Figure 14–3 Stage 2 of chemiosmotic coupling. an atp synthase (yellow) embedded in the lipid bilayer of a membrane harnesses the electrochemical proton gradient across the membrane, using it as a local energy store to drive atp synthesis. the red arrows show the direction of proton movement through the atp synthase.
Figure 14–4 Electron-transport processes. (a) the mitochondrion converts energy from chemical fuels. (B) the chloroplast converts energy from sunlight. In both cases, electron flow is indicated by blue arrows. Each of the protein complexes (green) is embedded in a membrane. In the mitochondrion, fats and carbohydrates from food molecules are fed into the citric acid cycle and provide electrons to generate the energy-rich compound NaDh from NaD+. these electrons then flow down an energy gradient as they pass from one complex to the next in the electron-transport chain, until they combine with molecular O2 in the last complex to produce water. the energy released at each stage is harnessed to pump h+ across the membrane. In the chloroplast, by contrast, electrons are extracted from water through the action of light in the photosystem II complex and molecular O2 is released. the electrons pass on to the next complex in the chain, which uses some of their energy to pump protons across the membrane, before passing to photosystem I, where sunlight generates high-energy electrons that combine with NaDp+ to produce NaDph. NaDph then enters the carbon-fixation cycle along with CO2 to generate carbohydrates.
light energy and power the transfer of electrons, not unlike a photocell in a solar panel. The chloroplasts drive electron transfer in the direction opposite to that in mitochondria: electrons are taken from water to produce O2, and these electrons are used (via NADPH, a molecule closely related to the NADH used in mitochondria) to synthesize carbohydrates from CO2 and water. These carbohydrates then serve as the source for all other compounds a plant cell needs.
Thus, both mitochondria and chloroplasts make use of an electron-transfer chain to produce an H+ gradient that powers reactions that are critical for the cell. However, chloroplasts generate O2 and take up CO2, whereas mitochondria consume O2 and release CO2 (see Figure 14–4).
Mitochondria occupy up to 20% of the cytoplasmic volume of a eukaryotic cell. Although they are often depicted as short, bacterium-like bodies with a diameter of 0.5–1 μm, they are in fact remarkably dynamic and plastic, moving about the cell, constantly changing shape, dividing, and fusing (Movie 14.1). Mitochondria are often associated with the microtubular cytoskeleton (Figure 14–5), which determines their orientation and distribution in different cell types. Thus, in highly polarized cells such as neurons, mitochondria can move long distances (up to a meter or more in the extended axons of neurons), being propelled along the tracks of the microtubular cytoskeleton. In other cells, mitochondria remain fixed at points of high energy demand; for example, in skeletal or cardiac muscle cells, they pack between myofibrils, and in sperm cells they wrap tightly around the flagellum (Figure 14–6).
Mitochondria also interact with other membrane systems in the cell, most notably the endoplasmic reticulum (ER). Contacts between mitochondria and ER define specialized domains thought to facilitate the exchange of lipids between the two membrane systems. These contacts also appear to induce mitochondrial fission, which, as we discuss later, is involved in the distribution and partitioning of mitochondria within cells (Figure 14–7).