[smghz]

I was studying cellular respiration and photosynthesis and learnt that redox is the basis of these reactions. Take CR, for instance.



So, glucose is oxidized in both glycolysis and citric acid cycle, and in electron transport chain oxygen gas accepts the low-energy electrons.

But it felt weird to me. Why is it that when glucose is oxidized, it loses atoms (I don't know particularly the number of each atoms lost but definitely all), and why is it that when oxygen gas receives electrons (just electrons, mind you) that all of a sudden it becomes water? Does it have to do with the resulting charges of oxidation-reduction? I was perplexed by it; I know I cannot completely understand cellular respiration and photosynthesis without knowing how redox works. thanks.

[Borek]

Sorry, but your questions are so basic they are almost impossible to answer without writing half a GenChem101 book. This is simply how these things work.

why is it that when oxygen gas receives electrons (just electrons, mind you) that all of a sudden it becomes water?

That's not true. It is not that oxygen receives "just electrons", the reaction is much more complicated (and takes many steps). The reaction equation you posted is just an overall summary of what is happening.

[smghz]

all right, so the reaction has many more steps and thus atoms are eventually rearranged in the way we see it in this simple formula. thank you!

[AWK]

For details see wikipedia
https://en.wiki2.org/wiki/Cellular_respiration+Newton

[Enthalpy]

It would indeed need lengthy explanations... A few hints:

On Earth, atoms use to organize in molecules. There are very few lone atoms.

Chemical reactions are rearrangements of the atoms, destroying some molecules (or more generally species) to form other ones. When the bonds in the new molecules are stronger, energy (like heat) is released.

Reactions are possible when an easy path exists, often over many intermediate species, to avoid every intermediate that needs a high energy hence would be highly improbable. For instance, you won't see any atomic C in the reaction from glucose to carbon dioxide. Chemical synthesis is much the art of finding such paths. Living matter is incredibly efficient at conducting reactions, sometimes very difficult ones, under very lenient conditions. Writing one arrow from glucose to carbon dioxide neglects all these intermediates.

O=O has rather weak bonds, weaker than N≡N, C=O or two H-O for instance, and this explains much why the combustion of hydrocarbons or sugars releases energy. O₂ must first be produced, and exists on Earth thanks to photosynthesis, using light provided by the Sun. Making cheap O₂ from light and CO₂, H₂O is just one example of a reaction badly difficult outside biology.

Up-to-date theories tell that atoms share electrons in molecules, possibly with the electronic density increasing at some atoms, but not necessarily. Though, the older ideas of atoms losing or gaining electrons are still interesting because they're simpler hence more fertile.

The (older but useful) oxido-reduction would tell that in 6×O₂, the electron-avid oxygen atoms receive no electron from their identical partners while in H₂O and CO₂ they do, so some C and H atoms get oxidized in the reaction, and this shall explain the produced heat.

Molecular orbitals instead tell that the orbitals formed in H₂O for instance are more favourable than in hydrocarbons or sugars and in O₂.