What is the main difference between active transport and passive transport?

• A. Active transport moves substances down their concentration gradient, while passive transport moves them against it.
• B. Active transport involves the movement of water, while passive transport involves the movement of ions.
• C. Active transport requires energy input, while passive transport does not.
• D. Active transport occurs only in animal cells, while passive transport occurs in plant cells.

Answer: (C)

Verified step by step guidance

1. Understand the basic definitions: Active transport is the movement of molecules across a cell membrane from a region of lower concentration to a region of higher concentration, requiring energy. Passive transport is the movement of molecules across a cell membrane from a region of higher concentration to a region of lower concentration, without the need for energy.
2. Identify the role of energy: Active transport requires energy, often in the form of ATP, because it moves substances against their concentration gradient. In contrast, passive transport does not require energy as it moves substances down their concentration gradient.
3. Consider the types of molecules involved: Active transport can involve ions, nutrients, and other molecules that need to be concentrated within a cell or organelle. Passive transport typically involves small molecules like oxygen, carbon dioxide, and water that can easily diffuse across membranes.
4. Examine the cellular structures involved: Active transport often involves specific transport proteins or pumps, such as the sodium-potassium pump, which actively move ions across the membrane. Passive transport can occur through simple diffusion, facilitated diffusion via channel proteins, or osmosis.
5. Clarify the biological contexts: Active transport is crucial for maintaining concentration gradients across membranes, which are essential for processes like nerve impulse transmission and nutrient absorption. Passive transport is important for maintaining equilibrium and allowing cells to acquire necessary substances without expending energy.