AP Biology Chapter 8 Introduction to Metabolism

Metabolism The chemistry of life is organized into metabolic pathways. The chemistry of life is organized into metabolic pathways. Metabolism is the collection of chemical reactions that occur in an organism. Most involve enzymes. Metabolism is the collection of chemical reactions that occur in an organism. Most involve enzymes. Catabolic pathways break down molecules to release energy. Catabolic pathways break down molecules to release energy. Anabolic pathways build molecules and consume energy to do so. Anabolic pathways build molecules and consume energy to do so.

Energy Energy is the capacity to do work by moving matter. Energy is the capacity to do work by moving matter. Kinetic energy is energy of motion. Kinetic energy is energy of motion. Potential energy is stored in the location or structure of matter. This includes chemical energy stored in chemical bonds. Potential energy is stored in the location or structure of matter. This includes chemical energy stored in chemical bonds. Energy can change form. Energy can change form.

Energy Transformation

Laws of Thermodynamics The study of the energy transformations that occur in a collection of matter is called thermodynamics. The study of the energy transformations that occur in a collection of matter is called thermodynamics. Organisms are open systems; energy can be transferred between the system and its surroundings. Organisms are open systems; energy can be transferred between the system and its surroundings. First Law of Thermodynamics: Energy can be transferred and transformed, but it can be neither created nor destroyed. First Law of Thermodynamics: Energy can be transferred and transformed, but it can be neither created nor destroyed. Second Law of Thermodynamics: Every energy transfer or transformation increases the entropy (disorder) of the universe. Second Law of Thermodynamics: Every energy transfer or transformation increases the entropy (disorder) of the universe.

Biological Order Biological order requires energy to maintain its complexity. Biological order requires energy to maintain its complexity. This does not violate the 2 nd Law of Thermodynamics. This does not violate the 2 nd Law of Thermodynamics. In open systems, organisms can increase their order at the expense of the order of their surroundings. In open systems, organisms can increase their order at the expense of the order of their surroundings. Matter can become more ordered only if the surroundings become more disordered. Matter can become more ordered only if the surroundings become more disordered.

Free Energy Organisms live at the expense of free energy. Organisms live at the expense of free energy. A living system’s free energy is energy that can do work under cellular conditions. A living system’s free energy is energy that can do work under cellular conditions. Free energy (G) is related directly to total energy (H) and to entropy (S). Free energy (G) is related directly to total energy (H) and to entropy (S). In an exergonic (spontaneous) chemical reaction, the products have less free energy than the reactants. In an exergonic (spontaneous) chemical reaction, the products have less free energy than the reactants. Endergonic (nonspontaneous) reactions require an input of energy. Endergonic (nonspontaneous) reactions require an input of energy.

Free energy, cont. In cellular metabolism, exergonic reactions power endergonic reactions (energy coupling). In cellular metabolism, exergonic reactions power endergonic reactions (energy coupling). The addition of starting materials and the removal of end products prevent metabolism from reaching equilibrium. The addition of starting materials and the removal of end products prevent metabolism from reaching equilibrium.

ATP ATP powers cellular work by coupling exergonic reactions to endergonic reactions. ATP powers cellular work by coupling exergonic reactions to endergonic reactions. Removing a phosphate group from ATP produces ADP and free energy. Removing a phosphate group from ATP produces ADP and free energy.

ATP, cont. ATP drives endergonic reactions by transfer of the phosphate to specific reactants, making them more reactive. Cells can carry out work, such as movement or building molecules (anabolism). ATP drives endergonic reactions by transfer of the phosphate to specific reactants, making them more reactive. Cells can carry out work, such as movement or building molecules (anabolism). Catabolic pathways drive the regeneration of ATP from ADP and phosphate. Catabolic pathways drive the regeneration of ATP from ADP and phosphate.

Enzymes Enzymes speed up metabolic reactions by lowering energy barriers. Enzymes speed up metabolic reactions by lowering energy barriers. Enzymes, which are proteins, are biological catalysts. The speed up reactions by lowering activation energy (energy needed for reactions to begin). Enzymes, which are proteins, are biological catalysts. The speed up reactions by lowering activation energy (energy needed for reactions to begin).

Enzymes, cont. Enzymes are substrate specific. Enzymes are substrate specific. Each type of enzyme has a unique active site that combines specifically with its substrate, the reactant molecule on which it acts. Each type of enzyme has a unique active site that combines specifically with its substrate, the reactant molecule on which it acts. The enzyme changes shape slightly when it binds the substrate (induced fit). The enzyme changes shape slightly when it binds the substrate (induced fit).

How Enzymes Work The active site is an enzyme’s catalytic center. The active site is an enzyme’s catalytic center. The active site can lower activation energy by orienting substrates correctly, providing a microenvironment that favors the reaction. The active site can lower activation energy by orienting substrates correctly, providing a microenvironment that favors the reaction. The enzyme forms an enzyme/substrate complex, but is not used up in the reaction; it is released to be used again. The enzyme forms an enzyme/substrate complex, but is not used up in the reaction; it is released to be used again.

Enzymes: Optimum conditions As proteins, enzymes are sensitive to conditions that influence their 3-D structure. As proteins, enzymes are sensitive to conditions that influence their 3-D structure. Each has an optimal temperature and pH. Outside of these favorable ranges, the protein can change shape (denature) and no longer function effectively, if at all. Each has an optimal temperature and pH. Outside of these favorable ranges, the protein can change shape (denature) and no longer function effectively, if at all.

Cofactors and Coenzymes Many enzymes require nonprotein helpers in order to be active. These are cofactors. Many enzymes require nonprotein helpers in order to be active. These are cofactors. Cofactors of some enzymes are inorganic, such as metal ions (Zn, Cu, or Fe). Cofactors of some enzymes are inorganic, such as metal ions (Zn, Cu, or Fe). Cofactors that are organic molecules (such as vitamins) are called coenzymes. Cofactors that are organic molecules (such as vitamins) are called coenzymes.

Enzyme Inhibitors Certain chemicals can inhibit the activity of specific enzymes. Certain chemicals can inhibit the activity of specific enzymes. Some inhibitors resemble the normal substrate and compete for the active site. These are competitive inhibitors, and reduce enzyme activity by blocking the substrate from entering the active site. Some inhibitors resemble the normal substrate and compete for the active site. These are competitive inhibitors, and reduce enzyme activity by blocking the substrate from entering the active site. Noncompetitive inhibitors prevent enzymatic reactions by binding to another part of the enzyme, which causes the enzyme to change shape. Noncompetitive inhibitors prevent enzymatic reactions by binding to another part of the enzyme, which causes the enzyme to change shape.

Control of Metabolism: Allosteric regulation Regulatory molecules change an enzyme’s shape and function by binding to an allosteric site, a location different from the active site. Regulatory molecules change an enzyme’s shape and function by binding to an allosteric site, a location different from the active site. Some molecules are activators, which stabilizes the active site and stimulates enzyme activity. Some molecules are activators, which stabilizes the active site and stimulates enzyme activity. Allosteric inhibitors stabilizes the inactive form of the enzyme. Allosteric inhibitors stabilizes the inactive form of the enzyme. The molecules are weakly bonded to the enzyme, and are basically reversible noncompetitive inhibitors. The molecules are weakly bonded to the enzyme, and are basically reversible noncompetitive inhibitors.

Allosteric Regulators

Feedback Inhibition Feedback inhibition is the switching off of a metabolic pathway by its end- product. Feedback inhibition is the switching off of a metabolic pathway by its end- product. The product acts as an inhibitor of an enzyme within the pathway. The product acts as an inhibitor of an enzyme within the pathway. Feedback inhibition prevents the cell from wasting chemical resources to synthesize more product than is needed. Feedback inhibition prevents the cell from wasting chemical resources to synthesize more product than is needed.

Cooperativity Cooperativity is a mechanism that amplifies the response of enzymes to substrates. Cooperativity is a mechanism that amplifies the response of enzymes to substrates. A substrate molecule binding to one active site of a multi-subunit enzyme activates the other subunits. A substrate molecule binding to one active site of a multi-subunit enzyme activates the other subunits.

Localization of Enzymes Localization of enzymes within a cell helps order metabolism. Localization of enzymes within a cell helps order metabolism. Some enzymes are grouped into complexes, some are incorporated into membranes, and others are contained inside organelles. Some enzymes are grouped into complexes, some are incorporated into membranes, and others are contained inside organelles. For example, mitochondria are membrane- bound organelles in eukaryotic cells that have a series enzymes working to carry out cell respiration. For example, mitochondria are membrane- bound organelles in eukaryotic cells that have a series enzymes working to carry out cell respiration.