Consider statements about human diseases: 1. Typhoid is caused by a bacterium. 2. Malaria is caused by a protozoan. 3. Dengue is caused by a virus. Which are correct? MCQ with Answer and Explanation

Explanation:
The skin and mucous membranes form the first line of defense (innate immunity), acting as physical barriers that prevent pathogens from entering the body. White blood cells (leukocytes), including phagocytes and lymphocytes, are the cellular components of the immune system that identify, attack, and destroy pathogens. Antibiotics are drugs specifically designed to kill or inhibit the growth of bacteria by targeting their unique structures or metabolic pathways; they are completely ineffective against viruses. Thus, statements 1 and 2 are correct.

Explanation:
Biological nitrogen fixation, carried out by symbiotic (Rhizobium) and free-living (Azotobacter) bacteria, reduces atmospheric N₂ to ammonia. Nitrification converts ammonia to nitrates, denitrification returns N₂ to air, ammonification releases ammonia from organic matter.

Explanation:
Chitin is a polymer of N-acetylglucosamine, forming the tough exoskeleton of arthropods and the cell wall of fungi. Cellulose is a plant polysaccharide, starch and glycogen are energy storage polysaccharides in plants and animals respectively. Chitin provides protection and support.