B3.1 (Summarized, SL Content)

Gas Exchange and Transpiration Notes

1. Gas Exchange

• All organisms absorb and release gases from the environment.

• Plants absorb CO₂ for photosynthesis and release O₂.

• Humans absorb O₂ for respiration and release CO₂.

Relationship Between Surface Area and Gas Exchange

• Small organisms have a large surface area to volume ratio, allowing for direct gas exchange.

• Larger organisms require specialized structures for gas exchange (e.g., lungs, gills, or leaf structures).

Features of Gas Exchange Surfaces

1. Permeable – Allows free diffusion of gases.

2. Large Surface Area – Maximizes gas exchange.

3. Moist – Helps gases dissolve for efficient diffusion.

4. Thin – Short diffusion distance ensures rapid gas exchange.

Importance of Concentration Gradients in Gas Exchange

• Gases diffuse down concentration gradients.

• Oxygen moves into organisms where O₂ levels are lower.

• CO₂ diffuses out as cellular respiration increases its concentration.

• Ventilation helps maintain concentration gradients.

2. The Lungs and Gas Exchange

• Mammals use lungs for gas exchange.

• Airways include bronchi, bronchioles, and alveoli.

Lung Adaptations for Efficient Gas Exchange

1. Airways – Ensure ventilation.

2. Large Surface Area – 300 million alveoli in human lungs.

3. Extensive Capillary Beds – Blood supply maximizes diffusion.

4. Short Diffusion Distance – Thin alveolar and capillary walls.

5. Moist Surface with Surfactant – Keeps alveoli open and allows gases to dissolve.

Image: Human lung tissue

3. Lung Structure and Ventilation

• Inhalation:
  
  

  • External intercostal muscles contract, expanding the ribcage.

  • Diaphragm contracts and moves down.

  • Lung volume increases, reducing pressure, drawing air in.

• Exhalation:
  
  

  • Internal intercostal muscles contract, pulling ribs inward.

  • Diaphragm relaxes and moves up.

  • Lung volume decreases, increasing pressure, pushing air out.

Image: Lung ventilation mechanism

4. Measuring Lung Volumes Using Spirometry

• Ventilation rate – Number of breaths per minute.

• Tidal volume – Volume of air inhaled/exhaled per breath.

• Vital capacity – Maximum air exhaled after deep inhalation.

• Inspiratory reserve volume – Additional air inhaled after tidal breath.

• Expiratory reserve volume – Additional air exhaled beyond tidal breath.

Image: Spirometry graph of lung volumes

5. Leaves Adapted for Gas Exchange

• Leaves have large, moist surfaces for efficient gas exchange.

• Key Adaptations:

  • Waxy cuticle – Reduces water loss.

  • Guard cells – Control stomatal openings.

  • Air spaces – Facilitate diffusion.

  • Spongy mesophyll – Provides a large moist surface.

  • Veins – Transport water for evaporation.

Image: Structure of a leaf showing gas exchange adaptations

6. Leaf Tissues

• Plan diagrams illustrate leaf tissues:

  • Upper epidermis – Protective outer layer.

  • Palisade mesophyll – Site of photosynthesis.

  • Spongy mesophyll – Facilitates gas diffusion.

  • Lower epidermis – Contains stomata.

  • Xylem and phloem – Transport water and nutrients.

Image: Lower epidermis of a leaf with stomata

7. Transpiration

• Loss of water vapor from leaves through stomata.

• Factors affecting transpiration:

  • Temperature – Increases evaporation.

  • Humidity – Decreases transpiration.

  • Wind – Prevents saturation near stomata, increasing transpiration.

Image: Transpiration process

8. Measuring Stomatal Density

• Methods:

  1. Peel-off epidermis and examine under a microscope.

  2. Apply clear nail varnish, peel off, and examine.

  3. Take a photograph and count stomata.

Formula:

Stomatal density=mean number of stomataarea of field of view (mm²)\text{Stomatal density} = \frac{\text{mean number of stomata}}{\text{area of field of view (mm²)}}

Image: Stomatal density measurement

9. The Bohr Shift

• Effect of CO₂ on Hemoglobin’s Oxygen Affinity:
  
  

  • Increased CO₂ lowers blood pH, reducing hemoglobin’s oxygen affinity.

  • CO₂ binds to hemoglobin, forming carbaminohemoglobin with lower oxygen affinity.

• Bohr Shift:
  
  

  • At high CO₂ levels, hemoglobin releases more oxygen.

  • In the lungs (low CO₂, high pH), hemoglobin rebinds oxygen efficiently.

Image: Oxygen dissociation curve showing the Bohr shift