Energy
Energy is the capacity to perform work. Work is done whenever an object is moved against an opposing force and moves things in directions in which they would not have moved if left alone. The body needs energy and since the body is composed of cells, it only makes sense that cells are the biggest users of energy in the body. Cells need energy to make complex molecules from monomer building blocks in anabolic reactions. Cells also need energy to break down macromolecules in catabolic reactions to obtain energy to do all of the activities they need to do every day.
Energy cannot be destroyed nor created. This is the tenet of the First Law of
Thermodynamics or the law of energy conservation. (Thermodynamics is the study of energy transformations). Energy can be converted from one form into another. Plants convert the energy in sunlight into chemical energy that life forms can use to perform the activities of life.
There are two basic forms of energy: kinetic and potential . Kinetic energy is the energy of motion. As an example as you ride a bike up a hill your leg muscles contract and push the bike’s pedals to move the bike up that hill. Sunlight and heat are other forms of kinetic energy. Potential energy is stored energy that an object has as a result of its locomotion or structure. As an example as you walk up a hill your locomotion which is kinetic movement is stored as potential energy; that potential energy is released and converted back into kinetic energy as you walk back down the hill. Another example of potential energy is a coil in a compressed position. As the coil is released its potential energy is converted into kinetic energy. The coil has stored potential energy due to its specific structure. In the same way the electrons of an atom have stored potential energy due to their arrangement into different energy shells of an atom in which they reside. Chemical energy is the potential energy of molecules due to the configuration of the atoms in their structures. This chemical energy can be converted into energy needed for cells to perform work.
Energy can be converted from one type into another but cannot be created or destroyed. You may wonder why then we can’t just use energy over and over again. The reason is because each time there is an energy transfer some energy becomes unusable.
Energy conversions generate heat. Heat is a form of kinetic energy or energy which comes from random movement of atoms and molecules. This energy is difficult to reuse to perform work. It is lost to the surroundings. It is the energy of aimless molecular movement, a measure of disorder or randomness. The amount of disorder in a system is called entropy.
The Second Law of Thermodynamics states that energy conversions increase entropy in a system and reduce order.

Energy flows into our ecosystem as sunlight. This kinetic energy is transformed into chemical energy, in the form of the potential energies of food and fuels by photosynthesis.
Animals consume the food products to provide ATP or energy for cells to perform work. The potential to perform work lies in the arrangement of atoms in the food and fuel molecules.
Reactions that yield potential energy are called endergonic reactions and require an input of energy. Energy is absorbed as the reaction takes place so the products have more energy than the reactants. The energy is stored in the covalent bonds of the products. Photosynthesis is an endergonic reaction. Reactions that release energy are termed exergonic.
These reactions begin with reactants whose covalent bonds contain more energy than those in the products and release energy to the environment. Exergonic reactions occur in the cells of the body. Cells release energy by cellular respiration.
In this process oxygen is used to convert the chemical energy stored in fuel molecules (glucose) to chemical energy (ATP) the cell uses to carry on its processes. Glucose + O
2

CO
2
+ H
2
O.
Every working cell in the body carries out both exergonic and endergonic reactions. The sum of these reactions is called cellular metabolism . In cells energy released from exergonic reactions is used to drive endergonic reactions. This is called energy coupling and is an essential ability of all cells.
ATP-Adenosine Triphosphate
ATP is the molecule that provides energy for cellular work. Once again we see how the structure of this molecule is responsible for its function. ATP consists of adenine, a nitrogenous base and ribose, a five carbon sugar. This part of the molecule is called adenosine.
3 PO
4
groups are attached providing the triphosphate part of the molecule. Each PO
4
group carries a negatively charge. Like charges repel. They are crowded into a small space which contributes to the molecule’s potential energy. This makes phosphate bonds unstable and thus can be easily broken by hydrolysis in an exergonic reaction. Each PO
4 group released from an
ATP molcule yields 7Kcal of energy. When one phosphate group is removed from a molecule of ATP  ADP + p i
+ 7Kcal of energy (adenosine diphosphate + inorganic phosphate + energy. When another phosphate is removed  AMP + p i
+
7Kcal of energy(adenosine monophosphate). PO
4
groups don’t just fly off; they are transferred to other molecules. This transfer is called phosphorylation.
ATP functions in energy coupling that is it transfers energy from processes that give off energy by the breakdown of organic molecules to processes that uses energy such as muscle contractions. Cells perform 3 types of work: mechanical such as muscle movement, transport perhaps to pump ions across a membrane and chemical to make molecules by dehydration synthesis. ATP cannot be stored. It is made continuously.

Factors in Chemical Reactions-Enzymes
Many of the molecules in our bodies store a tremendous amount of potential energy. These molecules do not spontaneously break down into smaller components. Metabolism, all the chemical reactions occurring in an organism needs to proceed at a fairly rapid rate. To initiate reactions reactants need to overcome an energy barrier . An energy barrier is the amount of energy called activation energy that a compound must absorb before a chemical reaction can begin. This requires that molecules absorb energy from their surroundings usually in form of heat. Body chemistry is cold.
There is not enough heat for reactions to take place spontaneously and do not occur without a catalyst that is something to speed the rate of a reaction.
Enzymes speed the rate of chemical reactions without being permanently changed or consumed. They do so by lowering the energy of activation. Enzymes allow metabolic reactions to occur at cooler temperatures by reducing the activation energy needed to break the bonds of reactant molecules. Activation energy is a barrier; enzymes lowers that barrier by binding to reactant molecule and putting them under physical or chemical stress making it easier to break bonds and start reactions.
Enzymes are globular proteins. Their names typically end in –ase. Each has a unique 3-D shape. Shape determines which reactions an enzyme can catalyze. A specific reactant for a specific enzyme is called a substrate . A substrate fits into a specific area of the enzyme called the active site which is a groove or pocket on the enzyme into which the substrate fits like a key in a lock. Enzymes are specific because they catalyze only 1 type of reaction and therefore can accommodate only
1 type of substrate. This is determined by the ability of enzymes to recognize shapes of substrates. This ability depends on the enzyme’s shape.
Once the substrate binds to the active site on the enzyme there is a temporary change in the shape of the enzyme which makes the substrate fit even more tightly. This is called an induced fit and ensures the enzyme remains with the substrate until product is formed. Once product forms it detaches from the active site and the enzyme is free to start another reaction.
A saturation limit controls how fast enzymatic reactions can occur.
The rate of an enzymatic reaction is proportional to the concentration of both substrate and enzyme. An enzyme must meet with a specific amount of substrate before catalysis can begin. Higher substrate concentrations more frequently encounter the enzyme.
When substrate concentrations are high enough so every enzyme molecule is cycling through its reaction sequence at top speed  a further increase in substrate concentration will not effect the rate of reaction unless more enzyme is added. The substrate concentration at which the rate of a reaction is maximum is the saturation limit .

Enzymatic reactions have several factors in place to regulate them. Many variables turn enzymes on and off and control reaction rates. Since enzymes are proteins and protein shapes can be changed by the environment, the environment is a key regulatory factor for enzymatic reactions. Every enzyme possess conditions at which it is most effective.
Temperature is important in enzymatic reactions. Temperature affects the motion of molecules. At faster temperatures molecules move faster. At an enzyme’s optimal temperature the highest rate of contact beween substrate and enzyme will occur. Most human enzyme work best at body temperature. If temperatures become to high enzymes will denature which destroys the strucuture of the enzyme and thus renders it ineffective.
Other substances important in enzymatic activity are salt concentration and pH.
Salt ions interfere with the chemical bonds that maintain the structure of proteins.
Most enzyme do no function well at high salt concentrations. The same holds for pH. Too many hydrogen ions, or at lower pHs structure of the enzyme will be altered and thus its function will be altered or diminished. Enzymes work best at pHs from 6-8.
Cofactors are important in enzyme function as well. Many enzymes will not work without helpers called cofactors. Cofactors are ions or molecules that bind to an enzyme before the substrate can bind. These allows enzymes to be active sometimes and inactive at other times. Several important inorganic cofactors are zinc, iron and copper. Organic cofactors are called coenzymes . Most coenzymes are made from vitamins or are vitamines.
There are chemicals that interfere with enzyme fuction. These are called inhibitors.
These inhibit by binding to the enzyme and disrupting its function. Some inhibitors are imposters or competitive inhibitors that fit into the active sites so real substrate cannot. Other inhibitors or noncompetitive inhibitors bind at sites other than the active site which serves to change the enzyme’s shape and causes the active site to no longer recognize its substrate.
Inhibition can be reversible or irreversible. Reversible inhibitors can serve a regulatory function turning the enzyme on when needed and turning it off when not. Irreversible inhibitors kill enzyme function.they include things like poisons and block metabolic processes that are essential to survival. Penicillin, for example inhibits enzymes in bacteria needed to make cells walls. Since humans do not have this enzyme penicillin can be used to kill the bug without effecting the human cells.