Which of the following describes the movement of ions across a membrane in response to a concentration gradient?

Which of the following describes the movement of ions across a membrane in response to a concentration gradient?

• A. Active Transport
• B. Facilitated Transport
• C. Passive Diffusion
• D. Secondary Active Transport

Answer: (C)

The movement of ions across a membrane in response to a concentration gradient is best described as passive diffusion. This process occurs when ions move from an area of higher concentration to an area of lower concentration without the need for additional energy. The driving force behind passive diffusion is the concentration gradient itself, which is a natural tendency for substances to spread out evenly in available space. During passive diffusion, ions can move directly through the lipid bilayer of the membrane or, for larger or charged ions, may involve passing through specialized protein channels. However, what distinguishes this process is that it does not require energy input from ATP or any cellular energy source, as it relies solely on the concentration gradient. In contrast, other mechanisms such as active transport require energy to move ions against their concentration gradient, while facilitated transport involves the use of proteins to help molecules cross the membrane but still occurs down the concentration gradient and does not require energy. Secondary active transport uses the energy from primary active transport processes but still does not describe the simple passive movement of ions along their concentration gradient. Therefore, passive diffusion accurately characterizes the ion movement described in the question.

The movement of ions across a membrane in response to a concentration gradient is best described as passive diffusion. This process occurs when ions move from an area of higher concentration to an area of lower concentration without the need for additional energy. The driving force behind passive diffusion is the concentration gradient itself, which is a natural tendency for substances to spread out evenly in available space.

During passive diffusion, ions can move directly through the lipid bilayer of the membrane or, for larger or charged ions, may involve passing through specialized protein channels. However, what distinguishes this process is that it does not require energy input from ATP or any cellular energy source, as it relies solely on the concentration gradient.

In contrast, other mechanisms such as active transport require energy to move ions against their concentration gradient, while facilitated transport involves the use of proteins to help molecules cross the membrane but still occurs down the concentration gradient and does not require energy. Secondary active transport uses the energy from primary active transport processes but still does not describe the simple passive movement of ions along their concentration gradient. Therefore, passive diffusion accurately characterizes the ion movement described in the question.