Cell Structure and Function: Prokaryotic and Eukaryotic Cells

Chapter 6: Cells

Cell Theory

The Cell Theory is a fundamental concept in biology stating that all organisms are composed of cells, which are the simplest units of life. The structure of each cell is closely related to its function, and understanding this relationship is key to studying biology.

• All living things are made of cells.

• Cells are the basic units of structure and function in living organisms.

• All cells arise from pre-existing cells.

Example: Human tissues such as muscle and nerve are composed of specialized cells that perform specific functions.

Learning Objectives

• Compare prokaryotic and eukaryotic cells.

• Identify differences between animal and plant cells.

• Recognize types of organelles and describe their functions.

• Understand basic microscopy techniques.

Comparing Prokaryotic and Eukaryotic Cells

Basic Features of All Cells

Despite their differences, all cells share several key features:

• Plasma membrane: A selective barrier that encloses the cell.

• Cytosol: A semifluid substance inside the cell where organelles are suspended.

• Chromosomes: Structures that carry genetic information (DNA).

• Ribosomes: Complexes that synthesize proteins.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as either prokaryotic or eukaryotic based on their internal structure.

• Prokaryotic cells: Lack a nucleus and membrane-bound organelles. Their DNA is located in a region called the nucleoid. Examples include Bacteria and Archaea.

• Eukaryotic cells: Have a true nucleus enclosed by a nuclear envelope and possess various membrane-bound organelles. Examples include Animal, Plant, Fungi, and Protist cells.

Example: Escherichia coli is a prokaryote; human skin cells are eukaryotes.

Comparison Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	Absent (DNA in nucleoid)	Present (DNA in nucleus)
Membrane-bound organelles	Absent	Present
Size	Generally smaller (1-10 μm)	Generally larger (10-100 μm)
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists

Advantages of Prokaryotes and Eukaryotes

• Prokaryotes: Rapid growth and reproduction, simple structure allows adaptation to diverse environments.

• Eukaryotes: Compartmentalization enables complex functions, multicellularity, and specialization.

Additional info: Prokaryotes often thrive in extreme environments due to their simplicity; eukaryotes can form multicellular organisms with specialized tissues.

Plasma Membrane

Structure and Function

The plasma membrane is a selective barrier that regulates the passage of oxygen, nutrients, and waste into and out of the cell. It is composed of a double layer of phospholipids with embedded proteins and carbohydrates.

• Phospholipid bilayer: Provides fluidity and flexibility.

• Integral and peripheral proteins: Facilitate transport and communication.

• Glycoproteins and glycolipids: Involved in cell recognition.

• Cholesterol: Modulates membrane fluidity.

Example: The plasma membrane of red blood cells contains specific proteins that determine blood type.

Diagram: Major Components of the Plasma Membrane

Component	Function
Phospholipid bilayer	Structural framework, barrier
Integral proteins	Transport, signaling
Peripheral proteins	Support, cell shape
Glycoproteins	Cell recognition
Cholesterol	Membrane fluidity

Key Formula: Surface Area to Volume Ratio

The efficiency of the plasma membrane is influenced by the cell's surface area to volume ratio:

$\text{Surface Area to Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

Smaller cells have a higher ratio, allowing for more efficient exchange of materials.

Summary

• All living organisms are composed of cells, which are classified as prokaryotic or eukaryotic.

• Cells share common features but differ in complexity and organization.

• The plasma membrane is essential for maintaining cellular integrity and regulating interactions with the environment.