IB Biology B3.1 Gas Exchange - Comprehensive Exam
2026
150 Questions with Rationales
SECTION A: PRINCIPLES OF GAS EXCHANGE & DIFFUSION
(Questions 1-30)
1. What is the fundamental process by which gases are exchanged between living
organisms and their environment?
A. Osmosis
B. Active transport
C. Diffusion
D. Facilitated diffusion
Rationale: C. Gas exchange takes place by the process of diffusion, where gases move
down their concentration gradient from an area of high concentration to low
concentration. This is the driving mechanism for oxygen uptake and carbon dioxide
release in all organisms. The rate of diffusion is determined by the size of the respiratory
surface, concentration gradient, and diffusion distance .
2. Which of the following correctly distinguishes gas exchange from respiration?
A. Gas exchange is the release of energy in cells, while respiration is the diffusion of
gases
B. Gas exchange refers to the diffusion of oxygen and carbon dioxide across a
respiratory surface, while respiration is a chemical process occurring in all living cells
C. Gas exchange only occurs in animals, while respiration occurs in all organisms
D. Respiration involves the movement of gases, while gas exchange involves ATP
production
Rationale: B. Gas exchange refers to the diffusion of oxygen and carbon dioxide across a
respiratory surface, while respiration is the chemical process occurring in all living cells
,that releases energy in the form of ATP. Students often confuse these two terms, making it
a common exam trap .
3. Single-celled organisms such as Amoeba can rely on diffusion for gas exchange
because they have:
A. A low metabolic rate
B. Specialised respiratory organs
C. A large surface area to volume ratio and a short diffusion distance
D. A thick cell wall that traps gases
Rationale: C. Small, unicellular organisms have a large surface area compared to the
volume of cytoplasm and a short diffusion distance. This means that the rate of diffusion is
sufficient to supply the organism with enough oxygen to function without the need for
specialised gas exchange surfaces .
4. As an organism increases in size, the challenges of gas exchange become greater
because of:
A. A larger surface area to volume ratio
B. A smaller surface area to volume ratio and a greater diffusion distance
C. An increased metabolic rate
D. A thicker cell membrane
Rationale: B. As an organism increases in size, its surface area to volume ratio decreases,
meaning there is proportionally less surface area available for gas exchange. Additionally,
the diffusion distance from the external environment to internal cells increases, making
diffusion alone insufficient .
5. Which of the following is NOT a required property of an effective gas exchange
surface?
A. Permeability to gases
B. A thick tissue layer to prevent gas leakage
C. Moisture to allow gases to dissolve
D. Large surface area
Rationale: B. Effective gas exchange surfaces require a thin tissue layer to create a short
diffusion distance for oxygen and carbon dioxide. A thick tissue layer would impede
diffusion. Other required properties include permeability, large surface area, and
moisture .
6. Why must gas exchange surfaces be moist?
A. To prevent the exchange surface from drying out
B. Gases must dissolve in water before they can diffuse across cell membranes
,C. To increase the surface area
D. To maintain the concentration gradient
Rationale: B. Gas exchange surfaces must be moist because gases must dissolve in water
before they can diffuse across cell membranes. This is why the respiratory surfaces in
animals are kept moist and why leaves have internal air spaces with moist cell surfaces .
7. Which of the following factors increases the rate of diffusion across a gas
exchange surface?
A. A longer diffusion distance
B. A smaller surface area
C. A steeper concentration gradient
D. A drier exchange surface
Rationale: C. A steeper concentration gradient increases the rate of diffusion. A shorter
diffusion distance and larger surface area also increase diffusion rate. Diffusion distance
and surface area are determined by the structure of the gas exchange surface .
8. In which type of organism would diffusion alone be sufficient for gas exchange?
A. A large mammal
B. A fish
C. A single-celled organism
D. An insect
Rationale: C. Single-celled organisms such as Amoeba have a large surface area to volume
ratio and short diffusion distance, making diffusion alone sufficient for gas exchange.
Large, multicellular organisms require specialised gas exchange organs .
9. What is the main challenge faced by large, multicellular organisms in obtaining
oxygen?
A. They have too few cells
B. They have a smaller surface area to volume ratio and greater diffusion distance
C. They have a higher surface area to volume ratio
D. They lack cell membranes
Rationale: B. Large, multicellular organisms have a smaller surface area to volume ratio,
meaning the external surface area is insufficient for gas exchange with all cells. The
diffusion distance from outside to inner cells is also greater, requiring specialised
respiratory surfaces .
10. Why is the external surface of large organisms often unsuitable for gas
exchange?
, A. It is too moist
B. It is designed to provide protection and is therefore not suitable as a respiratory
surface
C. It has too large a surface area
D. It lacks blood vessels
Rationale: B. The external surface of large organisms is designed to provide protection to
the underlying tissues and is therefore not suitable as a respiratory surface. This is why gas
exchange surfaces are often internal (e.g., lungs) or have specialised structures (e.g., gills) .
11. How is a concentration gradient for oxygen maintained at a gas exchange
surface?
A. By reducing the surface area
B. By a dense network of blood vessels with continuous blood flow
C. By increasing the diffusion distance
D. By drying out the exchange surface
Rationale: B. A dense network of blood vessels provides a large surface area for diffusion
and ensures that oxygen is constantly transported away from the gas exchange surface.
This maintains a steep concentration gradient for oxygen to continue diffusing into the
blood .
12. What role does ventilation play in maintaining concentration gradients in the
lungs?
A. Ventilation decreases the concentration gradient
B. Ventilation brings oxygen close to the gas exchange surface and removes carbon
dioxide
C. Ventilation increases the diffusion distance
D. Ventilation reduces the surface area for gas exchange
Rationale: B. Ventilation constantly refreshes the air in the lungs, bringing oxygen close to
the gas exchange surface and removing carbon dioxide. This maintains steep
concentration gradients that drive the diffusion of gases across the respiratory surface .
13. Which of the following is a property of gas exchange surfaces that maximises
diffusion rate?
A. A thick tissue layer
B. A dry surface
C. Large surface area and thin tissue layer
D. A small surface area
2026
150 Questions with Rationales
SECTION A: PRINCIPLES OF GAS EXCHANGE & DIFFUSION
(Questions 1-30)
1. What is the fundamental process by which gases are exchanged between living
organisms and their environment?
A. Osmosis
B. Active transport
C. Diffusion
D. Facilitated diffusion
Rationale: C. Gas exchange takes place by the process of diffusion, where gases move
down their concentration gradient from an area of high concentration to low
concentration. This is the driving mechanism for oxygen uptake and carbon dioxide
release in all organisms. The rate of diffusion is determined by the size of the respiratory
surface, concentration gradient, and diffusion distance .
2. Which of the following correctly distinguishes gas exchange from respiration?
A. Gas exchange is the release of energy in cells, while respiration is the diffusion of
gases
B. Gas exchange refers to the diffusion of oxygen and carbon dioxide across a
respiratory surface, while respiration is a chemical process occurring in all living cells
C. Gas exchange only occurs in animals, while respiration occurs in all organisms
D. Respiration involves the movement of gases, while gas exchange involves ATP
production
Rationale: B. Gas exchange refers to the diffusion of oxygen and carbon dioxide across a
respiratory surface, while respiration is the chemical process occurring in all living cells
,that releases energy in the form of ATP. Students often confuse these two terms, making it
a common exam trap .
3. Single-celled organisms such as Amoeba can rely on diffusion for gas exchange
because they have:
A. A low metabolic rate
B. Specialised respiratory organs
C. A large surface area to volume ratio and a short diffusion distance
D. A thick cell wall that traps gases
Rationale: C. Small, unicellular organisms have a large surface area compared to the
volume of cytoplasm and a short diffusion distance. This means that the rate of diffusion is
sufficient to supply the organism with enough oxygen to function without the need for
specialised gas exchange surfaces .
4. As an organism increases in size, the challenges of gas exchange become greater
because of:
A. A larger surface area to volume ratio
B. A smaller surface area to volume ratio and a greater diffusion distance
C. An increased metabolic rate
D. A thicker cell membrane
Rationale: B. As an organism increases in size, its surface area to volume ratio decreases,
meaning there is proportionally less surface area available for gas exchange. Additionally,
the diffusion distance from the external environment to internal cells increases, making
diffusion alone insufficient .
5. Which of the following is NOT a required property of an effective gas exchange
surface?
A. Permeability to gases
B. A thick tissue layer to prevent gas leakage
C. Moisture to allow gases to dissolve
D. Large surface area
Rationale: B. Effective gas exchange surfaces require a thin tissue layer to create a short
diffusion distance for oxygen and carbon dioxide. A thick tissue layer would impede
diffusion. Other required properties include permeability, large surface area, and
moisture .
6. Why must gas exchange surfaces be moist?
A. To prevent the exchange surface from drying out
B. Gases must dissolve in water before they can diffuse across cell membranes
,C. To increase the surface area
D. To maintain the concentration gradient
Rationale: B. Gas exchange surfaces must be moist because gases must dissolve in water
before they can diffuse across cell membranes. This is why the respiratory surfaces in
animals are kept moist and why leaves have internal air spaces with moist cell surfaces .
7. Which of the following factors increases the rate of diffusion across a gas
exchange surface?
A. A longer diffusion distance
B. A smaller surface area
C. A steeper concentration gradient
D. A drier exchange surface
Rationale: C. A steeper concentration gradient increases the rate of diffusion. A shorter
diffusion distance and larger surface area also increase diffusion rate. Diffusion distance
and surface area are determined by the structure of the gas exchange surface .
8. In which type of organism would diffusion alone be sufficient for gas exchange?
A. A large mammal
B. A fish
C. A single-celled organism
D. An insect
Rationale: C. Single-celled organisms such as Amoeba have a large surface area to volume
ratio and short diffusion distance, making diffusion alone sufficient for gas exchange.
Large, multicellular organisms require specialised gas exchange organs .
9. What is the main challenge faced by large, multicellular organisms in obtaining
oxygen?
A. They have too few cells
B. They have a smaller surface area to volume ratio and greater diffusion distance
C. They have a higher surface area to volume ratio
D. They lack cell membranes
Rationale: B. Large, multicellular organisms have a smaller surface area to volume ratio,
meaning the external surface area is insufficient for gas exchange with all cells. The
diffusion distance from outside to inner cells is also greater, requiring specialised
respiratory surfaces .
10. Why is the external surface of large organisms often unsuitable for gas
exchange?
, A. It is too moist
B. It is designed to provide protection and is therefore not suitable as a respiratory
surface
C. It has too large a surface area
D. It lacks blood vessels
Rationale: B. The external surface of large organisms is designed to provide protection to
the underlying tissues and is therefore not suitable as a respiratory surface. This is why gas
exchange surfaces are often internal (e.g., lungs) or have specialised structures (e.g., gills) .
11. How is a concentration gradient for oxygen maintained at a gas exchange
surface?
A. By reducing the surface area
B. By a dense network of blood vessels with continuous blood flow
C. By increasing the diffusion distance
D. By drying out the exchange surface
Rationale: B. A dense network of blood vessels provides a large surface area for diffusion
and ensures that oxygen is constantly transported away from the gas exchange surface.
This maintains a steep concentration gradient for oxygen to continue diffusing into the
blood .
12. What role does ventilation play in maintaining concentration gradients in the
lungs?
A. Ventilation decreases the concentration gradient
B. Ventilation brings oxygen close to the gas exchange surface and removes carbon
dioxide
C. Ventilation increases the diffusion distance
D. Ventilation reduces the surface area for gas exchange
Rationale: B. Ventilation constantly refreshes the air in the lungs, bringing oxygen close to
the gas exchange surface and removing carbon dioxide. This maintains steep
concentration gradients that drive the diffusion of gases across the respiratory surface .
13. Which of the following is a property of gas exchange surfaces that maximises
diffusion rate?
A. A thick tissue layer
B. A dry surface
C. Large surface area and thin tissue layer
D. A small surface area
