USABO Plant Physiology and Photosynthesis 2

Question 1

What triggers the conversion of the inactive phytochrome form (Pr) to the active form (Pfr)?

Accepted Answer

Red light at 660 nm

Answer

Phytochrome Pr absorbs red light at ~660 nm and undergoes a conformational change to the biologically active Pfr form, promoting germination and photomorphogenic responses.

Question 2

Which plant hormone primarily promotes cell elongation by activating plasma membrane H+-ATPases to loosen cell walls via expansins?

Accepted Answer

Auxin (IAA)

Answer

Auxin (IAA) activates H+-ATPases that acidify the apoplast, activating expansins that break hydrogen bonds in the cell wall and allow turgor-driven elongation.

Question 3

What is the primary role of abscisic acid (ABA) in plant responses to water deficit?

Accepted Answer

Triggering stomatal closure by inducing K+ efflux from guard cells

Answer

ABA is synthesized in wilted leaves and signals guard cells to efflux K+ and anions, reducing turgor and causing stomatal closure to limit transpirational water loss.

Question 4

According to the cohesion-tension theory, what property of water is most critical for maintaining continuous water columns in xylem?

Accepted Answer

High cohesion between water molecules via hydrogen bonds

Answer

Water's strong intermolecular hydrogen bonds create cohesion, allowing transpiration at the leaf to pull unbroken water columns up through xylem against gravity.

Question 5

What is the primary function of the Casparian strip in the root endodermis?

Accepted Answer

Blocking apoplastic water movement and forcing symplastic passage

Answer

The hydrophobic suberin lamellae of the Casparian strip block the apoplastic pathway, forcing water and ions to cross the endodermal plasma membrane where selective transport can occur.

USABO - USA Biology Olympiad Practice Test

USABO - USA Biology Olympiad Practice Test

USABO Plant Physiology and Photosynthesis 2