Which of the following best describes the main difference between active and passive transport across a cell membrane?

• A. Both active and passive transport require energy input from the cell.
• B. Passive transport moves substances against their concentration gradient, while active transport moves substances down their concentration gradient.
• C. Active transport only occurs in plant cells, while passive transport only occurs in animal cells.
• D. Active transport requires energy input (usually ATP), while passive transport does not require energy.

Answer: (D)

Verified step by step guidance

1. Step 1: Begin by understanding the concept of transport across a cell membrane. Transport mechanisms are ways substances move in and out of cells, and they are categorized into active and passive transport.
2. Step 2: Define passive transport. Passive transport is the movement of substances across the cell membrane without the use of cellular energy. It relies on the concentration gradient, moving substances from areas of high concentration to areas of low concentration. Examples include diffusion, osmosis, and facilitated diffusion.
3. Step 3: Define active transport. Active transport is the movement of substances across the cell membrane that requires energy input, typically in the form of ATP. It moves substances against their concentration gradient, from areas of low concentration to areas of high concentration. Examples include the sodium-potassium pump and endocytosis.
4. Step 4: Compare the two mechanisms. The key difference is that active transport requires energy input from the cell, while passive transport does not. Passive transport relies on the natural movement of molecules down their concentration gradient, whereas active transport works against the gradient and requires energy.
5. Step 5: Eliminate incorrect options based on the definitions. For example, the statement 'Both active and passive transport require energy input from the cell' is incorrect because passive transport does not require energy. Similarly, 'Passive transport moves substances against their concentration gradient' is incorrect because passive transport moves substances down the gradient. The correct answer is: 'Active transport requires energy input (usually ATP), while passive transport does not require energy.'