SAT Subject Biology Practice Question 51: Answer and Explanation

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Question: 51

5. Which of these increase the movement of sap within the xylem?

I. Water moves into guard cells’ vacuoles.

II. Humidity increases in surrounding air.

III. Root endodermis prevents ions from entering xylem.

A. I only
B. II only
C. I and III
D. II and III
E. I, II, and III

Correct Answer: A


Water moving into guard cells increases turgor pressure, causing them to swell. Swollen guard cells open the stomatal pore, allowing water vapor to exit the leaf via transpiration. This, in turn, draws water up from the roots. In contrast, increasing humidity decreases water loss from the leaves. Root endodermis increases root pressure by pumping ions into the xylem.

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