Cell Membrane Structure and Function

Surrounds the cell, forming an envelope or cover
Invisible by light microscopy (LM) due to its thinness (8-10 nm)
Can be stained by silver (Ag) or periodic acid-Schiff (PAS)
Appears as three parallel lines under electron microscopy (EM), two dark layers separated by a light layer (trilamellar membrane)
Outer covering rich in carbohydrates called the cell coat
Composed of lipids, proteins, and carbohydrates
- Lipids: Phospholipids and cholesterol
- Proteins: Intrinsic (integral) and extrinsic
  - Intrinsic: Present as small particles or large globules extending across the membrane, acting as pathways for water-soluble substances
  - Extrinsic: Small molecules loosely attached to both surfaces, forming a non-continuous layer
- Carbohydrates: Oligosaccharides attached to protein (glycoprotein) or lipid (glycolipid), forming the cell coat or glycocalyx
Functions:
- Maintaining the cell's internal composition
- Cell coat: Cell adhesion, recognition, protection, and immunity
- Allowing passage of substances through the membrane:
  - Passive diffusion (e.g., gases and water)
  - Facilitated diffusion (e.g., glucose)
  - Active transport (e.g., sodium pumps)
  - Selective permeability (through receptors)
  - Bulk transport (vesicular transport): Macromolecules enter/leave through vesicle formation or fusion with the membrane
    - Exocytosis: Substances leave the cell
    - Endocytosis: Substances enter the cell
    - Phagocytosis: Solid substances
    - Pinocytosis: Fluid

Membranous cell organelle
Powerhouse of the cell, responsible for cell respiration and energy production
Number varies with cell activity (e.g., liver cells contain 1000-2000/cell)
Present in all cells except red blood cells (RBCs)
Site: At the location of highest activity (e.g., apical part in ciliated cells)
LM: Appear as granules, rods, or filaments, staining dark blue with iron hematoxylin and green with Janus green stain
EM: Vesicle-shaped, rounded or oval, covered with a double membrane separated by an intermembranous space
- Outer membrane: Smooth
- Inner membrane: Forms folds, shelves, or cristae
Mitochondrial matrix: Fills the internal cavity, containing lipids, proteins, carbohydrates, Ca, Mg, DNA, and RNA
Oxidative enzymes attached by heads to the cristae
Functions:
- Energy production in the form of ATP
- Protein synthesis and division due to the presence of DNA and RNA

Membranous organelle formed of flattened communicating vesicles and tubules networking within the cytoplasm
Classified into:
- Rough (granular) ER: Contains ribosomes
- Smooth (agranular) ER: Lacks ribosomes

Merocrine: Most common, secretion released by exocytosis, no changes in the cell (e.g., pancreas)
Apocrine: Secretion released surrounded by a part of the cytoplasm and cell membrane, usually at the cell's apex (e.g., mammary gland and some sweat glands)
Holocrine: Secretion accumulates within the cell, cell swells and ruptures, releasing secretions with cell components (e.g., sebaceous glands)

Simple: Single non-branching duct
Simple branched: Single non-branching duct with a branched secretory portion
Compound: Branching duct system

Tubular:
- Simple tubular (e.g., intestinal glands)
- Simple branched tubular (e.g., fundic glands of the stomach)
- Simple coiled tubular (e.g., sweat glands)
- Compound tubular (e.g., kidney, liver)
Alveolar:
- Simple alveolar (e.g., sebaceous glands)
- Simple branched alveolar (e.g., sebaceous glands)
- Compound alveolar (e.g., mammary gland)
Tubulo-alveolar:
- Simple tubulo-alveolar: Not present humans
- Simple branched tubulo-alveolar (e.g., lingual, labial glands - minor salivary glands)
- Compound tubulo-alveolar (e.g., major salivary glands - parotid, pancreas)

Specialized epithelium functioning as a receptor
Contains three types of cells:
- Hair cells: Receive sensation
- Supporting cells: Provide support
- Basal cells: Act as stem cells for regeneration
Examples:
- Taste buds: Located in the tongue, responsible for taste sensation
- Organ of Corti: Found in the ear, responsible for hearing

Length: 15-20 cm
Parts:
- Prostatic: 3 cm, surrounded by the prostate gland, receives ducts from prostate and ejaculatory ducts
  - Upper part enclosed by the internal urethral sphincter (involuntary muscle, autonomic nerve innervation)
- Membranous: 2cm, passes through the perineum, surrounded by the external urethral sphincter (voluntary muscle, somatic nerve innervation)
- Penile: 10-15 cm, passes within the corpus spongiosum of the penis

Scrotum:
- Skin pocket containing the testes
- Extension of the anterior abdominal wall
- Suspends the testes to regulate temperature, lacks fat
Testes:
- Dimensions: 4 x 2 x 1 cm
- Located in the scrotum
- Divided into approximately 200 compartments, each containing about 2 seminiferous tubules
- Seminiferous tubules are about 2 feet (60 cm) long, responsible for sperm formation (takes about 2 months)
Epididymis:
- 6 meter long, single coiled duct
- Located in the scrotum
- Sperms pass through in about 2 weeks, developing motility
- Coiled shape, divided into head, body, and tail
Vas Deferens:
- 45 cm long duct
- Thick muscular wall, palpable during examination
- Continuation of the tail of the epididymis
- Found in the scrotum, spermatic cord, inguinal canal, and pelvis
- Ends by forming the ampulla (dilatation) for sperm storage
- The ampulla unites with the seminal vesicle to form the ejaculatory duct
Spermatic Cord:
- Sheath formed by extensions of the anterior abdominal wall
- Encloses the vas deferens, testicular artery and vein, lymphatic vessels, and nerves