Chapter 2. Carbon Metabolism: Rate of Photosynthesis

General Purpose

This pre-lab will present some of the general concepts related to the measurement of the net rate of photosynthesis at different light intensities by measuring carbon dioxide consumption.

Learning Objectives

General Purpose

Conceptual

Procedural

Background Information

The net amount of CO₂ used by leaf tissue is a function of the rate of CO₂ used by photosynthesis (specifically the Calvin cycle) and the rate of CO₂ generated by cellular respiration. In the presence of light both processes will be at work. Depending on a number of factors the rate of photosynthesis will slow in the dark. It should be noted however that even though the light reactions quickly come to a halt in the absence of light, the Calvin cycle could proceed for some time in the dark because the intracellular ATP and NADPH pools are not immediately depleted.

Photosynthesis rates and the rate of CO₂ used by photosynthesis is directly proportionate to the light available at low light intensities. This linear relationship is the result of the rate of photosynthesis being limited by the rate of the light reactions. Under lower light conditions the pigments in the leaf receive insufficient photons to produce enough ATP and NADPH to sustain maximum photosynthetic rates. At higher light intensities there is less of an increase in photosynthetic rate per unit increase in light intensity. Eventually photosynthesis reaches light saturation at the higher light intensities where light is no longer a limiting factor for the process of photosynthesis.

Changes in the levels of CO₂ for the experiments will be measured using the Qubit CO₂ Analysis laboratory package. This instrument measures CO₂ levels using an infrared gas analyzer (IRGA). The initial CO₂ concentration in the gas provided to the specimen chamber can be measured. The gas can then be pumped through the specimen chamber at a known flow rate and the concentration of CO₂ in the effluent gas from the chamber can be measured using the IRGA. The difference between effluent concentration and the input gas can be used to measure the net photosynthetic rate.

Using LED lamps the amount of light the specimen receives can be controlled and varied. The effect of this varied amount of light on the net rate of photosynthesis (as measured by CO₂ consumption) will be measured.

To prepare for the first exercise of the laboratory, make a copy of Table 8-1 in your laboratory notebook.

Table 8-1. CO₂ consumption rates.

Pre-Lab Quiz

Proceed to the Pre-Lab Quiz