The cell has mechanisms that transfer energy between the four currencies. The conversions are summarized in Figure 12.4. In a typical animal cell oxidation of fuel molecules in the mitochondria (by the Krebs cycle, page 283) tops up the supply of NADH. The cell then converts this energy currency into the other three. All the interconversions are reversible. Figure 12.3 shows where each conversion mechanism is located.
Consider what happens if a few sodium ions move out of the extracellular fluid and into the cytosol of a eukaryotic cell—for example, when a nerve cell transmits the electrical signal called an action potential (Chapter 15). The sodium gradient has been slightly depleted: the cell holds less of this energy currency than it did before. However, the cell still has plenty of energy in the form of ATP that it can convert into energy as a sodium gradient. It does this using the sodium/potassium ATPase. This protein is located in the plasma membrane. Its function is to move 3Na+ ions out of the cell and to move 2K+ ions into the cell. For this to happen ATP is hydrolyzed to ADP thus giving up energy that is used to push Na+ out of the cytosol to its higher energy state in the extracellular medium.
The cell has now used some ATP, but it still holds plenty of energy in another currency— the H+ gradient across the inner mitochondrial membrane. The enzyme ATP synthase is located in the inner mitochondrial membrane. The protein interconverts the two energy currencies: as H+ ions move into the mitochondrion they give up energy that is then used by ATP synthase to make ATP from ADP and inorganic phosphate.
The energy of the H+ gradient is now depleted. However, the cell still has plenty of energy in its NADH account. The electron transport chain allows interconversion of energy as NADH to energy as H+ gradient. NADH is used to reduce molecular oxygen to
water, releasing energy that is used to push H+ ions out of the mitochondrion to their higher energy state in the cytosol.
Each of the three energy conversion systems is a protein structure called a carrier because it carries solute across the membrane. The sodium/potassium ATPase carries sodium and potassium ions, while both ATP synthase and the electron transport chain carry H+ ions. There are many carriers in the cell with a wide variety of functions, some of which we will meet later in this book. The three that convert between the energy currencies are vital and are evolutionarily ancient.
Although by no means all bacteria are motile, some, such as Escherichia coli and Salmonella typhimurium, can swim. Their small size makes the process very different from swimming as we experience it. If you, the reader, dive into a swimming pool, your momentum will carry you some distance because your mass is great relative to the density of the water. For a bacterium whose mass is extremely small, water is a highly viscous medium. A bacterium swimming in water experiences roughly what a human swimmer in treacle might feel! Despite this, motile bacteria can achieve speeds of up 100 /m s-1 by the use of a structure called the bacterial flagellum. This consists of a rigid helical filament, 20-40 nm in diameter and up to 10 /m long, composed of a single protein, flagellin. The bacterial flagellum operates like a boat's propeller, pushing the bacterium along as it is turned by the motor at its base.
The flagellar motor consists of a series of rings that allow the motor to rotate within the complex layers of membranes and cell wall that make up the bacterial cell surface. Like ATP synthase, the flagellar motor allows H+ ions to flow in across the plasma membrane. The energy released as the H+ ions flow down their electrochemical gradient is used to turn the rotor of the motor and hence the flagellum at up to 100 Hz (=100 times per second).
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