Plant Biology: Photosynthesis and Reproduction Notes

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.

The correct answer is B, the anther. A stamen is the male reproductive unit of a flower and has just two parts: a stalk called the filament and a swollen head called the anther, in which pollen grains are produced. All the stamens of a flower together make up the whorl called the androecium. A is wrong because the style is the slender tube of the carpel that connects the stigma to the ovary, so it belongs to the female whorl. C is wrong for the same reason: the stigma is the sticky tip of the carpel that receives pollen, and candidates who remember only that pollen touches the stigma often pick it by mistake. D is wrong because a sepal is a member of the calyx, the outermost green whorl that protects the bud; it has no reproductive role. The pair to fix in memory is filament and anther for the stamen, stigma, style and ovary for the carpel.

The correct answer is D, angiosperms, the flowering plants. Their pollen tube carries two male gametes into the ovule; one fuses with the egg to form the diploid zygote, which is syngamy, and the other fuses with the two polar nuclei to form the triploid primary endosperm nucleus, which is triple fusion. Two fusions in a single ovule make the event double fertilisation, and it is found nowhere else in the plant kingdom. A is wrong because bryophytes such as mosses have no pollen tube at all and need water for their motile male gametes to swim to the egg. B is wrong because pteridophytes such as ferns also release swimming gametes and bear no seeds. C is wrong because gymnosperms do form pollen tubes and seeds, but only one gamete fuses with the egg and their nutritive tissue is haploid female tissue, not a triploid endosperm.

The correct answer is B, the ovary. Once fertilisation is complete, the ovary enlarges and ripens into the fruit, and its wall becomes the pericarp that may be fleshy, as in a mango, or dry, as in a pea pod. A is wrong, and is the single commonest error in this question, because each ovule inside the ovary becomes a seed, not the fruit; the number of seeds in a fruit therefore tells you how many ovules the ovary held. C is wrong because the style, like the stigma, usually withers and falls away after pollination, having served only to carry the pollen tube down to the ovule. D is wrong because petals are display organs that attract pollinators and drop off soon after pollination. Note also that a fruit formed from the ovary alone is a true fruit, while apple and pear, whose flesh comes from the thalamus, are false fruits.

The correct answer is B, a stem. The potato is an underground stem tuber, and the proof is on its skin: the hollows called eyes are axillary buds with tiny scale leaves, and buds of this kind are never found on a true root. That is why a cut piece of potato with an eye grows into a whole new plant. A is wrong because the familiar modified roots are the sweet potato, carrot, radish, turnip and beetroot, none of which carries buds. C is wrong because modified leaves give us the fleshy scales of an onion bulb, the tendrils of a pea and the spines of a cactus, but not the potato. D is wrong because the potato is not a reproductive structure in the floral sense; the potato plant does bear flowers separately. Group the modified stems for revision: potato tuber, ginger and turmeric rhizomes, onion and garlic bulbs, colocasia corm.

The correct answer is C, ethylene, the only plant hormone that is a gas. It hastens the ripening of fruit, encourages leaves and flowers to drop and is used commercially in the form of ethephon to ripen mangoes, bananas and tomatoes evenly; a ripe fruit kept with unripe ones ripens them because it gives off ethylene. A is wrong because auxin is the hormone of cell elongation, bending towards light and apical dominance, and in high doses it works as a weedkiller. B is wrong because gibberellin lengthens stems, breaks seed dormancy and causes bolting in rosette plants; it is associated with growth in height rather than ripening. D is wrong because cytokinin promotes cell division, delays the ageing of leaves and helps shoots form in tissue culture. The hormone to pair with stress is abscisic acid, which closes the stomata and enforces dormancy.