Mini Lesson/Activity

Evaluating Plants as Energy Stores

Mini Lesson

Plants capture and store solar energy through photosynthesis. During photosynthesis, living plants convert carbon dioxide in the air into sugar molecules they use for food. In the process of making their own food, plants also provide the oxygen we need to breathe. Thus, plants provide the energy and air required by most life forms on Earth.

Did you know that you can estimate the amount of energy that the plants absorb for any given location on Earth using NASA data? We call that the "energy efficiency" of photosynthesis.  This is the ratio of the amount of energy stored to the amount energy of light absorbed.

You can use these data to evaluate and model photosynthesis efficiency.

Energy per square meter of plants per second = Flux (flow of energy) * Leaf Area Index* Photosynthesis efficiency

Check out this example:

  • Flux (flow of energy) = 95 W/m^2 (link to dataset)
    • This quantity describes the total flow of energy from the Atmosphere to the surface by shortwave radiation, which is the visible light coming from the Sun. This is the flow of energy that warms the Earth's surface during the daytime. Clouds affect the rate of this energy flow, and the rate also varies with the seasons (higher rate in summer and lower rate in winter). These data have a grid spacing of 1 degree longitude and 1 degree latitude.
    • UNITS: The units of these data are Watts per square meter, which is the flow of energy spread out over an area. Five Watts per square meter is equivalent to the power used by a standard charger (5 Watts) passing through a square piece of paper with length and width of 1 meter.
  • Leaf Area Index (unitless value)= 0.86 (link to dataset)
    • This quantity measures the amount of leaves present at each location on the map relative to the total land area at that location. Values less than one mean that part of the location covered with leafy plants. Values greater than one mean that the location is covered with several layers of leaves, as in a forest canopy. The leaf area index is directly related to the health and primary productivity of plants, as plants are more productive with more leaves. These data have a grid spacing of 0.5 degrees longitude and 0.5 degrees latitude.
    • UNITS: The leaf area index is a ratio, so it has no units (dimensionless).
  • Photosynthesis Efficiency = 0.046 (value estimated by scientists for a typical plant)

Energy taken in by plants per second, per square meter of ground covered by plants = 95 (W/m^2) * 0.86 * 0.046 = 3.76 W for every square meter of ground.

This means that a plant that covers 1 square meter of ground gets 3.76 J of energy every second from photosynthesis. For reference, most cell phone chargers use about 5 J or energy every second (5 W).

There are other factors that affect the growth of plants, too.  These include:

  • Carbon dioxide - The concentration of this atmospheric gas is measured in parts per million.  Currently, the concentration of carbon dioxide in Earth's atmosphere is 400 parts per million. In general, the higher the concentration of carbon dioxide, the more efficiently the plant gains energy by photosynthesis. 
  • Freshwater - Received through rain and/or irrigation methods. Water is required for photosynthesis to take place.
  • Nutrients- These nutrients include potassium, nitrogen, and phosphorous found in the soil. In general, the higher the amount of nutrients available, the more efficiently the plant gains energy by photosynthesis.
  • Temperatures - Photosynthesis is more efficient at higher temperatures. Temperatures must be above freezing, since plants need liquid water to use photosynthesis.

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