2.1 Cell Theory, Prokaryotes vs. Eukaryotes, and Cellular Organization

2.1 Cell Theory, Prokaryotes vs. Eukaryotes, and Cellular Organization

Quick Summary: All living organisms consist of one or more cells, the fundamental structural and functional units of life. Cells arise exclusively from preexisting cells through division, refuting spontaneous generation. Cellular life is divided into prokaryotes (lacking membrane-bound nuclei and organelles) and eukaryotes (possessing a true nucleus, compartmentalized organelles, and linear chromosomes). In multicellular organisms, cells specialize and organize hierarchically into tissues, organs, organ systems, and whole organisms.

The Foundations of Cell Theory

Before seventeenth-century microscopy, the origin of life was attributed to spontaneous generation (abiogenesis)—the belief that living organisms arise from non-living matter, such as maggots from decaying meat.

Early doubt arose when Francesco Redi (1668) proved that maggots developed only on uncovered meat accessible to flies, not on sealed meat. However, belief in microbial abiogenesis persisted until systematic microscopy established Cell Theory through key empirical discoveries:

• Robert Hooke (1665): Observed dead cork tissue under a compound microscope, coining the term cell after monastic chambers.
• Anton van Leeuwenhoek (1674): Observed living, motile microorganisms in pond water and dental plaque, naming them animalcules.
• Matthias Schleiden (1838): Concluded that all plant tissues are composed of cells.
• Theodor Schwann (1839): Extended this finding to animals, unifying botany and zoology under a common cellular foundation.
• Rudolf Virchow (1855): Proposed the principle "Omnis cellula e cellula"—all cells arise from preexisting cells.

Louis Pasteur (1859) empirically disproved spontaneous generation using swan-neck flasks. Sterilized broth remained sterile until airborne microbes were admitted, demonstrating biogenesis: life arises only from preexisting life.

The Three Tenets of Classical Cell Theory

1. All organisms are composed of one or more cells. Organisms can be unicellular (bacteria) or multicellular (plants, animals).
2. The cell is the basic structural and functional unit of life. Cells are the smallest autonomous entities capable of performing all life processes.
3. All cells arise from preexisting cells. Cells reproduce exclusively via division (binary fission or mitosis/meiosis).

Modern additions establish that cellular metabolism occurs within cells, DNA is passed from parent to daughter cells, and all cells share basic biochemical compositions.

Prokaryotes vs. Eukaryotes

All organisms are grouped into two fundamental cellular designs: prokaryotes (Bacteria, Archaea) and eukaryotes (Protists, Fungi, Plants, Animals).

Prokaryotic Cellular Architecture

Prokaryotes are ancient, single-celled organisms:

• Size: Typically 0.1 to 5.0 µm, maintaining a high surface-area-to-volume ratio for rapid nutrient uptake.
• Nucleoid: DNA forms a single circular chromosome in an unbound region called the nucleoid. Small circular plasmids carry accessory genes (e.g., antibiotic resistance).
• Internal Structure: Lack membrane-bound organelles. ATP synthesis occurs across specialized foldings of the plasma membrane.
• Ribosomes: Contain smaller 70S ribosomes (30S and 50S subunits).
• Cell Wall: Most bacteria have a peptidoglycan cell wall, often surrounded by a capsule and flagella for motility.
• Reproduction: Divide asexually through binary fission.

Eukaryotic Cellular Architecture

Eukaryotes exhibit pronounced internal specialization:

• Size: Typically 10 to 100 µm, requiring active internal transport mechanisms.
• Nucleus: DNA is packaged into multiple linear chromosomes associated with histone proteins inside a double-membrane nuclear envelope.
• Compartmentalization: Membrane-enclosed organelles isolate incompatible biochemical reactions and optimize local catalytic conditions.
• Ribosomes: Cytoplasmic translation utilizes larger 80S ribosomes (40S and 60S subunits).
• Division: Replicate via mitosis (somatic growth) and meiosis (gamete production).

Universal Cellular Features

All living cells share four universal components:

1. Plasma Membrane: Phospholipid bilayer regulating transport.
2. Cytoplasm: Aqueous cytosol supporting metabolic reactions.
3. DNA Genome: Hereditary biological instructions.
4. Ribosomes: Protein translation machinery.

Hierarchical Organization of Multicellular Life

Multicellular organisms exhibit an increasing hierarchy of structural complexity:

1. Atoms & Molecules: Form biological macromolecules (proteins, nucleic acids, lipids, carbohydrates).
2. Organelles: Supramolecular assemblies performing specialized intracellular functions.
3. Cells: Fundamental living units. Cells undergo differentiation, selectively expressing specific genes to form specialized cell types.
4. Tissues: Coordinated groups of similar cells performing a shared role. Animals possess four primary tissues: epithelial (protective coverings/linings), connective (structural support and matrix), muscle (contractile movement), and nervous (signal conduction).
5. Organs: Structures of two or more integrated tissue types performing coordinated tasks (e.g., the stomach contains epithelial lining, smooth muscle, connective tissue, and nervous tissue).
6. Organ Systems: Groups of organs executing broad bodily functions (e.g., cardiovascular, digestive).
7. Organism: The integrated individual maintaining physiological homeostasis.

HiSET Scientific Inquiry: Interpreting Cellular Evidence

• Microscope Identification: A cell with a cell wall and ribosomes but lacking a nucleus is bacterial (prokaryotic). A cell possessing linear chromosomes or mitochondria is eukaryotic.
• Differential Gene Expression: All somatic cells in a multicellular organism share an identical genome; phenotypic specialization results from selective gene transcription.