Plant Physiology: questions with answers

The exclusive-leaf claim is too narrow because other green plant parts also photosynthesise. The site assignments are reversed: photochemical reactions use the membrane system, while enzymatic carbon reactions occur in the stroma.

The term means that these reactions are not directly driven by light. They can occur in light and depend indirectly on light because they consume ATP and NADPH produced by the photochemical reactions.

All three contributions reinforce a proton gradient: protons are released or transferred into the lumen while stromal protons are removed. Thus the lumen has a higher proton concentration and a lower pH than the stroma.

Water splitting releases protons into the lumen, an electron-linked carrier transfers protons from stroma to lumen, and reduction of NADP+ consumes stromal protons. Together these processes make the lumen proton-rich and lower its pH relative to the stroma.

Controlled leaf tests require light, green chlorophyll-containing tissue and carbon dioxide for starch formation. Air restoration requires sunlight and green tissue, and Engelmann's aerobic bacteria report oxygen evolution, chiefly in blue and red light.

Starch formation in the leaf requires green tissue and light. Ingenhousz's comparison makes sunlight and green tissue conditions of the oxygen-releasing plant process, while Engelmann's aerobic bacteria indicate sites of greatest oxygen evolution rather than carbohydrate formation.

Chief does not mean sole: chlorophyll b, xanthophylls and carotenoids also absorb light and transfer energy to chlorophyll a. Their contribution explains why the action spectrum broadly resembles but does not completely overlap chlorophyll-a absorption.

Water splitting is associated with Photosystem II and replaces electrons lost from P680. Electron transfer then reaches Photosystem I and ends in reduction of NADP+, so this is a non-cyclic route from Photosystem II to Photosystem I.

Non-cyclic electron flow starts when P680 in Photosystem II is excited. Water splitting associated with Photosystem II supplies replacement electrons, and electrons then pass through Photosystem I before reducing NADP+; they do not return to the starting reaction centre.

Five-carbon RuBP accepts carbon dioxide through RuBisCO, and the unstable six-carbon intermediate yields two three-carbon PGA molecules. Reduction follows, and ATP-dependent regeneration restores RuBP so carbon fixation is cyclic rather than linear.

RuBP, not PGA, is the carbon-dioxide acceptor; PGA is the first stable three-carbon product. ATP-dependent regeneration restores RuBP, and the Calvin cycle occurs in both C3 and C4 plants, though its cellular location differs.

In mesophyll, three-carbon PEP accepts carbon dioxide through PEPCase to form four-carbon OAA, which may form malate or aspartate. A four-carbon acid moves to bundle sheath and releases carbon dioxide for the Calvin cycle; the remaining three-carbon molecule returns to mesophyll and regenerates PEP.

This feature—large bundle-sheath cells arranged around the vascular bundles—is part of Kranz anatomy and supports identifying the leaf as C4 from internal anatomy. In a C4 leaf, mesophyll cells perform initial PEPCase fixation, while bundle-sheath cells contain RuBisCO and carry out the Calvin cycle.

Water stress mainly lowers photosynthesis indirectly: stomata close and reduce carbon-dioxide entry, while wilting reduces leaf area and metabolic activity. The same temperature also need not be equally favourable to both plants because temperature optimum depends on habitat adaptation.

At low light, photosynthetic rate rises approximately linearly with incident light. For the marked plateau: More light no longer raises the rate because another factor has become limiting.