3.2(13)

Learning Objectives

• By the end of this section, students will be able to:

  • Identify the two broad categories of cells: prokaryotic and eukaryotic.

  • Provide examples of prokaryotic and eukaryotic organisms.

  • Compare and contrast prokaryotic cells and eukaryotic cells.

  • Describe the relative sizes of different kinds of cells.

Prokaryotic Cells

• Definition: Simple, single-celled (unicellular) organisms without a nucleus or any membrane-bound organelles.

• Common Components:

  1. Plasma Membrane: Outer covering that separates a cell’s interior from the environment.

  2. Cytoplasm: Jelly-like region in which cellular components are found.

  3. DNA: Genetic material located in the nucleoid region.

  4. Ribosomes: Particles that synthesize proteins.

• DNA Location: The central part of the cell, in a darkened region called the nucleoid.

Differences from Eukaryotic Cells

• Prokaryotic cells lack a nucleus and other membrane-bound organelles.

• Prokaryotic cells are predominantly found in the domains Bacteria and Archaea.

Components of Bacterial Cells

• Cell Wall:

  • Made of peptidoglycan (sugars and amino acids).

  • Provides protection, maintains shape, and prevents dehydration.

• Capsule: A polysaccharide layer that helps bacteria adhere to surfaces in their environment.

• Appendages:

  • Flagella: Used for locomotion.

  • Pili & Fimbriae: Used primarily for genetic material exchange during conjugation.

Eukaryotic Cells

• Definition: Cells with a membrane-bound nucleus and organelles, which have specialized functions.

• Terminology:

  • "Eukaryotic" means "true nucleus" alluding to the presence of the nucleus.

  • "Organelle" means "little organ"; these structures perform specialized cellular functions.

Relationship Between Form and Function

• The principle "form follows function" describes how the shape of a cell or organism helps it perform its functions.

  • Example: Streamlined bodies of birds and fish allow efficient movement through air and water respectively.

Cell Size Comparison

• Prokaryotic Cells: Ranges from 0.1–5.0 µm in diameter.

• Eukaryotic Cells: Ranges from 10–100 µm in diameter.

• Transport Efficiency: The small size of prokaryotes allows for quicker diffusion of ions and molecules within the cell and removal of wastes.

• Structural Adaptations in Eukaryotes: Eukaryotic cells developed adaptations to enhance transport mechanisms due to their larger size.

• Surface Area vs. Volume:

  • As cell size increases, volume increases faster than surface area, limiting the ability to acquire sufficient materials for cellular processes.

Visual Reference

• Figure 3.6 shows the relative sizes of different kinds of cells and cellular components, including a human for comparison.