Compartmentation: Cells and Tissues – Human Physiology Study Notes

Compartmentation: Cells and Tissues

Table of Contents

• Functional Compartments of the Body

• Biological Membranes

• Intracellular Compartments

• Tissues of the Body

• Tissue Remodeling

• Organs

Functional Compartments of the Body

Major Body Cavities

The human body is organized into distinct anatomical and functional compartments, each serving specialized roles in protection, support, and physiological regulation.

• Cranial cavity: Contains the brain.

• Thoracic cavity: Contains the heart and lungs.

• Abdominopelvic cavity: Contains the stomach, intestines, spleen, and reproductive organs (not kidneys).

• These cavities are separated by bones and tissues and are lined with tissue membranes.

Additional Fluid-Filled Compartments

• Circulatory system

• Eyes

• Cerebrospinal fluid (CSF)

• Pleural and pericardial sacs

Lumens and External Environment

Hollow organs such as the heart, lungs, blood vessels, and intestines create additional compartments called lumens.

• The lumen is the interior of any hollow organ and may be filled with air or fluid.

• For some organs (e.g., digestive tract), the lumen is considered an extension of the external environment.

• Example: The digestive tract is like a hole through a bead, where the inside is technically outside the body until absorption occurs.

Body Fluid Compartments

Functionally, the body is divided into three main fluid compartments:

• Extracellular fluid (ECF): Fluid outside the cell, subdivided into:

  • Plasma: Fluid portion of the blood.

  • Interstitial fluid: Fluid surrounding or between cells.

• Intracellular fluid (ICF): Fluid within the cell.

Biological Membranes

Structure and Function

The cell membrane (plasma membrane) separates the cell from its environment and is essential for maintaining homeostasis.

• Physical isolation: Acts as a barrier between ICF and ECF.

• Regulation of exchange: Controls movement of nutrients and waste.

• Communication: Membrane proteins detect and respond to environmental changes.

• Structural support: Provides scaffolding for the cell and connects to the extracellular matrix.

Fluid Mosaic Model

The membrane is described by the fluid mosaic model:

• Fluid: Components (phospholipids, proteins) are mobile and can move laterally.

• Mosaic: Diverse mix of lipids, proteins, and carbohydrates embedded in the bilayer.

Membrane Components

• Lipids: Phospholipids (major component), sphingolipids, and cholesterol.

• Proteins: Integral (transmembrane and lipid-anchored) and peripheral proteins.

• Carbohydrates: Glycoproteins and glycolipids, found only on the external surface, forming the glycocalyx.

Membrane Lipids and Structures

• Phospholipid bilayer: Hydrophilic heads face outward, hydrophobic tails inward.

• Micelles: Small droplets with a single layer of phospholipids, important in fat digestion.

• Liposomes: Spheres with bilayer walls and aqueous core, used for drug delivery.

Membrane Proteins

• Integral proteins: Tightly bound, include transmembrane and lipid-anchored proteins.

• Peripheral proteins: Loosely bound, can be removed without disrupting the membrane.

• Functions: Signal transduction, transport, enzymatic activity, cell recognition, and structural support.

Membrane Carbohydrates

• Attach to both lipids (glycolipids) and proteins (glycoproteins).

• Form the glycocalyx, a protective layer on the cell surface.

Intracellular Compartments

Cell Differentiation and Structure

Cells specialize through differentiation, leading to unique shapes and functions (e.g., red blood cells vs. muscle cells).

• Cell membrane: Boundary of the cell.

• Cytoplasm: All material inside the cell except the nucleus, including:

  • Cytosol: Intracellular fluid (ICF).

  • Inclusions: Non-membranous organelles (e.g., ribosomes, glycogen granules, lipid droplets).

  • Cytoskeleton: Protein fibers for structure and movement.

  • Organelles: Membrane-bound compartments (e.g., mitochondria, ER, Golgi apparatus).

Cytoskeleton

• Provides cell shape, internal organization, and facilitates movement.

• Composed of three main types of protein fibers:

  • Microfilaments (actin): Thinnest, involved in contraction and movement.

  • Intermediate filaments (keratin, neurofilament): Provide mechanical strength.

  • Microtubules (tubulin): Thickest, involved in organelle movement and cell division.

Type of Protein	Functions
Actin (globular)	Cytoskeleton; contraction
Keratin, neurofilament	Cytoskeleton, mechanical strength
Tubulin (globular)	Movement of organelles, cell division

Motor Proteins

• Convert ATP into movement along cytoskeletal fibers.

• Myosins: Bind with actin for muscle contraction.

• Kinesins and dyneins: Move vesicles along microtubules; dyneins also move cilia and flagella.

Organelles

• Mitochondria: Site of most ATP production.

• Endoplasmic reticulum (ER):

  • Rough ER (RER): Protein synthesis (with ribosomes).

  • Smooth ER (SER): Synthesis of fatty acids, steroids, and lipids.

• Golgi apparatus: Modifies, sorts, and packages proteins.

• Lysosomes: Break down bacteria and old organelles.

• Peroxisomes: Break down fatty acids and toxic substances.

Nucleus

• Control center containing DNA.

• Bound by a double nuclear envelope with pores for communication with cytosol.

• Contains chromatin (DNA + proteins) and nucleoli (site of ribosomal RNA synthesis).

Tissues of the Body

Histology and Tissue Types

Histology is the study of tissue structure and function. Tissues are classified by cell shape, arrangement, connections, and extracellular material.

• Epithelial tissue

• Connective tissue

• Muscle tissue

• Nerve tissue

Extracellular Matrix (ECM)

• Material synthesized and secreted by tissue cells.

• Composed of proteoglycans (glycoproteins) and insoluble protein fibers (collagen, fibronectin, laminin).

Cell Junctions

• Gap junctions: Communicating junctions allowing chemical and electrical signals to pass (connexins).

• Tight junctions: Occluding junctions restricting movement between cells (claudins, occludins).

• Anchoring junctions: Attach cells to each other (cadherins) or to the ECM (integrins).

Name	Examples
Cadherins	Cell-cell junctions (adherens, desmosomes), calcium-dependent
Integrins	Cell-matrix junctions, also function in cell signaling
Immunoglobulin superfamily CAMs	NCAMs, nerve cell growth
Selectins	Temporary cell-cell adhesions

Types of Tissues

Epithelial Tissue

• Protects internal environment and regulates exchange.

• Covers exposed surfaces (e.g., skin, GI lining).

• Classified by layers (simple, stratified) and cell shape (squamous, cuboidal, columnar).

• Separated from underlying tissue by the basal lamina (basement membrane).

Functional Categories of Epithelia

• Exchange epithelium: Thin, flat cells for gas exchange (e.g., blood vessels, lungs).

• Transporting epithelium: Selectively moves non-gaseous materials (e.g., GI tract, kidneys). Characterized by microvilli, tight junctions, and many mitochondria.

• Ciliated epithelium: Moves fluid and particles (e.g., respiratory tract, female reproductive tract).

• Protective epithelium: Stratified, may secrete keratin, protects against mechanical and chemical stress (e.g., skin, mouth).

• Secretory epithelium: Produces and secretes substances; forms exocrine (via ducts) and endocrine (hormones into blood) glands.

Connective Tissue

• Provides structural support, protection, and insulation.

• Composed of cells (fixed and mobile) and ECM (proteoglycans, water, protein fibers).

• Types of matrix proteins:

  • Collagen: Most abundant, provides strength.

  • Elastin: Stretch and recoil.

  • Fibrillin: Forms elastic fibers with elastin.

  • Fibronectin: Connects ECM, important in healing.

Types of Connective Tissue

• Loose connective tissue: Elastic, supports small glands.

• Dense connective tissue: Provides strength (tendons, ligaments).

• Supporting connective tissue: Cartilage (flexible, no blood supply), bone (calcified).

• Adipose tissue: Fat storage (white and brown fat).

• Blood: Fluid matrix (plasma) with cells and cell fragments.

Muscle and Neural Tissue

• Muscle tissue: Contractile, responsible for movement (cardiac, smooth, skeletal).

• Neural tissue: Generates and propagates electrical signals (neurons and glial cells).

Tissue Remodeling

• Necrosis: Cell death from injury, causes inflammation.

• Apoptosis: Programmed cell death, no inflammation (e.g., intestinal epithelium replaced every 2-5 days).

• Stem cells: Undifferentiated cells that can divide and specialize.

  • Totipotent: Can become any cell type.

  • Pluripotent: Can become many, but not all, cell types.

  • Multipotent: Can become a limited range of cells within a tissue.

Organs

• Organs are groups of tissues with related functions.

• Example: Skin incorporates all four tissue types and serves multiple functions (protection, sensation, temperature regulation).

Summary Table: Characteristics of the Four Tissue Types

Tissue Type	Main Function	Key Features
Epithelial	Protection, exchange, secretion	Cell layers, tight junctions, basal lamina
Connective	Support, binding, transport	ECM, protein fibers, variable cell types
Muscle	Movement	Contractile proteins, excitable
Nerve	Communication	Neurons, glial cells, excitable