Chapter 3: Cells – Structure, Function, and Diversity

Cells: The Fundamental Unit of Life

The Cell Theory

The cell theory is a foundational concept in biology, describing the properties and significance of cells in living organisms.

• Cells are the building blocks of all plants and animals.

• Cells are the smallest functioning units of life, meaning all vital functions of an organism occur within cells.

• Cells are produced through the division of preexisting cells, ensuring continuity of life.

• Each cell maintains homeostasis, regulating its internal environment to sustain life.

Types of Cells

Prokaryotic vs. Eukaryotic Cells

Cells are classified into two main types based on their structural characteristics and complexity.

• Prokaryotic cells:

  • No true nucleus; genetic material is not enclosed within a membrane.

  • Lack membrane-bound organelles.

  • Usually smaller and unicellular (e.g., bacteria).

• Eukaryotic cells:

  • Have a true nucleus surrounded by a nuclear membrane.

  • Contain membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

  • Larger in size; most are multicellular (e.g., plants, animals, fungi), though some are unicellular (e.g., protists).

Example: Human cells are eukaryotic.

The Diversity of Cells in the Human Body

Specialized Cell Types

The human body contains a wide variety of cell types, each specialized for particular functions.

• Smooth muscle cells: Involved in involuntary movements (e.g., in the digestive tract).

• Blood cells: Include red blood cells (oxygen transport) and white blood cells (immune defense).

• Bone cells: Provide structure and support.

• Neurons: Specialized for communication via electrical and chemical signals.

• Fat cells (adipocytes): Store energy as fat.

• Cells lining the intestinal tract: Involved in nutrient absorption.

Structure of Eukaryotic Cells

Main Components

Eukaryotic cells have three primary structural components:

• Cell membrane (plasma membrane): Encloses the cell, controlling entry and exit of substances.

• Nucleus: Contains genetic material (DNA) and directs cellular activities.

• Cytoplasm: Gel-like substance containing organelles and cytosol.

Organelles are specialized structures within the cytoplasm that perform specific functions, often referred to as "little organs." Examples include mitochondria, endoplasmic reticulum, and Golgi apparatus.

The Cell Membrane

Structure and Function

The cell membrane, also known as the plasma membrane, is a dynamic barrier that separates the cell from its environment.

• Functions:

  • Contains cell contents.

  • Acts as a barrier for substances entering and leaving the cell.

  • Facilitates signal transduction and communication.

  • Adheres to other cells for tissue formation.

• Selectively permeable: Regulates the entry and exit of substances, allowing some molecules to pass while restricting others.

• Phospholipid bilayer: The fundamental structure of the membrane, consisting of two layers of phospholipids.

  • Hydrophilic (polar) heads face outward toward water.

  • Hydrophobic (nonpolar) tails face inward, away from water.

Fluid Mosaic Model

The cell membrane is described as a "fluid mosaic" because it is flexible and composed of various molecules that move laterally within the layer.

• Proteins are embedded within the bilayer, serving as channels, carriers, receptors, and enzymes.

• Cholesterol molecules help stabilize membrane fluidity.

• Carbohydrate chains are attached to proteins and lipids, playing roles in cell recognition and signaling.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No true nucleus	True nucleus present
Membrane-bound organelles	Absent	Present
Size	Smaller	Larger
Cellularity	Unicellular	Mostly multicellular (some unicellular)
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists

Additional Info

• Homeostasis refers to the maintenance of a stable internal environment within the cell, essential for proper function.

• Signal transduction is the process by which cells respond to external signals via receptors in the membrane.

• Organelle functions (not detailed in the images but relevant):

  • Mitochondria: Site of ATP (energy) production.

  • Endoplasmic reticulum: Synthesizes proteins (rough ER) and lipids (smooth ER).

  • Golgi apparatus: Modifies, sorts, and packages proteins and lipids.

  • Lysosomes: Contain digestive enzymes for breaking down waste.