Cell Theory and Cell Types: Prokaryotic vs. Eukaryotic Cells

Cell Theory

Fundamental Principles of Cell Theory

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

• All organisms are made of one or more cells. Every living thing, from bacteria to plants and animals, is composed of cells.

• The cell is the structural and functional unit of all organisms. Cells carry out essential life processes and are considered the basic unit of life.

• Cells arise by division of pre-existing cells. New cells are produced by the division of existing cells, ensuring continuity of life.

• Cells contain hereditary material, which they pass to their offspring. Genetic information (DNA) is stored in cells and transmitted during cell division.

Key Terms

• Cell: The smallest unit of life, capable of independent existence and reproduction.

• Hereditary Material: Genetic information, typically DNA, that is passed from parent to offspring.

Types of Cells

Prokaryotic Cells

Prokaryotic cells are among the simplest forms of life, lacking a nucleus and most internal organelles. They are found in domains Bacteria and Archaea.

• No nucleus: Genetic material is located in a region called the nucleoid, not enclosed by a membrane.

• Cell components: Prokaryotic cells typically have a plasma membrane, cytoplasm, ribosomes, and sometimes a cell wall and capsule.

• Organelles: Lack membrane-bound organelles such as mitochondria or endoplasmic reticulum.

• Reproduction: Usually reproduce by binary fission.

• Domains: Includes Bacteria and Archaea.

Key Structures of Prokaryotic Cells

• Plasma Membrane: Phospholipid bilayer that controls entry and exit of substances.

• Cell Wall: Provides structural support and protection; composition varies between bacteria and archaea.

• Capsule: Sticky outer layer that helps with protection and adherence to surfaces.

• Ribosomes: Free-floating structures for protein synthesis.

• Nucleoid: Region containing circular DNA.

• Plasmids: Small, circular DNA molecules carrying additional genes.

• Pili: Hair-like structures for attachment and DNA exchange.

• Flagella: Tail-like structures for movement.

Examples

• Escherichia coli (E. coli) – a common bacterium found in the intestines of animals.

• Streptococcus – bacteria responsible for strep throat.

Comparison: Bacteria vs. Archaea

Bacteria and Archaea are both prokaryotic, but they differ in several key features.

Feature	Bacteria	Archaea
Cell Wall Composition	Contains peptidoglycan	No peptidoglycan; may have pseudopeptidoglycan
Membrane Lipids	Unbranched fatty acids	Branched hydrocarbons
RNA Polymerase	One type	Several types
Antibiotic Sensitivity	Inhibited by streptomycin and chloramphenicol	Not inhibited by these antibiotics
Chromosome Shape	Circular	Circular

Eukaryotic Cells

Eukaryotic cells are more complex than prokaryotic cells, containing a nucleus and various membrane-bound organelles. They are found in plants, animals, fungi, and protists.

• Nucleus: Membrane-bound compartment containing DNA.

• Organelles: Specialized structures such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and peroxisomes.

• Cell Types: Can be autotrophic (plant-like) or heterotrophic (animal-like).

• Compartmentalization: Internal membranes create specialized environments for cellular processes.

• Cell Differentiation: Cells can specialize for different functions in multicellular organisms.

Key Structures of Eukaryotic Cells

• Nucleus: Contains genetic material; surrounded by nuclear envelope.

• Ribosomes: Site of protein synthesis; can be free or attached to endoplasmic reticulum.

• Endoplasmic Reticulum (ER): Network for protein and lipid synthesis.

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

• Mitochondria: Site of cellular respiration and energy production.

• Chloroplasts: (in plants and algae) Site of photosynthesis.

• Vacuoles: Storage and structural support (large central vacuole in plant cells).

• Cytoskeleton: Provides cell shape, organization, and movement.

• Cell Membrane: Controls movement of substances in and out of the cell.

Examples

• Plant cells – contain chloroplasts and a cell wall.

• Animal cells – lack cell walls and chloroplasts, have centrioles.

• Fungal cells – have cell walls made of chitin.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No	Yes
Organelles	No membrane-bound organelles	Many membrane-bound organelles
Cell Size	Generally smaller (1-10 μm)	Generally larger (10-100 μm)
DNA Location	Nucleoid region	Nucleus
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Additional info:

• Cell differentiation allows multicellular organisms to develop specialized tissues and organs.

• Autotrophic cells (e.g., plant cells) produce their own food via photosynthesis, while heterotrophic cells (e.g., animal cells) obtain food from other organisms.

• Prokaryotic cells can exchange genetic material via plasmids and pili, contributing to genetic diversity.