CBSE Class 9 Science Exemplar Chapter 5: The Fundamental Unit of Life - NCERT Solutions

Viruses are unique biological entities that exhibit characteristics of both living and non-living matter. They lack the cellular machinery necessary for independent metabolism and reproduction. Consequently, outside a host cell, viruses can be purified and concentrated into crystalline structures, much like chemical compounds. This ability to form crystals is a key indicator of their non-living nature in the extracellular state.

This scenario describes a situation where the surrounding medium has a higher water potential (meaning a lower concentration of solutes and thus more free water molecules) compared to the inside of the cell. According to the principle of osmosis, water molecules will move from the region of higher concentration (the surrounding medium) to the region of lower concentration (inside the cell) across the semipermeable cell membrane. This influx of water causes the cell to increase in volume and swell up.

Chromosomes are the structures within the nucleus of eukaryotic cells that carry the genetic information. They are complex structures composed of DNA (Deoxyribonucleic Acid) tightly wound around a group of proteins called histones. This association of DNA and proteins allows for the compact packaging of the long DNA molecule into the nucleus and plays a crucial role in gene regulation.

Ribosomes are cellular organelles responsible for protein synthesis, earning them the nickname 'protein factories'. Therefore, they are involved in the manufacture of protein molecules (i) and enzymes, as enzymes are typically proteins (ii). However, ribosomes are not involved in the synthesis of hormones (iii) or starch molecules (iv). Hormones are produced by endocrine glands or specialized cells, and starch is a polysaccharide synthesized in plants.

The Endoplasmic Reticulum (ER) is a network of membranes within the cytoplasm that plays vital roles in protein and lipid synthesis, modification, and transport. It acts as a transport channel for proteins moving between the nucleus and cytoplasm and facilitates the movement of various materials within the cytoplasm. Additionally, it serves as a site for certain biochemical activities. However, the primary site for cellular energy generation through respiration is the mitochondria, not the ER.

Osmosis is a specific type of diffusion that involves the net movement of water molecules (the solvent) across a selectively permeable or semipermeable membrane. This movement occurs from an area where water is more concentrated (i.e., lower solute concentration) to an area where water is less concentrated (i.e., higher solute concentration). Option (b) describes diffusion in general. Option (c) is incorrect because it mentions a permeable membrane and solvent movement without specifying the concentration gradient relative to the membrane. Option (d) describes active transport or a different process, not osmosis, as it involves solute movement against a concentration gradient.