Which of the following is true about the movement of ions across excitable living membranes?

• A. Ions can freely diffuse across the lipid bilayer without assistance.
• B. Ions move across excitable membranes primarily through specific protein channels.
• C. All ions move in and out of the cell at the same rate regardless of membrane potential.
• D. The movement of ions across membranes does not require any energy input.

Answer: (B)

Verified step by step guidance

1. Understand the structure of the lipid bilayer: The lipid bilayer is composed of hydrophobic tails and hydrophilic heads. Due to its hydrophobic nature, the bilayer acts as a barrier to charged particles like ions, preventing them from freely diffusing across the membrane.
2. Recognize the role of protein channels: Ions move across excitable membranes primarily through specific protein channels embedded in the membrane. These channels are selective and allow the passage of certain ions based on size, charge, and other properties.
3. Consider the influence of membrane potential: The movement of ions is influenced by the membrane potential, which is the electrical gradient across the membrane. Different ions move at different rates depending on the electrochemical gradient.
4. Evaluate energy requirements: Some ion movements are passive and occur without energy input, driven by concentration gradients or membrane potential. However, active transport requires energy, often in the form of ATP, to move ions against their concentration gradient.
5. Eliminate incorrect statements: Based on the above understanding, eliminate the incorrect options. Ions cannot freely diffuse across the lipid bilayer without assistance, and not all ions move at the same rate. Additionally, some ion movements do require energy input, especially during active transport.