10

key concepts

10.1

Photosynthesis Uses Light to Make Carbohydrates

10.2

Photosynthesis Converts Light Energy into Chemical Energy

10.3

Chemical Energy Trapped in Photosynthesis Is Used to Synthesize Carbohydrates

10.4

Plants Have Adapted Photosynthesis to Environmental Conditions

10.5

Photosynthesis Is an Integral Part of Plant Metabolism

Saving FACE

In July 2014, a satellite was launched into space to measure photosynthesis over all of Earth. The Orbiting Carbon Observatory has instruments aboard that map not only the concentration of carbon dioxide, but also the presence of the green plant pigment chlorophyll. Why these measurements, and why now?

As you may know, green plants use the pigment chlorophyll in photosynthesis, which has the general equation

The orbiting satellite is measuring chlorophyll, a reflection of the presence of green plants, and CO₂ over time because the global atmosphere and biological communities are changing. Over the past 200 years, the concentration of atmospheric CO₂ has increased—from 280 parts per million (ppm) in the year 1800 to 400 ppm in 2016—and the increase will probably continue for some time. Carbon dioxide is a “greenhouse gas” that traps heat in the atmosphere, and the rising CO₂ level is resulting in global climate change. Plant biologists are being asked two questions about the rise in CO₂: will it lead to an increased rate of photosynthesis, and if so, will it lead to increased plant growth?

To answer these questions, scientists developed a way to expose plants to high levels of CO₂ in the field. Free-air concentration enrichment (FACE) uses rings of pipes to release CO₂ into the air surrounding plants in fields or forests. Wind speed and direction are monitored by a computer, which constantly controls which pipes release CO₂. These experiments confirm that photosynthetic rates increase as the concentration of CO₂ rises, and indicate that as atmospheric CO₂ rises globally, there will be an increase in photosynthesis.

Will this increase in photosynthesis result in an increase in plant growth? Keep in mind that plants, like all organisms, use carbohydrates as an energy source. They perform cellular respiration with the general equation

Carbohydrates + O₂ → CO₂ + H₂O + energy

The challenge facing plant biologists is to determine the balance between photosynthesis and respiration and how this affects the rate of plant growth. The FACE experiments indicate that plant growth and crop yields increase under higher CO₂ concentrations, suggesting that the overall increase in photosynthesis is greater than the increase in respiration.

What is the chemistry of photosynthesis, and how will it be affected by rising atmospheric CO₂?