Leaves exchange gases mainly through stomata, pores controlled by guard cells. Carbon dioxide diffuses into photosynthesising leaves through air spaces to moist cell surfaces, while oxygen can diffuse out. Gases move through the leaf primarily by diffusion rather than being transported in xylem or phloem. Chloroplast-rich mesophyll cells perform much leaf photosynthesis.
Water evaporates from internal surfaces and water vapour diffuses out, producing transpiration. Opening stomata supports carbon dioxide entry but can increase water loss. A waxy cuticle, reduced exposed surface or regulated stomatal opening can conserve water. Stomatal density is number of stomata divided by measured area. Water-loss rate is change in water quantity divided by elapsed time, with units stated.
Example 1
A 0.5mm2 leaf sample contains 60 stomata. Calculate density.
Density =060.5=120 stomata/mm2.
Example 2
A plant loses 18g of water over 3 hours. Calculate mean loss rate.
318=h6g. This is an average over the stated interval.
Watch out:Carbon dioxide enters mainly through stomata and diffuses through leaf air spaces; xylem transports water and mineral nutrients.
Leaves, stomata and water loss practice | Year 11 Biology | Ascent Arcade