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According to Dr. Curt Stager of Paul Smith’s College, the first step in determining whether or not plants have blood is to define ‘blood.’ Though plants don’t have the red blood that humans do, they contain green chlorophyll in their leaves. The structure of a chlorophyll molecule is very similar to a hemoglobin molecule, which makes human blood red.
“The main difference would be that there’s a metal atom in the middle of each of those molecules, and in the case of plant chlorophyll, it’s magnesium that gives it a green color. In our case, it’s a red color,” said Stager. The molecules do different things in plants and humans. Chlorophyll’s job is to trap sunlight, and hemoglobin’s job is to trap or let go of oxygen.
Another purpose of blood is to transport fluids, such as water, around the body. Sap in trees functions in the same way that human blood does in this respect. A key difference is that human blood circulates, while sap goes into leaves and then evaporates out. Stager said, “Another thing blood will do is move food around. Like when we eat our food, blood comes from our digestive system and gets pumped around to feed out cells. And you could say plant sap does that too.”
“Actually, speaking of the hemoglobin thing, there are some plants that have basically a type of hemoglobin,” said Stager. He recently discovered this when one of his colleagues was preparing a lab to show nitrogen-fixing bacteria in the nodules of legume plants such as beans, alfalfa and clover. The nodules look like little lumps, and the plants make them to encourage soil bacteria to live there so that it will create nitrogen and fertilize the plant. Stager said, “I looked, and one of them was red, like bright, bloody-looking red! She had cut one open and it almost looked like meat. ”
The nodules contain a kind of hemoglobin that is related to the hemoglobin found in human blood. “The reason is kind of the same reason we have hemoglobin in our blood: to bind oxygen,” said Stager. “If the bacteria are going to do their job of making this nitrogen fertilizer that the plant wants, one of the problems the bacteria face is oxygen in the soil spaces will destroy their cellular machinery that they need to make the nitrogen fertilizer.”
The hemoglobin grabs the oxygen and hold onto it to prevent it from damaging the bacteria.