The proton-motive force is a combination of a difference in proton (H+ ion) concentrations across a membrane, and the resulting electrical potential. All prokaryotic cells (Bacteria and Archaea) maintain a proton gradient (pH gradient) across their plasma membranes. Mitochondria maintain a proton gradient across the inner mitochondrial membrane. The interior of the bacterial cell (or the mitochondrial matrix) is relatively alkaline, whereas the exterior periplasmic space (or the mitochondrial intermembrane space) is relatively acidic. Because protons are positively charged, an imbalance of protons also creates an electrical charge difference across the membrane. This proton motive force is a form of stored energy, and protons returning across the membrane down their concentration and electrical charge gradients release free energy that can be harnessed by ATP synthase to make ATP. The lipid bilayer membrane is almost impermeable to protons, so the proton gradient is stable and normally does not discharge except via ATP synthase, or via proton channels that may open in the membrane.
The location of the electron transport chain on the inner mitochondrial membrane, and the pyruvate oxidation and citric acid cycle in the mitochondrial matrix, makes sense in light of the endosymbiont theory for the origin of mitochondria. These locations correspond to the plasma membrane and cytoplasm of the aerobic bacterial endosymbiont, most likely an alpha-proteobacterium, that was the ancestor of mitochondria. The outer mitochondrial membrane derived from the endosomal membrane that originally engulfed the endosymbiont.
We find that all cells – Bacteria, Archaea, Eukarya – use the energy released via ATP hydrolysis (ATP –> ADP + Pi; Pi = inorganic phosphate; ΔG = -7.3 kcal/mol) to perform most of the cell’s work. How do cells make ATP? Cells can make ATP in either of two ways: either by of ADP, or by of ADP.
In eukaryotic cells, glycolysis and fermentation reactions occur in the cytoplasm. The remaining pathways, starting with pyruvate oxidation, occur in the mitochondria. Most eukaryotic mitochondria can use only oxygen as the terminal electron acceptor for respiration. In the presence of oxygen, pyruvate enters the mitochondrial matrix and is oxidized to acetyl-CoA, and then to CO2 via the citric acid cycle. The electron transport chain and ATP synthase are located on the mitochondrial inner membrane.
In prokaryotic cells, all the metabolic pathways occur in the cytoplasm, except for chemiosmosis and oxidative phosphorylation, which occur on the plasma membrane. Prokaryotic cells are capable of anaerobic respiration using alternative electron acceptors such as nitrate and sulfate, although they prefer oxygen as the terminal electron acceptor to drive chemiosmotic ATP synthesis. In the absence of any suitable electron acceptor, they use fermentation pathways.
Substrate-level phosphorylation means that a phosphate is transferred to ADP from a high-energy phosphorylated organic compound. We will see in the section on metabolic pathways that a couple of the enzymes in make ATP through substrate-level phosphorylation, as well as an enzyme in the . However, only a small amount of ATP is made this way in cells undergoing respiration.
In this class, we will take an evolutionary approach that begins with concepts and processes fundamental to all living cells, that must have been present in the last universal common ancestor (LUCA).
Energy metabolism in eukaryotic cells, from Wikipedia. This is a highly condensed illustration. Students should identify the various compounds and talk through (explain) this diagram.
Oxidative phosphorylation synthesizes the bulk of a cell’s ATP during cellular respiration. A , in the form of a large proton concentration difference across the membrane, provides the energy for the membrane-localized (a molecular machine) to make ATP from ADP and inorganic phosphate (Pi). The proton gradient is generated by a series of oxidation-reduction reactions carried out by protein complexes that make up an electron transport chain in the membrane. The term oxidative phosphoryation, then, refers to phosphorylation of ADP to ATP coupled to oxidation-reduction reactions.
Oxidative phosphorylation uses the energy from a membrane proton gradient to power ATP synthesis from ADP and inorganic phosphate . Image from Wikimedia Commons
The standard freshman biology textbook presentation focuses narrowly on glucose metabolism by animal cells, barely touches on fats and amino acids, and ignores most of the metabolic diversity of life. Moreover, the standard textbook version of how this elaborate metabolic network evolved beginning with is probably wrong, accordingly to the compelling essay by ).
Further evidence to support the endosymbiont theory is that mitochondria have their own DNA, in the form of a circular chromosome that is topologically like bacterial chromosomes. The sequence of the mitochondrial DNA most closely resembles the sequences of genes in alpha-proteobacteria. Mitochondrial ribosomes are structurally more similar to bacterial ribosomes than to eukaryotic ribosomes. Mitochondria reproduce in eukaryotic cells by fission, again resembling bacterial cell division.
The electron transport chain takes electrons from reduced electron carriers (NADH) and passes them to a terminal electron acceptor (O2), and uses the free energy released to generate a membrane proton gradient. Note that the ATP synthase is not part of the electron transport chain, but is shown here because it uses the proton gradient to power ATP synthesis. The ETC builds up the proton gradient, while the ATP synthase discharges the proton gradient in the process of making ATP.
The Nobel Prize winner Otto Warburg observed that many, and perhaps most, cancer cells derive most of their energy from glycolysis and lactic acid fermentation, even when oxygen is plentiful (see review by Liberti and Locasale, 2016). Several explanations have been proposed. One is that cancer cells can promote biosynthesis and cell growth by NOT respiring organic carbon to CO2, and using the organic carbon to build cellular biomolecules, instead. Another hypothesis is that ramping up glycolysis allows tumor cells to out-compete normal cells or immune system cells for glucose. A third hypothesis is that lactic acid secretion causes changes in the tumor cells’ environment that favors tumor cell growth and spread.