Compared to areas of a plant that are in the shade, parts the plant that are in sunlight are likely to show

Question

BlackTom AI · Accepted Answer

Sun-exposed leaf regions produce more sugars by photosynthesis, so companion cells that load those sugars into the phloem must work harder.

1) Relevant knowledge points and concepts
- Light increases photosynthetic rate in chloroplast-containing cells, producing triose phosphates that are converted to sucrose.
- Phloem transports sugars from source (photosynthetic tissues) to sinks. Sucrose loading into sieve-tube elements is an active process mediated by companion cells and proton pumps.
- Companion cells have abundant mitochondria to supply ATP for active transport. Xylem primarily transports water and mineral ions, not sugars. Respiration (oxygen consumption) occurs continuously, but net O2 output increases with photosynthesis in light.

2) Step-by-step reasoning
- Sunlight-exposed tissue: higher light reactions → more ATP and NADPH → Calvin cycle fixes more CO2 → increased production of sucrose.
- That sucrose must be moved into the phloem: companion cells use ATP-driven H+ pumps and sucrose transporters to load sucrose into sieve-tube elements.
- Increased sucrose production increases the demand for active loading, so companion cells increase metabolic activity (more ATP consumption, more transporter activity).
- The resulting higher solute concentration in phloem raises turgor pressure and enhances bulk flow toward sinks (so phloem flow increases, not decreases).

3) Intermediate steps (conceptual)
- Light → ↑ triose phosphates → conversion to sucrose.
- Sucrose concentration at source ↑ → companion cells activate H+-ATPase → cotransport of sucrose into phloem → ↑ phloem pressure and flow.

4) Why the correct answer is correct
Because companion cells carry out the energy-dependent loading of sugars into the phloem, parts of the plant in sunlight that make more sugar require higher companion-cell activity to export that sugar. Thus "higher activity in companion cells" directly follows from increased photosynthesis.

5) Why the other options are incorrect
- "Decreased bulk flow in phloem.": Wrong — more sugar loading increases osmotic pressure and typically increases, not decreases, phloem bulk flow.
- "Decreased oxygen consumption in photosynthetic cells.": Incorrect — net O2 production increases in light; cellular respiration continues, so oxygen consumption is not characteristically decreased by light exposure.
- "Increased transport of sugar through the xyem.": Incorrect — xylem transports water/minerals; phloem transports sugars.

Conclusion: The best choice is "higher activity in companion cells." This matches the provided correct answer.

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