Inside the Cell

Cell Theory

The cell theory is a foundational concept in biology, describing the properties and significance of cells in living organisms. It was developed through the work of several scientists:

• Theodor Schwann & Matthias Jakob Schleiden (1839): Proposed that the cell is the most basic unit of life and that all living organisms are composed of one or more cells.

• Rudolf Virchow (1855): Added that all cells arise from pre-existing cells (Omnis cellula e cellula).

Modern cell theory expands on these ideas:

• Cells are the fundamental unit of life.

• All living things are comprised of one or more cells.

• All cells come from pre-existing cells.

• The activity of a living thing is a result of the total activity of its cells.

• Energy flow occurs within cells.

• Hereditary information is passed from cell to cell.

• All cells have the same basic molecular composition.

Example: The discovery of cells by Robert Hooke and the observation of "animalcules" by Anton van Leeuwenhoek were pivotal in the development of cell theory.

Tree of Life: Domains & Cell Types

All life is classified into three domains based on cellular characteristics and molecular evidence:

• Bacteria: Prokaryotic cells with unique ribosome and membrane biochemistry.

• Archaea: Prokaryotic cells with distinct molecular features, often found in extreme environments.

• Eukarya: Eukaryotic cells with complex internal organization and membrane-bound organelles.

Cell types are differentiated by their internal organization:

• Prokaryotes: Lack a membrane-bound nucleus; DNA is stored in a nucleoid region.

• Eukaryotes: Have a membrane-bound nucleus and numerous organelles.

Example: The phylogenetic tree illustrates the evolutionary relationships among the domains.

Major Differences Between Prokaryotic and Eukaryotic Cells

Example: Generalized diagrams of animal, plant, and prokaryotic cells highlight these differences.

Cell Structures Found in All Organisms

Despite their diversity, all cells share certain fundamental structures:

• Plasma (cell) membrane: A phospholipid bilayer that encloses the cell, controlling the movement of substances in and out.

• Ribosomes: Sites of protein synthesis, composed of protein and RNA, and not membrane-bound.

• Cytoskeleton: A network of protein filaments that provides structural support and facilitates movement.

Example: The plasma membrane's structure and function are essential for cell viability.

Ribosomes: Translate mRNA into polypeptides, consisting of two subunits.

Cytoskeleton: Provides cell shape, organization, and movement.

Structures Found in Some Prokaryotes

Some prokaryotes possess specialized structures:

• Photosynthetic membranes (thylakoids): Internal membranes for photosynthesis.

• Fimbriae: Hair-like appendages for attachment.

• Flagella: Whip-like structures for movement.

Example: Electron micrographs show fimbriae and flagella on bacterial cells.

Structures Found in Some Prokaryotes & Some Eukaryotes

Cell walls are present in many organisms, providing structural support and protection:

• Plants: Cell wall made of cellulose.

• Fungi: Cell wall made of chitin.

• Bacteria: Cell wall made of peptidoglycan.

• Diatoms: Cell wall made of silica.

Example: Comparative diagrams of cell walls in different organisms.

Structures Found in All Eukaryotes

Eukaryotic cells contain several essential organelles:

• Nucleus: Enclosed by a double membrane, contains DNA, site of transcription, and ribosome assembly in the nucleolus.

• Endomembrane System: Includes the endoplasmic reticulum (ER), Golgi apparatus, and transport vesicles, responsible for synthesis, modification, and transport of molecules.

• Mitochondria: Site of aerobic cellular respiration, converts energy in organic molecules to ATP, contains its own DNA and ribosomes.

Example: Diagrams of the nucleus and endomembrane system.

Mitochondria: Double membrane structure, inner membrane highly folded for increased surface area.

Structures Found in Some Eukaryotes

Certain organelles are found only in specific eukaryotic groups:

• Vacuoles: Large storage organelles in plants and fungi.

• Lysosomes: Digestive organelles in animals.

• Chloroplasts: Site of photosynthesis in plants and some protists, contains its own DNA and ribosomes.

Example: Diagram of chloroplast structure.

Endosymbiotic Theory: Origin of Mitochondria and Chloroplasts

The endosymbiotic theory explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells. Evidence includes:

• Both organelles have double membranes.

• Both contain their own circular DNA and ribosomes similar to those of bacteria.

• Both replicate independently within the cell.

Example: Diagrams and micrographs illustrating the structure and evidence for endosymbiosis.