Chemical reactions occur when molecules interact and chemical bonds between them are formed or broken. Some reactions will occur just by putting two substances in close proximity. For example, iron in the presence of oxygen will form iron oxide, or rust. Other reactions require energy to get the reaction started. Once the activation energy is added, the reaction will continue if the final energy state is lower than the initial energy state. A good example is a lightning strike that starts a forest fire which, once started, will continue to burn until the fuel is used up. Show
In biology, chemical reactions are often aided by enzymes, biological molecules made of proteins which can be thought of as facilitators or catalysts. Enzymes speed the reaction, or allow it to occur at lower energy levels and, once the reaction is complete, they are again available. In other words, they are not used up by the reaction and can be re-used. Enzymes are designed to work most effectively at a specific temperature and pH. Outside of this zone, they are less effective. At very high temperatures, enzymes, because they are made of protein, can be denatured or destroyed. The material on which the enzyme will act is called the substrate. The enzyme attaches to the substrate molecule at a specific location called the active site. When the enzyme has attached to the substrate, the molecule is called the enzyme-substrate complex. For example, the sugar found in milk is called lactose. With the aid of the enzyme, lactase, the substrate, lactose, is broken down into two products, glucose and galactose. People who don't make enough lactase have trouble digesting milk products and are lactose intolerant. Children are usually lactose tolerant, but many people lose the ability to digest milk sugars as they grow older. Your probability of being lactose intolerant is correlated with whether your ancestors raised milk cows. The commercial product, Lactaid, contains lactase. Enzyme action can be blocked by molecules that obstruct the enzyme's active site. Herbicides and pesticides often work in this way. The active site of an enzyme has a very specific 3-dimensional shape. Therefore, enzymes are specific to particular substrates, and will not work on others with different configurations.
Some examples of enzymes and their specific substrates. There are several factors that can increase the rate of a reaction. Raising the temperature can speed a reaction because the molecules have more energy and therefore bump into each other more frequently. The same effect can be obtained by physically stirring the ingredients. A reaction can also be speeded by increasing the concentration of reactants, the chemicals that are necessary for the reaction to proceed; this is called the Law of Mass Action, or by decreasing the concentration of products, the chemicals that result from the reaction. Some reactions can even run in both directions depending on the concentration of molecules. For example, carbonic anhydrase can catalyse the conversion of bicarbonate, a blood pH buffer, into water and carbon dioxide, or can catalyse the reaction in the opposite direction when water and carbon dioxide are more abundant. The graph below shows that the rate or velocity (V) of a reaction depends on substrate (K) concentration up to a limit. KM is the substrate concentration midway to the maximum rate, and is a useful value to note since the reaction is non-linear, and return on substrate investment diminishes as we approach the maximum rate (Vmax). If the substrate is valuable, we can think of KM as the optimal amount of substrate to invest. If the substrate is inexpensive, then saturating the reaction with substrate ensures the most product in the shortest period of time. As we approach Vmax, more and more of the enzyme is involved with substrate, so no further increase in substrate concentration can speed the reaction further. Lab Source: http://www.learnnc.org/lp/pages/3398 Lab Activity: Testing the action of the enzyme, lactase, on lactose and sucrose, two disaccharide sugars. Introduction: Lactose is a disaccharide sugar found in milk, and is composed of two simpler sugars, glucose, a six-sided molecule, and galactose, another six-sided molecule. The enzyme, lactase (enzyme names often end in -ase) breaks lactose into its two monosaccharide components. Sucrose, or table sugar, is another common sugar composed of glucose and fructose, a five-sided molecule. In this lab, we will use the enzyme lactase to attempt to break down both of these disaccharides. Materials:
Solution Preparation:
Procedures:
Circle the + if glucose was detected, or the - if glucose is not detected. Questions:
What is the relationship between the reaction rate and the substrate concentration?The relationship between rate of reaction and concentration of substrate depends on the affinity of the enzyme for its substrate. This is usually expressed as the Km (Michaelis constant) of the enzyme, an inverse measure of affinity.
Why does reaction rate increase with enzyme concentration?If there is insufficient enzyme present, the reaction will not proceed as fast as it otherwise would because all of the active sites are occupied with the reaction. Additional active sites could speed up the reaction. As the amount of enzyme is increased, the rate of reaction increases.
What is the relationship between enzyme concentration and enzyme activity?The relationship between enzyme concentration and enzyme activity is directly proportional. On a graph showing reaction rate versus enzyme concentration, this direct proportional relationship looks like a straight line with a slope of one.
What is the relationship between substrate concentration and the reaction rate quizlet?As the substrate concentration increases the reaction rate does the same, because there is more substrate for the enzyme to react with. This is in fact the linear relationship.
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