6.3 Mitochondria and chloroplasts

Mitochondria are the primary energy-producing organelles in eukaryotic cells, generating most of a cell's ATP through oxidative phosphorylation. Chloroplasts, found in plants and algae, capture light energy and convert it to chemical energy through photosynthesis. Both organelles share a fascinating evolutionary origin explained by the endosymbiotic theory, and understanding their structure is key to understanding how cells manage energy.

Mitochondria and Cellular Energy Production

Structure and function of mitochondria

Mitochondria are double-membrane organelles, and that double membrane is central to how they work.

• The outer membrane is smooth and relatively permeable to small molecules like ions and nutrients.
• The inner membrane is highly folded into structures called cristae. These folds dramatically increase the surface area available for the protein complexes that drive energy production. Embedded in this membrane are the electron transport chain (ETC) complexes (I through IV) and ATP synthase.
• The matrix is the innermost compartment, enclosed by the inner membrane. It contains the enzymes that run the citric acid cycle (also called the Krebs cycle), including pyruvate dehydrogenase and citrate synthase.
• The intermembrane space (between the two membranes) is where protons accumulate during oxidative phosphorylation, which is critical for ATP synthesis.

Mitochondria are the primary site of cellular respiration. The full breakdown of glucose involves three stages that span different locations:

1. Glycolysis occurs in the cytosol (not in the mitochondrion itself).
2. The citric acid cycle runs in the mitochondrial matrix.
3. Oxidative phosphorylation takes place at the inner membrane.

Together, these processes allow mitochondria to produce roughly 90–95% of a cell's ATP.

Process of oxidative phosphorylation

Oxidative phosphorylation is the final and most productive stage of aerobic respiration. It takes place at the inner mitochondrial membrane and can be broken into two coupled processes: the electron transport chain and chemiosmosis.

Electron Transport Chain (ETC):

1. NADH and FADH₂, generated during glycolysis and the citric acid cycle, donate their electrons to the ETC. NADH feeds electrons into Complex I, while FADH₂ feeds them into Complex II.
2. Electrons pass through Complexes I, III, and IV (with mobile carriers ubiquinone and cytochrome c shuttling electrons between complexes). As electrons move through these complexes, energy is released.
3. That released energy is used to pump protons ($H^+$) from the matrix into the intermembrane space, building up an electrochemical gradient called the proton motive force ($\Delta p$).

Chemiosmosis and ATP Synthesis:

1. The accumulated protons in the intermembrane space flow back down their concentration gradient through ATP synthase, a channel-like enzyme complex.
2. This flow drives the physical rotation of ATP synthase's subunits ($F_0$ in the membrane, $F_1$ in the matrix), and that rotation catalyzes the reaction:

$ADP + P_i \rightarrow ATP$

The theoretical maximum yield is about 30–32 ATP per glucose molecule (older textbook estimates of 36–38 don't account for the energy cost of transporting molecules across mitochondrial membranes). Either way, oxidative phosphorylation produces the vast majority of ATP in aerobic organisms.

Endosymbiotic Theory and Organelle Structure

Endosymbiotic theory for organelles

The endosymbiotic theory proposes that mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by an ancestral eukaryotic cell. Instead of being digested, these engulfed cells survived and eventually became permanent organelles. Mitochondria likely descended from α-proteobacteria (aerobic bacteria), while chloroplasts descended from cyanobacteria (photosynthetic bacteria).

Several lines of evidence support this theory:

• Own DNA: Both organelles contain their own circular DNA, similar to bacterial chromosomes, separate from the cell's nuclear DNA.
• Own ribosomes: Their ribosomes are 70S (the bacterial type), not the 80S ribosomes found in the eukaryotic cytoplasm.
• Binary fission: Both organelles replicate by dividing in two, the same way bacteria reproduce, rather than being built from scratch by the cell.
• Membrane composition: The inner mitochondrial membrane and the chloroplast thylakoid membrane contain cardiolipin and other phospholipids characteristic of bacterial membranes.

Over evolutionary time, the relationship became obligate: the host cell provided protection and nutrients, while the endosymbionts supplied energy (ATP from mitochondria) or organic compounds (sugars from chloroplasts). Many genes originally belonging to the endosymbionts were transferred to the host cell's nuclear genome, a process called endosymbiotic gene transfer. This is why most mitochondrial and chloroplast proteins are now encoded by nuclear DNA and imported into the organelle after translation.

Mitochondria vs chloroplasts in eukaryotes

These two organelles share a common evolutionary pattern but serve very different functions. Here's how they compare:

Similarities:

• Both are double-membrane organelles with their own circular DNA and 70S ribosomes
• Both produce energy for the cell (ATP or sugars)
• Both originated through endosymbiotic events
• Both replicate by binary fission

Differences:

One detail worth noting: chloroplasts have an extra level of internal organization. Their thylakoid membranes form flattened discs that stack into grana, and these are the site of the light-dependent reactions. The surrounding fluid, the stroma, is where the light-independent reactions (Calvin cycle) take place. Mitochondria lack this kind of internal membrane stacking; their cristae are simply folds of the inner membrane that project inward.