General Science Previous Year Questions (PYQ) with Answers – Page 21

Biogas is mostly methane, with carbon dioxide as the second gas and only small amounts of nitrogen and others. Biogas forms when cattle dung, kitchen waste or crop residue is broken down by anaerobic bacteria, that is, without air, inside a digester.

Guard cells are specialised epidermal cells that surround each stoma on the surface of a leaf. Each stoma is a tiny pore bordered by two guard cells, which are bean-shaped in most plants and dumb-bell shaped in grasses.

Mendel worked out the laws of inheritance by crossing garden pea plants, Pisum sativum. Gregor Johann Mendel, an Austrian monk, carried out his hybridisation experiments between 1856 and 1863 in the monastery garden at Brno and published the results in 1866.

The eye changes the focal length of its lens to focus and changes the size of its pupil to control the light.

Diffusion is the spontaneous mixing of the particles of two substances, caused by the constant motion of those particles. Particles of matter are always moving and have spaces between them.

Red biotechnology is the branch applied to medicine and health care, including the making of insulin, antibiotics and vaccines. Biotechnology is often divided by colour according to its field of use.

Cerium oxide (CeO2) strongly absorbs ultraviolet rays while letting visible light through, so it is added to glass to make it UV-protective.

Photosynthesis absorbs energy from sunlight to build glucose, so it is an endothermic process. An endothermic process takes in heat or light from its surroundings, while an exothermic one gives energy out.

The atomic number (Z) is defined as the number of protons in the nucleus of an atom. Every atom of an element has the same number of protons, so the atomic number identifies the element: hydrogen has 1, carbon 6 and oxygen 8.

Dissolved salts make sea water less compressible, so sound travels faster in it, about 1,530 m/s at 25 °C, than in pure or drinking water.

The endodermis is the innermost layer of the cortex that surrounds the vascular tissue; it is not a part of the vascular bundle itself.

Physics recognises four fundamental forces: gravitational, electromagnetic, strong nuclear and weak nuclear, and the question lists all four. Every other force we meet is a form of these.

Adenosine triphosphate is the energy currency of the cell: breaking its last phosphate bond releases energy for cell work. ATP is made mainly in the mitochondria during cellular respiration, and in the chloroplasts of green plants during photosynthesis.

Charge equals current multiplied by time, and here that is 15 A × 600 s = 9000 C. Electric current is the rate of flow of charge, I = Q/t, so Q = I × t. The SI unit of charge, the coulomb, is the charge carried by one ampere…

Ligaments are bands of dense connective tissue that join one bone to another at a joint. Ligaments are rich in tough, slightly elastic fibres, so they hold the bones of a joint together while still allowing movement.

Glass is made by melting silica sand with fluxes and stabilisers, and this option lists the standard batch. Silica, from pure sand, forms the glassy network.

Ethyne, C2H2, has its two carbon atoms joined by a triple bond, each carbon also holding one hydrogen. Carbon has a valency of four.

Water molecules are strongly polar, so they can pull apart the positive and negative ions of a salt and surround them.

Xerophytes live where water is scarce, so they have few stomata to cut down water loss. Xerophytes are plants of deserts and dry habitats, such as cacti, Opuntia, Calotropis and Acacia.

The melting point is the minimum temperature at which a solid melts to become a liquid at atmospheric pressure, not the maximum.