Compartmentalization in Cells and Tissues: Structure and Function (Chapter 3 Study Notes)

Functional Compartments of the Body

Major Body Cavities

The human body is organized into distinct anatomical compartments that serve specialized functions and help protect internal organs.

• Cranial cavity: Contains the brain, protected by the skull.

• Thoracic cavity: Houses the heart and lungs, protected by the rib cage.

• Abdominopelvic cavity: Contains digestive, urinary, and reproductive organs.

Fluid-Filled Compartments

In addition to anatomical cavities, the body contains several fluid-filled compartments essential for physiological processes.

• Circulatory system: Blood vessels and heart, transporting blood throughout the body.

• Cerebrospinal fluid (CSF): Surrounds the brain and spinal cord, providing cushioning and nutrient transport.

• Pleural and pericardial sacs: Enclose the lungs and heart, respectively, reducing friction during organ movement.

Biological Membranes

Structure and Function of Cell Membranes

Biological membranes are critical for maintaining cellular integrity and compartmentalization.

• Physical isolation: Separates the cell from its external environment.

• Regulation of exchange: Controls movement of substances into and out of the cell.

• Communication: Facilitates signaling between the cell and its environment.

• Structural support: Maintains cell shape and anchors cytoskeletal elements.

Membranes are primarily composed of lipids (phospholipids, cholesterol) and proteins (integral, peripheral).

Fluid Mosaic Model

The fluid mosaic model describes the dynamic nature of biological membranes, where proteins and lipids move laterally within the bilayer.

• Phospholipid bilayer: Provides a semi-permeable barrier.

• Membrane proteins: Include integral (span the membrane) and peripheral (attached to the surface) proteins.

• Lipid rafts: Microdomains rich in sphingolipids and cholesterol, involved in cell signaling.

Intracellular Compartments

Cellular Organization

Cells are divided into specialized compartments to optimize biochemical processes.

• Cytoplasm: Includes cytosol, organelles, and inclusions.

• Cytosol: Fluid portion containing dissolved ions and molecules.

• Inclusions: Non-membranous structures for nutrient storage (e.g., glycogen granules, lipid droplets).

• Cytoskeleton: Network of protein fibers providing structural support and facilitating movement.

• Organelles: Membrane-bound structures (e.g., nucleus, mitochondria, lysosomes).

Cytoskeleton

The cytoskeleton is composed of protein fibers of varying sizes, each with distinct functions.

• Microfilaments (actin): Involved in cell movement and shape.

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

• Microtubules (tubulin): Facilitate intracellular transport and cell division.

Associated motor proteins include myosin, kinesin, and dynein.

Organelles

• Nucleus: Control center of the cell, contains DNA and nucleolus (site of ribosome synthesis).

• Mitochondria: Powerhouse of the cell, site of ATP production.

• Endoplasmic reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.

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

• Lysosomes and peroxisomes: Involved in digestion and detoxification.

Tissues of the Body

Histology and Extracellular Matrix

Histology is the study of tissue structure and function. Tissues are composed of cells and an extracellular matrix (ECM).

• Proteoglycans and glycoproteins: Provide structural and functional support.

• Insoluble protein fibers: Collagen, fibronectin, laminin.

Cell Junctions

Cell junctions hold cells together and facilitate communication.

• Gap junctions: Allow direct communication between cells.

• Tight junctions: Prevent passage of substances between cells.

• Anchoring junctions: Provide mechanical stability (adherens junctions, desmosomes, hemidesmosomes).

Major Cell Adhesion Molecules (CAMs)

Name	Examples
Cadherins	Cell-cell junctions such as adherens junctions and desmosomes. Calcium-dependent.
Integrins	Primarily found in cell-matrix junctions. Also function in cell signaling.
Immunoglobulin superfamily CAMs	NCAMs (nerve-cell adhesion molecules). Responsible for nerve cell growth during nervous system development.
Selectins	Temporary cell-cell adhesions.

Types of Tissues

• Epithelial tissue: Covers surfaces, lines cavities, regulates exchange.

• Connective tissue: Provides support, connects other tissues.

• Muscle tissue: Enables movement through contraction.

• Nervous tissue: Transmits electrical signals.

Epithelial Tissue

Structure and Classification

Epithelia consist of one or more layers of cells separated from underlying tissue by a basal lamina. They are classified by layers and cell shapes:

• Layers: Simple (one layer) or stratified (multiple layers).

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

Functional Categories of Epithelia

• Exchange epithelium: Thin, flattened cells for rapid gas exchange (e.g., alveoli in lungs, endothelium in blood vessels).

• Transporting epithelium: Cuboidal or columnar cells with membrane modifications for selective transport. Apical membrane faces lumen; basolateral membrane faces ECM. Many mitochondria for active transport.

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

• Protective epithelium: Multiple layers to prevent exchange and protect against mechanical/chemical stress (e.g., skin).

• Secretory epithelium: Produces and releases substances. Includes exocrine glands (serous and mucous secretions, goblet cells) and endocrine glands (hormones).

Connective Tissue

Structure and Types

Connective tissue consists of cells embedded in an extracellular matrix (ECM) composed of ground substance and fibers.

• Fixed cells: Maintenance, repair, energy storage (e.g., fibroblasts).

• Mobile cells: Defense (e.g., immune cells).

• Matrix fibers: Collagen, elastin, fibrillin, fibronectin.

Types of Connective Tissue

• Loose connective tissue: Elastic tissues under skin, support for glands.

• Dense connective tissue: Regular (tendons, ligaments) and irregular types for strength and flexibility.

• Supporting connective tissue: Cartilage (nose, ears, trachea; lacks blood supply), bone (calcified).

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

• Blood: Plasma, blood cells, and cell fragments.

Muscle and Neural Tissue

Muscle Tissue

Muscle tissue is specialized for contraction, producing force and movement.

• Skeletal muscle: Voluntary movement.

• Cardiac muscle: Heart contraction.

• Smooth muscle: Involuntary movement in organs.

Neural Tissue

• Neurons: Transmit electrical signals.

• Glial cells (neuroglia): Provide support and protection for neurons.

Tissue Remodeling

Cell Death and Renewal

Tissues undergo constant remodeling through cell death and renewal.

• Necrosis: Cell death due to injury.

• Apoptosis: Programmed cell death (cell suicide).

• Stem cells: Capable of mitosis and differentiation.

• Plasticity: Ability of stem cells to differentiate into multiple cell types.

Stem cell types:

• Totipotent: Can become any cell type, including extraembryonic tissues.

• Pluripotent: Can become any cell type within the body.

• Multipotent: Can become a limited range of cell types.

Organs: The Skin

Layers of the Skin

The skin is a complex organ composed of multiple layers, providing protection and regulating exchange with the environment.

• Epidermis: Outer layer, primarily epithelial cells.

• Dermis: Middle layer, connective tissue, blood vessels, nerves.

• Hypodermis: Deepest layer, adipose tissue.

Summary

• Functional compartments of the body

• Biological membranes

• Intracellular compartments

• Tissues of the body

• Tissue remodeling

• Organs