Photosynthesis · Paper 2 — Free Biology HL Practice Questions

Auxin (indole-3-acetic acid, IAA) is a plant hormone that regulates phototropism. In a classic experiment, coleoptile tips of oat (Avena sativa) were removed and placed on agar blocks. The blocks were then placed asymmetrically on decapitated coleoptiles, and the angle of curvature was measured.

Cytokinins are plant hormones that promote cell division and delay senescence. They are produced primarily in roots and transported to shoots. A research team investigated the effect of the synthetic cytokinin 6-benzylaminopurine (BAP) on chlorophyll retention in detached leaves of Phaseolus vulgaris (bean). - Leaf discs ($1\,\text{cm}$ diameter) were floated on BAP solutions of varying concentration. - After 5 days in darkness, chlorophyll content was measured spectrophotometrically at $663\,\text{nm}$. Treatment — Absorbance at $663\,\text{nm}$ Control (distilled water) — $0.12$ $10\,\mu\text{M}$ BAP — $0.35$ $100\,\mu\text{M}$ BAP — $0.48$

Endocrine disruptors are environmental chemicals that interfere with hormone signalling pathways. Bisphenol A (BPA) is a monomer used in polycarbonate plastics that leaches from plastic waste into aquatic ecosystems. BPA is structurally similar to oestradiol, a lipid-soluble steroid hormone that crosses the plasma membrane and binds to an intracellular receptor; the resulting hormone–receptor complex acts a transcription factor. Researchers measured BPA concentration in water and in the tissues of organisms at successive trophic levels in a contaminated lake. Sample — BPA concentration Lake water — $2.5\ \mu\text{g}\,\text{L}^{-1}$ Algal tissue — $15.3\ \mu\text{g}\,\text{kg}^{-1}$ Zooplankton tissue — $48.7\ \mu\text{g}\,\text{kg}^{-1}$ Small fish tissue — $162.4\ \mu\text{g}\,\text{kg}^{-1}$ Additional information: - BPA has a lower binding affinity for oestradiol receptors than oestradiol itself. - BPA is more resistant to metabolic breakdown in animal tissues than oestradiol.