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 illustrates a schematic mitochondrion, containing two membranes (inner and outer mitochondrial membrane) and two compartments (matrix and intermembrane space). The matrix contains many enzymes of the citric acid cycle (also called Krebs cycle or TCA cycle). Electron transport and phosphorylation occur in the inner mitochondrial membrane. The region bounded by the blue box is enlarged in the next figure to illustrate the functions of the electron transport system.
The animation shows a cartoon view of the cell membrane, illustrating certain parts of the electron transport machinery. Electron carriers are clustered in several protein complexes in the membrane; other mobile carriers move electrons between these complexes. 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.
- Protons are translocated across the membrane, from the matrix to the intermembrane space
- Electrons are transported along the membrane, through a series of protein carriers
- Oxygen is the terminal electron acceptor, combining with electrons and H+ ions to produce water
- As NADH delivers more H+ and electrons into the ETS, the proton gradient increases, with H+ building up outside the inner mitochondrial membrane, and OH- inside the membrane.
Animation of ATP synthesis in Mitochondria
The schematic diagrams illustratesa mitochondrion. In the animation, watch as H+ ions accumulate in the outer mitochondrial compartment whenever NADH is made from oxidation reactions, generating a proton gradient (Animation 2). Protons re-enter the cell through the ATP synthase complex, generating ATP (Animation 3).
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. Energy in this gradient can be coupled to ATP synthesis.
The figure illustrates a cartoon mitochondrion, showing the matrix, inner and outer mitochondrial membranes, and intermembrane space. Each time NADH is oxidized, protons are moved across the membrane during electron transport. Notice the gradual build up 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 electron transport system.
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.
The figure and animations can be used to demonstrate to students the steps involved in cellular respiration.
Animation 3 shows: Step 2: Protons (indicated by + charge) enter back into the mitochondrial matrix through channels in ATP synthase enzyme complex. This entry is coupled to ATP synthesis from ADP and phosphate (Pi)
Key points of Animations 2 and 3:
- Protons are translocated across the membrane, from the matrix to the intermembrane space, as a result of electron transport resulting from the formation of NADH by oxidation reactions. (See Animation 1.) The continued buildup of these protons creates a proton gradient.
- ATP synthase is a large protein complex with a proton channel that allows re-entry of protons.
- ATP synthesis is driven by the resulting current of protons flowing through the membrane:
ADP + Pi ---> ATP