Prenol lipids are synthesized from the five-carbon-unit precursors isopentenyl diphosphate and dimethylallyl diphosphate that are produced mainly via the mevalonic acid (MVA) pathway.  The simple isoprenoids (linear alcohols, diphosphates, etc.) are formed by the successive addition of C5 units, and are classified according to number of these terpene units. Structures containing greater than 40 carbons are known as polyterpenes. Carotenoids are important simple isoprenoids that function as antioxidants and as precursors of vitamin A .  Another biologically important class of molecules is exemplified by the quinones and hydroquinones , which contain an isoprenoid tail attached to a quinonoid core of non-isoprenoid origin.  Vitamin E and vitamin K , as well as the ubiquinones , are examples of this class. Prokaryotes synthesize polyprenols (called bactoprenols ) in which the terminal isoprenoid unit attached to oxygen remains unsaturated, whereas in animal polyprenols ( dolichols ) the terminal isoprenoid is reduced. 
5. Signal transduction – Proteins responsible for detecting stimuli are generally termed receptors, although in some cases the term sensor is used. The changes elicited by ligand binding in a receptor give rise to a cascade of events along a signalling pathway. When signalling pathways interact with one another they form networks, which allow cellular responses to be coordinated and these molecular events are the basic mechanisms controlling cell growth, proliferation, metabolism and many other processes. In multicellular organisms, signal transduction pathways have evolved to regulate cell communication in a variety of ways. Each component of a pathway is classified according to the role it plays with respect to the initial stimulus. Ligands are termed first messengers, while receptors are the signal transducers, such effectors are often linked to second messengers, which can activate secondary effectors, and so on. Depending on the efficiency of the nodes, a signal can be amplified, as with other signals, the transduction of biological signals is characterised by delay, noise and interference, which can range from negligible to pathological. With the advent of computational biology, the analysis of signalling pathways and networks has become a tool to understand cellular functions. The basis for signal transduction is the transformation of a certain stimulus into a biochemical signal, traditionally, signals that reach the central nervous system are classified as senses. These are transmitted from neuron to neuron in a process called synaptic transmission, many other intercellular signal relay mechanisms exist in multicellular organisms, such as those that govern embryonic development. The majority of signal transduction pathways involve the binding of signalling molecules, known as ligands, the combination of a signaling molecule with a receptor causes a change in the conformation of the receptor, known as receptor activation. Most ligands are soluble molecules from the medium which bind to cell surface receptors. These include growth factors, cytokines and neurotransmitters, components of the extracellular matrix such as fibronectin and hyaluronan can also bind to such receptors. In addition, some such as steroid hormones are lipid-soluble. In the case of hormone receptors, their stimulation leads to binding to the promoter region of steroid-responsive genes. Not all classifications of signalling molecules take into account the nature of each class member. For example, odorants belong to a range of molecular classes, as do neurotransmitters. Moreover, some molecules may fit more than one class, e. g. epinephrine is a neurotransmitter when secreted by the central nervous system
Steroid isolation , depending on context, is the isolation of chemical matter required for chemical structure elucidation, derivitzation or degradation chemistry, biological testing, and other research needs (generally milligrams to grams, but often more  or the isolation of "analytical quantities" of the substance of interest (where the focus is on identifying and quantifying the substance (for example, in biological tissue or fluid). The amount isolated depends on the analytical method, but is generally less than one microgram.  [ page needed ] The methods of isolation to achieve the two scales of product are distinct, but include extraction , precipitation, adsorption , chromatography , and crystallization . In both cases, the isolated substance is purified to chemical homogeneity; combined separation and analytical methods, such as LC-MS , are chosen to be "orthogonal"—achieving their separations based on distinct modes of interaction between substance and isolating matrix—to detect a single species in the pure sample. Structure determination refers to the methods to determine the chemical structure of an isolated pure steroid, using an evolving array of chemical and physical methods which have included NMR and small-molecule crystallography .  : 10–19 Methods of analysis overlap both of the above areas, emphasizing analytical methods to determining if a steroid is present in a mixture and determining its quantity.