The correct sequence of the concentration of cations inside the cell in decreasing order is
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Toggle navigationCELL BIOLOGY BY THE NUMBERS
WHAT ARE THE CONCENTRATIONS OF DIFFERENT IONS IN CELLS?
Beginning biology students are introduced to the macromolecules of the cell (proteins, nucleic acids, lipids and carbohydrates) as being the key players in cellular function. What is disturbingly deceptive about this picture is that it makes no reference to the many ion species without which cells could not function at all. Ions have a huge variety of roles in cells. Several of our favorites include the role of ions in electrical communication (Na+, K+, Ca2+), as cofactors in dictating protein function with entire classes of metalloproteins (constituting by some estimates at least ¼ of all proteins) in processes ranging from photosynthesis to human respiration (Mn2+, Mg2+, Fe2+), as a stimulus for signaling and muscle action (Ca2+), and as the basis for setting up transmembrane potentials that are then used to power key processes such as ATP synthesis (H+, Na+).
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Figure 1: Ionic composition in mammalian organisms. Three distinct regions are characterized: the cellular interior (“intracellular fluid”), the medium between cells (“intercellular fluid”) and the blood plasma that is outside the tissue, beyond the capillary wall. The y-axis is in units of ionic concentration called Eq for “equivalents”, which are equal to the ion concentration multiplied by its absolute charge. These units make it easy to see that the total amount of positive and negative charge is equal in each compartment, in line with the principle of electro-neutrality. Even though it is not evident from the figure, the total free solute concentrations (sum of concentrations of both positive and negative components not taking into account their charge) are the same in the intracellular and intercellular fluid. This reflects that the two compartments are in osmotic balance. (Adapted from O. Andersen, “Cellular electrolyte metabolism” in Encyclopedia of Metalloproteins, Springer, pp. 580-587, 2013, BNID 110754.
A census of the ionic charges in a mammalian tissue cell as well as in the surrounding intercellular aqueous medium in the tissue is shown in Figure 1 left and middle panels. The figure also shows the composition of another bodily fluid, the blood plasma, which is separated from tissues through the capillary walls. The figure makes it clear that in each region the sum of negative ion charges equals the sum of positive charges to a very high accuracy. This is known as the law of electroneutrality. The relatively tiny deviations we might expect are quantified in the vignette on “What is the electric potential difference across biological membranes?”. Figure 1 also shows that blood ionic composition is very similar to that of the interstitial fluid. Yet, the composition of the cell interior is markedly different from the milieu outside the cell. For example, the dominant positive ion within the cell is potassium with a concentration that is more than 10-fold higher than that of sodium. Outside the cell the situation reverses with sodium as the dominant positive ion. These and the other differences are carefully controlled by both channels and pumps and we discuss some of their functional importance below.
Table 1: Ionic concentrations in sea water, a bacterial and yeast cell, inside a mammalian cell and in the blood. Concentrations are all in units of mM. Values are rounded to one significant digit. Unless otherwise noted, concentration is total including both free and bound ions. Note that concentrations can change by more than an order of magnitude depending on cell type and physiological and environmental conditions such as the medium osmolarity or external pH. Na+ concentrations are especially hard to measure due to trapping and sticking of ions to cells. Most Mg2+ ions are bound to ATP and other cellular components. More BNIDs used to construct table: 104083, 107487, 110745, 110754.