Why Potassium is Essential to Life

In life forms, in between cells, no other positive ion is more prevalent than potassium (K+) . The easiest way to collect it is from ashes. If you let water pass through a cloth holding ashes, the water will dissolve some of their compounds, especially potassium carbonate and create a yellowish solution. Evaporating the water will leave behind potash. This is where potassium got its name. If you dip litmus into a potash solution, it turns blue indicating the presence of a basic or alkaline substance. The K+ ion is not responsible for this. It’s the carbonate which snatches an H+ from water, forming bicarbonate and hydroxide. But since the metal will react with water to also liberate hydroxide, it is appropriate to call potassium an alkali metal.

Alqali (اَلْقِلْي ) in modern Arabic means frying. Most sources link it to saltwort, a plant whose ashes are rich in potassium carbonate. In any case, alkali is also the source of the German word for potassium, kalium, which lends its “K” to the symbol of the element and K+ to the symbol of the ion in ashes and rocks.

The concentration of potassium ions (shown as red dots) changes, depending on whether pores are open or closed. Source: Wikipedia commons

i.                   Why do plants have K+ ?

K+ is involved in the opening and closing of a plant’s pores(stomata). The concentration of potassium ions rises inside guard cells when stomata are open and drop when pores are closed. The ion also affects several enzyme systems and affects the shape of proteins, which is crucial to the function they serve.

Along with nitrogen and phosphorus, potassium is one of three elements most likely to be missing from agricultural soils and limit plant growth. Plants deficient in potassium ion have a hard time polymerizing sugars and amino acids into large molecules of carbohydrates and protein.  Depending on the species, leaves could go yellow or brown, their edges curl or their stalks become weak. Root growth can be stunted, and fruits and seed yield may diminish.

Why is the ion also essential to animals? Well, without it we can’t move, think or hear. Let’s see why.

ii.                 K+ ‘s Role in Nerve Cells

I once asked my dentist how an anesthetic works, and he did not know. That does not make him a bad dentist. So long as he knows how much lidocaine or articane to use and where and how to inject it, ultimately, it’s what matters. It is however another reminder of how scientifically uninquisitive we are as a society, regardless of the education level reached by an individual.

Potassium ion plays an important role in the transmission of nerve impulses, which are of course interrupted by an anesthetic . The axon, the long stem-like section of a nerve cell, is nothing like a copper  wire.  An axon does not easily allow electrons to flow through it. In fact, it does not rely on them at all to transmit impulses, which move more slowly than electricity.

The thick axon (0.5 to 1.5 mm) of the squid led to a classic study of how a nerve impulse  is sent along the length of the nerve cell. An oscilloscope was connected to microelectrodes to keep track of any build up of either positive or negative charges, which when separated create a voltage, also known as a potential difference. When both were inserted in different regions outside the axon, there was no voltage. But if one microelectrode was inserted inside the axon, it revealed a voltage of -70mV, revealing that the inside of the cell was negatively charged and that the outside was positive. This is known as the resting potential, the voltage in the absence of a stimulus.

A potassium channel has ATP-powered molecules than can draw K+ ions away from water’s pull and move K+ in and out of the cell. In contrast, Na+, which is smaller than K+, cannot be pulled through. Source: Max Planck Institute.

When the axon is stimulated by applying pressure, a small region experiences a reversal of polarity: the inside becomes more positively charged and the outside becomes more negatively charged. If the stimulus is strong enough to overcome a threshold level, that region will experience a short-lived spike in voltage that will be strong enough to cause neighboring regions to go through a similar experience, we have a so-called action potential.    

What is happening at the level of ions? What does potassium have to with it? In the resting state there is 30 times more potassium inside the cell than outside. The K+ ions are free to move in and outside the cell through potassium channels in the membrane, but an excess of chloride, negatively charged proteins and other organic ions keep the amount of K+ at steady state and keep the inside negative. There is also another channel that’s narrower and which potassium ions cannot cross. It’s the sodium channel, which is wide enough for smaller sodium ions to cross. But in the resting state, it is closed, and the exterior of the cell is stuck with 10 times as much Na+, which is why the outside is positive.

A stimulus changes the membrane’s permeability to Na+. With the “doors” briefly open, a small amount of Na+ ions (1 in a million) move in, drawn by the excess negative ions(protein  ions, hydrogen  phosphate and others) on the inside. During the short-lived inflow of sodium, known as depolarization , a very few potassium ions get repelled and pumped out of the cell, but overall the Na+-deprived exterior becomes more negatively charged, and the Na+-enriched interior becomes more positively charged. 

The sodium gates re-shut. But before the cell’s pump can restore the original concentrations of sodium and potassium to that section of the axon, some of the excess positive ions from the inside move towards the negative organic ions of a neighboring region. This change of voltage opens the adjacent Na+ gates leading to an action potential next door. The cascade of events is replayed until the nerve impulse  travels to the dendrites at the end of the neuron. Why does it move in only one direction? In each region, after a surge in negative voltage, the potential difference drops below the original resting potential. This region’s extra positive charge inside the cell makes sure that the nerve impulse does not travel backwards. 

In vertebrates, a “design”-modification makes the impulse travel faster. The myelin sheath has nodes, which are the only parts get polarized. Effectively, the impulses jump from one node to the next, skipping over parts of the axon. This means a shorter length of the axon must be repolarized; the ATP -powered Na+/K+ pump works less, and less energy is consumed.

At the nerve terminal, the impulse eventually causes the release of neurotransmitters. They move across the gap between nerve cells, and when they reach the receptors of the neighboring cell, they affect the permeability of the membrane. The permeability can either be increased, which will cause an action potential; or if the interaction is of an inhibitory nature, it will decrease permeability, preventing a stimulus from being effective.

Each of the dentist’s common anesthetics, lidocaine or articaine, temporarily block the sodium channel. Although inflicted pain still opens the floodgates, the sodium ion cannot enter and cause an action potential. Without an impulse, no pain is felt by the patient while the dentist is drilling and not thinking about the theory he learned back in college. 

iii.       Why We Need K+  in the Inner Ear

The ear is a beautifully intricate organ. The outer ear, which consists of the recognizable outer part and the external auditory canal, focus air pressure disturbances on the tympanic membrane, the so-called eardrum . The middle ear has three small bones that evolved from the jawbones of ancient reptiles. The area of contact between the eardrum and of those bones(malleus) is much smaller than the eardrum. The combination of that factor with the leverage of the malleus and the incus allow for very little force to do significant work on the oval window, the boundary between the bones of the middle ear and the fluid of the inner ear’s cochlea, a structure shared by all mammals.

Along the length of the coiled cochlea there are three canals.  The following figure reveals three cross sections of those canals. Notice how wide they are at the base, which is closer to the round window,

and how they get progressively narrower as they move towards the apex of the coil—to the left of the other two in the diagram. Between two of the chambers there is the basilar membrane which plays a crucial role in an organism’s reception of sound waves.  If you could roll it out onto a flat surface, despite the progressively narrowing canals of the cochlea, the basilar membrane becomes gradually wider. The base is stiffer, and the membrane gets displaced by higher sound frequencies, whereas the membrane at the apex is sensitive to bass sounds of lower frequencies. Intermediate frequencies affect corresponding parts in between.  But how do the stimulated parts of the membrane get their message to the brain?

If we continue our game of Russian dolls, where we find one doll within another, magnification of the surface of the basilar membrane reveals the Organ of Corti.  This includes two types of hair cells, which are connected to two types of nerve fibers whose message flow in opposite directions to and from the brain. Embedded in the tectorial membrane,  are the hair cells’ stereocilia. As sound frequencies move the fluid inside the cochlea, disturbing specific sections of the basilar membrane, the interconnected stereocilia of its hair cells get bent. 

Finally, we come to potassium ion (K+)’s role. The membrane’s hair cells are in the middle of the 3 chambers, known as the cochlear chamber, and it is filled with aqueous fluid rich in K+and with lower concentrations of sodium ion. If the cilia get bent towards the tallest of the structures, it opens the gates of the coiled structures (tip links) connecting the cilia. Potassium ion flows into the hair cells, depolarizing them. By a mechanism we shall reveal in a future blog, depolarization open calcium channels, which stimulate the release of neurotransmitters. As the latter flow to auditory nerve fibers, they get an impulse that’s relayed to the brain.

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