Tissue Overview

A tissue is defined as a group of structurally and functionally related cells that cooperate to perform a specific biological function. In multicellular organisms, tissues represent an intermediate level of biological organization between cells and organs.In the human body, tissues are classified into four primary tissue types:

Muscle Tissue

Muscle Tissue
Muscle tissue is specialized for contraction and movement. It is subdivided into three categories:

• Skeletal muscle, responsible for voluntary body movements

• Cardiac muscle, forming the contractile tissue of the heart

• Smooth muscle, present in the walls of internal organs and blood vessels

Nervous Tissue

• Nervous tissue consists of neurons and supporting glial cells. Its primary function is the generation, transmission, and processing of electrical signals, enabling communication and coordination throughout the body.

Epithelial Tissue

Epithelial tissue forms protective coverings and linings of body surfaces and internal cavities. It is composed of cells with different morphological forms, including:

• Squamous cells

• Cuboidal cells

• Columnar cells

These tissues perform functions such as protection, absorption, secretion, and filtration.

Organ

An organ is a structurally distinct biological unit composed of two or more primary tissue types working together to perform specialized physiological functions. In many organs, all four basic tissue types are present.

Organs can be classified into two general categories:

• Tubular (hollow) organs, which possess an internal lumen

• Compact (parenchymal) organs, composed mainly of functional tissue

Examples of organs in the human body include:

• Brain

• Heart

• Lungs

• Stomach

• Liver

• Pancreas

• Kidneys

Organ Systems Overview

An organ system consists of multiple organs that interact to perform complex physiological functions necessary for maintaining homeostasis.

Major organ systems include:

• Digestive system : responsible for food digestion and nutrient absorption

• Excretory system : eliminates metabolic waste products

• Cardiovascular system : transports oxygen, nutrients, hormones, and metabolic products

• Respiratory system : facilitates gas exchange, including oxygen uptake and carbon dioxide removal

• Reproductive system : ensures species reproduction and continuity

• Endocrine system : regulates growth, metabolism, and physiological balance through hormones

• Musculoskeletal system : provides structural support and enables movement

• Nervous system : controls body activities, movement, and cognitive functions

Cell Structure

The cell represents the basic structural and functional unit of life. Each cell consists of a central region known as the cell body, enclosed by a protective boundary called the cell membrane.

The cell body contains two principal components:

• The nucleus, which stores the genetic material (DNA) and regulates cellular activities

• The cytoplasm, a semi-fluid matrix surrounding the nucleus that contains organelles and metabolic components

Thus, the three fundamental structural elements of a cell are:

1. The cell membrane

2. The cytoplasm

3. The nucleus

Cell Membrane

The cell membrane, also known as the plasma membrane or plasmalemma, forms the outer boundary of the cell. It separates the intracellular environment from the external environment.

The extracellular environment contains extracellular fluid (ECF), whereas the internal cellular environment contains intracellular fluid (ICF).

The plasma membrane exhibits selective permeability, meaning it regulates the passage of substances between the intracellular and extracellular compartments while maintaining cellular integrity.

Cell Membrane Composition

The plasma membrane is composed of three main molecular components:

• Proteins: approximately 55%

• Lipids: approximately 40%

• Carbohydrates: approximately 5%

These components interact to produce a dynamic and multifunctional biological membrane.

Cell Membrane Architecture

The plasma membrane is often described as a unit membrane or trilaminar membrane due to its characteristic layered appearance under electron microscopy.

Microscopic observations reveal three distinct layers:

• Two electron-dense outer layers

• One electron-lucent central layer

The central region mainly consists of a lipid bilayer, while the outer layers contain membrane-associated proteins. Carbohydrate molecules are also present and contribute to cell recognition and signaling.

Evolution of Cell Membrane Models

1. Danielli–Davson Model

Danielli–Davson Model

This early model proposed that the membrane consisted of a lipid bilayer sandwiched between two protein layers, forming a rigid structure.

2. Unit Membrane Model

In 1957, J. David Robertson introduced the Unit Membrane Model, based on observations from electron microscopy. This model refined the structural interpretation of the membrane as a three-layered structure.

3. Fluid Mosaic Model

The currently accepted membrane model was proposed in 1972 by S. J. Singer and Garth L. Nicolson.

The Fluid Mosaic Model describes the plasma membrane as a dynamic lipid bilayer in which proteins are embedded and capable of lateral movement, rather than forming rigid external layers. This model emphasizes the fluidity and structural heterogeneity of biological membranes.

Lipid and Protein Architecture of the Cell Membrane

The cell membrane is a complex and dynamic structure composed of lipids, proteins, and carbohydrates, each contributing to its functional properties. Its organization ensures both structural integrity and selective permeability.

Lipid Bilayer

The central framework of the plasma membrane is the lipid bilayer, consisting of two closely packed layers of lipid molecules. This bilayer exhibits fluidity, allowing membrane components to move laterally and creating a flexible, adaptive barrier.

Lipid Composition

The primary lipids in the membrane are:

• Phospholipids

• Cholesterol

Phospholipids are organized into a double-layered arrangement:

• Hydrophilic heads face outward toward the extracellular fluid (ECF) or inward toward the intracellular fluid (ICF), interacting with aqueous environments.

• Hydrophobic tails align within the membrane core, creating a water-repellent barrier.

This organization allows:

• Fat-soluble molecules ( oxygen, carbon dioxide, alcohol) to diffuse freely across the membrane

• Water-soluble molecules ( glucose, urea, ions) to require specialized transport mechanisms

Membrane Proteins

The lipid bilayer is coated on both surfaces by protein layers, which appear electron-dense under microscopy. Proteins serve multiple structural and functional roles:

Structural and Transport Functions

• Integral proteins provide structural stability to the membrane

• Channel proteins enable passive diffusion of water-soluble molecules

• Carrier (transport) proteins facilitate both passive and active transport

• Pumps actively move ions against concentration gradients using energy

Signaling and Recognition

• Receptor proteins bind hormones or neurotransmitters for cellular signaling

• Enzymatic proteins catalyze metabolic reactions at the membrane

• Antigenic proteins serve as immune markers

• Cell adhesion molecules (CAMs) anchor cells to neighbors or the basal lamina, supporting tissue structure

Carbohydrates in the Membrane

Carbohydrate chains are covalently attached to proteins (glycoproteins) or lipids (glycolipids) and are essential for:

• Cell recognition and signaling

• Formation of the glycocalyx, which stabilizes cell-to-cell adhesion

• Acting as specific binding sites for hormones or other signaling molecules

Negatively charged carbohydrate groups also act as electrostatic barriers, preventing the passage of other negatively charged molecules.