Gas Exchange and Transport

IGCSE Edexcel Biology
2.46–2.66 Gas Exchange, Transport and Circulation
Key Concepts: Gas exchange occurs by diffusion in alveoli. Ventilation depends on the thorax, intercostal muscles and diaphragm. Transport systems move substances around large organisms, including xylem and phloem in plants and the circulatory system in humans.

Section A: Ventilation and Thorax

1. Name the main structures of the thorax involved in ventilation. [6]
2. Explain how the intercostal muscles and diaphragm bring about inhalation and exhalation. [6]

Section B: Gas Exchange

3. State three adaptations of the alveoli for gas exchange. [3]
4. Describe how oxygen and carbon dioxide move between alveoli and blood. [4]

Section C: Smoking and Health

5. Describe three biological consequences of smoking on the lungs and circulatory system. [6]

Section D: Transport in Plants

6. Explain why unicellular organisms can rely on diffusion. [2]
7. Explain why multicellular organisms need transport systems. [2]
8. Describe the roles of xylem and phloem. [4]

Section E: Blood and Circulation

9. List the main components of blood and give one function of each. [8]
10. State what substances are transported in plasma. [4]
11. Describe three adaptations of red blood cells. [3]
12. Explain how phagocytes and lymphocytes defend the body. [4]
13. Describe the structure and function of the heart. [4]
14. Explain how and why heart rate changes during exercise. [3]
Total marks: 59

Mark Scheme

1. Any six: ribs, intercostal muscles, diaphragm, trachea, bronchi, bronchioles, alveoli, pleural membranes [6]
2. Inhalation: external intercostals contract, ribs move up/out, diaphragm contracts/flattens, volume increases, pressure decreases, air enters [3]; Exhalation: muscles relax, ribs move down/in, diaphragm domes, volume decreases, pressure increases, air leaves [3]
3. Large surface area; thin walls; rich blood supply; moist surface (any three) [3]
4. Oxygen diffuses from alveoli to blood; carbon dioxide diffuses from blood to alveoli; diffusion down concentration gradients; across thin walls [4]
5. Any three with effects: tar damages cilia/mucus buildup; emphysema reduces surface area; carbon monoxide binds haemoglobin reducing oxygen transport; smoking increases risk of coronary heart disease; nicotine increases heart rate/blood pressure [6]
6. Short diffusion distances/large surface area to volume so diffusion is sufficient [2]
7. Large body size means diffusion too slow; transport systems move substances over long distances efficiently [2]
8. Xylem transports water and mineral ions from roots; phloem transports sucrose and amino acids around the plant [4]
9. Red blood cells: carry oxygen; white blood cells: defense; platelets: clotting; plasma: transports substances (1 each) [8]
10. Carbon dioxide, digested food, urea, hormones, heat energy (any four) [4]
11. Biconcave shape, no nucleus, contains haemoglobin (any three) [3]
12. Phagocytes engulf/digest pathogens; lymphocytes produce antibodies/antitoxins (any two details) [4]
13. Four chambers with valves; right side pumps to lungs, left side pumps to body; muscular walls for pumping (any four) [4]
14. Heart rate increases during exercise to deliver more oxygen/glucose and remove CO₂; returns to normal afterwards [3]