and nervous tissue.

The Integumentary System

Overview and Functions

The integumentary system protects the body and maintains homeostasis. It includes the skin, hair, nails, sweat glands, and sebaceous glands.

• Skin: Largest organ; barrier and sensory interface.

• Hair: Provides protection and sensory input.

• Nails: Protect fingertips and aid in fine tasks.

• Sweat Glands: Regulate temperature and excrete waste.

• Sebaceous Glands: Secrete sebum to lubricate and protect skin and hair.

Skin Structure

• Epidermis: Superficial, epithelial tissue; avascular.

• Dermis: Deeper, fibrous connective tissue; vascular.

• Hypodermis (Subcutaneous Layer): Not part of skin but shares functions; mostly adipose tissue for shock absorption and insulation; anchors skin to muscles.

Layers of the Epidermis (from deep to superficial)

1. Stratum Basale: Mitotic stem cells, melanocytes.

2. Stratum Spinosum: Desmosomes, melanin, dendritic cells.

3. Stratum Granulosum: Keratinization begins, lamellar granules.

4. Stratum Lucidum: Only in thick skin (palms, soles); dead keratinocytes.

5. Stratum Corneum: Dead, keratinized cells; barrier function.

Mnemonic: Come Let's Get Sun Burned

Cells of the Epidermis

• Keratinocytes: Produce keratin for protection.

• Melanocytes: Produce melanin, protect against UV radiation.

• Dendritic (Langerhans) Cells: Immune defense.

• Tactile (Merkel) Cells: Sensory receptors for touch.

Dermis

• Cells: Fibroblasts, macrophages, mast cells, white blood cells.

• Structures: Nerves, blood vessels, hair follicles, oil and sweat glands.

• Layers:

  • Papillary: Areolar connective tissue, dermal papillae, tactile corpuscles.

  • Reticular: Dense irregular connective tissue, cleavage lines, important for surgical incisions.

Skin Color

• Melanin: Made by melanocytes; protects DNA from UV damage.

• Carotene: Yellow-orange pigment; vitamin A precursor.

• Hemoglobin: Pink hue in fair skin.

• Clinical Signs: Cyanosis (blue), pallor (pale), erythema (red), jaundice (yellow), hyperpigmentation.

Hair and Nails

• Hair: Vellus (fine, body hair), terminal (coarse, scalp, eyebrows). Growth influenced by hormones and nutrition. Hair cycle includes active and regressive phases.

• Nails: Protect fingertips, aid in fine tasks.

Glands of the Skin

• Eccrine (Merocrine) Sweat Glands: Most numerous; thermoregulation; sweat is 99% water, salts, vitamin C, antibodies, dermcidin.

• Apocrine Sweat Glands: Axillary/genital areas; milky sweat with proteins and lipids; begin at puberty; may cause body odor.

• Modified Apocrine Glands: Ceruminous (earwax), mammary (milk).

• Sebaceous (Oil) Glands: Secrete sebum; lubricate and protect skin and hair; associated with hair follicles; active at puberty; can cause acne if blocked.

Functions of the Integumentary System

• Protection: Physical, chemical, and biological barriers (keratin, acid mantle, dendritic cells, macrophages).

• Body Temperature Regulation: Insensible and sensible perspiration; vasodilation and vasoconstriction.

• Cutaneous Sensation: Exteroreceptors, free nerve endings.

• Metabolic Functions: Vitamin D synthesis, hormone activation, collagenase production.

• Blood Reservoir: Holds up to 5% of blood volume.

• Excretion: Nitrogenous wastes, water, salts.

Skin Disorders and Clinical Considerations

• Skin Cancer:

  • Melanoma: Most dangerous; ABCD rule (Asymmetry, Border irregularity, Color variation, Diameter >6 mm).

  • Basal Cell Carcinoma: Most common; arises from stratum basale.

  • Squamous Cell Carcinoma: Arises from keratinocytes of stratum spinosum.

• Burns: Tissue damage from heat, chemicals, or radiation; risk of dehydration and infection. Critical burns: >25% body with second-degree, >10% with third-degree, or burns to face, hands, feet.

Table: Comparison of Epithelial Tissue Types

Additional info: Some content was expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.