Figure 14–1 The membrane systems of bacteria, mitochondria, and chloroplasts are related. Mitochondria and chloroplasts are cell organelles that have originated from bacteria and have retained the bacterial energy-conversion mechanisms. Like their bacterial ancestors, mitochondria and chloroplasts have an outer and an inner membrane. Each of the membranes colored in this diagram contains energy-harvesting electron-transport chains. the deep invaginations of the mitochondrial inner membrane and the internal membrane system of the chloroplast harbor the machinery for cellular respiration and photosynthesis, respectively.
(“osmotic”). The chemiosmotic process occurs in two linked stages, both of which are performed by protein complexes in a membrane. Stage 1: High-energy electrons (derived from the oxidation of food molecules, from pigments excited by sunlight, or from other sources described later) are transferred along a series of electron-transport protein complexes that form an electron-transport chain embedded in a membrane. Each electron transfer releases a small amount of energy that is used to pump protons (H+) and thereby generate a large electrochemical gradient across the membrane (Figure 14–2). As discussed in Chapter 11, such an electrochemical gradient provides a way of storing energy, and it can be harnessed to do useful work when ions flow back across the membrane.
Stage 2: The protons flow back down their electrochemical gradient through an elaborate membrane protein machine called ATP synthase, which catalyzes the production of ATP from ADP and inorganic phosphate (Pi). This ubiquitous enzyme works like a turbine in the membrane, driven by protons, to synthesize ATP (Figure 14–3). In this way, the energy derived from food or sunlight in stage 1 is converted into the chemical energy of a phosphate bond in ATP.
Electrons move through protein complexes in biological systems via tightly bound metal ions or other carriers that take up and release electrons easily, or by special small molecules that pick electrons up at one location and deliver them to another. For mitochondria, the first of these electron carriers is NAD+, a water-soluble small molecule that takes up two electrons and one H+ derived from food molecules (fats and carbohydrates) to become NADH. NADH transfers these electrons from the sites where the food molecules are degraded to the inner mitochondrial membrane. There, the electrons from the energy-rich NADH are passed from one membrane protein complex to the next, passing to a lower-energy compound at each step, until they reach a final complex in which they combine with molecular oxygen (O2) to produce water. The energy released at each step as the electrons flow down this path from the energy-rich NADH to the low-energy water molecule drives H+ pumps in the inner mitochondrial membrane, utilizing three different membrane protein complexes. Together, these complexes generate the proton-motive force harnessed by ATP synthase to produce the ATP that serves as the universal energy currency throughout the cell (see Chapter 2).
Figure 14–4 compares the electron-transport processes in mitochondria, which harness energy from food molecules, with those in chloroplasts, which harness energy from sunlight. The energy-conversion systems of mitochondria and chloroplasts can be described in similar terms, and we shall see later in the chapter that two of their key components are closely related. One of these is the ATP synthase, and the other is a proton pump (colored green in Figure 14–4).
Among the crucial constituents that are unique to photosynthetic organisms are the two photosystems. These use the green pigment chlorophyll to capture
Figure 14–2 Stage 1 of chemiosmotic coupling. Energy from sunlight or the oxidation of food compounds is captured to generate an electrochemical proton gradient across a membrane. the electrochemical gradient serves as a versatile energy store that drives energy-requiring reactions in mitochondria, chloroplasts, and bacteria.