IB Biology Past Paper - 2025 Gas Exchange

First Exams 2025 B3.1 Gas Exchange Theme: Form and Function Level of Organisation: Organisms SL and HL Combin ed Content SL and HL Content From IB Guiding Questions the IB How are multi...

First Exams 2025 B3.1 Gas Exchange Theme: Form and Function Level of Organisation: Organisms SL and HL Combin ed Content SL and HL Content From IB Guiding Questions the IB How are multicellular organisms adapted to carry out gas exchange? What are the similarities and differences in gas exchange between a flowering plant and a mammal? SL and HL Content From SL & HL Content: B3.1: Gas the IB Exchange B3.1.1: Gas exchange as a vital function in all organisms B3.1.2: Properties of gas-exchange surfaces B3.1.3: Maintenance of concentration gradients at exchange surfaces in animals B3.1.4: Adaptations of mammalian lungs for gas exchange B3.1.5: Ventilation of the lungs SL and HL Content From SL & HL Content: B3.1: Gas the IB Exchange B3.1.6: Measurement of lung volumes B3.1.7: Adaptations for gas exchange in leaves B3.1.8: Distribution of tissues in a leaf B3.1.9: Transpiration as a consequence of gas exchange in a leaf B3.1.10: Stomatal density SL and HL Content SL & HL Key Terms Gas Exchange Alveolus / Alveoli Thorax Diffusion Lungs External Intercostal Concentration Gills Muscles Gradient Surfactant Internal Intercostal Aerobic Respiration Muscles Ventilation Photosynthesis Abdominal Muscle Inspiration Trachea Tidal Volume Expiration Bronchus Inspiratory Reserve Diaphragm Bronchioles Expiratory Reserve Vital Capacity SL and HL Content SL & HL Key Terms Spirometer Guard Cells Waxy Cuticle Plan Diagram Epidermis Transpiration Vein Humidity Xylem Stomatal Density Phloem Quantitative Data Spongy Mesophyll Standard Deviation Stoma / Stomata Standard Error SL and HL Content From B3.1.1: Gas exchange as a vital the IB function in all organisms Students should appreciate that the challenges become greater as organisms increase in size because surface area-to-volume ratio decreases with increasing size, and the distance from the centre of an organism to its exterior increases. SL and HL Content Gas Exchange Cells carrying out aerobic respiration require oxygen to enter the cell, and the waste product carbon dioxide to exit the cell. Cells carrying out photosynthesis require carbon dioxide to enter the cell, and the waste product oxygen to exit the cell. Gas exchange is the exchange of carbon dioxide and oxygen gases at cells and tissues through the process of diffusion. Large animals need specialized gas exchange systems, and transport systems to provide cells with sufficient oxygen for respiration. SL and HL Content Specialized Gas Exchange Surfaces Read the linked article on gas exchange surfaces. ❓ Explain why large animals require a specialized gas exchange system. SL and HL Content Specialized Gas Exchange Surfaces Unicellular organisms have a small surface area to volume ratio, allowing them to exchange gases directly through the plasma As membrane animals increaseof incells. size, the surface area to volume ratio of the animals decreases, meaning there is less surface area for gas exchange relative to the size of the organism. The cells of the organism cannot obtain sufficient oxygen for respiration in all of the cells. Gases are exchanged by diffusion, which is a slow process. As an animal becomes larger, it takes too much time for oxygen to diffuse to all cells. Large animals require specialized gas exchange and transport systems to ensure they obtain sufficient oxygen for all of the SL and HL Content From B3.1.2: Properties of gas-exchange the IB surfaces Include permeability, thin tissue layer, moisture and large surface area. SL and HL Content Adaptations of Gas Exchange Surfaces ❓ Outline the properties of gas exchange surfaces SL and HL Content Adaptations of Gas Exchange Surfaces Adaptations of gas exchange systems include: Large surface area, which increases the quantity of gas particles exchanged. Very thin tissue layers, which reduces the distance gases must travel. Exchange tissues are often one cell thick. Permeable membranes, which allow the gases to diffuse through them. Concentration gradient for diffusing gases, allowing gases to diffuse from a high high concentration to low concentration. Exchange surfaces are covered in a layer of SL and HL Content From B3.1.3: Maintenance of the IB concentration gradients at exchange surfaces in animals Include dense networks of blood vessels, continuous blood flow, and ventilation with air for lungs and with water for gills. SL and HL Content Maintaining Concentration Gradients Gas exchange occurs through diffusion. ❓ Define Diffusion. Diffusion is the passive transport of particles from a region of high concentration to a region of low concentration. Animals need to maintain a high concentration gradient so that gases will diffuse rapidly. SL and HL Content Maintaining Concentration Gradients Adaptations to maintain high concentration gradients include: A dense network of capillaries surrounding tissues involved in gas exchange. Continuous blood flow through the capillaries surrounding the tissues involved in gas exchange. Animals with lungs for gas exchange ventilate the lungs with air, bringing a high concentration of oxygen to the alveoli, and removing carbon dioxide from the alveoli. Animals with gills move water through the gills, providing a high concentration of oxygen, and moving carbon dioxide away from the gills SL and HL Content Gas Exchange Through Gills Gills in fish are adapted for rapid Watch the linked video from 0:28 to 3:11 exchange of gases through having a large surface area for gas exchange, a continuous supply of blood flowing through the gills, and water continually moving through SL and HL Content From B3.1.4: Adaptations of mammalian the IB lungs for gas exchange Limit to the alveolar lungs of a mammal. Adaptations should include the presence of surfactant, a branched network of bronchioles, extensive capillary beds and a high surface area. SL and HL Content Gas Exchange, Ventilation and Respiration Gas exchange is exchange of gases at cells and tissues through diffusion. Gas exchange occurs at the alveoli in the lungs and at respiring tissues. Ventilation is the movement of air in and out of the alveoli in the lungs, facilitating gas exchange. Ventilation is breathing. Ventilation maintains concentration gradients of oxygen and carbon dioxide between air in alveoli and blood flowing in adjacent capillaries. Respiration is the release of ATP energy from organic compounds (food), which occurs in cells. SL and HL Content Lungs Lungs allow for the exchange of oxygen from the air into the bloodstream, and carbon dioxide from the bloodstream to the air. SL and HL Content Label the Diagram of the Lungs A ❓ Label the diagram of B the lungs C D E The Lungs SL and HL Content Diagram of the Lungs Trache a Alveoli are located at the Bronch end of the us bronchioles. Bronchiole s Right Lung Left Lung The Lungs SL and HL Content Adaptations of the Lungs The lungs are adapted to facilitate the rapid exchange of oxygen and carbon dioxide between alveoli and the bloodstream. Adaptations include: Branching bronchioles which connect to many alveoli. All of the alveoli in the lungs provide a very large surface area for gas exchange. Alveoli Surrounded by a Bed of Capillari es SL and HL Content Adaptations of the Lungs Alveoli secrete a surfactant which prevents the walls of the alveoli adhering to each other, and provides a moist surface for gas exchange Alveoli are surrounded by an extensive capillary bed, which maintains high concentration gradients for O2 and CO2 between the blood and alveoli. The capillaries provide a continuous supply of blood with low oxygen Alveoli Surrounded by a Bed of Capillari concentration and high carbon es SL and HL Content From B3.1.5: Ventilation of the lungs the IB Students should understand the role of the diaphragm, intercostal muscles, abdominal muscles and ribs. SL and HL Content Ventilation of the Lungs Ventilation has two stages: Inspiration (breathing in) Expiration (breathing out. Read the linked article. ❓ Explain ventilation of the lungs. SL and HL Content Ventilation: Inspiration Inspiration involves: The diaphragm contracts and moves downwards. The external intercostal muscles contract, moving the ribcage up and out. The volume in the thorax increases, decreasing the pressure in the lungs. Air passively moves from the surrounding air (with high pressure) Inspiration into the lungs where there is low pressure. SL and HL Content Ventilation: Expiration Expiration involves: The abdominal muscles contract and push the diaphragm upwards. The external intercostal muscles relax and the internal intercostal muscles contract, moving the ribcage down and inwards. The volume in the thorax decreases, increasing the pressure in the lungs. The high pressure in the lungs moves Inspiration air out of the lungs to the surrounding air, where pressure is lower. SL and HL Content From B3.1.6: Measurement of lung the IB volumes Application of skills: Students should make measurements to determine tidal volume, vital capacity, and inspiratory and expiratory reserves. SL and HL Content Lung Volumes ❓ Define: Tidal volume Inspiratory reserve Expiratory reserve Vital capacity SL and HL Content Lung Volumes Lung volumes include: Tidal volume: the volume of air that moves in and out of the lungs in a normal breath. Inspiratory reserve: the additional volume of air that can be inhaled with maximum effort. Expiratory reserve: the additional volume of air that can be exhaled with maximum effort. Vital capacity: the greatest volume of air that can be expelled from the lungs after the deepest possible breath. Vital capacity = tidal volume + inspiratory reserve + SL and HL Content Measuring Lung Volumes Spirometers are instruments used to measure air capacity of the lungs. Students should use spirometers to determine tidal volume, vital capacity, and inspiratory and expiratory reserves. Students could measure vital capacity using either balloons or water displacement. Instructions for carrying out determining vital capacity using balloons and water displacement are available from the Ontario Science Centre. Biology Corner has a guided investigation on SL and HL Content From B3.1.7: Adaptations for gas the IB exchange in leaves Leaf structure adaptations should include the waxy cuticle, epidermis, air spaces, spongy mesophyll, stomatal guard cells and veins. SL and HL Content Adaptation of Leaves Leaves carry out respiration and photosynthesis, and therefore need to be adapted to exchange oxygen and carbon dioxide. Leaves also need to be adapted to reduce water SL and HL Content Leaf Structure ❓ Explain how Waxy Upper the following Cuticle Epidermis help the leaf Palisade Mesophyll to carry out Xyle its function: m Spongy Mesophyll Vein Waxy Cuticle Epidermis Phloe m Lower Spongy Epidermis Mesophyll Guard Air Air Spaces Cells Sto Space s Stomata and ma Guard Cells The linked article includes information on leaf Veins adaptations SL and HL Content Adaptations of Leaf Structure Waxy cuticle: The waxy cuticle covers the epidermis cells and reduces the evaporation of water from the leaf. Epidermis: The epidermis provides protection for the mesophyll cells within the leaf. Epidermal cells are transparent, allowing light to reach the mesophyll cells where photosynthesis is carried out. Spongy mesophyll: The irregular shape of spongy mesophyll cells increases the surface area for gas exchange. The spongy mesophyll cells are surrounded by air spaces. SL and HL Content Adaptations of Leaf Structure Air spaces: Air spaces around spongy mesophyll cells facilitate the diffusion of gases between the surrounding atmosphere and the mesophyll cells. Stomata: Stomata are pores which allow gases to enter and exit the leaf. Stomata are usually more common on the lower epidermis of the leaf. The stomata are open and closed by guard cells. Veins: Veins provide support for the leaf. They contain xylem and phloem tissue. Xylem transports water and minerals from the roots. Phloem transports nutrients up and down the plant. SL and HL Content From B3.1.8: Distribution of tissues in a the IB leaf Students should be able to draw and label a plan diagram to show the distribution of tissues in a transverse section of a dicotyledonous leaf. SL and HL Content Drawing Leaf Structures ❓ Draw and label a plan diagram to show the distribution of tissues in a transverse section Note: of aPlan diagrams do dicotyledonous notleaf. show individual cells, only the distribution of tissues Diagram of a cross section of a leaf. - This is not a plan diagram. SL and HL Content Plan Diagram of a Leaf SL and HL Content From B3.1.9: Transpiration as a the IB consequence of gas exchange in a leaf Students should be aware of the factors affecting the rate of transpiration. SL and HL Content Transpiration Transpiration is the movement of water through a plant, and its evaporation from aerial parts of the plant such as leaves. Transpiration is an inevitable consequence of gas exchange, as water in mesophyll cells evaporates, and diffuses through the open stomata. Transpiration SL and HL Content Factors Affecting Transpiration Water diffuses from the high concentration of water particles within the air spaces of the spongy mesophyll to the low concentration of water particles in ❓the Explain how four atmosphere. factors affect the rate of transpiration.  Factors Affecting Transpiration The following factors affect the rate of transpiration: 1. Light intensity: As light intensity increases, more stomata open. If there are more open stomata, then more water can diffuse out of the leaf, increasing the rate of transpiration. 2. Temperature: As the temperature increases, the water particles gain kinetic energy, and move faster. Faster moving water particles diffuse through the stomata of the leaf at a faster rate. Additionally, higher temperatures increase the rate of evaporation, which also increases the rate of transpiration.  Factors Affecting Transpiration 3. Humidity: As humidity increases, the concentration of water outside the leaf increases. This decreases the concentration gradient between the inside and outside of the leaf. Water particles will diffuse slower, resulting in a slower rate of transpiration. 4. Air flow (wind): As air flows past the leaf, it moves water vapour away from the leaf, reducing the concentration of water outside the stomata of a leaf. This increases the concentration gradient, resulting in an increase in the rate of transpiration. SL and HL Content From B3.1.10: Stomatal Density the IB Application of skills: Students should use micrographs or perform leaf casts to determine stomatal density. Nature of Science: Reliability of quantitative data is increased by repeating measurements. In this case, repeated counts of the number of stomata visible in the field of view at high power illustrate the variability of biological material and the need for replicate trials. SL and HL Content Stomatal Density Stomatal density is the number of stomata per unit area of a leaf. Students should use microscopes to determine the stomatal density of leaves. A microscope with a graticule is required, so the the area of the leaf viewed under the microscope can be calculated. SAPS provide methods for Stomata viewed under a microscope using an eyepiece graticule. determining the stomatal density of a lea f , including suggestions for investigations SL and HL Content Reliability of Quantitative Data Nature of Science: Quantitative data is any data which involves numbers. The reliability of data is determined by the range of data collected, often determined by calculating standard deviation or standard error to determine the reliability of data collected. Carrying out more replicates provides the scientist with greater confidence regarding the variation within data collected. In this case, repeated counts of the number of stomata visible in the field of view at high power illustrate the SL and HL Content Review and Discuss: SL & HL Key Terms Gas Exchange Alveolus / Alveoli Thorax Diffusion Lungs External Intercostal Concentration Gills Muscles Gradient Surfactant Internal Intercostal Aerobic Respiration Muscles Ventilation Photosynthesis Abdominal Muscle Inspiration Trachea Tidal Volume Expiration Bronchus Inspiratory Reserve Diaphragm Bronchioles Expiratory Reserve Vital Capacity SL and HL Content Review and Discuss: SL & HL Key Terms Spirometer Guard Cells Waxy Cuticle Plan Diagram Epidermis Transpiration Vein Humidity Xylem Stomatal Density Phloem Quantitative Data Spongy Mesophyll Standard Deviation Stoma / Stomata Standard Error SL and HL Content From B3.1 Gas Exchange - IB Linking the IB Questions How do multicellular organisms solve the problem of access to materials for all their cells? What is the relationship between gas exchange and metabolic processes in cells? Additional HL Conte nt HL Content Only From Additional HL Content: the IB B3.1 Cell Exchange B3.1.11: Adaptations of foetal and adult haemoglobin for the transport of oxygen B3.1.12: Bohr shift B3.1.13: Oxygen dissociation curves as a means of representing the affinity of haemoglobin for oxygen at different oxygen concentrations HL Content Only HL Only Key Terms Oxygen Dissociation Bohr Shift Curve Enzyme Oxygen Affinity Carbonic Anhydrase Partial Pressure Chloride Shift Haemoglobin Haem Group Adult Haemoglobin Foetal Haemoglobin HL Content Only From B3.1.13: Oxygen dissociation curves the IB as a means of representing the affinity of haemoglobin for oxygen at different oxygen concentrations Explain the S-shaped form of the curve in terms of cooperative binding. HL Content Only Partial Pressures of Gases Partial pressure of a gas is the pressure exerted by a single gas when it is found in a mixture of gases. Partial pressure of a gas depends on: The total pressure exerted by all of the gases in a mixture. The concentration of the gas in the mixture of gases. Partial pressure of gases is correlated with the concentration of a gas in solution. Partial pressures are used when looking at oxygen and carbon dioxide in blood. HL Content Only Oxygen Dissociation Curve An oxygen dissociation curve shows the affinity of haemoglobin forlow At oxygen. partial pressures of oxygen, haemoglobin has a very low affinity for oxygen. As the partial pressure of oxygen increases, haemoglobin’s affinity for oxygen increases. At high partial pressures, the curve flattens, as most haemoglobin molecules have four Oxygen Dissociation Curve oxygen molecules attached. HL Content Only Oxygen Dissociation Curve Watch the linked video. ❓ Explain the shape of the oxygen dissociati on curve HL Content Only Oxygen Dissociation Curve The oxygen dissociation curve has a sigmoid shape. Four molecules of oxygen can bind to the four haem groups in haemoglobin. At low partial pressures of oxygen, haemoglobin has a low affinity for oxygen, demonstrated by the slow initial increase in saturation of haemoglobin. When one oxygen binds to haemoglobin, it causes a conformational change in the shape of haemoglobin, which increases the affinity of haemoglobin for oxygen. HL Content Only Oxygen Dissociation Curve This results in a rapid increase in the oxygen saturation of haemoglobin as the partial pressure of oxygen increases, which allows two additional oxygen molecules to bind to haemoglobin. This is an example of cooperative binding, as the attachment of one oxygen enhances the affinity of haemoglobin, allowing the addition of more oxygen. At high partial pressures, the curve flattens, as most haemoglobin molecules are saturated with oxygen. HL Content Only Oxygen Dissociation Curve ❓ Use the oxygen dissociation curve to explain what happens to oxygen in capillaries surrounding the alveoli of the lungs. In the lungs, there is a high partial pressure of oxygen which diffuses into the capillaries surrounding Haemoglobinthe hasalveoli. a high affinity for oxygen, due to the high partial pressure. Haemoglobin becomes saturated Oxygen Dissociation Curve with oxygen. HL Content Only Oxygen Dissociation Curve ❓ Use the oxygen dissociation curve to explain what happens to oxygen in capillaries near respiring Respiring tissue. tissue uses oxygen, therefore there is a very low partial pressure of oxygen around respiring tissue. At very low partial pressures, haemoglobin has a low affinity for oxygen. Haemoglobin releases oxygen, which diffuses into the respiring Oxygen Dissociation Curve tissue to be used for aerobic HL Content Only From B3.1.11: Adaptations of foetal and the IB adult haemoglobin for the transport of oxygen Include cooperative binding of oxygen to haem groups and allosteric binding of carbon dioxide. HL Content Only Haemoglobin ❓ Describe the four levels of organization of a conjugated protein using haemoglobin as an example. Haem The haem groups in Group s haemoglobin can bind to an oxygen molecule. Each hemoglobin molecule has four haem groups, so can bind to four oxygen Haemoglobin molecules. HL Content Only Cooperative Binding of Oxygen to Haemoglobin Cooperative binding of oxygen refers to the phenomenon where the binding of one oxygen molecule to a haemoglobin molecule facilitates the binding of additional oxygen molecules. If there is no oxygen attached to haemoglobin, its affinity for oxygen is low, meaning that the partial pressure of oxygen must be high for oxygen to bind. However, once a single oxygen molecule binds to one haem group the shape of the haemoglobin molecule changes, increasing haemoglobin’s affinity for oxygen. HL Content Only Carbon Dioxide and Haemoglobin Carbon dioxide is transported in three ways in the blood: A small amount of CO2 is dissolved in the blood plasma. Some CO2 is bound to haemoglobin. Most CO2 is reversibly converted to hydrogen carbonate The hydrogen ions ions andtohydrogen bind ions (H haemoglobin, + ) in red causing a blood cells. conformational change of the protein, which increases its affinity for oxygen. HL Content Only Adult and Foetal Haemoglobin Adult haemoglobin has Watch the linked video from 1:24 to 3:26 two alpha and two beta chains of polypeptides, while foetal haemoglobin has two alpha and two gamma chains of ❓ Explain how polypeptides. oxygen is transferred from maternal blood to foetal HL Content Only Foetal Haemoglobin There is a high partial pressure of oxygen in maternal blood, and a low partial pressure in foetal blood. Foetal haemoglobin has a greater affinity for oxygen than adult haemoglobin. Oxygen is more likely to be transferred from adult to foetal haemoglobin. The adult haemoglobin releases oxygen due to high partial Oxygen Dissociation Curves for Adult and Foetal Hae moglobin pressure of oxygen, while foetal haemoglobin binds to oxygen due HL Content Only From B3.1.12: Bohr shift the IB Students should understand how an increase in carbon dioxide causes increased dissociation of oxygen and the benefits of this for actively respiring tissues. HL Content Only The Bohr Shift High partial pressures of carbon dioxide reduce the affinity of haemoglobin for oxygen, which shift the oxygen dissociation curve to The shift the right. of the oxygen dissociation curve due to carbon dioxide partial pressures is known as the Bohr shift (or Bohr effect). The Bohr Shift HL Content Only The Bohr Shift ❓ Explain why haemoglobin Watch the linked video from 1:16 to 4:44 will release carbon dioxide around respiring tissues, but is likely to upload oxygen in the Refer to the Bohr lungs. shift and oxygen dissociation curves in your answer. HL Content Only Bohr Shift at Respiring Tissues Tissues use oxygen and produce carbon dioxide during aerobic respiration. Therefore there is a very low partial pressure of oxygen, which reduces the affinity of haemoglobin for oxygen. The high partial pressure of carbon dioxide causes a Bohr shift, which further reduces the affinity of haemoglobin for oxygen. Haemoglobin releases oxygen at respiring tissues. HL Content Only Bohr Shift at the Lungs There is a high concentration of oxygen, and a low concentration of carbon dioxide in the alveoli of the lungs. Oxygen diffuses from the alveoli into the blood, creating a high partial pressure of oxygen, resulting in high affinity of haemoglobin for oxygen. Carbon dioxide diffuses from the blood into alveoli, creating a low partial pressure in the blood. The low partial pressure causes a Bohr shift, resulting in a stronger affinity of haemoglobin for oxygen. High oxygen and low carbon dioxide partial pressures result in oxygen binding to haemoglobin at high saturation HL Content Only Carbon Dioxide in Red Blood Cells Watch the linked video from 1:49 to 4:41 Most of the carbon dioxide in the blood diffuses into red blood cells. HL Content Only Carbon Dioxide in Red Blood Cells Carbon dioxide diffuses into red blood cells. Carbon dioxide reacts with water to form carbonic acid CO2 + H2O → H2CO3 Carbonic acid dissociates to form hydrogen carbonate ions and hydrogen ions. This reaction is catalysed by Hydrogen Carbonate reactions in the blood carbonic anhydrase. H2CO3 → HCO3- + H+ HL Content Only Carbon Dioxide in Red Blood Cells The hydrogen carbonate ion leaves the cell and chloride ions (Cl-) enter the cell. This is known as the chloride shift. The hydrogen ion binds to haemoglobin, causing a conformational change, which increases the affinity of haemoglobin for oxygen. The reactions are reversible, releasing CO 2 when the partial pressure of carbon dioxide is low in the blood plasma. HL Content Only HL Only Key Terms Oxygen Dissociation Bohr Shift Curve Enzyme Oxygen Affinity Carbonic Anhydrase Partial Pressure Chloride Shift Haemoglobin Haem Group Adult Haemoglobin Foetal Haemoglobin SL and HL Content From B3.1 Gas Exchange - IB Linking the IB Questions What are the advantages of small size and large size in biological systems? 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