Question

Electrogenic Transporters A single-cell organism, Paramecium, is large enough to allow the insertion of a microelectrode, permitting the measurement of the electrical potential between the inside of the cell and the surrounding medium (the membrane potential). The measured membrane potential is \(-50 \mathrm{mV}\) (inside negative) in a living cell. What would happen if you added valinomycin to the surrounding medium, which contains \(\mathrm{K}^{+}\)and \(\mathrm{Na}^{+}\)?

Short answer

The membrane potential becomes more negative than \(-50 \mathrm{mV}\) due to increased \(\mathrm{K}^+\) efflux.

Step-by-step solution

Understand Valinomycin's Effect

Valinomycin is an ionophore that specifically increases the permeability of the cell membrane to potassium ions (\(\mathrm{K}^+\)). Adding valinomycin would facilitate the movement of \(\mathrm{K}^+\) across the cell membrane according to its concentration gradient.

Identify Ion Concentration Gradient

In typical cells, the concentration of \(\mathrm{K}^+\) is higher inside the cell compared to the outside, while \(\mathrm{Na}^+\) concentration is higher outside than inside. The existing gradient implies \(\mathrm{K}^+\) ions will move out of the cell when permeability increases.

Predict Change in Membrane Potential

As \(\mathrm{K}^+\) ions move out due to increased permeability, positive charge will leave the cell, further increasing the negativity of the membrane potential. This hyperpolarizes the cell, making the membrane potential more negative than \(-50 \mathrm{mV}\).

Conclude Effect on Membrane Potential

The addition of valinomycin enhances \(\mathrm{K}^+\) efflux from the cell, resulting in a more negative membrane potential. This hyperpolarizes the membrane potential beyond its original \(-50 \mathrm{mV}\).

Key concepts

Membrane Potential

Membrane potential is an essential electrical phenomenon in cells. It refers to the difference in electric potential between the inside and the outside of a cell. This difference is created by the distribution of ions across the cell membrane. Typically, in most resting cells, the inside of the cell maintains a negative voltage relative to the outside. For example, in the Paramecium, a living single-cell organism, the membrane potential is recorded at a e − 5 0 mV (inside negative).

The membrane potential is primarily established by differences in the concentrations of ions, like sodium ( N a ^ + ) and potassium ( K ^ + ) across the cell membrane. This potential is crucial for a variety of cellular functions, including signal transmission in nerves and muscle contractions.

Ion Transport

Ion transport involves the movement of ions across the cellular membrane through various mechanisms. These can be passive, like diffusion, where ions move along their concentration gradient, or active, requiring energy to move ions against their gradient. It refers to the process where specific transport proteins or channels facilitate the movement of ions such as K ^ + and N a ^ + across the cell membrane.

• The sodium-potassium pump (Na/K pump), for instance, is an active transporter that pumps three N a ^ + ions out of the cell and two K ^ + ions into the cell, contributing to the membrane potential.
• Additionally, when specific ion channels open, like those affected by molecules such as valinomycin, they allow ions like K ^ + to flow.

This ion transport is vital for maintaining cellular homeostasis and enabling neurotransmission and muscle contraction.

Valinomycin Effect

Valinomycin is a type of ionophore, a compound that can transport ions across the lipid bilayer of cell membranes. Specifically, valinomycin increases the permeability of the membrane to potassium ions ( K ^ + ). This shift significantly influences how ions are transported across the cell's boundary.

When valinomycin is present, it facilitates K ^ + ions moving along their concentration gradient from an area of higher concentration inside the cell to a lower concentration outside. This change disrupts the balance of charges across the membrane:

• As K ^ + ions exit the cell, they carry positive charges with them.
• This loss of positive charge results in an increased membrane potential, making it more negative— a process known as hyperpolarization.

By altering the K ^ + flow, valinomycin's effect explains the crucial link between ion permeability and changes in membrane potential.

Potassium Ion Permeability

Potassium ion permeability is pivotal in determining a cell's membrane potential. It refers to how easily K ^+ ions can move across the cell membrane. Under normal conditions, the cell membrane is selectively permeable, allowing certain ions to pass more easily than others.

Potassium ions typically have a higher concentration inside the cell compared to the outside. This gradient is maintained by specific pumps and channels in the cell membrane. However, permeability can change:

• When the permeability to K ^ + is increased, for instance through ionophores like valinomycin, K ^ + starts to flow out of the cell easily.
• This K ^ + efflux results in a shift in the membrane potential, leading the inside of the cell to become more negatively charged.

Therefore, potassium ion permeability is a deciding factor in cellular excitability, influencing functions like neural activity and muscle contractions.