Introduction to Cells

Cell Theory and Cellular Organization

The cell is the smallest living unit in the human body and serves as the fundamental building block of all organisms. The study of cells is central to understanding anatomy and physiology.

• Cell Theory:

  • All organisms are composed of cells.

  • All cells arise from the division of preexisting cells.

  • Cells are the smallest units that carry out life's essential physiological functions.

  • Each cell maintains homeostasis at the cellular level.

  • Homeostasis at higher levels (tissue, organ, organ system, organism) results from the combined feedback of many cells.

Cell Types in the Human Body

Cells in the human body are classified based on their function and role in reproduction.

• Sex cells (germ cells or reproductive cells):

  • Responsible for reproduction.

  • Male: sperm

  • Female: oocyte

• Somatic cells: All body cells except sex cells.

Histology is the study of the structure (anatomy) of cells and tissues, and is a branch of cell biology.

Plasma Membrane

Structure and Components

The plasma membrane (also called the cell membrane) forms the outer boundary of the cell and regulates the movement of substances in and out. Its main components are lipids and proteins.

• Phospholipid bilayer: The fundamental structure of the plasma membrane.

  • Hydrophilic heads: Face outward toward the watery environments of the extracellular fluid and the intracellular fluid (cytosol).

  • Hydrophobic tails: Form the inner core of the membrane, acting as a barrier to ions and water-soluble compounds.

  • Other lipids: Cholesterol, other steroids, and glycolipids. Cholesterol makes the plasma membrane less fluid and less permeable.

Functions of the Plasma Membrane

The plasma membrane is essential for maintaining cellular integrity and function.

• Physical isolation: Separates the inside of the cell (cytoplasm) from the surrounding extracellular fluid.

• Regulation of exchange with the environment: Controls the entry of ions and nutrients, elimination of wastes, and release of secretions.

• Sensitivity to the environment: Contains receptors that allow the cell to respond to chemical signals and environmental changes.

• Structural support: Anchors cells to each other and to extracellular materials, providing stability to tissues.

Key Terms and Concepts

• Phospholipid bilayer: Double layer of phospholipids forming the basic structure of the plasma membrane.

• Hydrophilic: Water-loving; describes the phosphate heads of phospholipids.

• Hydrophobic: Water-fearing; describes the fatty acid tails of phospholipids.

• Cholesterol: A lipid that stabilizes the membrane and affects its fluidity and permeability.

Example: Plasma Membrane Function

For example, the plasma membrane of intestinal cells contains specialized proteins that allow for the selective absorption of nutrients, while also protecting the cell from harmful substances in the digestive tract.

Additional info: The plasma membrane also contains proteins and carbohydrates that contribute to cell recognition, signaling, and intercellular interactions, which are covered in more detail in subsequent sections of the chapter.