Edexcel IGCSE Biology 复习笔记 2.7.3 Plants: Gas Exchange in Photosynthesis & Respiration

• The processes of respiration and photosynthesis both rely on the exchange of oxygen and carbon dioxide

Gas exchange during respiration

• All living cells respire
• The process requires the uptake of oxygen and the release of carbon dioxide

The respiration equation

• The oxygen diffuses down the concentration gradient from a high concentration (outside the leaf) to a low concentration (inside the leaf)
  • The cells use oxygen in respiration so the concentration is always low inside the respiring cells

   

• The carbon dioxide diffuses down the concentration gradient from a high concentration (inside the leaf) to a low concentration (outside the leaf)

In respiration, carbon dioxide diffuses out of the leaf and oxygen diffuses into the leaf

Gas exchange during photosynthesis

• Plant cells photosynthesize when there is enough sunlight
• The process requires the uptake of carbon dioxide and the release of oxygen

The photosynthesis equation

• The carbon dioxide diffuses down the concentration gradient from a region of high concentration (outside the leaf) to a region of low concentration (inside the leaf)
  • The cells use carbon dioxide in photosynthesis so the concentration is always low inside the photosynthesizing cells

   

• The oxygen diffuses down the concentration gradient from a high concentration (inside the leaf) to a low concentration (outside the leaf)

In photosynthesis, carbon dioxide diffuses into the leaf and oxygen diffuses out of the leaf

• Plants can only photosynthesize when they have access to light, however, cells respire all the time
• This means that gas exchange in plants varies throughout a 24 hour period
• During the daytime plants both respire and photosynthesize
  • The rate of photosynthesis tends to be higher than the rate of respiration (unless there is a low light intensity)
  • Therefore there is net diffusion of carbon dioxide into the plant and net diffusion of oxygen out of the plant during the day

   

• During the nighttime, plants only respire
  • This means that there is a net movement of oxygen into the plant and net diffusion of carbon dioxide out of the plant during the night time

   

• At low light intensities, the rate of photosynthesis is equal to the rate of respiration
  • This means that there is no net movement of oxygen or carbon dioxide in either direction

   

Plants photosynthesize and respire during the day but only respire at night time