Biology — PPSC previous year questions

Saffron is the dried stigma (with part of the style) of the flower of Crocus sativus; each flower yields only three stigmas, which is why it is the world's costliest spice. In India it is grown mainly at Pampore in Kashmir.

Photosynthesis is the process that removes carbon dioxide from the atmosphere: green plants fix CO₂ into carbohydrates using sunlight. Combustion of fossil fuels, decomposition and respiration by bacteria and other organisms all release CO₂ back into the air, so they complete the carbon cycle in the opposite direction.

Escherichia coli, the coliform bacterium of the human and animal gut, is the standard indicator organism. Its presence in a water sample shows faecal contamination. Other pathogens are then likely too. Entamoeba histolytica is itself a pathogen, not an indicator. Eichhornia (water hyacinth) and Lemna (duckweed) are aquatic plants. Their growth indicates nutrient enrichment, not sewage pollution as such.

Rice (Oryza sativa) was the first crop genome to be sequenced. Draft sequences of the indica and japonica subspecies were published in 2002. The International Rice Genome Sequencing Project released the complete map in 2005. Maize followed in 2009, sugar beet in 2014, and bread wheat's huge hexaploid genome only in 2018. Rice's small genome, about 430 million base pairs, made it the model cereal.

Bhang, ganja and charas (hashish) are all obtained from Cannabis sativa, commonly called Indian hemp. Bhang is prepared from the dried leaves, ganja from the flowering tops and charas from the resin exuded by the plant; the active compound is tetrahydrocannabinol (THC). The other options yield different drugs: the opium poppy gives morphine and heroin, henbane gives hyoscyamine, and Rauwolfia serpentina gives the antihypertensive alkaloid reserpine.

Abiotic components are the non-living physical and chemical factors of an ecosystem: sunlight, temperature, water, air (including carbon dioxide), soil and minerals. Green plants are living producers and belong to the biotic component along with consumers and decomposers.

Paraheliotropism is the movement that turns leaves parallel to the incoming sunlight, so they avoid direct sun. It guards against excess light, heat and water loss, and is seen in many legumes. Diaheliotropism is the opposite: leaves track the sun at right angles to its rays, as in the sunflower. Heliotropism is sun-tracking in general. Phototropism is directional growth towards light, driven by the hormone auxin.

Hydroponics is the technique of growing plants without soil, in a nutrient-rich water solution, often on inert substrates like cocopeat, rockwool or perlite inside greenhouses. It saves water, allows year-round cultivation and higher density. Related soil-less methods: aeroponics (roots suspended in air and misted with nutrients) and aquaponics (hydroponics combined with fish farming).

Corals and their zooxanthellae algae share mutualism (+/+): the algae photosynthesise and supply the coral polyps with food and oxygen, while the coral provides the algae shelter and CO2/nutrients. When stressed by warm water, corals expel the algae — 'coral bleaching'. Commensalism benefits one with no effect on the other (+/0), amensalism harms one with no effect on the other (-/0), and parasitism benefits one at the other's cost (+/-).

Bergmann's Rule says that within a species, birds and mammals in colder regions are larger in body size. A larger body has a smaller surface area compared to its volume, so it loses less heat; this is exactly why bigger bodies suit cold places. Both statements are true and Statement II explains Statement I.