Question

During translocation of photosynthates in plants from source to sink:

• A. The loading of photosynthates at source is by active transport and unloading at the sink is by passive transport.
• B. The loading of photosynthates at source is by passive transport and unloading at the sink is by active transport.
• C. Both loading at the source and unloading at the sink are by active transport.
• D. Both loading at the source and unloading at the sink are by passive transport.

Hint:

Loading needs energy to build a sugar gradient in the phloem, unloading can simply use that gradient.

Answer 1

The Correct Option is A

Step 1: Understanding the Question.
We are asked how sugars made during photosynthesis (photosynthates) move from the source, a leaf that makes sugar, to the sink, a part of the plant that uses or stores sugar, and whether loading and unloading are active or passive.

Step 2: Key Concept.
At the source, sucrose has to move into the phloem sieve tubes from surrounding leaf cells. This happens against a concentration gradient in many plants, using proton pumps and sucrose-H+ co-transporter proteins, so it needs ATP and is an active transport process. At the sink, the phloem already carries a high sugar concentration built up during loading, while sink cells keep using up sugar for growth, respiration or storage, keeping their own concentration low. Because of this gradient, sugar can simply diffuse out of the phloem into the sink cells without an extra active step, making unloading generally passive.

Step 3: Detailed Explanation.
This matches option 1, loading at the source is active, unloading at the sink is passive. Option 2 reverses this, which does not fit how the gradients are actually set up. Options 3 and 4 say both steps use the same mechanism, but the source must use active transport to build a high enough phloem concentration in the first place, so both being passive is wrong, and both being active ignores that the sink can simply use the gradient already built at the source.

Step 4: Final Answer.
Loading at the source is active transport, and unloading at the sink is passive transport. \[ \boxed{\text{Loading: active, Unloading: passive}} \]