Plant Physiology and Photosynthesis Flashcards
7 cards from real USABO practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Plant Physiology and Photosynthesis flashcards as text
What triggers the conversion of the inactive phytochrome form (Pr) to the active form (Pfr)?
Answer: Red light at 660 nm
Phytochrome Pr absorbs red light at ~660 nm and undergoes a conformational change to the biologically active Pfr form, promoting germination and photomorphogenic responses.
Which plant hormone primarily promotes cell elongation by activating plasma membrane H+-ATPases to loosen cell walls via expansins?
Answer: Auxin (IAA)
Auxin (IAA) activates H+-ATPases that acidify the apoplast, activating expansins that break hydrogen bonds in the cell wall and allow turgor-driven elongation.
What is the primary role of abscisic acid (ABA) in plant responses to water deficit?
Answer: Triggering stomatal closure by inducing K+ efflux from guard cells
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.
According to the cohesion-tension theory, what property of water is most critical for maintaining continuous water columns in xylem?
Answer: High cohesion between water molecules via hydrogen bonds
Water's strong intermolecular hydrogen bonds create cohesion, allowing transpiration at the leaf to pull unbroken water columns up through xylem against gravity.
What is the primary function of the Casparian strip in the root endodermis?
Answer: Blocking apoplastic water movement and forcing symplastic passage
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.
In photoperiodism, which molecular sensor in leaves measures the length of the critical dark period?
Answer: Phytochrome, which slowly converts from Pfr to Pr in darkness
Phytochrome in leaf tissue slowly reverts from Pfr to Pr during uninterrupted darkness; the length of the dark period (measured by Pfr depletion) determines flowering responses in photoperiodic plants.
Gibberellins promote stem elongation primarily by which mechanism?
Answer: Promoting both cell division and cell elongation by targeting DELLA repressors for degradation
Gibberellins trigger DELLA protein degradation via the GID1-SLY1/GID2 ubiquitin pathway, derepressing growth genes that promote both cell division in intercalary meristems and cell elongation.