Assertion (A): Viruses are considered obligate parasites. Reason (R): Viruses lack their own metabolic machinery and require a host cell to replicate. MCQ with Answer and Explanation

Explanation:
Antonie van Leeuwenhoek, using his hand-crafted single-lens microscopes, was the first to observe and describe living sperm cells, bacteria, protozoa, and red blood cells in the 1670s. Hooke saw dead cork cells, Pasteur and Koch later advanced microbiology with germ theory and pure culture techniques.

Explanation:
The plant cell wall is primarily made up of cellulose, hemicellulose, pectins, and proteins. Cellulose provides the main structural framework, while hemicellulose and pectins help in cross-linking and providing flexibility. Chitin is found in fungal cell walls, and peptidoglycan is found in bacterial cell walls.

Explanation:
Plant cells possess a rigid cell wall of cellulose, chloroplasts for photosynthesis, and a large central vacuole. Animal cells lack cell wall and chloroplasts, and have small temporary vacuoles if present. Centrioles are present in animal cells (centrosome) but absent in higher plant cells. Both have plasma membrane. The cell wall provides shape and protection. These differences are fundamental in cell biology.