In photosynthesis, oxygen is released from: MCQ with Answer and Explanation

Explanation:
Alveoli are tiny, thin-walled air sacs surrounded by pulmonary capillaries. Oxygen diffuses across the alveolar and capillary walls into the blood, while CO₂ diffuses out. The trachea, bronchi, and nasal cavity are air passages that do not participate directly in gas exchange.

Explanation:
Modern cell theory states that all living organisms are composed of cells and cell products, the cell is the basic unit of structure and function, and all cells arise from pre-existing cells (Omnis cellula e cellula). Not all cells have a nucleus (e.g., prokaryotes, mature RBCs). Cell theory was formulated by Schleiden, Schwann, and Virchow. Spontaneous generation was disproved.

Explanation:
Calcitonin, produced by parafollicular (C) cells of the thyroid gland, lowers blood calcium by inhibiting osteoclast activity and promoting calcium deposition in bones. Parathyroid hormone (PTH) raises calcium. Adrenal cortex secretes corticosteroids, pancreas secretes insulin/glucagon. Calcitonin opposes PTH.