Compartmentation: Cells and Tissues – Study Notes for Animal Physiology (BIO 360)

Compartmentation in Animal Physiology

Introduction to Compartmentation

Compartmentation refers to the organization of the body into distinct spaces separated by membranes. This allows for specialized environments and processes within cells and tissues, essential for physiological function.

• Compartmentation enables separation of biochemical processes that may otherwise conflict.

• Examples include the separation of the cell interior from the external environment and the division of organelles within cells.

• The lumen of hollow organs (e.g., intestines, veins, arteries) is considered part of the external environment.

Cell Membranes

Structure and Composition of Cell Membranes

Cell membranes, also known as the plasmalemma or phospholipid membrane, are critical for maintaining cellular integrity and regulating interactions with the environment.

• Phospholipid Bilayer: Composed of two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails inward.

• Membrane Components: Includes lipids (phospholipids, cholesterol), proteins (integral and peripheral), and carbohydrates.

• Fluid Mosaic Model: Proposed by Singer and Nicolson (1972), describes the membrane as a dynamic structure with proteins and lipids moving within the bilayer.

Functions of Cell Membranes

• Barrier: Isolates the cell from its environment.

• Selective Permeability: Regulates exchange of substances between intracellular and extracellular fluids.

• Cell Communication: Facilitates signaling between cells.

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

Phospholipids and Cholesterol in Membranes

• Phospholipids: Each molecule consists of two fatty acid chains, a glycerol backbone, and a phosphate group.

• Cholesterol: Present at approximately one molecule per phospholipid, cholesterol stabilizes the membrane, reduces deformability, and decreases permeability to small water-soluble molecules.

• Temperature Effects: At higher temperatures, the bilayer becomes more fluid, allowing increased movement of membrane components.

Membrane Proteins

• Integral (Transmembrane) Proteins: Span the membrane and function as channels, transporters, or receptors.

• Peripheral Proteins: Attached to the membrane surface, often involved in signaling or structural support.

• Glycoproteins: Proteins with carbohydrate groups, important for cell recognition and adhesion.

Cell Organelles and Protein Synthesis

Major Organelles

• Nucleus: Surrounded by a double membrane (nuclear envelope) with pores for communication with the cytoplasm. Contains DNA and nucleoli (sites of ribosome synthesis).

• Endoplasmic Reticulum (ER): Network of membranes involved in protein and lipid synthesis. Rough ER has ribosomes; smooth ER does not.

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

• Mitochondria: Spherical or oval organelles responsible for ATP production via cellular respiration.

Protein Synthesis Overview

• Transcription: DNA is transcribed into mRNA in the nucleus.

• Translation: mRNA is translated into protein by ribosomes in the cytoplasm or on the rough ER.

Cytoskeleton

Types and Functions of Cytoskeletal Elements

The cytoskeleton provides structural support, facilitates intracellular transport, and enables cell movement.

• Microfilaments: Composed of actin, involved in cell shape and movement.

• Intermediate Filaments: Provide mechanical strength (e.g., keratin in epithelial cells, neurofilaments in neurons).

• Microtubules: Hollow tubes made of tubulin, important for cell shape, organization, and transport of organelles and vesicles.

Motor Proteins and Intracellular Transport

• Kinesins: Move cargo antegrade (away from the nucleus).

• Dyneins: Move cargo retrograde (toward the nucleus).

• Movement occurs along microtubules, facilitating distribution of organelles and vesicles.

Cell Junctions

Types of Cell Junctions

Cell junctions are specialized structures that connect cells to each other and to the extracellular matrix, contributing to tissue integrity and communication.

Junction Type	Main Function	Key Components
Desmosomes	Cell-to-cell anchoring; provides mechanical strength	Glycoproteins, intermediate filaments
Gap Junctions	Communication; allows chemical and electrical signals to pass directly between cells	Connexin proteins (transmembrane)
Tight Junctions	Occluding; prevents movement of material between cells	Claudins, occludins

Tissues of the Body

Overview of Tissue Types

The body is organized into four primary tissue types, each with distinct functions and locations.

• Nervous Tissue: Responsible for internal communication; found in the brain, spinal cord, and nerves.

• Muscle Tissue: Contracts to produce movement; includes skeletal, cardiac, and smooth muscle.

• Epithelial Tissue: Forms boundaries, protects, secretes, absorbs, and filters; found on skin surface and lining of organs.

• Connective Tissue: Supports, protects, and binds other tissues; includes bone, tendons, fat, and soft padding tissue.

Epithelial Tissue

• Function: Forms boundaries between different environments, protects, secretes, absorbs, and filters.

• Examples: Skin surface (epidermis), lining of gastrointestinal tract and other hollow organs.

• Endothelial Tissue: A specialized type of epithelial tissue lining the inside of the heart and blood vessels. Endothelial cells release substances that regulate vascular relaxation/contraction, control blood clotting, and mediate platelet adhesion.

Additional info:

• All substances entering or leaving the internal environment of the body must cross an epithelial barrier.

• Combination of tight junctions and endothelial cells is crucial for selective permeability in blood vessels (e.g., blood-brain barrier).