Which one of the following process is NOT useful for purification of solid impurities?

The correct answer is C, Neither I nor II, because both pairs are the wrong way round. Iron is a micronutrient, needed by plants only in very small amounts, and calcium is a macronutrient, needed in large amounts. Macronutrients are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulphur, while the micronutrients are iron, manganese, boron, zinc, copper, molybdenum, chlorine and nickel. Iron works in chlorophyll formation and in the electron carriers of photosynthesis, so a shortage shows as yellowing between the veins of young leaves. Calcium builds calcium pectate in the middle lamella of the cell wall and holds cells together. A is wrong because the first pair calls iron a macronutrient. D is wrong because the second pair calls calcium a micronutrient. B is wrong because neither pair is correct. Exam tip: remember the nine macronutrients; anything outside that list is a micronutrient.

The correct answer is A, Arteries. Arteries are the blood vessels that carry blood away from the heart to the different parts of the body. Because the heart pushes blood into them under high pressure, arteries have thick, muscular and elastic walls and no valves, and they are placed deep in the body. The largest artery is the aorta, which leaves the left ventricle. One important exception is the pulmonary artery, which carries deoxygenated blood from the heart to the lungs, so arteries are defined by direction, not by the kind of blood. B is wrong because veins bring blood back to the heart, have thinner walls and carry valves to stop backflow. C is wrong because the septum is the wall of muscle that separates the right and left sides of the heart. D is wrong because capillaries are the very thin vessels where exchange of gases and nutrients takes place. Exam tip: arteries away from the heart, veins towards it.

The correct answer is A, Wild yak. Bos mutus is the scientific name of the wild yak, the long haired bovine of the high cold plateaus of Ladakh, Tibet and the Karakoram, which grazes above four thousand metres and is listed as Vulnerable on the IUCN Red List. The domestic yak is called Bos grunniens, the grunting ox, and mutus means mute, since the wild animal is far quieter. The two part naming system, with the genus first and the species second, was given by Carl Linnaeus. B is wrong because the horse is Equus caballus. C is wrong because the ox, meaning domestic cattle, is Bos taurus or Bos indicus for the humped Indian breeds. D is wrong because the Asiatic water buffalo is Bubalus bubalis. Exam tip: the genus Bos covers cattle and yak, Bos mutus wild yak and Bos grunniens domestic yak.

The correct answer is A, Lysosome. The lysosome is a small sac bound by a single membrane that is filled with digestive enzymes, so it breaks down food particles taken into the cell and clears out worn out parts of the cell itself. Because it can digest the whole cell when the membrane bursts, for example when the cell is injured or starving, it is often called the suicide bag of the cell, a name given by Christian de Duve, who discovered it. Its enzymes work best in an acidic setting, which the lysosome keeps inside itself. B is wrong because peroxisomes handle oxidation reactions and break down hydrogen peroxide rather than acting as the digestive system. C is wrong because glyoxysomes are found in fatty plant seeds and convert stored fat into sugar. D is wrong because a polysome is only a group of ribosomes reading one mRNA strand. Exam tip: lysosome, single membrane, suicide bag, de Duve.

The correct answer is B, London dispersion forces. These are the weak attractions that appear when the electron cloud of an atom becomes momentarily lopsided, creating a temporary dipole which then induces a matching dipole in a neighbouring atom. Named after Fritz London, they are the weakest of the van der Waals forces and fall away very sharply with distance, so they count only over a few hundred picometres. They exist between all atoms and molecules and are the only forces holding non-polar substances such as helium, argon and methane together, which is why such gases liquefy only at very low temperatures. Their strength grows as the electron cloud gets larger. A is wrong because dipole-dipole forces act between molecules that already carry permanent dipoles. C is wrong because dipole-induced dipole forces need one permanent dipole to begin with. D is wrong because a hydrogen bond is a far stronger attraction, formed where hydrogen is bonded to nitrogen, oxygen or fluorine. Exam tip: strength order - hydrogen bond, then dipole-dipole, then London dispersion.