BIOLOGY Volume 1 - A Guide to General Biology - 2004

9. UTILIZATION OF ENERGY

9.6. Gas exchange in flowering plants

Plants expend less energy per unit mass than animals because their metabolic rate is lower. Consequently, they do not require such intensive gas exchange as more complex animals do; air diffusing through intercellular spaces is sufficient. Plants lack any specialized ventilation mechanisms. In flowering plants, gas exchange occurs via diffusion through Stomata on leaves and green stems, as well as through lenticels and cracks in the bark of woody stems. Leaves are thin and have a large surface area, as already discussed in Chapter 7 (Fig. 7.5), so gas exchange takes place primarily through them. In dicotyledonous leaves, efficient diffusion is ensured by the spongy mesophyll with its large intercellular spaces. Fairly large air-filled cavities are also adjacent to the stomata (Fig. 7.5). Since the operation of this entire system relies on diffusion, it is clear that Water can also diffuse out of the plant into the external environment via the same pathway. This presents a certain hazard—excessive water loss causes plants to wilt. Even a mild water deficit can inhibit plant growth (and consequently reduce crop yields in cultivated plants). Plants possess protective mechanisms that enable them to close their stomata when water is scarce. Plant HORMONES, particularly Abscisic acid, are involved in this process.

Within the plant, the distribution of oxygen is determined by diffusion gradients within the air-filled intercellular spaces. Through these pathways, oxygen reaches the Cells and dissolves in the moisture coating The Cell walls. From there, it diffuses further into the cells. CO₂ moves through the plant via the same pathway, but in the opposite direction.

Photosynthesis complicates the situation. Oxygen is produced as a byproduct of this process within the METABOLISM/14.html">Chloroplasts. This oxygen can be used immediately for Respiration by the Mitochondria contained within the same cells. The situation is similar for the CO₂ produced during respiration: chloroplasts can utilize it for photosynthesis.

9.17. a) List (in tabular form) the main differences between photosynthesis and aerobic respiration. b) Outline the similarities between photosynthesis and aerobic respiration (including biochemical ones).