The disease caused by deficiency of niacin is: MCQ with Answer and Explanation

Explanation:
Robert Brown, a Scottish botanist, first described the nucleus in 1831 while studying epidermal cells of orchids. Hooke discovered cells, Leeuwenhoek observed living cells, Virchow proposed that cells arise from pre-existing cells. Brown's discovery was pivotal for understanding cellular organization.

Explanation:
COVID-19 is caused by SARS-CoV-2, a novel coronavirus. Tuberculosis is bacterial (Mycobacterium). Influenza is caused by influenza viruses (Orthomyxoviridae), not coronavirus. Pneumonia can be bacterial or viral, but the question specifies Streptococcus which is bacterial. COVID-19 emerged in 2019, causing a global pandemic, primarily affecting the respiratory system.

Explanation:
Chlorophyll a is the primary photosynthetic pigment that acts as the reaction center in photosystem I (P700) and II (P680), converting light energy to chemical energy. Accessory pigments like chlorophyll b, carotenes, and xanthophylls absorb light and transfer energy to chlorophyll a. Phytochrome is involved in photoperiodism. Without chlorophyll a, photosynthesis cannot occur.