The oxidative phosphorylation pathway
In oxidative phosphorylation, the electrons removed from food molecules in pathways such as the citric acid cycle (Krebs cycle) are transferred to oxygen and the energy released is used to make ATP.
This is done in eukaryotes by a series of proteins in the membranes of mitochondria called the electron transport chain (oxidative phosphorylation chain). In prokaryotes, these proteins are found in the cell’s inner membrane.
These proteins use the energy released from passing electrons from reduced molecules like NADH onto oxygen to pump protons across a membrane.
Pumping protons out of the mitochondria creates a proton concentration difference across the membrane and generates an electrochemical gradient.
This force drives protons back into the mitochondrion through the base of an enzyme called ATP synthase. The flow of protons makes the stalk subunit rotate, causing the active site of the synthase domain to change shape and phosphorylate adenosine diphosphate (ADP), turning it into ATP.
Energy from inorganic compounds in prokaryotes
Chemolithotrophy is a type of metabolism found in prokaryotes where energy is obtained from the oxidation of inorganic compounds. These organisms can use hydrogen, reduced sulfur compounds (such as sulfide, hydrogen sulfide and thiosulfate), ferrous iron (FeII) or ammonia as sources of reducing power and they gain energy from the oxidation of these compounds with electron acceptors such as oxygen or nitrite.
These microbial processes are important in global biogeochemical cycles such as acetogenesis, nitrification and denitrification and are critical for soil fertility.
Energy from light
The energy in sunlight is captured by plants, cyanobacteria, purple bacteria, green sulfur bacteria and some protists. This process is often coupled to the conversion of carbon dioxide (CO2) into organic compounds, as part of photosynthesis, which is discussed below.
The energy capture and carbon fixation systems can however operate separately in prokaryotes, as purple bacteria and green sulfur bacteria can use sunlight as a source of energy, while switching between carbon fixation and the fermentation of organic compounds.
In many organisms the capture of solar energy is similar in principle to oxidative phosphorylation, as it involves energy being stored as a proton concentration gradient and this proton motive force then driving ATP synthesis.
The electrons needed to drive this electron transport chain come from light-gathering proteins called photosynthetic reaction centres or rhodopsins.
Reaction centers are classed into two types depending on the type of photosynthetic pigment present, with most photosynthetic bacteria only having one type, while plants and cyanobacteria have two.
In plants, algae, and cyanobateria, photosystem II uses light energy to remove electrons from water, releasing oxygen as a waste product. The electrons then flow to the cytochrome b6f complex, which uses their energy to pump protons across the thylakoid membrane in the chloroplast.
These protons move back through the membrane as they drive the ATP synthase, as before. The electrons then flow through photosystem I and can then either be used to reduce the coenzyme NADP+, for use in the Calvin cycle which is discussed below, or recycled for further ATP generation.
See also
energy production