Questions: Root Water Uptake and Stomatal Regulation

Question 1

What is the main effect of the Casparian strip in a root?

• A. It speeds water flow through the cell walls into the xylem
• B. It forces water and dissolved minerals to pass through endodermal cell membranes before entering the xylem
• C. It stores starch for the root
• D. It pumps water upward by contraction

Answer: (B)

The Casparian strip is a band of suberin, a waxy material, in the walls of endodermal cells. It blocks the apoplastic route (through walls and spaces between cells). Water and solutes must enter the cytoplasm of endodermal cells, so membrane transport proteins can select which minerals reach the xylem.

Question 2

During a drought, the hormone abscisic acid causes stomata to close.

• T. True
• F. False

Answer: True

Abscisic acid (ABA) accumulates when roots and leaves are short of water. It triggers guard cells to lose potassium ions; water leaves by osmosis, the guard cells go limp, and the pore closes. This cuts water loss but also cuts carbon dioxide uptake.

Question 3

Guard cells pump potassium ions into their cytoplasm in the morning. What happens next?

• A. Water leaves the guard cells and the stoma closes
• B. Water enters the guard cells, they become turgid and bow apart, and the stoma opens
• C. The guard cells divide to make the pore larger
• D. Carbon dioxide is pumped out of the leaf

Answer: (B)

Taking up potassium lowers the water potential inside the guard cells, so water enters by osmosis. The cells swell. Cellulose fibers wrapped around each guard cell let it lengthen but not widen much, so the pair bows outward and the pore opens.

Question 4

Why does water move from the soil into a root and then up to the leaves without the plant spending energy to pump it?

Model answer: Water moves passively from higher to lower water potential. Soil water usually has the highest water potential, the root and stem xylem lower, the leaf lower still, and the dry air the lowest. Evaporation from the leaves keeps the xylem under tension, and cohesion between water molecules carries that pull down to the roots.

The energy that lifts water comes from the sun, which drives evaporation. The plant does spend energy on ion transport, which sets up some of the gradients, but the bulk flow of water itself is passive.