In plants, the atmospheric air moves in and out by simple diffusion that takes place through:
(a) the general body surface of the plant
(b) lenticels and
(c) stomata present in the leaves and young stems.
Plants do not need oxygen carrier since oxygen requirement is less and plants have a large surface area (leaves) to absorb the required amount of oxygen through diffusion. From the atmosphere gases enter the intercellular spaces inside the plants. As oxygen is utilized, more of it diffuses into the plant. Since carbon dioxide 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 being used for photosynthesis.
In plants, oxygen released during photosynthesis in day time is made available for respiration. However, rate of photosynthesis is greater than that of respiratin. Thus, plants give out excess oxygen in the daytime. However, they give out only CO2 at night as photosynthesis stops in the absence of sunlight.