A living cell has certain requirements to function.
It must have a source of fuel. That fuel can be sunlight, carbohydrate, fat or protein (but sometimes it can be odd things like sulfide molecules or metal ions).
It also needs a bag to hold it together. That bag is made of fat molecules in the same arrangement, called a bilipid layer. These fats arrange themselves in such a way that movement of chemicals in and out of the cell is controlled. All cells: plant cells, animal cells, fungus cells, bacteria and archaea have this same arrangement of fats.
The crossing guards for that bag are made of protein, and the machinery inside the cell is also made of protein. Protein is the way that DNA can “do” things. So every cell from every organism is made of fat, protein, carbohydrate and some other small bits.
Proteins can vary widely from organism to organism, but their basic constituents have to be largely the same, because the way these things consume fuel is highly conserved (meaning that all cells use largely the same sets of proteins to do their inside housekeeping).
So protein is a building block of all life, can’t be absent from a non-refined food and is present abundantly in all living cells. Are we clear so far?
Protein is protean. What that means is that protein can take almost any form. It can be the slick substance that makes you slip when you are walking in tidepools or the rough, hard keratin of bristles and nails.
So it is nothing short of bizarre when I am ordering a salad in an eating establishment and the food server asks me which protein I would like to have on said salad.
Protein is present in the leaves, the tomato, the onions and would be present in just about any food that I put on the salad. Protein is not a scarce resource that we are all failing to get. There are no populations of humans on any traditional diet that have any trouble getting protein.
There are people in New Guinea who eat 95% of their calories as carbohydrate and have done so for thousands of years. They are doing fine.
So why are they asking me what protein I want to put on my salad? I think it may have to do with the desire we have to distance ourselves from what we are actually eating. If someone asked you if you wanted a dead chicken, cow, pig or fish on your salad, you probably wouldn’t want it. But when they ask you about protein, you have positive associations with that.
In addition, when I said dead chicken, you imagined a feathered corpse with eyes and a beak and everything. That’s not the sanitized “protein” you get when you get it put on your lettuce leaves.
So the next time I’m asked what protein I want on my salad, I think I will say, “There’s already plenty of protein in the salad, but can I have beans?”