The sodium concentration in a cell is 10 times less than the concentration in the surrounding fluid. How can the cell move sodium out of the cell? (Explain your answer.)
a. Passive transport
b. Receptor-mediated endocytosis
c. Active transport
d. Facilitated diffusion

Verified step by step guidance

1. Understand the problem: The sodium concentration inside the cell is lower than the surrounding fluid, meaning sodium ions would naturally move into the cell down their concentration gradient. To move sodium out of the cell against this gradient, energy is required.
2. Review the transport mechanisms: Passive transport (a) does not require energy and moves substances down their concentration gradient. Receptor-mediated endocytosis (b) involves the intake of substances into the cell, not their removal. Facilitated diffusion (d) also moves substances down their concentration gradient without energy input.
3. Recognize that active transport (c) is the only mechanism that can move substances against their concentration gradient, which requires energy input, typically in the form of ATP.
4. Understand how active transport works: Active transport uses protein pumps, such as the sodium-potassium pump, to move sodium ions out of the cell against their concentration gradient. This process requires ATP to function.
5. Conclude that the correct answer is active transport (c), as it is the only mechanism capable of moving sodium out of the cell against its concentration gradient.

Key Concepts

Active Transport

Active transport is a cellular process that moves ions or molecules against their concentration gradient, from an area of lower concentration to an area of higher concentration. This process requires energy, typically in the form of ATP, because it is working against the natural tendency of substances to move from high to low concentration. In the context of sodium ions, active transport mechanisms, such as the sodium-potassium pump, are essential for maintaining the necessary concentration gradients across the cell membrane.

Concentration Gradient

A concentration gradient refers to the difference in the concentration of a substance between two areas, such as inside and outside a cell. In this scenario, the sodium concentration inside the cell is significantly lower than outside, creating a gradient that favors the movement of sodium ions into the cell. Understanding this gradient is crucial for determining how substances move across cell membranes, as it influences whether transport is passive or requires energy.

Passive vs. Active Transport

Transport mechanisms can be classified as passive or active. Passive transport occurs without the use of energy, allowing substances to move down their concentration gradient, while active transport requires energy to move substances against their gradient. In the case of sodium ions, since the cell's internal concentration is lower than that of the surrounding fluid, moving sodium out of the cell would necessitate active transport to maintain homeostasis and proper cellular function.