3. Micronutrients

Page created on March 7, 2018. Last updated on November 19, 2018 at 17:16

Summary

• The micronutrients are vitamins, minerals and trace elements
• The minerals are Na⁺, K⁺, Ca²⁺, Mg²⁺, PO₄^3- and Cl^–
• Many trace elements are needed, like iron, iodine, zinc, copper and fluorine

Sodium, chloride and potassium

The main role of sodium and potassium ions is to carry charge. By having different amounts of charge inside and out of the cell, we create a potential force called the action potential, because electrochemical forces want to even out these charges.

Because the strong electrochemical force makes ions leak across the cell membrane, the electrolyte gradient must constantly pump ions back to maintain the gradient. The proteins that do this are called ion pumps. Some molecules are absorbed into the cell by taking advantage of the electrolyte gradient, as seen in the case of glucose and amino acids in the previous chapter.

In the collecting duct of the nephron in the kidney, as well as in the colon, Na⁺ is reabsorbed from the filtrate into the bloodstream. The epithelial sodium channel, or eNaC, is an ion channel than allows sodium ions to enter the cell. Sodium is then pumped out from the cell into the blood by Na⁺/K⁺ ATPase. eNaC is inhibited by a drug called amiloride and is activated by mineralocorticoid hormones like aldosterone.

Another ion channel used in sodium absorption in both the digestive system and the kidney is the sodium-hydrogen antiporter 3, or NHE3. This pumps one Na⁺ into the cell, and one H⁺ out. Another membrane protein, called DRA, pumps one Cl^– into the cell and one bicarbonate out. The net reaction of these two transporters is that Na⁺ and Cl^– is absorbed while H⁺ and HCO₃^– is pumped out. The latter two combine into CO₂ in the carbonic anhydrase reaction, which diffuses across the cell membrane into the cell again. Na⁺/K⁺ ATPase pumps Na⁺ out of the cell into the blood, while Cl^– passively does the same, with a simple ion channel.

The Cystic fibrosis transmembrane conductance regulator, or CFTR channel, is an ion channel that lets Cl^– leave the cell during excretion of Na⁺ and Cl^–. Its name comes from the fact that defects in the channel will cause cystic fibrosis. CFTR is opened when phosphorylated by PKA. Na⁺ simply moves between the cells from the blood into the lumen.

Magnesium and calcium

Calcium is mostly stored in our body in bones, as part of a crystal called apatite. It’s also needed for blood clotting, muscle contraction and signal transduction.

Magnesium is needed in many biochemical processes. It stabilizes molecules like ATP, nucleic acids, and is needed for DNA polymerase reactions as well.