Introduction to Metabolism

 

Metabolism: the totality of an organisms chemical processes. It is divided into two parts: catabolism biological pathways used in the breaking down of substances (respiration, digestion, etc.) and anabolism biological pathways that build complex molecules (photosynthesis, protein synthesis).

Metabolism manages the material and energy resources of the cell.

 

Energy: the ability to do work. There are two types of energy: kinetic and potential. Kinetic (KE = 1/2 MV2) is energy of movement (thermal and light) while potential is energy of position or stored energy (chemical).

Energy is transformed (not cycled) from one form to another. The study of these transformations is called thermodynamics.

 

Laws of Thermodynamics: First law states that energy of the universe is constant. It may change from one form to another but cannot be created or destroyed. This is sometimes called the Law of conservation of energy.

The second law states that every process increases the entropy (S) of the universe. Entropy is the quantitive measure of disorder of a system. If we look at any organism as a system that needs and uses energy, we can see what would happen if the energy supply was cut off. The system would fail due to lack of the necessary energy to meet its needs. This type of system is considered a closed system. No new energy can enter, while the available energy is changed into useless heat, causing the system to fail.

Heat is a useless form of energy unless it is used to maintain temperature of a system, but it must move from a warm area to a cool one. The earth and its organisms are not closed systems. They are considered open, due to the fact that they can replenish their energy when needed. The sun and food are their means of replacement. If for some reason the energy supply stops they will become a closed system and fail. All open systems will eventually fail due to the process of energy turning into useless heat. The universe will eventually die due to a lack of useful energy and an abundance of heat. The universe is a closed system.

Chemical Energy: is stored in the bonds of the chemical they are holding together. Covalent bonds contain the most energy,while hydrogen bond much less. During a chemical reaction 2 thing must occur,

1). energy must be absorbed to break the bonds of the reactants. 2). Energy is released when new bond are formed. This is called bond energy.

 

CH4 + 2O2 --------------> CO2 + 2H2O

 

Each C-H bond contains 99 kcal / mole of stored energy. Since methane contains 4 of them its total bond energy is 396 kcal / mole. The double bonded oxygen contains 118 kcal / mole and there are 2, giving us 236 kcal / mole. Since these bonds must be broken the total energy absorbed is 633 kcal / mole.

On the products side( new molecules being formed), each C = O bond contains 174 kcal /mole since there are 2 each carbon dioxide contains 348 kcal / mole of energy. The O-H bond contains 111 kcal / mole, and there are 4 of them giving us a total of 444 kcal / mole. The total amount of energy released is 792 kcal / mole. 160 kcal / mole is the net energy released or the heat we feel from the reaction. This is the heat of the reaction or delta-H = - 160 kcal / mole. this refers to the stored energy being released. Always subtract the product answer from the reactant answer. If a reaction has a negative delta H, it is said to be exothermic.

Enthalpy : The total potential energy of a molecule. Enthalpy relates to the amount of heat energy released from a chemical reaction. In an exothermic reaction the enthalpy of the products is less than the reactants, hence you feel the heat. In an endothermic reaction the reverse is true, since the products have more enthalpy than the reactants. This occurs at the expense of its surroundings.

Spontaneous Reactions: is a reaction that will occur without any outside help. Specifically it can occur without the introduction of external energy. A nonspontaneous reaction cannot occur on its own. it will only happen if external energy is added.

Free Energy: The quantity that combines total energy (enthalpy) and entropy is free energy. Free energy is represented by the letter (G). Spontaneous reactions occur when the free energy of the system decreases. During nonspontaneous reactions the free energy of the system increases.

Based on free energy movement in a system the terms exergonic and endergonic are used to determine the direction of this free energy. An exergonic reaction will release energy from the reaction, while an endergonic reaction will absorb free energy from its surroundings.

In order for an endergonic reaction to go to completion it needs an outside source of energy. In the cell this energy source comes from the chemical compound ATP. ATP helps the cell carry on 3 main types of work. Mechanical (cell movement), chemical (anabolism),and transport (pumping materials into and out of the cell ). ATP (adenosine triphosphate) is constructed of a molecule of adenine attached to a molecule of ribose sugar which is attached to 3 phosphates. ATP is recycled 10,000,000 times/sec/cell.

 

ATP + H2O ---------------> ADP + P + ÆG = -7.3 kcal / mol

 

This occurs in a test tube. In the human cell -10 / -12 kcal / mol are given off.

 

ATP transfers this energy to what ever it is reacting with. When the new chemical receives the P it is said to be phosporylated. Phosphorylation has occurred.

 

ATP is renewable. ADP + P ------------------> ATP ÆG = + 7.3 kcal / mol (endergonic)

 

Enzymes: are catalysts that speed up a chemical reaction by lowering its activation energy. Enzymes are proteins that act on a substance called a substrate.

 

enzyme

substrate -------------> product

 

 


In the above reaction the enzyme's active site binds with the substrate. This active site is specific for each different type of substrate. Even the slightest change in the form of this site will alter the enzyme's function.

Factors affecting enzyme activity: Temperature, pH, and a particular chemical that specifically influences that enzyme. Enzymes work best at temperatures between 35o and 40oC in humans. pH range between 6 and 8 promotes optimum function. Salts inhibit enzyme action.

 

Coenzymes: are non protein chemicals that help enzymes act. Many of these coenzymes are vitamins.

 

Enzyme Inhibitors:

 

1. Competitive inhibitors: These chemicals mimic or resemble the normal substrate molecule.

 

2. Noncompetitive inhibitors: These chemicals attach themselves to the enzyme at another point and alters the enzyme's shape. This causes the enzyme's active site to become non receptive to the substrate.

 

3. Allosteric regulation: Most enzymes that are affected by this type of regulation are composed of 2 or more polypeptide chains. These enzymes fluctuate between an active and inactive substance.

The enzyme contains 2 sites the active site and the allosteric site, located away from the active site. The allosteric site must contain an activator substance that will allow the active site to remain open. If the activator is missing then an inhibitor occupies the space and inactivates the enzyme.

 

Feedback Inhibition: The most common form of metabolic control. The process involves the switching off of the metabolic pathway by its end product.