UWCSEA - IB Biology 2025 Topic B3.1 Gas exchange

B3.1.1—Gas exchange as a vital function in all organisms

Gas exchange is essential for cellular respiration, providing oxygen to cells and removing carbon dioxide.

As organisms increase in size, their surface area-to-volume ratio decreases.

A smaller surface area-to-volume ratio means less surface area available for gas exchange relative to the organism's volume.

The distance from the centre of an organism to its exterior increases with size, making efficient gas exchange more challenging.

B3.1.2—Properties of gas-exchange surfaces

Permeability: Gas-exchange surfaces are permeable to gases, allowing oxygen and carbon dioxide to pass through easily.

Thin Tissue Layer: They have a thin tissue layer, often only one cell thick, minimizing the distance for diffusion.

Moisture: Surfaces are moist to enable gases to dissolve before diffusing across membranes.

Large Surface Area: A large surface area increases the amount of gas that can be exchanged at any given time.

B3.1.3—Maintenance of concentration gradients at exchange surfaces in animals

Dense Networks of Blood Vessels: Provide continuous blood flow, maintaining a steep concentration gradient for gases.

Continuous Blood Flow: Removes oxygenated blood and brings deoxygenated blood to the exchange surface.

Ventilation with Air: In lungs, ventilation ensures fresh air with high oxygen concentration reaches the alveoli.

Ventilation with Water: In gills, water flow maintains concentration gradients necessary for gas exchange in aquatic animals.

B3.1.4—Adaptations of mammalian lungs for gas exchange

Alveolar Lungs: Composed of numerous alveoli that provide a large surface area for gas exchange.

Presence of Surfactant: Surfactant reduces surface tension in the alveoli, preventing collapse and aiding expansion.

Branched Network of Bronchioles: Ensures air is efficiently distributed to all alveoli.

Extensive Capillary Beds: Surround alveoli, facilitating close contact between air and blood for efficient exchange.

High Surface Area: The multitude of alveoli significantly increases the overall surface area for gas exchange.

B3.1.5—Ventilation of the lungs

Diaphragm: A muscle that contracts and moves downward during inhalation, increasing thoracic volume.

Intercostal Muscles:External intercostal muscles contract to lift the ribs up and out during inhalation.Internal intercostal muscles assist in forced exhalation by pulling the ribs down and in.

Abdominal Muscles: Contract during forced exhalation to push the diaphragm upward.

Ribs: Movement of the ribs changes thoracic cavity volume, aiding in the ventilation process.

B3.1.6—Measurement of lung volumes

Spirometry: Technique used to measure these lung volumes, providing insights into respiratory health.

Tidal Volume: The volume of air inhaled or exhaled in a normal breath.

Vital Capacity: The maximum volume of air that can be exhaled after a maximum inhalation.

Inspiratory Reserve Volume: Additional air that can be inhaled after a normal inhalation.

Expiratory Reserve Volume: Additional air that can be exhaled after a normal exhalation.

B3.1.7—Adaptations for gas exchange in leaves

Waxy Cuticle: A waterproof layer that reduces water loss while allowing light to penetrate.

Epidermis: The outer layer of cells that protects internal tissues and allows light through.

Air Spaces: Found in the spongy mesophyll, facilitate the diffusion of gases throughout the leaf.

Spongy Mesophyll: Contains loosely packed cells with air spaces for efficient gas exchange.

Stomatal Guard Cells: Regulate the opening and closing of stomata, controlling gas exchange and transpiration.

Veins: Composed of xylem and phloem, transport water, minerals, and sugars.

B3.1.8—Distribution of tissues in a leaf

Upper Epidermis: Protective layer on the top surface.

Palisade Mesophyll: Layer rich in chloroplasts for photosynthesis.

Spongy Mesophyll: Loosely arranged cells with air spaces.

Lower Epidermis: Contains stomata for gas exchange.

Veins: Vascular bundles containing xylem and phloem.

B3.1.9—Transpiration as a consequence of gas exchange in a leaf

Transpiration: The loss of water vapour from plant leaves through stomata.

Gas Exchange: Stomata must open for CO₂ uptake, resulting in water loss.

Factors Affecting Transpiration Rate:

Temperature: Higher temperatures increase evaporation rates.

Humidity: Lower humidity levels accelerate transpiration.

Wind: Wind removes moisture-rich air, enhancing transpiration.

Light Intensity: More light increases stomatal opening, raising transpiration.

B3.1.10—Stomatal density

Stomatal Density: The number of stomata per unit area on a leaf surface.

Micrographs: Use of microscopic images to count stomata and calculate density.

Leaf Casts: Creating impressions of the leaf surface to observe stomata.

Replicate Measurements: Conducting multiple counts to improve reliability of data.

Variability of Biological Material: Recognizing natural differences and the need for replicate trials to ensure accurate results.

B3.1.11—Adaptations of foetal and adult haemoglobin for the transport of oxygen

Haemoglobin Structure: Contains haem groups that bind oxygen molecules.

Cooperative Binding: Binding of one oxygen molecule increases haemoglobin's affinity for the next.

Foetal Haemoglobin: Has a higher affinity for oxygen than adult haemoglobin, facilitating oxygen transfer from mother to fetus.

Allosteric Binding of CO₂: Carbon dioxide binds to haemoglobin at a different allosteric site;

reducing affinity for oxygen and causing its release at higher partial pressures;

B3.1.12—Bohr shift

Bohr Effect: Increased carbon dioxide levels cause haemoglobin to release oxygen more readily.

by allosteric binding;

Lower pH: High CO₂ lowers blood pH, reducing haemoglobin's oxygen affinity.

Increased Dissociation of Oxygen: Beneficial for actively respiring tissues needing more oxygen.

Respiring Tissues: Produce CO₂, enhancing oxygen delivery where it's most needed.

IB. question -

a. Carbon dioxide concentration is higher in respiring tissues.

b. Carbon dioxide lowers the pH of the blood.

c. Carbon dioxide combines with water to form carbonic acid: CO₂ + H₂O → H₂CO₃.

d. Carbonic acid dissociates into hydrogen ions and hydrogencarbonate ions: H₂CO₃ → H⁺ + HCO₃⁻.

e. Hydrogen ions bind to haemoglobin, and the low pH changes the shape of haemoglobin, which causes oxygen to be released.

f. Carbon dioxide binds to haemoglobin, which causes oxygen to be released.

g. Haemoglobin's affinity for oxygen is reduced.

h. Haemoglobin gives up its oxygen more readily in the respiring tissues, where it is needed.

i. The dissociation curve shifts to the right when carbon dioxide levels increase.

j. At any given partial pressure of oxygen, the percentage saturation of haemoglobin is lower at higher levels of carbon dioxide.

k. The axes on a sketch of the graph illustrating the Bohr shift are correctly labelled.

l. The lines on a sketch of the graph illustrating the Bohr shift are correctly drawn and labelled.

B3.1.13—Oxygen dissociation curves as a means of representing the affinity of haemoglobin for oxygen at

different oxygen concentrations

Oxygen Dissociation Curve: Shows the percentage saturation of haemoglobin at different oxygen concentrations.

S-Shaped Curve: Results from cooperative binding;

initial oxygen binding is difficult, but subsequent molecules bind more easily.

Affinity Changes: Haemoglobin's affinity for oxygen increases with each molecule bound.

Cooperative Binding: Explains the curve's shape and haemoglobin's efficiency in oxygen uptake and release.