Most of us are taught in school that plants make their food out of sunlight. While this is true in some sense, it is easy to miss what is really going on inside plants. Plants are sometimes depicted as receiving sustenance directly from the sun, just as we do from eating food, but this is not correct. I recently encountered a person who, upon hearing that plants make their food out of sunlight, believed that plants harness Einsteinian physics to convert energy into matter. This is not such a strange thing to believe, given the ubiquity of the “plants eat sunlight” meme. I want to set the record straight in this post.
So what do plants eat? The short answer is that they eat glucose. Glucose is a simple sugar and it is the same thing a lot of other organisms eat for food. What’s different about plants (as well as some bacteria and protists) is that they make their own glucose. When animals want to eat glucose, they need to eat something that is already made of it. When we eat plants, a lot of the nutrition we are getting out of it is the glucose. Unlike animals, plants have the ability to build glucose out of other molecules — carbon dioxide and water. Once plants have built glucose, they can use it as a building material and they can us it as food.
But what is food really? Food is energy stored in chemical bonds. When the chemical bonds are broken, the energy is released and can be used to power different body functions (a few more steps are involved, but this is the short version). Carbon dioxide and water are relatively low-energy molecules. Glucose has much more energy, and turning carbon dioxide and water into glucose requires the addition of energy. Plants harness the sun’s energy to make this happen. Glucose molecules are therefore vaguely analogous to batteries. Breaking down glucose into carbon dioxide and water is like releasing the battery’s charge, and combining them back into glucose is like recharging the battery.
So do plants make their food out of sunlight? In one sense, glucose is made out of carbon dioxide and water, not sunlight. In another sense, the point of making glucose is to capture the energy from the sunlight. Water and carbon dioxide combining to form glucose is the vessel that carries that energy.
What about carnivorous plants, like the pitcher plant, venus fly-trap and sundew? These plants have evolved to trap, kill and digest animals. Carnivorous animals eat other animals for the protein and fat, but this is not the case for carnivorous plants. Plants need carbon dioxide and water to make glucose, and carnivorous plants are no different — but these are not the only molecules that plants need to thrive. Plants need to make proteins and other molecules that cannot be built using only carbon dioxide and water. These require nitrogen, phosphorous, potassium, calcium, and other molecules. Carnivorous plants live in environments where these other nutrients are rare, and it is these nutrients that carnivorous plants are after when they eat animals.
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