Biology Chapter 5: Membrane Dynamics Quiz

Course Information

• Faculty of Medicine
• Academic Year: 2024-2025
• Year: 1
• Semester: 1
• Module: Human Body Function (HBF) 102

Resting Membrane Potential

• Resting membrane potential (RMP) is the electrical potential difference across a cell membrane when the cell is at rest.
• The inside of the cell membrane is more negative than the outside.
• Normal range: -70 mV to -90 mV
• Important for neurons and muscle cells.

Objectives

• Define and list different membrane potentials.
• Recognize the driving forces behind the RMP development.
• Relate equilibrium potential of an ion to the RMP of the cell membrane.
• Identify the application of the Nernst equation in neurophysiology.
• Understand the role of the Na+/K+ pump in maintaining the RMP.

Cell Membrane

• The cell membrane acts as a selective filter.
• Allows the free movement of some molecules across it, but controls movement of others tightly.
• Uncharged molecules like oxygen (O2), carbon dioxide (CO2), urea, alcohol, and glucose move freely down their concentration gradient.
• Charged molecules (ions) cannot easily diffuse. They use ion channels, which are selective for specific ions.

Gibbs-Donnan Effect

• The Donnan effect is the phenomenon of a predictable unequal distribution of permeant charged ions on either side of a semipermeable membrane in the presence of impermeable charged ions.
• Ionic distribution across cell membranes, which is critical in maintaining the cell's resting membrane potential.

Ion Concentration

• Large anion proteins are not permeable to cell membranes.
• Small cations are attracted but are not bound to proteins.
• Small anions cross the capillary walls away from anionic proteins more readily than small cations.
• Intracellular concentration of K+ is much higher than extracellular. ( 150 mmol vs 5mmol )
• Extracellular concentration of Na+ is much higher than intracellular.(150mmol vs 15mmol ).
• Extracellular concentration of Cl- is much higher than intracellular. ( 125mmol vs 9mmol ).

The Donnan Effect on Ionic Distribution in Cells

• Cells contain impermeant anions (proteins/nucleic acids).
• These negatively charged ions create a tendency for positively charged ions, like potassium (K+), to enter the cell to balance the negative charges.
• If too many ions move across, osmotic balance is disrupted, leading to potential swelling (water entering).

Ion Movement

• Three factors induce ion movement through channels:
  • Concentration gradient
  • Electrical gradient
  • Ion channels

Interrelation of Donnan Effect and Resting Membrane Potential

• The Donnan effect influences the distribution of ions across the cell membranes.
• Large intracellular proteins (anions) cannot cross the membrane and contribute to overall negative charge inside the cell.
• This is essential in maintaining the resting membrane potential.

Resting Membrane Potential Values

• In most resting neurons, the potential difference across the membrane is about 30 to 90 mV.
• Inside the cell is more negative than outside.
• Excitable tissues (nerves and muscles) have higher potentials than other cells.
• Dead cells do not have membrane potentials.

Basic Physics of Resting Membrane Potential

• Contribution of K+ diffusion potential: The cell membrane tends to pump K+ out (positive charge) from high to low concentration.
• Electro-positivity outside; electro-negativity inside
• RMP is much more permeable to K+ than Na+.
• K+ diffusion contributes far more to resting membrane potential.

Basic Physics of Resting Membrane Potential

• Contribution of Na+ diffusion: A small amount of Na+ diffuses into the cell down its concentration gradient.
• The membrane is only slightly permeable to Na+ through K-Na leak channels.

Contribution of Na+/K+ Pump

• This is a powerful electrogenic pump on the cell membrane.
• Maintains concentration gradients of K+ and Na+ between the two sides of the membrane.
• Pumps 3 Na+ out and 2 K+ in, causing a net loss of positive charge from inside, returning the nerve fibre to the resting state.

Membrane Ion Channels

• Leak channels (passive channels) are always open and ions diffuse through them according to the ion concentration gradient.
• Voltage-gated channels open when the cell membrane is electrically activated.
• Chemically-gated channels open when a neurotransmitter binds to the receptor.

Equilibrium Potential

• The electrical potential difference across the cell membrane that exactly balances the concentration gradient for an ion is known as the equilibrium potential.
• The Nernst equation is used to calculate the equilibrium potential of a particular ion.

The Potassium Nernst (Equilibrium) Potential

• At rest, the intracellular K+ concentration (inside the cell) is significantly higher than the extracellular K+ concentration (outside the cell).
• K+ leak channels are much more permeable to K+ than Na+ (K+ molecules are smaller).
• This results in a net outward diffusion of K+ and negative charge inside. (-94mV).

The Sodium (Equilibrium) Potential

• The inside-to-outside ratio of Na+ is much less inside vs. outside (0.1).
• Na+ leak channels are relatively less permeable.
• This results in a net positive potential for Na+, as it would rush into the cell (+61mV).

Action Potential

• A series of brief, self-propagated electrical changes in excitable tissue when stimulated by adequate stimuli.
• Action potentials are responsible for communication between neurons and other cells,
• Action potentials exhibit multiple phases which include: depolarization, repolarization, and hyperpolarization.

Electrical Activities of Excitable Tissues

1. Resting membrane potential.
2. Electrotonus
3. Action potential

Clinical Significance of Action Potentials

• Nerve impulse transmission.
• Excitation-contraction coupling.
• Excitation-secretion coupling.
• Medical diagnosis (ECG, EEG, EM).
• Research in neuroscience and artificial intelligence (AI) fields.

Question Answers

• Most permeable ion at rest is potassium (K+).
• Blocking potassium channels leads to depolarization (positive).
• Sodium (Na+) has a positive equilibrium potential.

Test your knowledge on membrane dynamics with this quiz focusing on the Gibbs-Donnan model, resting membrane potential, and the functions of ion channels. Answer questions on the sodium-potassium pump and the equilibrium potential of various ions. Ideal for students studying cell biology or physiology.