How Photosynthesis Works: Out of Biology Class and Into the Garden

What’s really going on inside your garden plants as they soak up sun and water?

| June 2014

While planting the first crops of spring, you may envision the luscious products of photosynthesis, but you likely won’t ponder the process responsible for your anticipated harvest. Sure, you know that most fruits and vegetables need at least six hours of sunlight each day, preferably more, but understanding the how and why of the sun’s effects on plant physiology can improve your garden’s production — or your over-the-fence garden chats.

Blue Light Special

So let us start with the source — the sun. Sunlight comes in a variety of wavelengths, only some of which are useful to plants — some more useful than others. Plant pigments, located in the chloroplasts of plant cells, absorb energy from wavelengths that make up the visible spectrum of sunlight — red, orange, yellow, green, blue, indigo and violet. The most abundant plant pigment, chlorophyll, absorbs energy from red and blue wavelengths and is responsible for most of the plant’s energy absorption. Most of the green wavelength is reflected rather than absorbed, which is why leaves appear predominantly green. Some pigments pick up other wavelengths, including green, to enhance energy absorption. Come fall, when the chlorophyll begins to degrade, these other pigments shine through, absorbing the green wavelengths and reflecting the yellows, reds and oranges we “ooh and ahh” over. Chlorophyll’s affinity for red and blue wavelengths is one of the reasons growers use fluorescent lights for starting plants indoors. Your everyday incandescent light produces plenty of the red and infrared (heat) wavelengths, but fluorescent lights produce more blue wavelengths, which increase vegetative growth and are cooler. LED (light emitting diode) lights, however, are all the rage. LEDs are not only more energy-efficient, they can also be tuned to specific wavelengths — customized sunlight for each crop! In fact, designers of large, futuristic urban and suburban farms speak of glowing pink towers where growers tend plants indoors under an exclusive LED mix of blue and red light.

Whether from fancy fuchsia LEDs or the sun, what happens when light falls on leaves? The packets of energy, or photons, that make up wavelengths of light excite electrons in the chlorophyll molecule, which kicks off a fascinating — if complex — chain of events that results in a simple sugar and our ability to breathe.

Here is the basic biology class formula for how photosynthesis works:

6H2O (water) + 6CO2 (carbon dioxide) → sunlight → C6H12O6 (sugar) + 6O2 (oxygen)

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