All Free Biology MCQs with Answers

661. A dangerous fall in core body temperature below about 35 degrees Celsius is called

• A. hyperthermia
• B. fever
• C. hypothermia
• D. heat stroke

Explanation: Hypothermia slows enzymes and eventually the heart, and it is treated by gradual rewarming. Fever and heat stroke lie at the hot end of the scale, the opposite problem.

662. Excretion is best defined as the removal of

• A. undigested food from the intestine
• B. excess water only
• C. dead cells from the skin surface
• D. metabolic waste products formed in the cells

Explanation: True excretion eliminates substances made by metabolism, such as urea and CO2. Undigested food never entered the cells, so its removal is egestion, a distinction examiners test regularly.

663. Carbon dioxide, the waste product of respiration, is excreted mainly by the

• A. lungs
• B. kidneys
• C. liver
• D. skin

Explanation: CO2 diffuses into the alveoli and is breathed out, so the lungs are the principal organ of its removal. The kidneys handle nitrogenous wastes instead, dividing excretory labour between the two systems.

664. In humans, ammonia is converted into urea in the

• A. kidney
• B. liver
• C. lungs
• D. intestine

Explanation: The liver runs the ornithine cycle, turning toxic ammonia from amino acid breakdown into urea, which the kidneys then filter out. Without the liver this conversion, ammonia would accumulate fatally.

665. Ammonia is not allowed to accumulate in the body but is rapidly converted to urea because ammonia is

• A. too large a molecule to transport
• B. highly toxic even in low concentrations
• C. needed for protein synthesis
• D. insoluble in water

Explanation: Ammonia raises pH and poisons cells, particularly neurons, so it must be detoxified at once. Urea is far less toxic and soluble enough to travel safely to the kidney.

666. In addition to water, sweat contains mainly

• A. glucose and protein
• B. urea and hormones
• C. amino acids and fats
• D. mineral salts and a little urea

Explanation: Sweat is mostly water with dissolved salts and traces of urea, so sweating causes both water and salt loss that must be replaced. It is a cooling mechanism first and an excretory route second.

667. One way in which plants dispose of waste materials is by

• A. excreting urea through the stomata
• B. sweating from the leaves
• C. releasing ammonia through the roots
• D. storing wastes in leaves that later fall

Explanation: Plants lack excretory organs, so they deposit wastes as resins, crystals or in old leaves that are then shed. Falling autumn leaves remove some accumulated waste along with the organ.

668. Birds excrete uric acid as a semi solid paste because uric acid

• A. is almost insoluble in water, so very little water is lost
• B. is highly soluble in water
• C. is converted to urea in the cloaca
• D. is released as a gas

Explanation: Precipitating uric acid saves water and keeps the egg clean, since an embryo sealed in a shell cannot dilute soluble wastes. Both advantages explain why birds and reptiles chose this route.

669. Animals that excrete their nitrogenous waste mainly as urea are called

• A. uricotelic
• B. ammonotelic
• C. ureotelic
• D. phototelic

Explanation: Mammals and adult amphibians are ureotelic, balancing toxicity against water cost. Aquatic animals are ammonotelic because water is abundant, and birds are uricotelic for the reasons above.

670. A patient whose kidneys have failed completely can be kept alive by

• A. regular blood transfusions only
• B. a heart lung machine
• C. dialysis or a kidney transplant
• D. a high protein diet

Explanation: Dialysis artificially filters wastes from the blood while a transplant can restore real function. A high protein diet would worsen the accumulation of urea, so it is contraindicated.