Q4. Assertion: birds excrete uric acid as a paste. Reason: uric acid is the least toxic nitrogenous waste and can be removed with a minimum loss of water. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: birds are uricotelic and excrete uric acid as a pellet or paste, with a minimum loss of water (NCERT page 205).
Q5. In which form, and across which surfaces, do bony fishes excrete ammonia?
As ammonium ions, by diffusion across body surfaces or through the gill surfaces (NCERT page 205).
Q6. Why do some ureotelic animals keep some urea in the kidney matrix?
To maintain a desired osmolarity (NCERT page 205).