Q: Explain the TACT mechanism of water movement through plants.
A: TACT = Tension, Adhesion, Cohesion, Transpiration — four physical forces that pull water up xylem without any energy input from the plant. Transpiration: water evaporates from leaves through stomata, creating negative pressure (tension) at the top of the water column. Cohesion: water molecules are strongly attracted to each other (hydrogen bonds) — the water column doesn't break. Adhesion: water molecules adhere to xylem cell walls (also hydrogen bonds) — prevents the column from pulling away from walls. Tension: the negative pressure at the leaf end pulls the entire water column upward. This tension-cohesion-transpiration mechanism can move water 100+ meters up tall trees. A large tree may transpire 100+ gallons per day.
Q: Compare C3, C4, and CAM photosynthesis — when is each pathway advantageous?
A: C3 (most plant species — wheat, rice, soybeans, trees): CO2 fixed directly by RuBisCO into a 3-carbon compound. Problem in hot, dry conditions: stomata close → CO2 drops → RuBisCO fixes O2 instead of CO2 (photorespiration) → wasteful. C4 (corn/maize, sugarcane, sorghum): pre-fix CO2 in mesophyll cells using PEP carboxylase (high CO2 affinity) → concentrate CO2 in bundle sheath cells → suppress photorespiration. More efficient in hot, sunny climates. CAM (cacti, agave, pineapple): fix CO2 at night when stomata open (cooler, less water loss), store as organic acids, release CO2 during day for Calvin cycle with stomata closed. Most water-efficient — adapted for arid environments.
Q: How do plants detect and respond to photoperiod (day length) to control flowering?
A: Plants detect photoperiod through phytochrome pigments in leaves that respond to red and far-red light. Short-Day Plants (SDPs) flower when night length exceeds a critical minimum (actually they measure night length, not day length). Long-Day Plants (LDPs) flower when night length is below a critical maximum. Day-Neutral Plants (DNPs) flower regardless of day length. Signal transduction: leaves detect photoperiod → produce florigen protein (FT — Flowering Locus T) → travels through phloem from leaf to shoot apical meristem → FT activates floral integrator genes → flower development. Grafting experiments: a SDP leaf grafted onto a LDP plant in wrong-day conditions will still induce flowering — proving florigen is mobile and universal.
Q: What are the five key adaptations that allowed plants to colonize land?
A: Moving from aquatic to terrestrial environments required solutions to five major challenges: (1) Cuticle — waxy layer on aerial surfaces prevents desiccation (water loss); thicker in desert plants. (2) Stomata — pores that allow CO2/O2 exchange while minimizing water loss; opened/closed by guard cells. (3) Vascular tissue — xylem transports water up (lignin provides structural support against gravity), phloem transports sugars. (4) Seeds — protect embryo, provide food supply (endosperm), allow dormancy until conditions are favorable, enable dispersal away from parent. (5) Pollen — male gametophyte dispensed by wind or animals, making fertilization independent of water — key to land colonization. Each adaptation expanded the range of habitats plants could occupy.
Q: What do ABA (abscisic acid), IAA (auxin), GA (gibberellin), and ethylene do in plant signaling?
A: ABA (abscisic acid): stress hormone — rises during drought (closes stomata via guard cell K+ loss), maintains seed dormancy, inhibits germination. Antagonist to GA. IAA (auxin — indole-3-acetic acid): main growth hormone — promotes cell elongation. High in shoot tips, causes phototropism (bends toward light by elongating shaded cells). Inhibits root growth at high concentrations but promotes lateral root formation at low concentrations. GA (gibberellin): promotes stem elongation (deficient plants are dwarf), triggers seed germination, fruit development. GA antagonizes ABA. Ethylene: gaseous hormone — promotes fruit ripening, leaf/fruit abscission, response to wounding. 'One rotten apple spoils the barrel' — ethylene gas is why. Cytokinin: promotes cell division, delays leaf senescence, works with auxin to regulate growth.