<P> The nuclear membrane contains a lipid bilayer that encompass the contents of the nucleus . The endoplasmic reticulum (ER) is a synthesis and transport organelle that branches into the cytoplasm in plant and animal cells . The Golgi apparatus is a series of multiple compartments where molecules are packaged for delivery to other cell components or for secretion from the cell . Vacuoles, which are found in both plant and animal cells (though much bigger in plant cells), are responsible for maintaining the shape and structure of the cell as well as storing waste products . A vesicle is a relatively small, membrane - enclosed sac that stores or transports substances . The cell membrane is a protective barrier that regulates what enters and leaves the cell . There is also an organelle known as the Spitzenkörper that is only found in fungi, and is connected with hyphal tip growth . </P> <P> In prokaryotes endomembranes are rare, although in many photosynthetic bacteria the plasma membrane is highly folded and most of the cell cytoplasm is filled with layers of light - gathering membrane . These light - gathering membranes may even form enclosed structures called chlorosomes in green sulfur bacteria . </P> <P> The organelles of the endomembrane system are related through direct contact or by the transfer of membrane segments as vesicles . Despite these relationships, the various membranes are not identical in structure and function . The thickness, molecular composition, and metabolic behavior of a membrane are not fixed, they may be modified several times during the membrane's life . One unifying characteristic the membranes share is a lipid bilayer, with proteins attached to either side or traversing them . </P> <P> Most lipids are synthesized in yeast either in the endoplasmic reticulum, lipid particles, or the mitochondrion, with little or no lipid synthesis occurring in the plasma membrane or nuclear membrane . Sphingolipid biosynthesis begins in the endoplasmic reticulum, but is completed in the Golgi apparatus . The situation is similar in mammals, with the exception of the first few steps in ether lipid biosynthesis, which occur in peroxisomes . The various membranes that enclose the other subcellular organelles must therefore be constructed by transfer of lipids from these sites of synthesis . However, although it is clear that lipid transport is a central process in organelle biogenesis, the mechanisms by which lipids are transported through cells remain poorly understood . </P>

What are the two types of molecules that compose cellular membranes