Gas Exchange in Plants

• Waxy Cuticle

  • Thin, waterproof layer on the leaf surface
  • Reduces water loss but allows gas exchange through stomata
• Epidermis


  • Single layer of cells covering the leaf surface
  • Usually transparent to allow light penetration
• Air Spaces


  • Found within the mesophyll
  • Facilitate rapid diffusion of gases within the leaf
• Spongy Mesophyll


  • Loosely packed cells with large air spaces
  • Increases surface area for gas exchange
• Stomatal Guard Cells

  • Regulate the opening and closing of stomata
  • Control gas exchange and water loss
• Veins

  • Transport water and minerals to leaf cells
  • Remove products of photosynthesis
• Adaptations for Gas Exchange in Leaves


  Leaves are highly adapted structures for efficient gas exchange, particularly for photosynthesis.

  1. Waxy Cuticle
  2. Epidermis
  3. Air Spaces
  4. Spongy Mesophyll
  5. Stomatal Guard Cells
  6. Veins

  
• The distribution of tissues in a leaf is closely related to its function. For example, the palisade mesophyll is located near the upper surface to maximize light absorption for photosynthesis, while the spongy mesophyll below facilitates gas exchange.
• Transpiration as a Consequence of Gas Exchange in a Leaf

  Transpiration is the process of water loss from plants through evaporation, primarily through stomata. It's an inevitable consequence of gas exchange in leaves, as stomata must open for CO₂ uptake, simultaneously allowing water vapor to escape.
• Factors affecting transpiration rate:

  1. Light intensity
  2. Temperature
  3. Humidity
  4. Wind speed
  5. Soil water availability
• Light intensity


  • Higher light → increased stomatal opening → increased transpiration
• Temperature

  • Higher temperature → increased evaporation → increased transpiration
  • Relationship follows van 't Hoff's rule: Q10=(R2R1)10T2−T1Q10​=(R1​R2​​)T2​−T1​10​ Where RR is the rate and TT is the temperature
• Humidity


  • Lower humidity → steeper water vapor gradient → increased transpiration
• Wind speed

  • Higher wind speed → faster removal of water vapor → increased transpiration
• Soil water availability


  • Less soil water → stomata close → decreased transpiration
• Stomatal Density


  Stomatal density is the number of stomata per unit area of leaf surface. It's an important characteristic that influences gas exchange and water loss in plants.
• AOS (Application of Science): Determining Stomatal Density


  1. Using micrographs
  2. Leaf casts method:
• Using micrographs:

  • Observe a prepared slide or micrograph of a leaf epidermis
  • Count the number of stomata in the field of view
  • Measure the area of the field of view
  • Calculate density: Stomatal density=Number of stomataAreaStomatal density=AreaNumber of stomata​
• Leaf casts method:

  • Apply clear nail polish to leaf surface
  • Let it dry and peel off
  • Observe the peel under a microscope
  • Count stomata and calculate density as above
• Variability in biological material

  • Stomatal distribution isn't uniform across a leaf
  • Different leaves on the same plant may have different densities
• Need for replicate trials


  • Multiple counts reduce the impact of random errors
  • Allows calculation of mean and standard deviation
• Statistical significance

  • More replicates increase confidence in results
  • Can be expressed using standard error: SE=snSE=n​s​ Where ss is standard deviation and nn is sample size
• Stomatal density can vary significantly based on environmental conditions during leaf development. Plants grown in high light or CO₂-poor environments often have higher stomatal densities.