When sodium ion channels open sodium will always diffuse in

When sodium ion channels open, sodium will always diffuse _____ into the cell out of the cell via facilitated diffusion up its own concentration gradient down its own concentration gradient

Solution

Ans. Correct option. A. Into the cell. When sodium ion channels open, sodium ions always move into the cell causing depolarization of neurons.

Option B. Incorrect: see A

Option C. Incorrect. No doubt, movement of sodium ions through the ion channel does not require energy input (ATP), so it could be a resemblance of facilitated diffusion. However, there is a difference. Facilitated diffusion is the spontaneous movement of a molecule across a membrane along the gradient through a protein. That is, whenever, there is a concentration gradient across the membrane. Facilitated diffusion occurs spontaneously through the transporter protein. But sodium ions transport is highly regulated. These ion channels open and allows movement of ions only when stimulated (say, by electric potential), NOT spontaneously.

Therefore, keeping the ‘lack of spontaneity’, movement of sodium ions through sodium ion channel shall preferably not be a true facilitated diffusion process.

Option. D. Incorrect. See e

Option. E. Sodium ions move along the gradient (from high to low concentration), but the movement is always INTO the cell, never vice-versa. Movement down the gradient can mean movement into ot out the cell, depending on which side has higher concentration. However, through sodium ion channel, the ion transport is unidirectional, only into the cell, never vice versa.

So, option A specifying the unidirectional nature is more appropriate than ‘down the gradient’ that opens the possibility of bidirectional transport.