Maintaining an ideal environment in a cell requires the constant movement of molecules in and out of the cell through the plasma membrane. The membrane is made up almost entirely of proteins and phospholipids. There is a phospholipid bilayer with inwardly directed hydrophobic lipid tails, and outwardly directed hydrophilic heads. The bilayer is made up of a variety of protein molecules. The proteins that occur on or in only one of the layers are called "extrinsic" proteins. The proteins that extend across both layers are called "intrinsic" proteins. This phospholipid layer can move it is fluid, and in surface...
by "adenosine triphosphate" which is built up from the broken down respiratory substrates. When the molecules of adenosine triphosphate are hydrolysed, energy is released for reactions where it is needed in the cells. The process of active transport occurs through the carrier proteins that span the membrane. The proteins accept the molecule, and then the molecule enters the cell by a change in shape of the carrier molecule

All cells require materials to pass in and out through the plasma membrane. All of these transport processes are essential for reactions to take place, and therefore, for life to exist.