Mitochondria, Chloroplasts, and Endosymbiosis

Core Concept

Mitochondria and chloroplasts are the cell's energy-converting organelles. Mitochondria harvest chemical energy from food molecules through cellular respiration, producing the energy carrier adenosine triphosphate (ATP). Chloroplasts, found in plants and algae, capture light energy through photosynthesis and convert it into the chemical energy stored in sugars. Both organelles share a striking and unusual feature: they look less like typical organelles and more like small cells living inside a larger one.

That resemblance is no coincidence. According to the endosymbiotic theory, mitochondria and chloroplasts descend from free-living prokaryotes that were engulfed by an ancestral eukaryotic cell and, instead of being digested, became permanent, mutually beneficial residents. The evidence — double membranes, their own circular DNA, and bacteria-like 70S ribosomes — makes this one of the best-supported ideas in cell biology.

Key Concepts

Mitochondria: The Powerhouse

A mitochondrion has two membranes. The outer membrane is smooth; the inner membrane folds inward into cristae, which greatly increase its surface area for the reactions of cellular respiration. The space between the two membranes is the intermembrane space, and the innermost compartment is the matrix. During aerobic respiration, glucose is broken down and the energy released is used to pump protons across the inner membrane; their flow back drives ATP synthesis. Cells with high energy demands — muscle, liver, sperm — have many mitochondria. Notably, mitochondria carry their own small, circular DNA molecule and their own 70S ribosomes, and they divide by binary fission.

Chloroplasts: The Solar Panels

Chloroplasts also have a double membrane. Inside, an internal membrane system forms flattened sacs called thylakoids, which stack into columns called grana; the fluid surrounding them is the stroma. The pigment chlorophyll, embedded in the thylakoid membranes, absorbs light energy, which drives the synthesis of sugars from carbon dioxide and water. Like mitochondria, chloroplasts contain their own circular DNA and 70S ribosomes. (They are also members of a larger family of plant organelles called plastids.)

The Endosymbiotic Theory

The endosymbiotic theory proposes that a host eukaryotic cell engulfed an aerobic prokaryote (the ancestor of mitochondria) and, in photosynthetic lineages, a cyanobacterium-like prokaryote (the ancestor of chloroplasts). Over evolutionary time, these engulfed cells became interdependent organelles. The supporting evidence is strong and convergent:

• Double membranes — consistent with a prokaryote being wrapped in the host's membrane during engulfment.
• Circular DNA — both organelles have their own circular DNA molecule, like bacteria rather than the linear chromosomes of the eukaryotic nucleus.
• 70S ribosomes — their ribosomes resemble bacterial ribosomes, not eukaryotic 80S ribosomes.
• Binary fission — both organelles reproduce by dividing, independently of the cell cycle.
• Size and genetics — both are similar in size to bacteria and use bacterial-style gene sequences.

How It Works

The energy flow is a chain of cause and effect. In mitochondria, the breakdown of glucose (via glycolysis, the citric acid cycle, and the electron transport chain) releases electrons that are passed along membrane proteins, pumping protons into the intermembrane space. The resulting proton gradient drives an enzyme called ATP synthase to phosphorylate ADP into ATP — the cell's portable energy currency. In chloroplasts, light energy excites electrons in chlorophyll, which drives the synthesis of ATP and the reducing power needed to fix carbon dioxide into sugar in the Calvin cycle. The endosymbiotic origin explains why these organelles run these processes with their own bacterial-style machinery, semi-independently of the nucleus.

How it works

The energy flow is a chain of cause and effect. In mitochondria, the breakdown of glucose (via glycolysis, the citric acid cycle, and the electron transport chain) releases electrons that are passed along membrane proteins, pumping protons into the intermembrane space. The resulting proton gradient drives an enzyme called ATP synthase to phosphorylate ADP into ATP — the cell's portable energy currency. In chloroplasts, light energy excites electrons in chlorophyll, which drives the synthesis of ATP and the reducing power needed to fix carbon dioxide into sugar in the Calvin cycle. The endosymbiotic origin explains why these organelles run these processes with their own bacterial-style machinery, semi-independently of the nucleus.

Common confusions

• "Plants have mitochondria." Yes — plants have both chloroplasts (for making sugar) and mitochondria (for breaking sugar down into ATP); chloroplasts do not replace mitochondria.
• "Mitochondria and chloroplasts have linear chromosomes like the nucleus." Their DNA is circular, like bacterial chromosomes.
• "Endosymbiosis is just a hypothesis with little support." It is a well-supported theory backed by multiple independent lines of evidence.
• "Mitochondrial DNA comes from both parents." It is inherited almost entirely from the mother.
• "The cristae are for storage." The cristae increase membrane surface area for the electron transport chain and ATP synthesis.

Quick review

• Mitochondria (cristae, matrix) make ATP via cellular respiration.
• Chloroplasts (thylakoids, grana, stroma) make sugars via photosynthesis.
• Both have double membranes, circular DNA, and 70S ribosomes, and divide by binary fission.
• Endosymbiotic theory explains their origin from engulfed prokaryotes.
• mtDNA is maternally inherited and relevant to mitochondrial disease.
• Plants have both chloroplasts and mitochondria.