Question

How are the intracellular levels of $K^{+}$ higher than extracellular levels in animal cells?

Answer 1

Hint: The word intracellular is made up of two words, intra meaning inside and cellular refers to the cell. Therefore, the word intracellular means inside the cell. Similarly, the word extracellular is made up of two words, extra meaning outside and cellular refers to the cell. Therefore, the word extracellular means outside the cell.

Complete answer: The extracellular and intracellular fluid consists of various ions like sodium ions, potassium ions, chloride ions, calcium ions, etc. Some of these ions are positively charged, whereas other ions are negatively charged. The differential concentration of these ions across the axonal membrane polarizes it. During the resting potential, the axonal membrane is more permeable to the potassium ions. It is also almost impermeable to the sodium ions. Due to this reason, the axoplasm contains high concentrations of potassium ions and low concentrations of sodium ions. It is worth noting that the axoplasm forms the intracellular fluid. Also, the extracellular fluid contains high concentrations of sodium ions and very low concentrations of potassium ions. The gradients of these ions are maintained across the resting membrane by the active transport of the ions via the sodium-potassium pump. The pump transports three sodium ions outside into the ECF and two potassium ions inside the cell. This transportation occurs at the expenditure of one ATP. This makes the outer surface of the membrane of the axon positively charged whereas the inner surface becomes negatively charged.

Note: The resting potential is referred to as a condition in which the charge separation across the axonal membrane is maintained. Due to this reason, the neuron is said to be at rest. A resting neuron is always positively charged on the outside and negatively charged inside. This charge separation is known as the polarization due to which the resting neuron is called polarized.