Synthesis is that aspect of metabolism in which the components of protoplasm are built or synthesized from nutrient materials coming into the cell.

This involves the manufacture of structural and enzymatic proteins, nucleic acids, lipids, carbohydrates, and various other types of molecules. In all types of cells, synthesis involves the construction of the basic macro- molecules of living systems from nutrient substances, and the chemical reactions involved are primarily energonic, that is, they exhibit and P.

It is convenient to classify synthetic reactions according to the source of the energy involved. Photosynthesis includes those reactions occurring in cells containing chlorophyll in which an external energy source, sunlight, provides the energy. Such reactions combine the raw materials carbon dioxide and water in the formation of carbohydrates.

Chemosynthesis involves the oxidation of certain inorganic compounds such as H2S and NH3 as an energy source, and this energy is utilized the synthesis of complex compounds from simple raw materials.


It should be pointed out that while the synthetic reactions themselves are energy-consuming, and the reactions upon which we are basing our classification are energy-yielding, it is more meaningful to classify synthetic reactions from the standpoint of the decompositional reactions which provide the energy for synthesis.

This is a reflection of the intimate relation between synthesis and respiration; actually, it is difficult to discuss these two aspects of metabolism separately. Hence, much of what we say about synthesis will be applied later on to respiration.

Before we discuss the types of synthetic reactions we have introduced, let us degrees from this topic and discuss some basic molecules and types of reactions involved in energy transformations which occur within cells.

Energy changes in cells result from electron transfers which occur among certain molecules of the cell. Any process whereby electrons are removed from atoms or molecules is called an oxidation, and any process whereby electrons are added to atoms or molecules is called a reduction.


Of necessity, an oxidation is always accompanied by a reduction, that is, the atom or molecule which serves as an electron donor is oxidized, while that which serves as an electron receptor is reduced. In living systems, the removal or addition of electrons may be considered synonymous with the removal or addition of hydrogen atoms, since hydrogen is always involved in cellular oxidation-reduction.

Hence, an oxidation-reduction reaction might also be called a dehydrogenation-hydrogenation reaction. An entire oxidation-reduction reaction consists of two half-reactions, that is, the oxidation of one molecules and the reduction of another.

When it exists in the Fe++ condition, the cytochrome molecule is a reduced state; conversely, when it exists in the Fe++ condition, it is in an oxidized state. Thus, as an electron passes from a Fe++cytochrome b to a Fe++ cytochrome b is a oxidized to the Fe++condition and cytochrome c, cytochrome b is oxidized to the Fe++ condition and cytochrome c is reduced to the Fe++ condition.

The cytochromes are usually intermediate compounds in a system of electron and hydrogen transfer reactions, that is, they are neither the initial acceptors, nor the ultimate acceptors. Two other kinds of molecules may serve as initial acceptors; they are both members of a class of organic molecules called coenzymes, and they function in connection with enzymes which catalyze oxidation- reduction reactions.


Specifically; these two types of coenzymes are the flavones. The nicotimides usually serve as the initial hydrogen acceptors, and they are more common is cells than flavins. The two major nicotinamides are nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phospate.

The major flaking is flaking adenine dinucleotide; in some instances, it serves as an initial hydrogen acceptor, but usually, it is an intermediate acceptor between the nicotinamides and the cytochromes. Later on, when we discuss respiration, we shall illustrate the relationships which exist among these hydrogen acceptors, but for the present, it will be sufficient that we know of their existence.

The source of the hydrogen atoms which furnish electrons to oxidation-reduction sequences in cells is somewhat variable. In photosynthetic reactions, electrons are contributed by chlorophyll or water. In chemosynthetic reactions, they are contributed by certain inorganic compounds.

In organosynthetic reactions, hydrogen atoms are furnished by organic molecules. As a further example of electron transfer in living systems, let us consider the loss of hydrogen from an organic compound and its transfer to an electron transporting system.


Lactic acid is an important intermediate compound in certain metabolic systems. In its metabolism, it undergoes dehydrogenation to yield pyretic acid in the presence of the enzyme lactic dehydrogenate and the coenzyme NAD.

Note that in this reaction lactic acid is oxidized to pyretic acid while the initial hydrogen acceptor has been reduced to NAD.H2. The net reaction is energy- yielding and the end products are more stable than the reactants.

At this point, let us consider a very important aspect of electron transfer. If we follow the pathway taken by the electrons furnished by lactic acid in the example given above, we find that they are transferred to oxygen, but all along the line, they yield energy to the cell through successive oxidation-reduction reactions.

It is necessary that such energy be made available to the cell in small steps; otherwise, it would all take the form of heat. Living cells are not heat engines, and if energy were liberated suddenly in great amounts, a cell would literally burn up. In other words, a cell runs on chemical energy, not thermal energy.


When we speak of a reaction as “yielding”‘ energy, we do so for want of a better term. Actually, only a small part of the energy made available to a cell is yielded as heat; the remainder is transferred from one type of chemical bond to another. This transfer involves a compound called adenosine diphosphate, which is phosphorylated in the process to form adenosine triphosphate.

The terminal phosphate bond of ATP, which is formed by energy transfer, is called a high-energy bond, and we denote it by a special symbol. In a sense, it is ordinary bonding in a highly concentrated form. Whenever such a high-energy bond is formed, it is a result of exergonic reactions which occur in the cell. When it is broken and the energy is transferred to other bonds, the reactions are endergonic. During the process, ATP is dephosphorylated back to AD P. To turn to our original point, energy is not actually “yielded”, “released”, “utilized”, or “expended” in the cell; it is transferred and transformed.

First of all, the fuel molecule is phosphorylated through reaction with a phosphate source, that is, a radical containing a single atom of phosphorus is substituted on the molecule for a hydrogen atom or a hydroxyl group.

It then undergoes dehydrogenation and loses two hydrogen atoms. If will lose its others later on, but we will consider only these two at this point. The hydrogen atoms reduce NAD to Nad.H2 and the fuel molecule is this oxidized. This oxidization of the molecule causes some electrons to shift, and in the process, the phosphate bond becomes a high- energy bond. Almost simultaneously, this high-energy bond is transferred to ADP, thus forming ATP, and the oxidized fuel molecule is in a dephosphorylated state.


Meanwhile, NAD.H passes the hydrogens along to the nexc acceptor in the hydrogen-transport sequence. At this point, the electrons of the hydrogen atoms assume a lower energy level, and the resulting energy is transferred to a high- energy bond to ATP through the phosphorylation of ADP. At certain points in the sequence, more ATP is generated in this fashion.

Some energy dissipates as the process, but much of the energy is transferred to ATP. In a sense, ATP is the “usable currency” of the cell, and it may be used to phosphorylate more fuel or to drive the many endergonic reactions which take place in the cell.

Apparently, the ADP-ATP energy-transfer system is operative in all cells, and it’s important to the world of life can hardly be overestimated. In spite of the variety of forms that energy expenditure takes in organisms. ATP is usually involved in all such reactions.

The system is quite efficient, in as much as energy may be transferred from a fuel substance to ATP in a series of steps whereby heat productions is held to a minimum. It is worthy of note that living organisms have this mechanism in common, which is an indication that all of them are more nearly alike than superficial differences have often led us to believe.