Through a set of reactions that occur in the cytosol, energy derived from the partial oxidation of energy-rich carbohydrate molecules is used to form ATP, the chemical energy currency of cells (discussed in Chapter 2). But a much more efficient method of energy generation appeared very early in the history of life. This process is based on membranes, and it enables cells to acquire energy from a wide variety of sources. For example, it is central to the conversion of light energy into chemical bond energy in photosynthesis, as well as to the aerobic respiration that enables us to use oxygen to produce large amounts of ATP from food molecules.

The membrane that is used to produce ATP in procaryotes is the plasma membrane. But in eucaryotic cells, the plasma membrane is reserved for the transport processes described in Chapter 11. Instead, the specialized membranes inside energy-converting organelles are employed for the production of ATP. The membrane-enclosed organelles are mitochondria, which are present in the cells of virtually all eucaryotic organisms (including fungi, animals, and plants), and plastids—most notably chloroplasts—which occur only in plants. In electron micrographs the most striking morphological feature of mitochondria and chloroplasts is the large amount of internal membrane they contain. This internal membrane provides the framework for an elaborate set of electron-transport processes that produce most of the cell's ATP.

The common pathway used by mitochondria, chloroplasts, and procaryotes to harness energy for biological purposes operates by a process known as chemiosmotic coupling—reflecting a link between the chemical bond-forming reactions that generate ATP (“chemi”) and membrane-transport processes (“osmotic”). The coupling process occurs in two linked stages, both of which are performed by protein complexes embedded in a membrane:

• Stage 1. High-energy electrons (derived from the oxidation of food molecules, from the action of sunlight, or from other sources discussed later) are transferred along a series of electron carriers embedded in the membrane. These electron transfers release energy that is used to pump protons (H⁺, derived from the water that is ubiquitous in cells) across the membrane and thus generate an electrochemical proton gradient. As discussed in Chapter 11, an ion gradient across a membrane is a form of stored energy, which can be harnessed to do useful work when the ions are allowed to flow back across the membrane down their electrochemical gradient.
• Stage 2. H⁺ flows back down its electrochemical gradient through a protein machine called ATP synthase, which catalyzes the energy-requiring synthesis of ATP from ADP and inorganic phosphate (P_i). This ubiquitous enzyme plays the role of a turbine, permitting the proton gradient to drive the production of ATP (Figure 14-1).

Figure 14-1

Harnessing energy for life. (A) The essential requirements for chemiosmosis are a membrane—in which are embedded a pump protein and an ATP synthase, plus a source of high-energy electrons (e ^-). The protons (H⁺) shown are freely available from (more...)

The electrochemical proton gradient is also used to drive other membrane-embedded protein machines (Figure 14-2). In eucaryotes, special proteins couple the “downhill” H⁺ flow to the transport of specific metabolites into and out of the organelles. In bacteria, the electrochemical proton gradient drives more than ATP synthesis and transport processes; as a store of directly usable energy, it also drives the rapid rotation of the bacterial flagellum, which enables the bacterium to swim.

Figure 14-2

Chemiosmotic coupling. Energy from sunlight or the oxidation of foodstuffs is first used to create an electrochemical proton gradient across a membrane. This gradient serves as a versatile energy store and is used to drive a variety of energy-requiring (more...)

It is useful to compare the electron-transport processes in mitochondria, which convert energy from chemical fuels, with those in chloroplasts, which convert energy from sunlight (Figure 14-3). In the mitochondrion, electrons—which have been released from a carbohydrate food molecule in the course of its degradation to CO₂—are transferred through the membrane by a chain of electron carriers, finally reducing oxygen gas (O₂) to form water. The free energy released as the electrons flow down this path from a high-energy state to a low-energy state is used to drive a series of three H⁺ pumps in the inner mitochondrial membrane, and it is the third H⁺ pump in the series that catalyzes the transfer of the electrons to O₂ (see Figure 14-3A).

Figure 14-3

Electron transport processes. (A) The mitochondrion converts energy from chemical fuels. (B) The chloroplast converts energy from sunlight. Inputs are light green, products are blue, and the path of electron flow is indicated by red arrows. Each of the (more...)

The mechanism of electron transport can be compared to an electric cell driving a current through a set of electric motors. However, in biological systems, electrons are carried between one site and another not by conducting wires, but by diffusible molecules that can pick up electrons at one location and deliver them to another. For mitochondria, the first of these electron carriers is NAD⁺, which takes up two electrons (plus an H⁺) to become NADH, a water-soluble small molecule that ferries electrons from the sites where food molecules are degraded to the inner mitochondrial membrane. The entire set of proteins in the membrane, together with the small molecules involved in the orderly sequence of electron transfers, is called an electron-transport chain.

Although the chloroplast can be described in similar terms, and several of its main components are similar to those of the mitochondrion, the chloroplast membrane contains some crucial components not found in the mitochondrial membrane. Foremost among these are the photosystems, where light energy is captured by the green pigment chlorophyll and harnessed to drive the transfer of electrons, much as man-made photocells in solar panels absorb light energy and use it to drive an electric current. The electron-motive force generated by the chloroplast photosystems drives electron transfer in the direction opposite to that in mitochondria: electrons are taken from water to produce O₂, and they are donated (via NADPH, a compound closely related to NADH) to CO₂ to synthesize carbohydrate. Thus, the chloroplast generates O₂ and carbohydrate, whereas the mitochondrion consumes them (see Figure 14-3B).

It is generally believed that the energy-converting organelles of eucaryotes evolved from procaryotes that were engulfed by primitive eucaryotic cells and developed a symbiotic relationship with them. This would explain why mitochondria and chloroplasts contain their own DNA, which codes for some of their proteins. Since their initial uptake by a host cell, these organelles have lost much of their own genomes and have become heavily dependent on proteins that are encoded by genes in the nucleus, synthesized in the cytosol, and then imported into the organelle. Conversely, the host cells have become dependent on these organelles for much of the ATP they need for biosyntheses, ion pumping, and movement; they have also become dependent on selected biosynthetic reactions that occur inside these organelles.