Where Food Disappears To Inside the Cell

Following a Meal Through the Cell

The food on our plates is not simply burned the way logs burn in a stove. The cell takes carbohydrates, fats and protein apart into small units and, reaction by reaction, pulls electrons out of them. Those electrons travel along a string of proteins in the inner membrane of the mitochondria until, at the far end, they combine with oxygen and form water.

The energy set free on the way is not lost as heat; much of it ends up stored in a single molecule, adenosine triphosphate, known as ATP. That precise sequence – breakdown, electron handoff, ATP formation – is what biochemists have in mind when they talk about energy-yielding metabolism. It names a chemical process, not a sensation.

At several points along this route, trace elements appear as built-in parts of molecules. This page centers on two of them: iron, and after it iodine. The European Food Safety Authority (EFSA) reviewed scientific files on each, and the wording that resulted is listed entry by entry, one per element, in Regulation (EU) No 432/2012.

Two Profiles, Two Atomic Numbers

The elements are covered in turn. Every entry finishes with the sentence authorized for that particular element and no other.

1. Iron · Atomic Number 26

   One trait makes iron handy for cell chemistry: it flips easily between two charge states, Fe²⁺ and Fe³⁺. Every flip takes on or gives up precisely one electron. That is why, inside mitochondria, iron is lodged in proteins that hand electrons from one station to the next – either set into a heme group or tied to sulfur atoms in so-called iron-sulfur clusters.

   “Iron contributes to normal energy-yielding metabolism”

   EU-authorized wording · Regulation (EU) No 432/2012

2. Iodine · Atomic Number 53

   Iodine is a halogen and ranks among the heaviest elements the human organism puts to any use. It usually enters the body as the iodide ion. Once inside, it is found almost only in covalent form, attached to an amino acid scaffold that gives rise to the iodothyronines – molecules carrying three or four iodine atoms, whose signal travels as far as the cell nucleus.

   “Iodine contributes to normal energy-yielding metabolism”

   EU-authorized wording · Regulation (EU) No 432/2012

Zooming In on Where the Atoms Are Lodged

Not a single molecule is named in the authorized sentences. The sections below lay out which biochemical structures sit behind that wording, so you can follow what the text points to – and what lies outside it.

Heme Iron Inside the Cytochromes

Cytochromes are proteins built around a ring-shaped porphyrin framework with an iron atom lodged at its center. The inner mitochondrial membrane holds several of them: cytochromes b and c₁ in complex III, the mobile cytochrome c between complexes III and IV, and the heme groups a and a₃ in complex IV. Each of these iron centers picks up an electron, goes from Fe³⁺ to Fe²⁺ as it does, and passes the electron to its next partner.

Iron-Sulfur Clusters: Relay Points for Electrons

Besides the heme groups there is a second, more ancient kind of iron-containing center. Here, two or four iron atoms are linked with an equal number of sulfur atoms into a tiny cube or rhombus. Complex I on its own, the first big protein complex of the chain, holds a long line of these clusters in sequence, and electrons coming from NADH are relayed along them. Succinate dehydrogenase, which is part of the citric acid cycle at the same time, has three of them too.

• [2Fe-2S]: two iron atoms and two sulfur atoms, as in the Rieske protein
• [4Fe-4S]: a cube-shaped arrangement, present in aconitase and elsewhere
• [3Fe-4S]: a less common variant, seen in succinate dehydrogenase

Iodine Atoms Bonded Covalently Into Molecules

Iodine does not work as a free ion sitting on an enzyme; it gets incorporated into molecules. In the thyroid gland those molecules become tetraiodothyronine (T4) and triiodothyronine (T3). Close to two-thirds of the mass of T4 is accounted for by its four iodine atoms alone. In many cells of the body, T3 attaches to receptors in the nucleus and there affects how often certain genes are read – among them genes for mitochondrial proteins and for the sodium-potassium pump, one of the cell’s big ATP users.

Each of the two statements is independent of the other. Both refer to a chemical process as it runs when requirements are covered. Neither says anything about well-being, mood or sleep.