All Free Biology MCQs with Answers

1881. In animals excess of nitrogen is excreted primarily in form of___________?

• A. Creatinine
• B. Trimethyamine oxide
• C. Pyrimidine
• D. Ammonia

Explanation: Ammonia is the simplest nitrogenous waste and is formed when amino acids are deaminated; many aquatic animals excrete it directly because water rapidly dilutes its toxicity. The likely distractor is creatinine, which is a waste product of muscle metabolism rather than the primary nitrogenous waste in general animals.

1882. Which of the following excretory structure is associated with gut of the organism?

• A. Protonephridium
• B. Metanephridium
• C. Malpighian tubules
• D. Nephrons

Explanation: Malpighian tubules are insect excretory organs connected to the gut, where they release nitrogenous waste and help regulate salts and water. The likely distractor is nephrons, but nephrons are the excretory units of vertebrate kidneys and are not attached to the gut.

1883. The main nitrogenous waste formed in the body of earthworm is/are___________?

• A. Ammonia
• B. Urea
• C. Uric acid
• D. Trimethylamine oxide

Explanation: Earthworms are mainly ureotelic, so their principal nitrogenous waste is urea, although some ammonia may also be excreted depending on water availability. The likely distractor is ammonia, because it is the simplest nitrogenous waste, but earthworms generally convert more nitrogen into the less toxic urea.

1884. Each ach nephridium of earthworm opens to the exterior by__________?

• A. Nephrostome
• B. Nephridiopore
• C. Flame cell
• D. Anus

Explanation: A nephridiopore is the external opening through which an earthworm's nephridium discharges excretory fluid. The likely mistake is nephrostome, which is the internal ciliated funnel that collects coelomic fluid rather than the external opening.

1885. Amount of water required to excrete 1gN of Urea is_________?

• A. 1 ml
• B. 50 ml
• C. 100 ml
• D. 150 ml

Explanation: Urea requires about 50 mL of water for the excretion of 1 g of nitrogen, so the value is 50 mL per gram of nitrogen. The likely distractor is 100 mL, which may result from doubling the standard approximate value or confusing urea with a more water-demanding nitrogenous waste.

1886. Which of the following statement about conifers is correct?

• A. Stored excretory products in vacuoles, at a concentration that leads to crystal formation.
• B. Accumulate waste in leaves, which fall in autumn.
• C. Production of black wood in the center.
• D. Excrete some of the waste materials directly into soil, occasionally using them as a chemical weapon against other competing plants.

Explanation: Conifers can release certain chemical wastes into the soil through their roots, and some of these substances may inhibit competing plants, a form of chemical defence. The likely distractor is black wood at the centre, but that describes heartwood formation and is not the characteristic excretory process stated here.

1887. Which excretory product in produced in plants during autotrophic mode of life?

• A. CO2
• B. H2O
• C. O2
• D. Both A and B

Explanation: During autotrophic nutrition, plants carry out photosynthesis and release oxygen as a by-product when water is split during the light reactions. The likely distractor is carbon dioxide, but carbon dioxide is a raw material consumed in photosynthesis, not its net waste product.

1888. Which of the following marine fishes are considered direct descendents of fresh water ancestors?

• A. Hagfish
• B. Jelly Fish
• C. Star Fish
• D. Bony fishes

Explanation: Bony fishes are generally considered to have evolved from freshwater ancestors, although many lineages later adapted to marine environments. The likely distractor is hagfish, but hagfish belong to an ancient jawless lineage rather than being the standard example of bony fishes derived from freshwater ancestors.

1889. Most of the marine invertebrates are__________?

• A. Osmoconformers
• B. Osmoregulators
• C. Both depend upon the concentration of sea water
• D. None of these

Explanation: Most marine invertebrates are osmoconformers, meaning their internal osmotic concentration changes in close relation to that of seawater. The likely distractor is osmoregulators, which actively maintain a relatively constant internal osmotic concentration despite external changes.

1890. Which of the following represents osmoregulatory steps taken by hydrophytes?

• A. Increases transpiration by increasing the surface area of the leave.
• B. Small and thick leaves to decrease the surface area proportional to the volume of the leave.
• C. Stomata closing in sufficient supply, and opening in restricted supply of water.
• D. None of these

Explanation: Hydrophytes commonly have broad leaves with a large surface area, supporting rapid transpiration and gaseous exchange in their water-rich environment. The likely distractor is small, thick leaves, which are adaptations of xerophytes that reduce water loss in dry habitats.