Lipids A fat consists of a three-carbon backbone called glycerol attached to three fatty acids, which contain long hydrocarbon chains. Some fats are solid at room temperature. Other fats called oils are liquids at room temperature. In addition to storing energy for later use, fatty tissues cushion your organs and provide your body with insulation.

A saturated fat is a fat in which all three fatty acid chains contain the maximum possible number of hydrogen atoms. All the carbon atoms in the fatty acid chains form single bonds with each other, and the rest of their bonds are with hydrogen atoms. Most animal fats, such as lard and butter, are saturated. They are solid at room temperature.

An unsaturated fat, contains less than the maximum number of hydrogen atoms in one or more of its fatty acid chains because some of its carbon atoms are double-bonded to each other. The fats in fruits, vegetables, and fish are generally unsaturated, as are corn oil, olive oil, and other vegetable oils

Diets rich in saturated fats may be unhealthy. These diets may promote the buildup of lipid-containing deposits, called plaques, within the walls of blood vessels.

Steroids A lipid molecule in which the carbon skeleton forms four fused rings is called a steroid. All steroids have a core set of four rings, they differ in the kinds and locations of functional groups attached to the rings.

Steroids are classified as lipids because they are hydrophobic, but they are very different from fats in structure and function. Some steroids circulate in your body as chemical signals. The steroids estrogen, a female sex hormone, and testosterone, a male sex hormone, function in this way.

Cholesterol ( a steroid) is an essential molecule found in the membranes that surround your cells. It is also the starting point from which your body produces other steroids.

Protein A protein is a polymer of just 20 kinds of monomers called amino acids. Proteins are responsible for almost all of the day-to-day functioning of organisms.

Amino Acids Each amino acid monomer consists of a central carbon atom bonded to four partners. Three of the central carbon's partners are the same in all amino acids. One partner is a hydrogen atom. Two others are a carboxyl group and an amino group, functional groups.

What is different about each type of amino acid is the "side group" that attaches to the fourth bond of the central carbon. The side group, sometimes called the "R-group," is responsible for the particular chemical properties of each amino acid.

Building a Protein Cells create proteins by linking amino acids together into a chain called a polypeptide. Each link is created by a dehydration reaction between the amino group of one amino acid and the carboxyl group of the next amino acid in the chain. Proteins are composed of one or more polypeptide chains

Activation Energy To start a chemical reaction, it is first necessary to weaken chemical bonds in the reactant molecules. This "start-up" energy is called activation energy because it activates the reactants and triggers a chemical reaction

Cellular reactions depend on the assistance of catalysts, compounds that speed up chemical reactions. The main catalysts of chemical reactions in organisms are specialized proteins called enzymes.

Enzymes provide a way for reactions to occur at the cell's normal temperature. An enzyme doesn't supply activation energy to the reacting molecules, but instead lowers the energy requirement.

Each enzyme catalyzes a specific kind of chemical reaction. At any moment in the cell's life, the specific enzymes that are present and active determine which reactions occur.

How Enzymes Work The shape of each enzyme fits the shape of only particular reactant molecules. A specific reactant acted upon by an enzyme is called the enzyme's substrate. The substrate fits into a particular region of the enzyme, called the active site.

As with any other protein, an enzyme's structure and shape are essential to its function. Factors such as pH and temperature can greatly affect how well an enzyme works or if it can work at all. This is one reason why cells (and hence organisms) can only survive and function within certain ranges of conditions.