Plants take water, metals (calcium, iron, potassium, manganese, molybdenum, and zinc), phosphorous, and nitrogen from the soil with their roots. Plant roots release an acid into the soil which helps dissolve the metals so they can absorb it.
Plants take carbon from the air, and use sunlight to help with this.
Air is mostly nitrogen (78% nitrogen, 21% oxygen) but it’s very hard for a plant to take nitrogen from the air.
Except for water, these other compounds don’t exist in air, and are important for food.
Nitrogen is very important for all life, and it’s a major part of DNA.
Some plants, especially legumes (beans, peas) have bacteria on their seeds and roots that can take nitrogen from the air and give it to a plant. As a result beans and other legumes are commonly farmed as the bacteria on their roots fertilize the soil.
Nitrogen also happens to be extremely explosive, and it’s an important part of gun powder and explosives.
During WWI, the Nazis were unable to gen enough gun powder and nitrogen, so a German Scientist, Frits Haber, designed a machine that could take Nitrogen from the air using electricity. Today this machine is used to produce fertilizer, and it allowed farmers to produce more food than ever allowing the world’s population to expand.
Metabolism – the ability to turn all these basic into new things is an important part of life. It’s easier for scientists to use things like algae or plants to take sunlight, water, and carbon from the air and turn these into food and oil.
We eat not only for nutrition. Food is very much a part of our culture and social behavior. This isn’t easy to change. If all that was needed was nutrition and not flavor, texture etc we can grow nutrient rich food like algae much faster and using much less land, water and pesticides. Essentially we don’t need to create a machine – nature has created a perfectly adequate biological factory.
The reason we don’t do this is most people would find eating green mush rather unexciting and the cost of trying to make that green mush something that looks and tastes good would cost a lot.
Plants don’t produce food from water and sunlight. They require a variety of nutrients and such, plus CO2 and oxygen.
That aside, humans evolved to eat large and complex organic molecules. In fact, we evolved to eat the molecules produced by plants. A machine to produce these molecules would require a fair amount of energy and complex many-step chemistry. Why bother?
The problem is that water and CO2(and other nutrients, but the point still stands) can be combined in different ways, which leads to different products.
As an example, both cellulose(plant fibers) and starch are made of glucose chains. The difference is simply the way the individual sugar molecules are linked together.
So we’d need to manipulate the ressources on a molecular level. This *can* be done, but we’d essentially have to build artificial plant cells. And at that point, we might as well just use plants.
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