Compartmentation: Cells and Tissues – Study Notes for Animal Physiology

Compartmentation in Animal Physiology

Introduction

Compartmentation is a fundamental concept in animal physiology, referring to the organization of cells and tissues into distinct functional regions. This organization allows for specialized biochemical processes and efficient regulation of physiological functions.

Cell Membranes and Structure

Cell Membrane Composition and Function

The cell membrane, also known as the plasmalemma or phospholipid membrane, encloses the cell and separates its internal environment from the external surroundings. It is primarily composed of lipids, proteins, and carbohydrates.

• Phospholipid Bilayer: The basic structure consists of two layers of phospholipids, each with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Cholesterol: Interspersed within the bilayer, cholesterol stabilizes the membrane, reduces its deformability, and decreases permeability to small water-soluble molecules.

• Proteins: Membrane proteins serve as channels, transporters, and receptors, facilitating communication and transport across the membrane.

• Carbohydrates: Often attached to proteins or lipids, they play roles in cell recognition and signaling.

Functions of Cell Membranes:

• Act as barriers to isolate cellular compartments

• Regulate exchange of substances between intracellular and extracellular fluids

• Facilitate cell-to-cell communication

• Provide structural support

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

Membrane Dynamics and Permeability

• At higher temperatures, the bilayer becomes more fluid, allowing increased movement of molecules.

• Cholesterol immobilizes parts of the phospholipid chains, making the membrane less deformable.

Example: The lumen of the digestive tract is considered part of the external environment, separated from the body by membranous tissues.

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 proteins, and one or more nucleoli.

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

• Ribosomes: Sites of protein synthesis, found free in the cytosol or attached to the ER.

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

• Mitochondria: Spherical or rod-shaped organelles responsible for ATP production.

Protein Synthesis: Involves transcription of DNA to mRNA in the nucleus, followed by translation of mRNA to protein at ribosomes.

Cytoskeleton and Microtubules

Cytoskeletal Components

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

• Intermediate Filaments: Provide mechanical strength; examples include keratin and neurofilaments.

• Microtubules: Hollow tubes made of tubulin, essential for intracellular transport, cell division, and maintaining cell shape.

Functions of the Cytoskeleton:

• Maintains cell shape

• Organizes internal structures

• Facilitates intracellular transport

• Enables movement and assembly of cells into tissues

Motor Proteins: Kinesins move cargo away from the nucleus (anterograde), while dyneins move cargo toward the nucleus (retrograde).

Cell Junctions

Types and Functions of Cell Junctions

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

Junction Type	Main Function	Key Components
Gap Junctions	Allow chemical and electrical signals to pass directly from cell to cell	Connexin proteins
Tight Junctions	Prevent movement of material between cells; occluding junctions	Claudins, occludins
Anchoring Junctions (Desmosomes)	Provide strong mechanical attachments between cells	Cadherins, integrins

Example: Endothelial cells lining blood vessels use tight junctions to regulate permeability and maintain vascular integrity.

Tissues of the Body

Overview of Tissue Types

Tissues are groups of cells with similar structure and function. The four main types are:

Tissue Type	Main Function	Examples
Nervous Tissue	Internal communication	Brain, spinal cord, nerves
Muscle Tissue	Contracts to cause movement	Skeletal muscles, cardiac muscle, smooth muscle
Epithelial Tissue	Forms boundaries, protects, secretes, absorbs, filters	Skin (epidermis), lining of GI tract, hollow organs
Connective Tissue	Supports, protects, binds other tissues	Bones, tendons, fat, soft padding tissue

Epithelial Tissue and Endothelium

• Epithelial Tissue: Forms boundaries between different environments, regulates exchange, and provides protection.

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

Example: Substances entering or leaving the internal environment of the body must cross an epithelial barrier.

Homeostasis and Feedback Mechanisms

Negative and Positive Feedback

Homeostasis is maintained through feedback mechanisms that regulate physiological variables.

• Negative Feedback: A change in a controlled variable triggers a response that opposes the change, helping to maintain stability.

• Positive Feedback: A change in a variable triggers a response that amplifies the change, often leading to a specific outcome (e.g., childbirth).

Equation for Homeostatic Control:

$\text{Response} = -k \times \text{Change in Variable}$

Example: Regulation of blood glucose levels involves negative feedback mechanisms.

Additional info: Some details, such as the specific molecular components of junctions and the equation for homeostatic control, were inferred from standard academic sources to provide a complete and self-contained study guide.