Capitolo 5 Flashcards
(28 cards)
What is the purpose of electrical potentials across cell membranes?
Electrical potentials across cell membranes enable functions in various cell types. In nerve and muscle cells, they help transmit signals rapidly. In other cells, like glandular cells and macrophages, local changes in membrane potential activate specific cellular functions.
How are membrane potentials generated in nerve and muscle cells?
Membrane potentials are generated by differences in ion concentrations across the cell membrane. When ions like potassium or sodium diffuse through the membrane, they create an electrical charge difference across it, resulting in a membrane potential.
What happens when potassium ions diffuse outward through a selectively permeable membrane?
As potassium ions move outward due to their concentration gradient, they leave behind negative ions inside the cell. This causes the outside to become electropositive and the inside to become electronegative, creating a diffusion potential.
How does sodium ion diffusion affect membrane potential?
When sodium ions diffuse inward, they bring positive charges inside the cell, resulting in a membrane potential with positivity inside and negativity outside.
Why does further net diffusion of ions stop after a certain point?
The membrane potential rises to a level that blocks further net diffusion of the ions, balancing the electrical and concentration gradients. For potassium in nerve fibers, this potential is about -94 mV, while for sodium, it is +61 mV.
Perché l’ulteriore diffusione netta di ioni si ferma dopo un certo punto?
Il potenziale di membrana sale a un livello che blocca l’ulteriore diffusione netta di ioni, bilanciando i gradienti elettrici e di concentrazione. Per il potassio nelle fibre nervose, questo potenziale è di circa -94 mV, mentre per il sodio è di +61 mV.
What role do changing diffusion potentials play in nerve and muscle impulse transmission?
Rapid changes in diffusion potentials allow for the transmission of impulses along nerve and muscle membranes. These quick shifts in membrane potential are essential for signal propagation.
What is the Nernst potential, and how is it related to ion diffusion?
The Nernst potential is the diffusion potential across a membrane that exactly balances the net diffusion of a specific ion. It prevents any further net diffusion of that ion across the membrane by counteracting the concentration gradient with an opposing electromotive force (EMF).
How does the Nernst equation relate to ion concentration differences across a membrane?
The Nernst equation calculates the potential required to balance the ion concentration difference across a membrane. The greater the concentration ratio of the ion across the membrane, the larger the Nernst potential needed to counteract the ion’s diffusion.
What factors determine the sign and magnitude of the Nernst potential?
The sign of the Nernst potential depends on the charge of the ion and its direction of diffusion. If a positive ion diffuses from inside to outside, the Nernst potential is negative; if a negative ion diffuses outward, the potential is positive. The magnitude depends on the concentration ratio of the ion across the membrane.
How does body temperature influence the Nernst equation?
The Nernst equation is derived assuming a normal body temperature of 37°C (98.6°F). This temperature affects the constant in the equation, as temperature influences the kinetic energy and diffusion rates of ions across the membrane.
Why is the potential outside the membrane often considered zero in calculations?
Assuming the extracellular fluid has a potential of zero simplifies the calculation of the Nernst potential, making it easier to determine the relative potential inside the membrane needed to balance ion diffusion.
What would the Nernst potential be if the potassium concentration inside the membrane is ten times that outside?
If the potassium concentration inside is ten times higher than outside, the Nernst potential is calculated as -61 millivolts inside the membrane. This negative value prevents further diffusion of positively charged potassium ions outward.
What factors influence the diffusion potential in a membrane permeable to multiple ions?
The diffusion potential depends on the polarity of each ion’s electrical charge, the membrane’s permeability to each ion, and the concentration of each ion inside and outside the membrane.
Why are sodium, potassium, and chloride ions especially significant in creating membrane potentials?
Sodium, potassium, and chloride are the primary ions influencing membrane potentials because their concentration gradients and membrane permeabilities determine the membrane’s voltage, especially in nerve and muscle cells.
How does the permeability of the membrane to different ions affect the membrane potential?
The membrane potential is primarily determined by the ion to which the membrane has the greatest permeability. For example, if the membrane is only permeable to potassium, the membrane potential will closely match the Nernst potential for potassium.
What happens when there is a positive ion concentration gradient from inside to outside the membrane?
A positive ion gradient from inside to outside results in electronegativity inside the membrane. Positive ions diffuse outward, leaving behind non-diffusible negative ions inside, which makes the inside of the membrane electronegative.
How do concentration gradients of negative ions, like chloride, affect membrane potential?
A chloride gradient from outside to inside leads to negativity inside the cell. As chloride ions move inward, they leave excess positive ions outside, causing the membrane’s inside to become negative.
What role do changes in ion permeability play in nerve impulse transmission?
Rapid changes in sodium and potassium permeability enable signal transmission in neurons, as these ions shift to create quick alterations in membrane potential, crucial for nerve impulses. Chloride permeability, however, remains mostly unchanged during this process.
What is the main challenge in measuring membrane potential in cells?
The main challenge lies in the small size of cells and fibers, which requires highly precise instruments capable of detecting very low voltages with high resistance.
How is the potential difference between the inside and outside of the cell measured?
A micropipette filled with an electrolyte solution is inserted into the cell, and an indifferent electrode is placed in the extracellular fluid. The difference in potential is then measured using a voltmeter.
What role does the voltmeter play in measuring membrane potential, and why is it highly sophisticated?
The voltmeter measures small voltages across the cell membrane and is highly sophisticated because it must detect tiny voltage changes despite the high resistance at the micropipette’s tip.
What happens to the recorded potential as the electrode crosses the membrane?
As the electrode passes through the cell membrane’s voltage change area (the electrical dipole layer), the potential quickly drops to -70 millivolts and then stabilizes at that level within the cell.
How does the cell create a negative potential inside the membrane?
By transporting a small number of positive ions outward, the cell generates a negative resting potential of about -70 millivolts, requiring only a minimal shift in total positive charges.
How do rapid shifts in ion movement contribute to nerve signaling?
Fast movements of ions across the membrane can quickly change the potential from -70 millivolts to +35 millivolts in a fraction of a second, which is essential for transmitting nerve signals.
