Welcome to the final installment of the "Aliens and Digestion" mini-series. If you missed the first two posts, you can find them here and here. For this installment, I'm going to talk briefly about aliens whose biochemistry is completely different from ours. For this, I thought of two types of alien races: the silicon-based and pure energy forms. There may be other forms out there, but these two should illustrate how difficult it can be to speculate about something we know so little about.
Silicon-Based Life Forms: If you've ever taken organic chemistry, then you know how versatile the element carbon is. It has special properties that allow it to form the many types of molecules our bodies use. So naturally, if you want to create a new type of biochemistry, you might want to find another element that can take carbon's place. Silicon is part of the same group of elements that carbon belongs to, and it's the next-lightest element in the group after carbon. Although its structure is similar to carbon in some ways, it can't perform all the chemical tricks carbon can. Even though silicon is more common on our planet than carbon (rocks are made with silicon), the fact that life here is carbon-based rather than silicon-based suggests that silicon might be less suitable for life than we'd like. Some types of silicon molecules decompose in water, and if you combine silicon with oxygen, you get a solid instead of a gas like carbon dioxide. This means that a silicon-based life form that tried to breathe oxygen would soon suffocate. In order to have any silicon biochemistry, you'd have to find alternate molecules for water and oxygen. Some people have proposed ammonia and methane as possible alternates for water.
How would a silicon life-form digest its food? I think it would have to rely on physical processes like grinding or dissolving instead of chemical ones, since the chemistry of silicon is much more limited than that of carbon. I'm not sure if silicon could be used to make the equivalent of enzymes.
For more information, check out the following links:
Energy Beings: Although I've listed them (since they're part of popular culture), I find them very difficult to think about scientifically. All life forms that we know about have definitive boundaries. On the cellular level, these boundaries, called membranes, control what flows in and out of the cell. How could you get pure energy to cohere in any reasonable form? How could you keep it from radiating away? Could it keep itself alive if it moved away from an energy source? The only way I can think of an energy being "digesting" something would be for it to convert one type of energy into the type it uses. I have no idea how that would work without access to something material. You can learn more about energy beings at the links below:
I hope you enjoyed this mini-series. I found it fun to put together, and since I'm creating a couple of alien races for my next project, it was useful for me too. I post these science topics because I think science is cool and to demonstrate how to brainstorm ideas and fit them together into a unified whole. No matter what genre you write, that's still a useful skill.