Photosynthesis for Gardeners

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

| Summer 2017

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.

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