Cellular Organelles: Energy Converters and Cytoskeleton

Definition: This theory proposes that mitochondria and chloroplasts originated from prokaryotic cells living within larger host cells.
Ancestors:
- Mitochondria: Proposed ancestors were oxygen-using, nonphotosynthetic prokaryotes. These were engulfed by ancestral host cells.
- Chloroplasts: Proposed ancestors were photosynthetic prokaryotes. These were subsequently engulfed by a lineage of host cells that already contained mitochondria.
Evolutionary Process: Engulfing events led to a symbiotic relationship; over many generations, the prokaryotes evolved into organelles within the host cell.
Evidence Supporting the Theory:
- Membranes: The inner membranes of mitochondria and chloroplasts have enzyme systems and transport proteins similar to those found in the plasma membranes of prokaryotes.
- Genetic Material and Ribosomes: Like prokaryotes, both mitochondria and chloroplasts contain their own ribosomes and circular DNA molecules, which resemble bacterial chromosomes. This DNA programs the synthesis of some organelle proteins on the ribosomes located within the organelle.
- Autonomy: Mitochondria and chloroplasts are somewhat independent, growing and reproducing within the cell without direct input from the nuclear DNA for all their components, consistent with their probable evolutionary origins as independent cells.

Prevalence: Found in nearly all eukaryotic cells, including those of plants, animals, fungi, and most protists.
Number per cell: Varies significantly.
- Some cells have a single large mitochondrion.
- More often, a cell has hundreds or even thousands of mitochondria.
- The number directly correlates with the cell's level of metabolic activity (e.g., cells that move or contract have proportionally more mitochondria).
Structure:
- Enclosing Membranes: Surrounded by two phospholipid bilayers, each with a unique collection of embedded proteins.
  - Outer Membrane: Smooth.
  - Inner Membrane: Highly convoluted with infoldings called cristae (singular: crista).
- Internal Compartments: The inner membrane divides the mitochondrion into two main compartments:
  - Intermembrane Space: The narrow region between the inner and outer membranes.
  - Mitochondrial Matrix: Enclosed by the inner membrane.
- Contents of Matrix: Contains many different enzymes crucial for cellular respiration, as well as mitochondrial DNA and ribosomes.
- Inner Membrane Contents: Other proteins essential for respiration, including the enzyme that synthesizes ATP, are embedded in the inner membrane.
- Cristae Function: The highly folded cristae provide a large surface area for the inner mitochondrial membrane, significantly enhancing the efficiency and productivity of cellular respiration. This exemplifies the principle of