22-Agric-A1 Applied Plant, Animal or Human Physiology · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. 04-Agric-A1 Applied Plant Physiology, National Exams May 2015 — a three-hour closed-book examination; one of two approved calculator models (Casio or Sharp) is permitted. All six (6) printed questions constitute a complete exam paper totaling 100 marks, and all six are worked here.
Reference texts. L. Taiz, E. Zeiger, I.M. Møller and A. Murphy, Plant Physiology and Development, 6th ed. (plant tissue types, photosynthesis, water potential and osmosis, phytochrome-mediated photoperiodism); E. Runkle, Daily Light Integral: A Useful Tool for Greenhouse Growers, Michigan State University Extension (DLI, greenhouse light transmission, supplemental lighting design); ASABE Standards (American Society of Agricultural and Biological Engineers) (greenhouse environment engineering).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Photosynthesis converts light energy into the chemical energy of sugar, and it happens in two coupled stages inside the chloroplast. The light reactions take place in the thylakoid membrane. Light-harvesting pigment complexes (chlorophyll a/b, carotenoids) absorb photons and funnel that energy into photosystem II and photosystem I, which pass excited electrons down an electron transport chain. Photosystem II replaces the electrons it loses by splitting water (photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂), which is the source of the O₂ released by plants. The electron transport chain pumps protons across the thylakoid membrane, building a gradient that ATP synthase uses to generate ATP (photophosphorylation), while photosystem I donates its excited electrons to reduce NADP⁺ to NADPH. So the light reactions' job is to capture light energy and convert it into two portable chemical energy carriers, ATP and NADPH, while releasing O₂ as a by-product.
The Calvin cycle (light-independent reactions) runs in the stroma and uses that ATP and NADPH to build sugar from CO₂. It has three phases: (1) carbon fixation — the enzyme RuBisCO attaches CO₂ to the 5-carbon sugar RuBP, forming an unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA); (2) reduction — ATP phosphorylates 3-PGA and NADPH reduces it to glyceraldehyde-3-phosphate (G3P), the usable sugar-phosphate product; and (3) regeneration — most of the G3P is recycled, using more ATP, to regenerate RuBP so the cycle can continue, while one net G3P out of every three turns of the cycle (for one CO₂ fixed) is exported to build glucose, starch and other carbohydrates. The Calvin cycle's job is therefore to fix inorganic carbon into organic sugar, spending the ATP and NADPH the light reactions supplied and handing back their spent forms (ADP, NADP⁺) to be recharged.
