$Na^{+}$ and $K^{+}$ in Biological Systems

Location:

• $Na^{+}$: Primarily extracellular (outside cells) — blood plasma, interstitial fluid
• $K^{+}$: Primarily intracellular (inside cells)

Functions:

1. Nerve impulse transmission (action potential):

   • Resting potential: $Na^{+}$ outside, $K^{+}$ inside (maintained by $Na^{+}$-$K^{+}$ ATPase pump)
   • Depolarization: $Na^{+}$ rushes in (through voltage-gated $Na^{+}$ channels)
   • Repolarization: $K^{+}$ rushes out (through $K^{+}$ channels)
   • Pump: 3 $Na^{+}$ pumped out, 2 $K^{+}$ pumped in per ATP (active transport)

2. Osmotic balance: $Na^{+}$/$K^{+}$ concentration gradient maintains cell turgor and prevents lysis/crenation.

$Mg^{2+}$ in Biological Systems

1. Chlorophyll: $Mg^{2+}$ is the central metal in the chlorophyll porphyrin ring

   • Absorbs light energy at ~430 nm (blue) and ~680 nm (red)
   • Essential for photosynthesis and all plant life

2. Enzyme cofactor: $Mg^{2+}$ activates >300 enzymes including:

   • Kinases (ATP-dependent phosphoryl transfer)
   • DNA/RNA polymerases
   • Ribosomes ($Mg^{2+}$ stabilizes ribosomal structure)

$Ca^{2+}$ in Biological Systems

1. Bone and teeth mineralization: $Ca_{10}$($PO_{4}$){6}(OH){2} (hydroxyapatite) — provides mechanical strength
2. Blood clotting: $Ca^{2+}$ activates Factors VII, IX, X, and thrombin in the coagulation cascade
3. Muscle contraction: $Ca^{2+}$ binds troponin C → conformational change → allows actin-myosin interaction
4. Second messenger: $Ca^{2+}$ signals in cell signaling pathways (calmodulin, PKC activation)

Summary Table