A) Schematic representation of the gas exchange system LiCor. B) Basic equation for photosynthesis and transpiration. C) Light curve resulting from plotting rate of CO2 assimilation versus increasing light intensity (irradiance). Note that assimilation is first limited by the amount of light and then by the rate of carboxylation and recycling of the required precursors. Schemes courtesy of Susanne von Caemmerer, The Australian National University.
It is possible to determine rates of CO2 assimilation and water loss (transpiration) by measuring the flux of CO2 and water vapour from a leaf in a sealed chamber. This process, termed gas exchange (because CO2 is going in and water vapour is coming out) is more complicated than might be initially imagined. During photosynthesis, plants take up CO2 (which is converted to sugar) and produce oxygen. All the while they are respiring and releasing CO2 back into the cells. To make matters more complex, the enzyme that fixes CO2 (Rubisco) also ‘fixes’ oxygen, a reaction called photorespiration that releases CO2, but does not produce energy. Gas exchange is also influenced by light levels, because when more light is available, generally, more CO2 can be fixed.
Transpiration is the loss of water from a plant in the form of water vapor. Water is absorbed by roots from the soil and transported as a liquid to the leaves via xylem. In the leaves, small pores allow water to escape as a vapor and CO2 to enter the leaf for photosynthesis. Of all the water absorbed by plants, less than 5% remains in the plant for growth and storage following growth. This lesson will explain why plants lose so much water, the path water takes through plants, how plants might control for too much water loss to avoid stress conditions, and how the environment plays a role in water loss from plants.
At the completion of this lesson, students will be able to:
The Artificial Photosynthesis group is a close collaboration involving five principal investigators whose expertise covers various aspects of chemical science and who pursue the common goal of advancing fundamental knowledge of processes leading to efficient conversion of sunlight to viable chemical fuels. We design and study chemical systems whose reactivity is inspired by natural photosynthesis, in which green plants convert sunlight, water and carbon dioxide into oxygen and carbohydrates. The research efforts of the AP group are mainly funded by the U.S. Department of Energy Office of Science.