the same temperatures as humans. Extremophiles are organisms that grow best in extremely cold, acidic, basic or hot environments. P. fumarii is a hyperthermophile, indicating that this organism grows best at extremely high temperatures (70–125 °C). P. fumarii grows best at 106 °C. Due to the extremely high temperatures this archaea is subjected to, this organism must have extremely stable biomolecules to survive. Without increased stability in the membrane, the cell would fall apart, and too many molecules would flow in and out of the membrane, destroying the chemical gradients the cell uses as energy and letting all the proteins the cell has synthesized diffuse away, stopping its metabolic processes. To deal with this problem archaea have changed their membrane lipid compositions. They still contain phosphate groups and long fatty acid tails, but they also contain ether linkages instead of ester linkages. The ether linkages make the bonds between phosphate groups and hydrocarbons more stable because the carbon connecting the phosphate group and glycerol molecule is more electron-rich than it would be in an ester, making that carbon less electrophilic and therefore less chemically reactive. This lets the ether-linked phospholipid be more stable and less susceptible to breakdown from large amounts of increased thermal energy. This contributes to the archaea's ability to live in such extreme environments. Another membrane adaptation seen in some archaea is tetraether phospholipids. This specific adaptation has been found in P. fumarii along with other hyperthermophiles. A tetraether phospholipid is a molecule containing two hydrocarbon tails, each coming from one ester bond and one phosphate molecule. These phospholipids form monolayers instead of the typical bilayers seen in most bacteria and all eukaryotes. Therefore, instead two different molecules interacting with each other, only one molecule spans the entire width of the membrane. The monolayer then allows for tighter packing of molecules within the membrane because fewer molecules must fit into membrane, however these large molecules are less able to move within the membrane. This then decreases membrane fluidity, allowing the cell to keep more molecules from crossing the membrane. This is an extremely important adaptation because at such high temperatures, molecules move more quickly than at mesophilic temperatures. By decreasing the membrane fluidity, the cell can decrease movement of the phospholipid molecules, which stops unwanted movement of molecules across the membrane. Another extremely important membrane regulation modification that archaea use to control influx and efflux of solutes