Sodium ions - Na+
When referring to co-transport or nervous impulses, make sure to mention sodium IONS or Na+, not just sodium or Na
Both seawater and blood plasma are mixtures of many ions, but one of the principal ions is sodium. Common table salt is sodium chloride, so chloride ions (Cl-
) are also present in large amounts.
Sodium ions are involved in the uptake of glucose and amino acids in the ileum by co-transport
Initially, sodium ions are actively transported
out of epithelial cells called enterocytes which line the villi (fingerlike structures in the small intestine). 3 sodium ions are pumped out for every 2 potassium ions pumped in.
This is an energy-requiring process, driven by ATP, produced as a result of aerobic respiration (oxidative phosphorylation).
As a result, the concentration of sodium ions inside the epithelial cells is lower than the sodium ion concentration in the lumen of the small intestine.
There is a concentration gradient
between these two areas, separated by the cell membrane.
There are a variety of specific co-transport proteins
on the surface membrane of the epithelial cells. Incidentally, there are many intuckings (microvilli) on this surface, forming a 'brush border', and this increases the surface area for absorption.
Some co-transport proteins accept amino acids (there are different co-transport proteins for acidic, neutral and basic amino acids). Other co-transport proteins [Na+
/glucose cotransporter 1 (SGLT1)] accept glucose.
But all these co-transport proteins must also take in sodium ions, and discharge them, together with the other molecules, inside the epithelial cells.
They then diffuse around the cell in the cytoplasm.
On the other surface of the cell, near to a network of blood capillaries, there are specific carrier proteins
forming channels through which amino acids (and glucose) can pass.
As the blood is always taking away these nutrients (towards the liver and general circulation) these nutrients are in a lower concentration outside the cell than inside. So the amino acids (and glucose) diffuse down a concentration gradient and out through the carrier protein channels - facilitated diffusion
Sodium-dependent glucose cotransporters also exist in the proximal convoluted tubule of the kidney and they perform renal glucose reabsorption.
Sodium ions are also involved in the passage of nervous impulses along the axon of a neuron, and at synapses between them.
These processes also depend on the sodium-potassium ion pump.