Double fertilisation is a characteristic feature of which group of plants?

The correct answer is B, chemical. In photosynthesis a green plant absorbs sunlight through chlorophyll and uses that energy to join carbon dioxide and water into carbohydrate. The energy of the light does not disappear; it is locked into the chemical bonds of the glucose molecule, which is why the leaf is described as converting light energy into chemical energy. That stored energy is later released by respiration in the plant, or in the animal that eats it. A is wrong because although leaves do warm slightly in sunlight, heat is an incidental loss and not the product of photosynthesis. C is wrong because kinetic energy is the energy of a moving body and nothing in the leaf is set in motion by the process. D is wrong because physical energy is not a recognised category of energy at all; it is a vague term included to mislead a hurried reader.

The correct answer is B, magnesium. Chlorophyll is built on a ring structure with a single magnesium atom held at its centre, and because magnesium is needed to make the pigment, a plant short of magnesium develops chlorosis, the yellowing of leaves between the veins. A is wrong, and is the commonest error, because iron sits at the centre of the haem group of haemoglobin, the respiratory pigment of blood; iron is still needed by plants, but for enzymes and for chlorophyll synthesis rather than as part of the molecule. C is wrong because calcium in plants is used mainly in the cell wall as calcium pectate and in cell signalling. D is wrong because zinc is a micronutrient needed in trace amounts by several enzymes. A useful pair to memorise together is magnesium in chlorophyll, iron in haemoglobin, cobalt in vitamin B12.

The correct answer is B, water. In the light reaction, water absorbed by the roots is split in the presence of light and chlorophyll, a step called photolysis of water or the Hill reaction. The hydrogen freed is carried away to reduce NADP, and the oxygen atoms combine and escape as oxygen gas through the stomata. Experiments that fed plants water labelled with heavy oxygen found the label in the gas released, which settled the matter. A is wrong because the carbon and oxygen of carbon dioxide are built into the carbohydrate rather than released, though the name of the gas tempts candidates. C is wrong because glucose is a product of the process, not a raw material being broken down. D is wrong because chlorophyll only absorbs light and passes on the energy; it is a catalyst-like pigment that is not consumed and contributes no oxygen.

The correct answer is B, the stroma, the fluid-filled ground substance of the chloroplast that surrounds the membrane stacks and holds the enzymes of the Calvin cycle, including RuBisCO. Here the ATP and NADPH made in the light reaction are spent on fixing carbon dioxide into carbohydrate. A is wrong because the grana are the stacks of thylakoid membranes that carry the pigments and run the light reaction; placing the dark reaction there is the classic reversal in this question. C is wrong because the outer membrane of the chloroplast is only a boundary and has no role in either reaction. D is wrong because the thylakoid lumen is the space inside those membranes where protons accumulate during the light reaction to drive ATP synthesis, again part of the light phase. Remember the pair: light reaction in grana, dark reaction in stroma.

The correct answer is B, oxaloacetic acid, a four-carbon acid, which is exactly why the group is called C4. The gas is first fixed in the mesophyll cells by the enzyme PEP carboxylase on to a three-carbon acceptor, and the pathway is named after Hatch and Slack. Maize, sugarcane, sorghum and amaranthus follow it and show the special leaf anatomy called Kranz anatomy. A is wrong because 3-phosphoglyceric acid is the three-carbon first product of C3 plants such as wheat and rice; the two are swapped in almost every paper that asks this. C is wrong because pyruvic acid is a product of glycolysis in respiration and appears in the C4 cycle only later, when the acid is regenerated. D is wrong because ribulose bisphosphate is the five-carbon acceptor molecule in the Calvin cycle, not a product of fixation at all.