The chemical term lipids defines a class of distinct compounds; it also includes the "fats" as well as the sterols such as cholesterol. A better and simple way to class lipid compounds would be to state that these are substances including the fats, all oils or waxes that are not soluble in water but readily dissolves in alcohol. Like the other two primary organic compounds carbohydrates and proteins, the lipids are also made up of carbon, hydrogen and oxygen; they differ from carbohydrates mainly in that the proportion of oxygen in lipids tends to be much lower. All living cells require lipids as the lipids form an important structural part of the cell - besides performing other important roles inside the cell itself. All plant and animals cells contain lipids along with the other two primary organic compounds, namely proteins and carbohydrates. In fact, all cells are vital aggregations of these three organic compounds.
Lipids include the important biomolecule cholesterol as well as the triglycerides - the principal storage fats in adipose tissues. The body readily stores lipids - these high energy biomolecules are used as a store of energy by all cells. The two main roles of lipids are their structural role in the cell architecture and their storage role as a source of fuel. In general, the term lipids cover the fatty acids, the neutral fats, as well as the waxes and steroids like the important hormone cortisone. Lipids are classified into many types, one important type being the compound lipids in which one kind of lipid is combined or chemically bound with another type of organic or inorganic partner, these class of lipids are important and include the combination with proteins called lipoproteins, the glycolipids which are a combination with carbohydrates as well as the phospholipids.
Some common properties shared by all classes of lipids are the fact that they are not soluble in water - they are non-polar molecules that are only soluble in some weakly polar or non-polar solvents including chloroform and alcohols. The formation of cellular structures is an important function of lipids, all kinds of cells in the body depend on lipids to stems to form sheet like membranes, that when combined with proteins forms the cell or plasma membrane - the basis of cellular life itself and by extension the most vital requirement for cellular organization in living systems. In both the simplest prokaryotic cells like bacteria and the more complex eukaryotic cells - animals and plants - the plasma membrane separates the cellular contents that form the living organism from the external fluid environment. Therefore, this membrane formed by lipids and proteins permits any cell to function as the basic unit of life on earth. In addition to the outer plasma membrane, the more complex eukaryotic cells of plants and animals have internal membranes surrounding small organelles inside the cell, like that surrounding the ER - endoplasmic reticulum, a complex of tubes and secretory granules - those around the nucleus - the controlling center of the cell, those around the mitochondrion - the powerhouse of the cell in animals, as well as those found around the chloroplast - the powerhouse of the cell in plants. These internal membranes which are also made from lipids and proteins add complexity to the cells of animals and plants and act to further compartmentalize the cell into specific units dedicated to unique roles. Inside the cell, one more important role that lipids play is as efficient storage molecules that can be oxidized to produce energy.
Membrane based lipids are of three major kinds, the phospholipids, the glycolipids, and the sterols such as cholesterol. A structure known as a lipid bilayer is rapidly formed by the chemical association of both phospholipids and glycolipids when they are in a fluid medium - this bilayer is the basis for the cell membrane. Once formed, the plasma membranes of cells act like two dimensional, semi fluid structures, which permit the embedded protein molecules in the membrane to constantly move freely in the lipid bilayer by lateral diffusion - therefore, cell membranes are dynamic and not fixed forms. In the simpler membranes of prokaryotic cells - bacteria - the cell membranes fluidity or dynamism is regulated by the varying of the number of double bonds as well as the fatty acid chains forming the lipid molecules of the membrane. Membrane fluidity and dynamism is a function of the quantity of the sterol lipid cholesterol in the cells of higher animals.
One very important aspect of the plasma membrane is its role as a selective filter that is responsible for controlling the entry of nutrients and other molecules into the cell - the membrane is the barrier to and regulator of the kind of molecules and compound that will enters the cell from the external environment. It may allow the entry of molecules useful to cellular processes and stop the entry of other substances which are dangerous or not required by the cell. The membrane is thus a conduit or a gate for all substances needed by the cell; it also expels the waste products of chemical metabolism and passes it out into the external environment for disposal. The chemical nature and the composition of membranes make them unique barriers, for example, membranes have a low permeability for ions and most polar molecules, these molecules are only allowed to pass through protein channels formed by integral membrane proteins - a special type of membrane protein - this allows them to be regulated. Most substances are taken into the cell by a process of diffusion, where substances move into the cell from an area outside the cell where they are found in high volumes to the area inside the cell where they are in low volume - example, a scent diffuses about a room from one fixed point where it is sprayed, this is how most materials are taken in by the cell. If any substance is transported against the concentration gradient from an area of lower concentration outside the cell to an area of higher concentration inside the cell where its volume is already high, then some energy must be expended in a process called active transport - another way in which all cells take in substances they need.