Question

endocytosis and exocytosis are forms of active transport. what is active transport? (1 point)

the movement of molecules down a concentration gradient from high to low, using no energy

the movement of molecules up a concentration gradient from low to high, using no energy

the movement of molecules up a concentration gradient from low to high, using energy

the movement of molecules down a concentration gradient from high to low, using energy

Explanation:

To determine the correct definition of active transport, we analyze the key characteristics:

1. Active transport moves molecules against (up) the concentration gradient, i.e., from an area of low concentration to high concentration.
2. This process requires energy (usually in the form of ATP) because it is working against the natural tendency of diffusion (which is down the gradient and passive, using no energy).

Now let's evaluate each option:

• Option 1: Movement down the gradient (high to low) with no energy is passive transport (like diffusion), not active. Eliminate.
• Option 2: Moving up the gradient (low to high) but using no energy is impossible, as moving against the gradient requires energy. Eliminate.
• Option 3: Movement up the gradient (low to high) using energy matches the definition of active transport.
• Option 4: Movement down the gradient (high to low) using energy is incorrect, as moving down the gradient is passive and does not require energy input. Eliminate.

Answer:

C. the movement of molecules up a concentration gradient from low to high, using energy (assuming the third option is labeled C; adjust the label based on the actual numbering, but the text of the option is as above)