Photosynthesis for Gardeners

Photosynthesis isn’t nearly as efficient as you may think.

Leaf

In a simplified nutshell, photosynthesis is a natural process whereby light energy is harvested and converted to chemical energy.

Photo by Adobe Stock/itestro

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The other day, I was having an interesting conversation with a gardener who was marveling at the beauty of photosynthesis. She had inadvertently assumed that the process is highly efficient because it’s a natural one. When I replied that photosynthesis is only as efficient as it needs to be — and noted that energetically speaking, it’s actually highly inefficient — she seemed surprised, even a little doubtful.

In a simplified nutshell, photosynthesis is a natural process whereby light energy is harvested and converted to chemical energy. The process occurs mainly in green plants — which is my prime focus here — within a highly specialized organelle called “chloroplast.” The chloroplast absorbs light of specific wavelengths and converts it to electro-chemical energy. The electro-chemical energy is funneled to specific places within the plant where it helps convert carbon dioxide and water into oxygen and glucose, which is used to build much of what the plant needs. To produce a single molecule of glucose (C6H12O6 ), photosynthesis requires six molecules of carbon dioxide (CO2 ) and six molecules of water (H20). The “waste” products of this process are heat and the leftover oxygen molecules.

Within the chloroplast, a green class of pigments known as “chlorophylls” is the most prevalent and is arguably the most responsible for driving photosynthesis. There are other pigments at play, too. A large class of compounds called “carotenes” and another called “xyanthophylls” are also involved. These so-called "accessory pigments" are critical to light-energy harvest and are responsible for eventually transferring the energy to the chlorophylls that do the more specialized work. These accessory pigments are strongly antioxidant, and they help protect the photosynthetic machinery from getting overstimulated by light, which would destroy them (by bleaching). The accessory pigments and some of their breakdown products are also principally responsible for changing the leaf color when deciduous perennial plants go dormant.

The glucose (formed within the chloroplast) supplies fuel to another type of cellular organelle called the “mitochondria.” Mitochondria consume oxygen while breaking glucose back down into carbon dioxide and water. In the case of plants, the mitochondria are active during the day and they allow the plant to survive through the night and during other periods of low light. The process of burning glucose in this manner is called “respiration” and can be thought of as the reverse of photosynthesis.

Light Reactions and Dark Reactions

A closer look at the photosynthetic reactions reveals two distinct types: Those that are driven by light (so-called “light reactions”), and those that don’t require light (so-called “dark reactions”). During the light reactions, packets of light energy, called "photons," collide with pigment molecules, which make the pigment molecules energetically excited — like when you have that third cup of coffee in the morning and start bouncing around. Molecules don’t like to stay in an excited state for long, and so that excited pigment can pass the energy along to another molecule directly, or by emitting heat or releasing a photon of lower energy light.

Unfortunately, the light energy that hits a pigment molecule can get lost from the system and further complicate things. For example, if the excited pigment molecule reacts with, say, dissolved oxygen, it will become oxidized and more or less destroyed.

When working smoothly, light energy will eventually get transferred from pigment to pigment until it energizes specific molecules that can actually make use of that energy. Sugar is not made during light reactions, but light energy is successfully converted into chemical energy.

During the dark reactions, the chemical energy that was produced by the light reactions is used to combine CO2 molecules to produce sugars that the plant uses structurally and that fuel metabolism through the mitochondria. Additionally, these sugars are used to build starch, cellulose, and other carbon-containing compounds needed by the plant. While there are many nuances to photosynthesis, most green plants do it roughly the same way.

Let There Be Light

Now let’s get back to energy for a minute. The visible light spectrum includes everything from violet to red light and everything in between — blue, green, yellow, orange, etc. But the pigments used to collect photons in photosynthesis are particular about which wavelengths they trap. For most terrestrial plants, the colors of light that most drive photosynthesis are blue and red — that’s why plants appear green. The green light is either reflected away or transmitted through the leaf, and that’s why plants appear green in color. And yet, the leaf encounters all of the light energy wavelengths provided by the sun. So right there, only a fraction of the light that even hits the leaf will be used to drive photosynthesis — some light will pass through, some will be reflected away, some will not be valuable, some will be converted to heat, some will be re-emitted as light, and some will be converted to undesirable chemical reactions. 

Even if you only look at the color wavelengths that are utilized by photosynthesis, a theoretical maximum efficiency of about 26 percent is all that’s possible. In real life, however, plants store much less than 26 percent of the energy — in some estimates only about 3 percent or less. Wow! How can that be?

It turns out that life on Earth, as we know it, would not be possible without what amounts to an endless supply of solar energy to keep things orderly and running. Evolution doesn’t typically yield the most efficient engineering solutions — it yields solutions that are good enough. Next time you look at a green plant and marvel at the magic of photosynthesis, or the flexibility of its stem, or the strength of its thorns, know that those processes and structures are only as good as they need to be given the current environment. What’s good enough today may not cut it tomorrow should the environment change drastically in a short period of time.


Hank Will has an MS in plant physiology and a PhD in pigment biochemistry and genetics. He currently serves as Heirloom Gardener's Editor-in-Chief.