Question

The tendency for Na+ to move into the cell can be due to a. the higher numbers of Na+ outside the cell, resulting in a chemical concentration gradient. b. the net negative charge inside the cell attracting the positively charged Na+. c. the attractive force of K+ inside the cell pulling Na+ into the cell. d. all of the above. e. a and b only.

Short answer

The correct answer is e. a and b only.

Step-by-step solution

Evaluate the Options

Firstly, understand each of the options provided. a and b refer to two valid reasons for Na+ ions to move into the cell, a chemical concentration gradient and the presence of a net negative charge inside the cell which attracts positively charged ions like Na+. However, option c suggests that another positively charged ion, K+, would attract Na+ into the cell. This is however not true, as like charges repel, not attract.

Eliminate Incorrect Options

Option c, therefore, is incorrect. This means any answer choices including c, in this case, option d should also be ruled out.

Choose the Correct Option

After eliminating, we're left with options a, b and e. Given our evaluation in Step 1, we know that both a and b represent valid reasons for Na+ movement into the cell. So, the correct answer is the one that includes both of these, which is 'e. a and b only.'

Key concepts

Sodium Ion Movement

Sodium ions, or Na+, have specific reasons for wanting to move into a cell. This movement is part of a larger process related to how cells maintain their environment, and it happens because of two main factors.

• First, there's often a higher concentration of sodium ions outside the cell than inside. This difference creates a chemical concentration gradient that encourages sodium ions to move where there are fewer of them—inside the cell.


• Second, inside the cell, there's a net negative electrical charge. Sodium ions are positively charged, and opposite charges attract each other, much like a magnet. So, the negative charge inside the cell pulls sodium towards it.

Together, these factors lead to effective sodium ion movement across the cell membrane.

Chemical Concentration Gradient

A chemical concentration gradient is a fundamental concept in cell biology. It describes how molecules, like sodium ions, naturally move from areas of higher concentration to areas of lower concentration.

• In cells, this movement is driven by the natural tendency of the molecules to spread out evenly in their environment.


• When we talk about sodium ions, they tend to move into the cell because the concentration of sodium is typically higher outside than inside.
• This difference across the cell membrane creates a driving force for sodium ions to move inward, trying to balance the concentrations on either side of the membrane.

This movement is passive, meaning it doesn’t require energy from the cell.

Electrical Gradient

Besides the chemical concentration gradient, the electrical gradient also plays a pivotal role in ion movement. This involves the cell's tendency to maintain a certain charge difference between its inside and outside.

• Cells usually have a higher concentration of negatively charged ions inside than outside. This creates a net negative charge.


• Positively charged ions, like Na+, are attracted to these negatively charged spaces.
• This attraction forms an electrical gradient that encourages sodium ions to move into the cell. It's the electrical equivalent of a tug of war where the negative charge inside the cell is pulling in the positive sodium ions.

Therefore, the electrical gradient works alongside the chemical gradient to regulate sodium ion movement.

Cell Membrane Dynamics

The cell membrane is like a security system, controlling what comes in and goes out of the cell. It has a unique structure made up of a lipid bilayer with embedded proteins that guide ion movement.

• The membrane serves as a barrier, yet it is selectively permeable. This means that while it protects the cell, it also allows specific ions like sodium to pass when needed.


• The proteins within this membrane act as channels or gates that open and close as required, permitting sodium ions to move in according to the gradients mentioned before.
• By controlling the influx and efflux of ions, the cell maintains its internal environment, essential for functions like nerve impulse transmission and muscle contraction.

The cell membrane dynamics are key to keeping a balanced internal state and responding to extracellular changes.