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Energy
Cells must have a steady supply of energy to carry out all cellular functions. There are different forms of energy: mechanical energy, heat energy, energy resulting from motion (= kinetic energy) and energy stored in chemical compounds. One form of energy can be transformed into another. For example living organisms transform chemical energy into growth, motion, heat and metabolism. Plants can transform light energy into chemical energy.
Metabolizing cells are carrying out continuously chemical reactions, which either consume energy or release energy. In order to have some measure of how much energy a compound contains, it can be oxidized (burned) and the amount of heat evolved can be measured. There are two ways of measuring the evolved heat: under conditions where the volume is constant or under conditions where the pressure is constant. Calorie values listed on food items are measured under constant volumes, whereas reactions in living cell occur of course under constant pressure. The heat energy released or absorbed in a chemical reaction under constant pressure is called enthalpy and has the symbol H (delta H) . It is measured in calories ( 1 cal is the amount of energy necessary to heat 1 g of water by one degree. ). An alternative unit for the measurement of energy is Joule :

1 cal = 4.184 J     1 J = kg x m² / s² = N x m (N = Newton)

For example the oxidation of ethanol is a reaction in which heat is evolved

C ₂H₅OH + 3 O₂ ® 2 CO₂ + 3 H₂O     H = -1367 kJ/mol or -326.7 kcal/mol

H is the change in enthalpy, i.e. you could say it the difference in the heat content of the system before and after the reaction or the difference in heat content between the reactants (ethanol + oxygen) and the products (water + carbon dioxide). The negative sign shows that the system loses heat (positive values would indicate that the reaction consumes heat). The likelihood for a chemical reaction to proceed does not only depend on the enthalpy change but also on another thermodynamic parameter, called entropy . Entropy is a measure for disorder of a system. For example if diamond (which is a crystalline form of carbon) is burned to CO₂ the disorder of the system is greatly increased, since the carbon atoms in diamond were arranged in a highly ordered crystal, whereas in the gas CO₂ they are distributed randomly. Thus, the disorder has increased and the entropy change S has a positive sign (also measured in cal or J). If the order in a system increases S becomes negative.
Thus, the entire energy change in a chemical reaction is a function of H, S and also the temperature at which the reaction occurs. It can be formulated as:

G = H - TS

G is called the free energy change in a chemical reaction and T is the absolute temperature in ^oK).
A reaction is exergonic if the energy content of the reactants is higher then the energy content of the products. In such a case G is negative i.e. the reaction can proceed without energy input, actually energy is released in the reaction. If the energy content of the products is higher then the energy content of the reactants, then energy is required for the reaction to occur and G is positive . Such a reaction is said to be endergonic. Even if a chemical reaction is exergonic (energy is released by the reaction) it requires an initial energy input in form of an activation to start the reaction. This activation energy can be quite large and in chemical reactions in the laboratory it is usually achieved by raising substantially the temperature of the reactants. Since this is not possible in biological systems, the activation energy has to be lowered, which is done by biological catalyst, the enzymes . It is because of the enzymes that chemical reactions can occur at lower temperature and at fast rates.

Mechanism of enzymatic reactions: please review corresponding section in your text book

ATP
As mentioned above, the cell has to have a steady supply of energy. Most chemical reactions in the cell require energy. We talked about protein synthesis, a sequence of reactions which consumes much energy. You may remember transport systems such as the Na/K pump which consumes a lot of energy. How does the cell make this energy available? It is in the form of a high energy compound called ATP or adenosine triphosphate.

ATP is composed of a the nucleic acid base adenine, ribose and three phosphates. These phosphates connected to each other which makes ATP a high energy compound. For example water can easily hydrolyze one phosphate producing ADP

ATP + H₂O ® ADP + H₂PO₄^-

This reaction is very exergonic, i.e. it releases energy. G = - 7.3 kcal/mol.
Since ATP is continuously consumed, the cell must have mechanisms in place which produce ATP in the reverse reaction.

ADP + H₂PO₄^- ® ATP + H₂O .

The free energy change for this reaction must be positive i.e. the reaction requires energy and the amount is of course G =+ 7.3 kcal/mol. One of the major metabolic pathways leading to the formation of ATP is called glycolysis. Before we discuss this pathway, we should understand what oxidation and reduction reactions are.

Oxidation and Reduction
When you drive your car, the power is produced in an oxidation reaction, i.e. gasoline reacts with oxygen in a highly exergonic reaction. Living organisms also carry out such oxidations. When a compound is oxidized it "loses" electrons. Let’s try to understand what this means using the example of methane oxidation : CH₄ + 2 O₂ ® CO₂ + 2 H₂O

Assume for a minute, that we are stripping all bonds from the methane molecule. Since there are 4 bonds we will get 8 electrons. We have removed 4 electrons form the carbon and 4 electrons from 4 hydrogen atoms, the carbon has 4 positive charges (C⁺⁺⁺⁺) and each of hydrogens will have a positive charge (4 H⁺). Furthermore let's separate the double bond between the two oxygen in O₂ right in the middle, then each oxygen gets two of the bonding electrons. Since no electrons removed, the two oxygen atoms have no charge. Now, let us reassemble the molecule CO₂ which is one of the reaction products. As you can see above, using the 2 oxygen atoms and the C⁺⁺⁺⁺ we need only 4 electrons, thus 4 electrons along with 4 protons are left over. Now we understand the above statement which said : when a compound is oxidized it loses electrons. In our case carbon was oxidized from methane to carbon dioxide.
What happens to the electrons ? They must be accepted by something else. Oxygen can function as this electron acceptor. As you can see above, one oxygen atom and two protons (H⁺) can form water, if two electrons are supplied. Thus, oxygen has accepted electrons and we say oxygen has been reduced. In summary: Substances which are oxidized lose electron and substances which are reduced gain electrons.

In addition of using oxygen as electron acceptor in oxidation reactions, the cell has developed other substances that can also function as electron acceptors. One of them is called nicotine amide dinucleotide (NAD⁺)
NAD⁺ can accept 2 electrons (in the process it also accepts a H⁺) to form NADH . Thus, NADH is the reduced form of NAD ⁺.

Another electron acceptor is flavin adenine dinucleotide (FAD). FAD can also accept two electrons (in addition it accepts 2 H⁺) to form FADH₂.
Now let us look at this electron transfer, the electrons may come from a reduced substance like in the above case of methane. When methane is oxidized a lot of energy is released (methane is natural gas). You can think of this energy as being stored in the electrons which are given off. These high energy electrons can carry out work. The work which they are doing is a reduction. These electrons could for example reduce NAD⁺ to NADH. This reaction consumes a large amount of energy. Thus, the cell can harvest the energy given off in an oxidation reaction by producing a reducing substance such as NADH or FADH₂ ( oxidation of glucose )