gas exchange bio

HL IB Biology Gas Exchange

Contents

  • Gas Exchange in Organisms

  • Mammalian Lungs: Adaptations

  • Mechanism of Ventilation

  • Measuring Lung Volumes: Skills

  • Gas Exchange in Plants

  • Drawing Leaf Structure: Skills

  • Determining Stomatal Density: Skills

  • Haemoglobin & Oxygen (HL)

  • The Bohr Shift (HL)

  • The Oxygen Dissociation Curve (HL)

Gas Exchange in Organisms

  • Cellular respiration: Process in living cells releasing energy as ATP by oxidizing substrate molecules (e.g., glucose).

  • Aerobic respiration requires oxygen, producing carbon dioxide as a waste product.

  • Living organisms extract oxygen from the environment and return carbon dioxide.

  • Gas exchange: Process of oxygen uptake and carbon dioxide release, taking place via diffusion, influenced by:

    • Size of the respiratory surface - larger surfaces enhance diffusion.

    • Concentration gradient - steeper gradients lead to quicker diffusion.

    • Diffusion distance - shorter distances speed up diffusion.

  • Unicellular organisms like amoeba have a favorable surface area-to-volume ratio for diffusion.

Challenges of Gas Exchange in Organisms

  • Increased size poses challenges:

    • Reduced surface area to volume ratio.

    • Greater diffusion distance.

  • Larger multicellular organisms cannot rely solely on diffusion for oxygen supply.

  • External surfaces providing protection aren’t effective for respiration.

  • Higher metabolic demand in larger, active organisms requires specialized gas exchange organs.

  • Distinction: Respiration is a chemical process; gas exchange refers to diffusion across a respiratory surface.

Gas Exchange Surfaces: Properties

To maximize diffusion of gases:

  • Permeability: Enables gas movement across the surface.

  • Thin tissue layer: Reduces diffusion distance for gases.

  • Moisture: Facilitates gas dissolving for diffusion.

  • Large surface area: Allows numerous gas molecules to diffuse simultaneously.

Maintaining a Concentration Gradient

  • A steep concentration gradient enhances diffusion rates, allowing oxygen diffusion into and carbon dioxide diffusion out of organisms.

  • Maintaining gradients involves:

    • A dense network of blood vessels enhancing surface area for gas diffusion.

    • Continuous blood flow that ensures constant transport of gases.

    • Ventilation, which refreshes gases by bringing new oxygen close to exchange surfaces and removing carbon dioxide.

The Alveolus

  • The alveolus serves as the gas exchange surface in humans, ensuring concentration gradients for oxygen and carbon dioxide are maintained.

Mammalian Lungs: Adaptations

  • Air enters through the nose/mouth, goes down the trachea that splits into bronchi, serving each lung.

  • Trachea: Reinforced by cartilaginous rings for structural integrity and flexibility.

  • Bronchi bronchioles: Consist of smooth muscle for regulating airflow and are lined with ciliated epithelium to clear mucus.

  • Alveoli clusters: Surrounded by capillaries for enhanced gas exchange efficiency.

Properties of Mammalian Lungs

  • Each lung contains numerous small alveoli that increase surface area.

  • Alveoli clusters are evenly distributed throughout the lungs, enhancing gas exchange efficiency.

  • Surfactant production prevents alveoli collapse by lowering surface tension, ensuring optimal function during breathing.

Mechanism of Ventilation

Ventilation Process

  • Involves two main actions: inspiration (inhaling) and expiration (exhaling).

Inspiration

  • Chest volume increases and pressure decreases, allowing external air to enter.

  • Diaphragm contracts and flattens, while external intercostal muscles lift the ribcage.

Expiration

  • Mostly passive; relies on lung recoil from expansion during inhalation.

  • Volume of the chest reduces, thereby increasing pressure and forcing air out.

  • Can be active during forced breathing efforts.

Measuring Lung Volumes: Skills

  • A spirometer measures lung capacities: contains a water chamber capturing breathing movements.

  • Spirometer traces record ventilation parameters through drum motions or computer graphs.

Tidal Volume and Lung Capacities

  • Tidal Volume: Volume of air exchanged during normal breathing.

  • Inspired and Expiratory Reserve Volumes: Additional volumes of air inhaled/exhaled beyond tidal movement.

  • Vital Capacity (VC): Total air exhaled after deep inhalation: VC = Tidal Volume (TV) + Inspiratory Reserve Volume (IRV) + Expiratory Reserve Volume (ERV).

  • Ventilation Rate: Number of breaths taken in a minute increases with exercise.

Gas Exchange in Plants

Leaf Structure

  • Key leaf tissues:

    • Epidermal tissue: Protects internal structures.

    • Mesophyll tissue: Site of photosynthesis.

    • Vascular tissue: Xylem and phloem for substance transport.

  • Stomata: Small pores on the leaf for gas exchange, controlled by guard cells.

Adaptations for Gas Exchange in Plants

  • Waxy cuticle: Prevents excessive water loss by sealing air spaces.

  • Stomata: Primarily located in lower epidermis to minimize water loss and maximize gas exchange.

  • Air spaces: Facilitate gas diffusion between the internal air and mesophyll cells.

Transpiration: Consequence of Gas Exchange

  • Stomata open for gas exchange results in water vapor loss in a phenomenon known as transpiration.

  • Guard cells regulate stomatal openings to balance gas exchange and water retention.

  • Factors influencing transpiration:

    • Air movement: Increases transpiration rates via concentration gradient enhancement.

    • Temperature: Higher temperatures elevate evaporation rates, may cause stomatal closure.

    • Light intensity: Stomata open in light, increasing gas exchange.

    • Humidity: High humidity reduces transpiration rates due to a lack of gradient.

Measuring Transpiration Rates

  • Utilization of a potometer: Measures water uptake as a proxy for transpiration rates.

  • Variations in environmental conditions (like wind speed, humidity, light intensity, temperature) can be controlled and observed.

Drawing Leaf Structure: Skills

  • Identify key structures: Chloroplasts, cuticle, guard cells, stomata, epidermis, palisade mesophyll, spongy mesophyll, vascular bundles.

Determining Stomatal Density: Skills

  • Measure stomatal number to infer plant responses to environmental conditions.

  • Method involves creating a leaf cast using clear nail varnish and counting visible stomata under a microscope.

Haemoglobin & Oxygen (HL)

  • Haemoglobin: Globular proteins in red blood cells binding oxygen.

  • Each molecule can hold four oxygen molecules via iron-containing haem groups.

  • Oxygen dissociation: Cooperative binding affects oxygen uptake under different partial pressures.

  • Foetal Hemoglobin has higher oxygen affinity than adult haemoglobin, allowing effective oxygen transfer from mother.

The Bohr Shift (HL)

  • The Bohr effect describes reduced haemoglobin affinity for oxygen due to increased carbon dioxide levels, assisting in oxygen delivery to respiring tissues.

The Oxygen Dissociation Curve (HL)

  • Illustrates how oxygen binds to haemoglobin at varying partial pressures, demonstrating the effects of oxygen affinity and dissociation under physiological conditions.