Chapter 4: From Cells to Organ Systems – Human Biology

Introduction: Unicellular and Multicellular Life

Unicellular Organisms

Unicellular organisms are the earliest and simplest forms of life, consisting of only a single cell. Their survival depends directly on their immediate external environment, which can vary greatly.

• Definition: Organisms made up of one cell.

• Examples: Bacteria, protozoa.

• Key Properties: Direct interaction with environment; limited specialization.

Multicellular Organisms

Multicellular organisms are composed of many cells, allowing for increased size and complexity. Their cells can specialize, contributing to the overall well-being of the organism.

• Definition: Organisms made up of multiple cells.

• Key Properties: Cell specialization, ability to maintain favorable environments, larger size.

• Examples: Humans, plants, animals.

Tissues: Groups of Cells with a Common Function

Definition and Classification

Tissues are groups of specialized cells that are similar in structure and perform a common function. Multiple tissue types can combine to form organs.

• Epithelial tissues

• Connective tissues

• Muscle tissues

• Nervous tissues

Epithelial Tissues

Functions and Organization

Epithelial tissues are organized as sheets of cells, forming one or more layers. They serve to line body cavities, cover surfaces, and protect underlying tissues. Some epithelial cells are adapted to form glands.

• Glandular epithelia: Specialized for synthesis and secretion.

• Exocrine glands: Secrete products into hollow organs or ducts (e.g., sweat glands).

• Endocrine glands: Secrete hormones into the blood for distribution throughout the body (e.g., thyroid gland).

Classification According to Shape

Epithelial tissues are classified by the shape of their cells:

• Squamous: Flattened cells; form the outer surface of the skin and line blood vessels, lungs, mouth, throat, and vagina.

• Cuboidal: Cube-shaped cells; form the lining of kidney tubules and glandular tissue.

• Columnar: Tall, rectangular cells; line the digestive tract, certain reproductive organs, and the larynx. May include goblet cells that secrete mucus.

Classification Based on Number of Layers

The number of cell layers in epithelial tissue determines its function:

• Simple (single-layered): Adapted for diffusion across cell barriers; found lining glands and the respiratory, digestive, and reproductive systems.

• Stratified (multiple-layered): Provide protection; found on the skin surface.

Example Table: Epithelial Tissue Classification

Shape	Layering	Location	Function
Squamous	Simple or Stratified	Skin, blood vessels, lungs	Diffusion, protection
Cuboidal	Simple	Kidney tubules, glands	Secretion, absorption
Columnar	Simple	Digestive tract, reproductive organs	Absorption, secretion

Additional info: Epithelial tissues also play roles in sensation and selective permeability, and their structure is supported by a basement membrane (not shown in these slides).