Question

In the resting state of the neural membrane, diffusion due to concentration gradients, if allowed, would drive (a) \(\mathrm{K}^{+}\)into the cell (b) \(\mathrm{K}^{+}\)and \(\mathrm{Na}^{+}\)out of the cell (c) \(\mathrm{Na}^{+}\)into the cell (d) \(\mathrm{Na}^{+}\)out of the cell.

Short answer

Diffusion due to concentration gradients would drive (c) Na+ into the cell.

Step-by-step solution

Understand Resting Membrane Potential

The resting state of a neural membrane is characterized by its resting membrane potential, which is primarily established by the differences in concentration of ions, notably sodium (Na+) and potassium (K+), across the membrane. There is a higher concentration of K+ inside the cell and a higher concentration of Na+ outside the cell.

Identify Direction of Diffusion for K+

Due to the high internal concentration compared to the outside, K+ would diffuse out of the cell, down its concentration gradient, if channels were freely permeable. However, this is not an option, therefore K+ would not be driven into the cell.

Identify Direction of Diffusion for Na+

Similarly, due to the higher concentration of Na+ outside the cell compared to the inside, Na+ would diffuse into the cell, down its concentration gradient, if channels allowed for its movement.

Key concepts

Understanding the Neural Membrane

The neural membrane plays an instrumental role in the function of neurons. It's a biological barrier that separates the interior of the neuron from the extracellular space. Think of it like a security guard; it controls what enters and leaves the neuron. The unique structure of this membrane, composed of a double layer of lipids with embedded proteins, allows for selective permeability.

For a neuron to transmit signals, it relies on the difference in electric charge inside and outside of the cell, known as membrane potential. The membrane's resting potential, which is about -70 millivolts, is essential for the neuron's ability to fire an action potential. This difference in voltage is achieved by a delicate balance of ions, maintained by ion channels and pumps in the neural membrane.

These channels and pumps manage the movement of ions such as potassium (K+) and sodium (Na+), to maintain the neural equilibrium. Without this selective permeability and the careful maintenance of the resting membrane potential, our neurons would not be able to communicate, and our nervous system would not function effectively.

Ion Concentration Gradient: The Driving Force

When studying how neurons work, one fundamental concept is the ion concentration gradient. Imagine a crowd in a small room that sees an open door to a less crowded room — naturally, they'd want to spread out. In a similar manner, ions tend to move from areas of high concentration to areas of low concentration. This movement is what scientists call the concentration gradient.

For neurons, this involves primarily potassium (K+) and sodium (Na+). The cell membrane's natural state has a high concentration of K+ inside and Na+ outside. This differential creates a gradient for each ion, driving them to move to the side where they are less concentrated.

These ion concentration gradients are not just random occurrences; they are actively maintained by the cell's own energy-consuming ion pumps. The most famous is the sodium-potassium pump which keeps the inside of the cell high in K+ and low in Na+. The gradients are crucial because they store potential energy, which can be quickly converted into electrical signals when a neuron becomes active.

Potassium (K+) and Sodium (Na+) Diffusion Explained

Diffusion of ions across the neural membrane is like a dance of particles, with potassium (K+) and sodium (Na+) moving across the floor— the membrane. For potassium, which is more concentrated inside the neuron, the natural move is to diffuse outwards. This is the body's way of trying to balance concentrations on both sides of the membrane.

However, for sodium (Na+), which is more plentiful outside the neuron, its diffusion would naturally occur inward, as Na+ ions follow their gradient towards the area of lower concentration inside the neuron.

This dual movement of K+ and Na+ is what primarily shapes the resting membrane potential. Ion channels, most of which are gated, can open or close to allow these ions to move across the membrane. When the neuron is at rest, most sodium channels are closed, preventing Na+ from moving inward, while some potassium channels remain open, letting K+ slowly exit. This selective diffusion helps maintain the negative charge inside the cell, crucial for the neuron's readiness to fire an action potential.