In the universe, oxygen is the third most abundant element after hydrogen and helium. Oxygen is synthesized at the end of the life of massive stars, when helium is fused into carbon, nitrogen, and oxygen nuclei. Stars burn out, explode, and expel the heavier elements into interstellar space. Later, oxygen plays a crucial role in the emergence of life. Oxygen is not always reactive to the same extent. The oxygen atom (O) is more reactive than the oxygen molecule (O₂). To understand this, we will review some of the basics of oxygen. The oxygen atom is depicted in Fig. 9.1.

The eight electrons of the oxygen atom fill the “s” and “p” orbitals. The names “s” and “p” indicate the orbital shape and are used to describe the electron configurations. S orbitals are spherically symmetric around the nucleus, whereas p orbitals are rather like two identical balloons tied at the nucleus. The electron configuration for the oxygen atom reads as follows: 1s² 2s² 2p⁴. There are two electrons in the first shell and six in the second (Fig. 9.2). In the second shell, two electrons occupy an s-type orbital and four occupy p-type orbitals. Given that a p-type orbital has a capacity of six electrons, the oxygen atom falls short by two electrons of its “wanting” to fill its outermost shell to its full natural capacity.

This explains the high reactivity of the oxygen atom. Oxygen is, after fluorine, the most electronegative element. The electronegativity of an element describes its “electron hunger.” Atoms or molecules with an unpaired electron in their outer shell are called free radicals. The oxygen atom is a free radical. Since the two 2p orbitals (each containing a lone electron) are not full, the oxygen atom tries to become stable by reacting with other atoms and trying to add the electron of the other atom to its own shell. This makes the oxygen atom highly reactive. In nature, an oxygen atom typically steals away an electron from one or two other atoms to form a molecule, such as water (H₂O). To form an oxygen molecule, each oxygen atom donates two electrons to the other oxygen atom. In the case of the formation of water, each hydrogen atom “donates” one electron to the oxygen (Fig. 9.3).

This is an example of a redox reaction where hydrogen atom is oxidized (“loses electrons”) and oxygen is reduced (“gains electrons”). This electron transfer sets energy free; in other words, it releases heat, which is why this reaction is called exothermic. We can, therefore, also say that water is formed when hydrogen is “burned” by oxygen.

Similar to the oxygen atom, molecular oxygen also has two unpaired electrons in its last orbital that have the same spin (Fig. 9.4). Interestingly, however, the oxygen molecule, although a bi-radical, is only minimally reactive because the unpaired electrons in the oxygen molecule have the same spin. Thus, for the oxygen molecule to be able to react, it would need another molecule or atom with two unpaired electrons with a parallel spin opposite to that of the oxygen molecule. The latter will only rarely occur. Hence, the oxygen molecule is minimally reactive (spin restriction).

As oxygen is almost always needed in biological energy metabolism, we shall discuss the role of oxygen in more detail.

Oxygen is one of the most important molecules required for human life. One of the key ways for a cell to gain useful energy is by aerobic respiration (in other words, by oxidizing high-energy molecules). In environments, however, where oxygen is not sufficiently available, both primitive organisms and cells in our body can metabolize glucose by fermentation without using oxygen. This process also yields energy, though in lower amounts, and produces byproducts such as lactic acid.

Combustible material (e.g., sugar or wood) can be found anywhere on earth. Fortunately, most atmospheric oxygen (molecular oxygen) is inert, meaning that it reacts very sluggishly with other molecules. The oxygen in the air we breathe is normally in its “ground” (not energetically excited) state. However, for use in mitochondria (see Sect. 14.3), it does not need to be activated since mitochondria have special machinery that donates electrons to the oxygen molecule. If, however, oxygen reacts outside the oxidative phosphorylation pathway, it needs to be activated. This activation of oxygen may occur by two different mechanisms: either through the absorption of sufficient energy to reverse the spin on one of the unpaired electrons or through monovalent reduction (Fig. 9.5).

Thus, if ground-state oxygen absorbs sufficient energy to reverse the spin of one of its unpaired electrons, the two unpaired electrons now have opposite spins. In this more reactive form of oxygen, namely singlet oxygen (¹O₂), one of these unpaired electrons has a changed spin direction (Fig. 9.6).

Singlet oxygen, being much more reactive than ground-state oxygen, reacts destructively with molecules with double bonds.

The second mechanism of activation is by the stepwise monovalent reduction of oxygen that gives rise to superoxide anion radical (O₂^•−), hydrogen peroxide (H₂O₂), and, finally, water (H₂O). The situation is similar to that of a burning fire. To make a fire, we first need to transfer heat to a combustible material because the fire contains a cloud of electrons that facilitates the transfer of electrons to oxygen.

During combustion (burning), electrons are transferred from, for example, hydrogen or carbon to oxygen, resulting in the final product of water or carbon dioxide, respectively. We inhale oxygen and exhale carbon dioxide (the carbon comes from sugar or fat). To close the circuit and reach the “steady state” of these molecules in the atmosphere, oxygen must be regenerated by plants with the help of photosynthesis.

The term “photosynthesis” simply implies “synthesis” with the necessary energy coming from light (“photo”). The energy from sunlight is absorbed by green plants with the aid of the green pigment (chlorophyll) (Fig. 9.7).

Light absorbed by the chlorophyll is in the visible spectrum, which is a small part of the electromagnetic spectrum, as shown in Fig. 9.8.

The longer the wavelength of visible light is the more red the color will be. Likewise, the shorter wavelengths are toward the violet side of the spectrum. Since not all wavelengths of visible light are absorbed by chlorophyll, the leaves of plants appear green rather than black. Chlorophyll plant pigments are capable of absorbing light. Their light absorption spectrum is shown in the illustration in Fig. 9.9.

To regenerate oxygen, the chlorophyll molecules in plants need to take away an electron from the oxygen atom bound within a molecule, which in the case of photosynthesis is water. Prior to this, however, the chlorophyll molecule itself needs to lose an electron, as only the oxidized chlorophyll can take an electron away from oxygen. Chlorophyll is oxidized with the help of sunlight energy. However, one photon of light does not give the chlorophyll molecule enough energy to lose an electron. For this reason, the energy of several photons is summed up in the so-called “antenna” of chlorophyll molecules (Fig. 9.10).