External Respiration or Gaseous Exchange
In plants, the atmospheric air moves in and out by simple diffusion that takes place through
- (a) the general body surface of the plant (stems, roots, fruits and seeds)
- (b) lenticels (openings in the bark of the tree trunk
- (c) stomata present in the leaves and young, green parts of the stems
Plants do not need O2 carrier (in contrast to animals where O2 is carried by blood). This is because O2 requirement is less in plants than in animals since the plants have a large surface area (leaves) to absorb the required amount of O2 through diffusion.
From the atmosphere, gases enter the intercellular spaces inside the plants. As O2 is utilized, more of it diffuses into the plant. Since CO2 is being continuously formed, its concentration in tissue spaces becomes higher than in the surrounding air. As a result, it diffuses out of the plant, specially when it is not being used for photosynthesis.
Why during the day, plants give out O2 instead of taking it up for respiration?
In plants, O2 released during photosynthesis in day time is made available for respiration. However, the rate of photosynthesis is higher than that of respiration. Thus, plants give out excess O2 in the daytime. However, these release only CO2 at night as photosynthesis stops in the absence of sunlight. Animals give out CO2 at all times.