Gas Exchange: Plants and Humans

Gas Exchange in Plants

  • Definition: The process of taking in oxygen and releasing carbon dioxide from the respiratory surface.
  • Location: Gas exchange in plants primarily occurs in the leaf.

Key Points on Gas Exchange in Plants

  1. Gases Exchanged: CO2, oxygen, and water vapor.

  2. Relationship Between Photosynthesis and Respiration:

    • Photosynthesis uses carbon dioxide and water as raw materials:

      [ 6CO2 + 6H2O \rightarrow C6H{12}O6 + 6O2 ]

    • Respiration uses oxygen and glucose as raw materials:

      [ C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O ]

  3. Photosynthesis and Respiration Timing:

    • Respiration occurs both day and night, while photosynthesis occurs only in daylight.
    • Net exchange of gases depends on light intensity.

Structure of the Leaf

Mechanism of Gas Exchange
  • Stomata: Tiny openings on the leaf surface.
    • Regulated by guard cells that open to allow gas exchange and close to prevent water loss.
  • Spongy Mesophyll Cells: Main gas exchange surface.
    • Absorb CO2 and release oxygen and water vapor.
  • Air Spaces: Facilitate the diffusion of gases within the leaf.
Adaptations of Spongy Cells
  • Loosely packed with intercellular spaces, allowing gas collection and mixing.
  • Fewer chloroplasts than palisade cells hence less photosynthesis conducted.

Importance of Large Surface Area in Gas Exchange

  • Gas exchange surfaces, such as alveoli in lungs and gills of fish, have a large surface area relative to their volume, increasing diffusion efficiency.

Stomata's Role in Gaseous Exchange

  • Allow free diffusion of CO2 into leaf and O2 out of leaf.

Additional Adaptations of Leaves for Gas Exchange

  • High Stomata Density: Increases gas exchange capacity.
  • Thin Structure: Reduces diffusion distance for gases.
  • Flat Shape: Maintains a high concentration gradient for efficient diffusion.
  • Moist Internal Surfaces: Essential for gas absorption and release.

Gas Exchange Experiment

Experiment 1: Effect of Light on Gas Exchange

  • Materials: Hydrogen carbonate indicator, four test tubes, three leaves, black paper, tissue paper, corks.
  • Procedure: Place a leaf in a stoppered boiling tube with hydrogen carbonate indicator.
Indicator Color and CO2 Concentration
  • Color Change Table:
    • Highest CO2: Yellow
    • Low CO2: Purple
    • Atmospheric level: Red/Orange
Control and Results
  • Established control with non-living leaf.
  • Higher light increases rate of photosynthesis; light blockage reduces photosynthesis.

Gas Exchange in Humans

  • Exchange of gases occurs in the lungs:
    • Oxygen diffuses from alveoli into blood.
    • Carbon dioxide diffuses from blood into alveoli for exhalation.
  • Gas Exchange System: Comprises lungs and associated structures.

Features and Adaptations of Human Gas Exchange System

  1. Nostrils: Filter dust; warm and moisten inhaled air.
  2. Pharynx: Directs air towards the larynx; blocks nasal cavity during swallowing.
  3. Larynx: Voice production through vibration.
  4. Trachea: Contains cartilage rings for support.
  5. Bronchi: Divides into the lungs, delivering air.
  6. Bronchioles and Alveoli: Site of gas exchange; surrounded by capillaries for diffusion.
  7. Pleural Membranes: Allow lungs to expand and contract smoothly.
  8. Diaphragm: Major muscle aiding in ventilation.
  9. Ribs and Intercostal Muscles: Support the thoracic structure; facilitate breathing.
  10. Ciliated Epithelial Cells: Keep airways clean by sweeping mucus and trapped particles.

Mechanism of Ventilation

Inhalation (Inspiration)
  • Diaphragm contracts, increasing thoracic volume, lowering pressure, drawing air in.
Exhalation (Expiration)
  • Diaphragm relaxes, thoracic volume decreases, and air is expelled.

Effective Gas Exchange in Alveoli

  • Oxygen moves from alveoli (high concentration) to blood (low concentration); CO2 moves in the opposite direction.

Adaptations of Alveoli

  • Thin Walls: Shortened diffusion distance.
  • Large Surface Area: Enhances gas exchange efficiency.
  • Moist Lining: Facilitates gas dissolution and diffusion.
  • Rich Blood Supply: Maintains concentration gradients.

Smoking Effects on Respiratory System

Short-Term Effects

  • Impaired cilia function;
  • Increased heart rate and blood pressure due to nicotine.
  • Formation of carboxyhaemoglobin, reducing oxygen transport.

Long-Term Effects

  • Chronic Bronchitis: Excess mucus and infection.
  • Emphysema: Alveoli damage reducing surface area for gas exchange.
  • Lung Cancer: Tar leads to abnormal cell growth.

Disease from Nicotine

  • Coronary Heart Disease: Leads to atheroma and risk of clots.

Effects of Exercise on Breathing

  • Increases rate and depth of breathing.
  • Heart rate increases to enhance oxygen delivery.
  • Measured by recording breaths per minute, using spirometers for volume.

Exercise Experiment

  • Materials: Treadmill, stopwatch.
  • Procedure: Measure resting and post-exercise breathing rates.

Carbon Dioxide Content Experiment

  • Materials: Test tubes, T-tube, indicator.
  • Procedure: Observe CO2 changes using limewater or hydrogen carbonate indicator.