All Free Biology MCQs with Answers

2001. When abdomen expands, the number of spiracles open is_____________?

• A. 2
• B. 3
• C. 6
• D. 8

Explanation: When the cockroach abdomen expands during inspiration, the eight abdominal pairs of spiracles open to admit air. The two thoracic pairs are generally associated with expiration in the usual textbook description. Students often choose two by counting only the thoracic spiracles and overlooking the eight abdominal pairs.

2002. Number of spiracles present in cockroach is___________?

• A. 15
• B. 10
• C. 20
• D. 30

Explanation: Cockroaches possess 10 pairs of spiracles, two pairs on the thorax and eight pairs on the abdomen. Therefore, the total number is 10 x 2 = 20 spiracles. Students may choose 10 by counting pairs rather than individual spiracles.

2003. Arterial blood contains _________ of CO2 per 100 ml of blood

• A. 54 ml
• B. 60 ml
• C. 64 ml
• D. 50 ml

Explanation: Arterial blood carries about 48 ml of carbon dioxide per 100 ml of blood, commonly rounded to 50 ml per 100 ml in textbook questions. This is less than venous blood because body tissues add carbon dioxide to the blood. Students may choose 60 or 64 ml by confusing arterial CO2 content with the higher CO2 content of venous blood.

2004. Percentage of CO2 carried as carboxyhaemoglobin is_________?

• A. 5%
• B. 70%
• C. 20%
• D. 10%

Explanation: About 20% to 23% of transported carbon dioxide forms carbaminohaemoglobin by binding to haemoglobin, so 20% is the suitable option. Most carbon dioxide, about 70%, is transported as bicarbonate ions, while about 7% remains dissolved in plasma. Students often choose 70% because it is the largest percentage, but that represents bicarbonate transport, not carbaminohaemoglobin.

2005. The increase in pH of blood has ________ effect on the oxygen carrying capacity of haemogolobin?

• A. plus ve
• B. - ve
• C. No
• D. All of these

Explanation: An increase in blood pH increases haemoglobin's affinity for oxygen, so its oxygen-carrying capacity rises. This is part of the Bohr effect, in which lower pH promotes oxygen unloading in active tissues and higher pH favours oxygen binding. Students may choose negative because they remember that increased carbon dioxide lowers pH, but the question asks about the direct effect of increased pH.

2006. Important factors which affect the capacity of haemoglobin to combine with oxygen is

• A. Temperature
• B. Carbon dioxide
• C. pH
• D. All of these

Explanation: Temperature, carbon dioxide concentration and pH all affect haemoglobin's affinity for oxygen. Increased temperature, carbon dioxide and acidity generally promote oxygen unloading from haemoglobin, so students who select only pH overlook the effects of the other two factors.

2007. Haemoglobin can be almost completely oxygenated by an oxygen pressure of __________ of mercury.

• A. 10 mm
• B. 60 mm
• C. 10 cm
• D. 100 mm

Explanation: At an oxygen partial pressure of about 100 mm Hg, haemoglobin is almost completely saturated because the oxygen-haemoglobin dissociation curve is near its plateau. Students may choose 60 mm Hg because it is a substantial oxygen pressure, but saturation is not yet nearly maximal at that value.

2008. The maximum amount of oxygen which normal human blood absorbs and carries at sea level is ________ per 100 ml of blood.

• A. 10 ml
• B. 20 ml
• C. 30 ml
• D. 40 ml

Explanation: Each 100 ml of normal arterial blood carries about 20 ml of oxygen, mainly bound to haemoglobin and a small amount dissolved in plasma. Students may choose 10 ml because they count only dissolved oxygen and ignore the much larger haemoglobin-bound component.

2009. During rest the breathing occurs rhythmically at the frequency of _________ times per minute in human?

• A. 16-30
• B. 15-20
• C. 5?10
• D. 6?12

Explanation: A resting human normally breathes about 15 to 20 times per minute, with the rhythm controlled by respiratory centres in the brain in response to blood carbon dioxide levels. Students may choose 6 to 12 breaths per minute because that range can occur during unusually slow breathing, but it is below the usual resting range given here.

2010. Lungs are covered with double layered thin membranous sac called___________?

• A. Disphragm
• B. Pleura
• C. Pericardium
• D. Rib cage

Explanation: The lungs are enclosed by the pleura, a double-layered membrane with a thin fluid-filled space that reduces friction during breathing. Students may choose the diaphragm because it moves the lungs during ventilation, but it is a muscle below the lungs rather than their membranous covering.