All Free Biology MCQs with Answers

771. Which of the following is NOT a characteristic of an efficient respiratory surface?

• A. It is dry and thick
• B. It is thin
• C. It is kept moist
• D. It has a large surface area

Explanation: Gases must dissolve before diffusing, and distance must be tiny, so efficiency demands a moist, thin, extensive surface. A dry thick barrier would stop diffusion, which is exactly why land animals keep their lungs internal.

772. Between the air in an alveolus and the blood, oxygen must cross

• A. one cell layer
• B. two cell layers
• C. four cell layers
• D. eight cell layers

Explanation: Only the single alveolar epithelium and the single capillary endothelium stand in the way, a combined thickness of about one micrometre. This minimal barrier is what makes rapid diffusion possible.

773. During gaseous exchange in the lungs, carbon dioxide moves

• A. from the alveolar air into the blood
• B. from red blood cells into haemoglobin
• C. from the blood into the alveolar air
• D. from the plasma into the lymph

Explanation: Venous blood arrives with a higher CO2 pressure than alveolar air, so the gas diffuses out to be exhaled. The opposite direction would defeat the entire purpose of ventilation.

774. The sticky brown substance in tobacco smoke that carries most of the carcinogens is

• A. nicotine
• B. carbon monoxide
• C. ash
• D. tar

Explanation: Tar condenses in the airways and contains the chemicals that mutate DNA and cause cancer, while nicotine addicts and carbon monoxide starves tissues of oxygen. Each component has its own signature harm.

775. Which of the following is NOT caused by smoking?

• A. Malaria
• B. Lung cancer
• C. Emphysema
• D. Chronic bronchitis

Explanation: Malaria is transmitted by the Anopheles mosquito and has nothing to do with tobacco. Cancer, emphysema and chronic bronchitis form the classic trio of smoking diseases.

776. Babies born to mothers who smoke during pregnancy often have

• A. a higher than average birth weight
• B. a low birth weight
• C. extra ribs
• D. immunity against respiratory infection

Explanation: Carbon monoxide and nicotine reduce the oxygen and blood supply to the fetus, restricting growth and raising the risk of premature birth. This is one of the strongest arguments against smoking in pregnancy.

777. Damage to the cilia of the airways by smoking leads to

• A. improved filtering of dust
• B. faster removal of mucus
• C. accumulation of mucus and debris, producing the smoker's cough
• D. widening of the trachea

Explanation: Healthy cilia sweep mucus and trapped particles upwards, and when smoke paralyses them the debris collects instead. The cough is the body's substitute for the lost escalator.

778. Giving up smoking is particularly difficult mainly because

• A. tobacco is inexpensive
• B. cigarettes contain essential vitamins
• C. withdrawal produces no symptoms
• D. nicotine produces strong physical and psychological dependence

Explanation: Nicotine reaches the brain within seconds and alters reward pathways, so stopping brings cravings and withdrawal. Dependence, not price, is why most smokers need repeated attempts to quit.

779. The risk of lung cancer in a smoker rises with

• A. the brand of cigarette
• B. the time of day the cigarettes are smoked
• C. the length of the filter
• D. the number of cigarettes per day and the number of years of smoking

Explanation: Cancer risk follows cumulative dose: each cigarette adds exposure to carcinogens over years of tissue damage. No brand or filter makes the dose safe.

780. Smoking destroys the walls of the alveoli irreversibly, permanently reducing the respiratory surface. This disease is called

• A. emphysema
• B. asthma
• C. pneumonia
• D. bronchitis

Explanation: The merged, ragged air spaces of emphysema cannot transfer enough oxygen, leaving the patient breathless even at rest. Unlike bronchitis, the structural damage cannot be undone by drugs.