HOTS Science Primary 5 Systems

A healthy potted plant is placed inside a completely dark, sealed chamber for 48 hours. The temperature and water supply remain unchanged. A student makes two predictions about what will happen inside the plant during this period: Prediction 1: The rate of water transport through the xylem will decrease. Prediction 2: The phloem will stop transporting glucose to the roots. Which of the following correctly evaluates BOTH predictions and explains the reasons?

A Both predictions are correct: darkness causes stomata to close, reducing transpiration and slowing the transpiration pull that drives xylem water flow; and without light, photosynthesis stops so no glucose is produced in the leaves, leaving the phloem with no glucose to transport downward.
B Prediction 1 is correct but Prediction 2 is wrong: stomata close in the dark, slowing xylem flow; but the phloem can still transport stored glucose that was produced before the lights went off, so glucose transport to the roots continues normally.
C Both predictions are wrong: xylem transport is driven by root pressure which does not depend on light, so the rate does not change; and phloem transport continues because the plant uses cellular respiration to break down stored starch back into glucose for transport.
D Prediction 1 is wrong but Prediction 2 is correct: xylem transport actually increases in the dark because without photosynthesis the plant needs to absorb more minerals from the soil; but phloem transport stops because glucose requires light energy to move through the phloem tubes.
Show Worked Solution

Worked Solution

Step 1: Evaluate Prediction 1 — xylem transport in the dark The main driving force for water movement through xylem in most plants is the transpiration pull: water evaporates from leaf cells through stomata → this creates tension (low pressure) in the xylem → water is pulled upward from roots. In the dark, guard cells close stomata to conserve water. Closed stomata = reduced transpiration = reduced transpiration pull = slower xylem flow. Prediction 1 is CORRECT. Step 2: Evaluate Prediction 2 — phloem transport in the dark Phloem transports glucose (and other sugars) from the leaves (source) to the roots and other sinks. This transport is driven by a concentration gradient: photosynthesis continuously produces glucose in the leaves, keeping glucose concentration high there. Without light, photosynthesis stops. Glucose concentration in the leaves drops as it is consumed by cellular respiration. The concentration gradient that drives phloem transport weakens. After 48 hours of complete darkness, essentially no new glucose is being loaded into the phloem from the leaves. Prediction 2 is CORRECT. Step 3: Match to the correct option Option A correctly identifies both mechanisms: stomata closure → reduced transpiration pull → slower xylem; and no photosynthesis → no new glucose in leaves → phloem transport effectively stops. Both predictions are correct.

Correct answer: Both predictions are correct: darkness causes stomata to close, reducing transpiration and slowing the transpiration pull that drives xylem water flow; and without light, photosynthesis stops so no glucose is produced in the leaves, leaving the phloem with no glucose to transport downward.

Want more questions like this? Superholic Lab has 10,000+ MOE-aligned questions with full worked solutions.

Start Free Trial — 7 Days Free