Figure 1.
Respiration is a process in which electrons are transferred sequentially through a series of membrane-bound protein carriers, the electron transport chain. Electrons are removed from membrane carriers by reducing some terminal electron acceptor such as oxygen (aerobic respiration) or nitrogen, sulfate, or carbon dioxide (anaerobic respiration). This process occurs in mitochondria in most eukaryotic cells, or in the cell membrane of prokaryotic cells.

The figure can illustrate to students a schematic bacterial cell, showing the location of the cell membrane, cell wall, and cytoplasm. The small section surrounding cell membrane, boxed in blue, is enlarged in the following image to demonstrate the organization of electron transport in bacterial membranes.

Animation 1.
The animation shows a cartoon view of the cell membrane, illustrating certain parts of the electron transport machinery. The area shown in the animation is magnified from the blue square shown in Figure 1. The exact organization of enzymes will vary among different bacteria; this illustration is only intended to demonstrate the basic features of the process, not to accurately represent any particular bacterial cell. In the animation, watch as NADH transfers H+ ions and electrons into the electron transport system. Electron transport typically involves the following stages:

1. Electron transport begins when electron carriers such as reduced nicotinamide adenine dinucleotide (NADH) release electrons (typically in the form of hydrogen atoms) to membrane-bound electron carriers, Enzyme Complex I in this diagram.
2. Protons are translocated across the cell membrane, from the cytoplasm to the periplasmic space just outside the membrane. As protons accumulate outside the membrane, hydroxyl ions accumulate inside the membrane.
3. Electrons are transported along the membrane, through a series of protein carriers.
4. Oxygen is the terminal electron acceptor, combining with electrons and H+ ions to produce water.

Key points:

1. Protons are translocated across the cell membrane, from the cytoplasm to the periplasmic space
2. Electrons are transported along the membrane, through a series of protein carriers
3. Oxygen is the terminal electron acceptor, combining with electrons and H⁺ ions to produce water
4. As NADH delivers more H⁺ and electrons into the ETS, the proton gradient increases, with H⁺ building up outside the cell membrane, and OH^- inside the membrane.

Animations of Chemiosmotic ATP synthesis in Bacteria

Animation 2.
In respiration, electron transport is not directly coupled to ATP synthesis. Instead, as electrons flow through the electron transport chain, protons are simultaneously translocated across the membrane. As more electrons flow, more protons accumulate just outside the membrane, resulting in a substantial proton gradient. This gradient can be used for a variety of purposes, such as ATP synthesis, transport, or flagellar motion.

The figure illustrates a cartoon bacterial cell, showing the location of the cell membrane, cell wall, and cytoplasm. Each time NADH is oxidized, protons are moved across the membrane. Notice the gradual buildup of a proton gradient, coupled to the synthesis of ATP which provides needed electrons. The small section of cell membrane boxed in blue is enlarged in the following image to demonstrate how ATP synthesis can be powered by a proton gradient.

Animation 2 shows:

Step 1: Proton gradient is built up as a result of NADH (produced from oxidation reactions) feeding electrons into the electron transport system. As these electrons are transported through a series of electron carriers, protons are translocated to the outside face of the cell membrane.

Animation 3.
Proton gradients store energy, both because of charge separation and concentration differential, and protons would rapidly cross the membrane to restore equilibrium if allowed. The cell membrane is impermeable to protons, except through protein complexes called ATP synthases. When protons move through these complexes, energy released by their passage is coupled to synthesis of ATP from ADP and phosphate (Pi). The exact details of this coupling are not yet clear, although it is known that part of the ATP synthase complex rotates during transport. The animation is not intended to be an accurate scale model of the details of the process, only to sketch the major concept, that entry of protons provides the energy needed to make ATP. This process is often called oxidative phosphorylation, since oxygen is a frequent electron acceptor, but is more accurately called chemiosmotic phosphorylation, since phosphorylation is coupled to the discharge of a chemiosmotic gradient.

Animation 3 shows:
Step 2: Protons (indicated by + charge) enter back into the bacterial cytoplasm through channels in ATP synthase enzyme complex. This entry is coupled to ATP synthesis from ADP and phosphate (P_i)

Key notes for Animations 2 and 3:

1. Protons are first translocated across the membrane, from the cytoplasm to the periplasmic space, as a result of electron transport resulting from the formation of NADH by oxidation reactions. (See animation of electron transport (Animation 1) if you don't understand this step.) The continued buildup of these protons creates a proton gradient.
2. ATP synthase is a large protein complex with a proton channel that allows re-entry of protons.
3. ATP synthesis is driven by the resulting current of protons flowing through the membrane:
   ADP + P_i ---> ATP