is the addition of cyclopentane rings within the hydrocarbon tails of the ester-linked phospholipids. The addition of these rings into the membrane allows for even tighter packing of the membrane molecules. These cyclopentane rings can exist in tetraether lipids or diether lipids. By increasing the number of atoms in the middle of the membrane, there is less space for solutes to move in or out of the cell. This helps again to control the amount of solutes moving in and out of the cell. Cyclopentane rings help to crowd the membrane's inner structure making it more difficult for the solutes to get through the membrane to the other side of cell. This is so important for the cell because at hyperthermophilic conditions, the solutes travel very fast, carrying a lot of thermal energy from the environment. If the cell did not have these rings, too many unwanted molecules would likely pass through the membrane either into or out of the cell. This would result in the slowing or complete stop of metabolic processes resulting in cell death. While these cyclopentane rings are extremely useful for keeping unwanted solutes from entering or leaving the cell, not all archaea use them. They are even seen in psychrophiles, which are archaea that require very cold conditions to survive (−15 °C or below). This is counterintuitive because cyclopentane molecules help to make the membrane more rigid, which is something that happens naturally. It is unclear why these rings are seen at both ends of the temperature spectrum, but it is clear that they serve functions other than simply slowing molecules from entering or leaving a cell. Metabolism Because organisms like P. fumarii live in such harsh environments, these archaea have needed to devise unusual ways to gather energy from the environment and protect themselves against heat stress. P. fumarii, like plants, are able to harvest CO2 from the environment to build their biomolecules, but unlike plants, they take electrons from H2 instead of H2O and transfer those electrons to NO3−, SO42− or O2. This type of metabolic process is classified as chemolithoautrophism, meaning their carbon comes from an inorganic source, their final electron acceptor is not O2 and they produce and consume their own food. Another way that hyperthermophiles ensure their proteins' proper function is by using heat shock proteins (HSPs). While these HSPs are not unique to extremophiles, they are extremely important to