Gas Exchange in Flowering Plants

• Gas exchange in plants occurs through tiny pores in the leaves, mostly on the underside, called stomata
• During photosynthesis, carbon dioxide enters the leaf and oxygen exits as a net gas exchange
• In respiration, oxygen enters the leaf and carbon dioxide exit the leaf
• If respiration and photosynthesis are equal and opposite, there wouldn't be any net gas exchange
• Plants respire 24 hours/day at a low level, but during sunlight, they do more photosynthesis, leading to a net exchange of gases in favor of photosynthesis
• When it's dark, the net exchange of gases is in favor of respiration
• Leaf adaptations for gas exchange:

  • a large surface area (for diffusion)
  • thin structure for quick diffusion
  • air spaces within the spongy mesophyll layer for really good diffusion
  • loads of stomata, so lots of points for gases to come in and out
  • guard cells that can open and close the stomata depending on conditions.
• Hydrogen carbonate indicator can be used to measure CO2 levels in plants, showing a color change from yellow to purple as CO2 is used up during photosynthesis
• Experiments can be set up to investigate the effect of light on net gas exchange in plants using hydrogen carbonate indicator and different light intensities
• Setting up experiments with leaves in tubes with hydrogen carbonate indicator exposed to different light intensities can show the impact of photosynthesis and respiration on gas exchange in plants
• 1. Leaves are broad, maximizing diffusion.
  2. They're thin, minimizing the gas travel distance to cells.
  3. Air spaces inside the leaf aid gas exchange.
  4. The lower surface has stomata for CO2/O2 exchange and transpiration.
  5. Stomata close in darkness to prevent water loss.
  6. They also close when water supply decreases, preventing dehydration.
  7. Stomata opening and closing are controlled by guard cells changing shape and volume.