1.1.3  CO2 diffusion to chloroplasts

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Leaves are covered with a barrier or ‘cuticle’ on the outer walls of epidermal cells that is impermeable to both water and CO2. Accordingly, CO2 used in leaf photosynthesis gains entry via stomata (Figure 1.5), and as CO2 molecules diffuse inwards they encounter an opposite flux of H2O molecules rushing outwards that is three to four orders of magnitude stronger. Leaves control this gas exchange by adjusting the aperture of stomata which can vary within minutes in response to changes in several environmental variables including light, humidity and CO2 concentration (see Chapter 15 for more details). Air-spaces inside leaves are effectively saturated with water vapour (equivalent to 100% relative humidity at that leaf temperature) and because air surrounding illuminated leaves is almost universally drier, water molecules diffuse in response to this concentration gradient from leaf to air.

The diffusion pathway for H2O is usually divided into two parts, namely the boundary layer of still air at the leaf surface and stomatal pores (Figure 1.5). Boundary layer thickness depends on windspeed, leaf dimensions and the presence of surface structures (e.g. hairs in Figure 1.1). Positioning of stomata also varies between species. Leaves of terrestrial plants always have stomata on their lower (abaxial) surface but many species have stomata on both surfaces, especially if they have high photosynthetic rates and are in sunny locations such as pendulant leaves of eucalypts. Adaptations for arid environments include having surface structures like hairs and waxes, which increase the thickness of the boundary layer, and leaf rolling and encryption of stomata by placing them in crevices in the leaf surface. While these features restrict water loss, they also impose an increased resistance (decreased conductance) to CO2 uptake.

CO2 molecules diffusing inwards from ambient air to chloroplasts encounter restrictions additional to boundary layer and stomata (Figure 1.5). CO2 must also diffuse from substomatal cavities throughout the mesophyll, dissolve in wet cell walls, enter the cytosol across a plasmalemma, diffuse into chloroplasts across a double membrane (outer envelope in Figure 1.7) and finally reach fixation sites within the stroma of those chloroplasts.

There is considerable variation in leaf anatomy and hence potential restriction to CO2 diffusion, but in general leaves with high rates of photosynthesis tend to have more permeable leaves (e.g. tobacco in Figure 1.2) and this complex anatomy ensures a greatly enlarged surface area for diffusion across interfaces. Indeed the total mesophyll cell wall area can be 20 times that of the projected leaf surface.

Diffusion to chloroplasts is further enhanced by their tendency to appress in clusters against cell walls adjacent to intercellular spaces (Figure 1.2 C, D), while carbonic anhydrase within chloroplasts speeds up diffusion of CO2 by catalysing interconverion of CO2 and bicarbonate within the stroma of chloroplasts. Although CO2 rather than HCO3 is the substrate species for Rubisco, the presence of carbonic anhydrase enables bicarbonate ions, more abundant under the alkaline conditions (pH 8.0) that prevail inside chloroplasts, to diffuse to Rubisco in concert with diffusion of CO2. By sustaining a very rapid equilibration between CO2 and HCO3 immediately adjacent to active sites on Rubisco, carbonic anhydrase enhances inward diffusion of inorganic carbon.

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