⚡ Energy Organelles — Mitochondria & Plastids

Mitochondria

The mitochondria are arguably the most important organelles for energy production. They convert the chemical energy stored in food molecules into ATP (adenosine triphosphate), the universal energy currency of the cell.

• Called the "powerhouse of the cell" — site of aerobic respiration and ATP synthesis.
• Discovered by Richard Altmann (1890); named "mitochondria" by Carl Benda (1898).
• Double membrane-bound organelle. The double membrane is a key characteristic shared with chloroplasts and the nucleus.
• Contains its own circular DNA (mtDNA) and 70S ribosomes — supports the Endosymbiotic Theory (proposed by Lynn Margulis). This theory states that mitochondria were once free-living prokaryotic organisms that were engulfed by an ancestral eukaryotic cell and evolved into a symbiotic relationship.
• Maternally inherited (mitochondria come from the egg cell). During fertilization, the sperm contributes almost no cytoplasm, so virtually all mitochondria in the offspring come from the mother.

Structure

• Outer membrane — smooth, porous (permeable to small molecules). Contains porins that allow molecules up to 5,000 daltons to pass freely.
• Inner membrane — highly folded into cristae (increases surface area); contains ETC enzymes and ATP synthase (F₀-F₁ particles). The folds dramatically increase the surface area available for ATP production — more cristae means more ATP can be generated.
• Intermembrane space — between outer and inner membranes; accumulates H⁺ ions during ETC. This buildup of protons creates the proton gradient that drives ATP synthesis through chemiosmosis.
• Matrix — innermost compartment; contains enzymes for Krebs cycle, circular DNA, 70S ribosomes. The matrix is where pyruvate is broken down and CO₂ is released.

Functions

1. Aerobic respiration — oxidation of pyruvate to CO₂ and H₂O.
2. ATP synthesis via oxidative phosphorylation. This is the most efficient method of ATP production, generating ~34–36 ATP molecules per glucose molecule.
3. Krebs cycle (TCA cycle) occurs in the matrix.
4. ETC and chemiosmosis occur on inner membrane (cristae).
5. Apoptosis — release of cytochrome c triggers programmed cell death. Mitochondria play a central role in deciding whether a damaged cell should live or die.
6. Synthesis of some amino acids and fatty acids.

Endosymbiotic Theory — Evidence

The evidence supporting the endosymbiotic theory for mitochondria includes: - Mitochondria have their own **circular DNA** (similar to bacterial DNA) - They have **70S ribosomes** (same as bacteria, not 80S like eukaryotic cytoplasm) - They **replicate independently** by binary fission - They have a **double membrane** (the inner membrane may be the original bacterial membrane) - Their size is **similar to bacteria** (~1–10 μm)

Plastids

Plastids are a family of double membrane-bound organelles unique to the plant kingdom. They play roles ranging from photosynthesis to storage to giving color to flowers and fruits.