I'd never really given this much thought, but how come plants can contain proteins, but animals are a hundred percent noncarbohydrates?

[Update a while later]

A commenter points out I'm mistaken. OK, but that still seems like trace amounts, relative to how much protein that you can get from, say, soy. And when I look at any package of dead animal in the supermarket, it always has zero grams of carbs. So even if it's not a hundred percent, there still seems to be a big disparity. Also, the example given, blood sugar, really part of the animal? I mean, yes, it can't function without it, but it's produced by absorbing food and has to be continually replenished. I was thinking about the animal itself. It seem like, for the most part, structurally, we're meat, fat and bone, not sugar and spice and everything nice. (And does that mean that little girls contain more carbs than little boys, what with the snips and snails and puppy dog tails?)

Uh, because you're wrong? Fauna have large amounts of, eg, glucose and glycogen. Not to mention more complex sugars, glucosaccharides, etc.

(Um, er, "blood sugar"?)

And remember, little boys who drink soy milk grow up to be gay. Its all in those sugar hormones.

As a guess, I'd say because plants don't have to carry their energy stores. Fat is twice as energy dense as carbs, so it's lighter to take with you while hunting the wily flora.

Plants need protein for DNA and other cell nuclear structures to govern growth and functioning.

We don't need to make our own carbos - that's what plants are for! Animals evolved to take advantage of those non-moving sources of food.

Actually, we can and do make carbs (a process called gluconeo-genesis) from non-carb precursors. And glycogen was mentioned above, which is the animal version of starch and is mainly found in the liver and muscle.

Note there is normally no hyphen in that word in my last post, but your system took issue with the stringing together of the 3 letters immediately preceding the hyphen and the 3 letters immediately after. Weird.

I suspect much of the difference is due to the demands being placed on the materials. In plants, carbohydrates are largely static and structural. In animals, proteins are largely used in muscles and enzymes. Proteins have positive and negatively charged side groups that enable them to catalyze specific chemical reactions.

Proteins are also more evolutionarily maleable, since making a new protein is just a matter of changing a single base pair in the DNA, while making a new carbohydrate involves changing an entire synthetic pathway.

Perhaps the question should be, not why do animals use proteins as structural elements, but why don't plants? The reason may be shortage of available nitrogen.

I am a diabetic, so I am well aware that my body is full of carbs, I am fighting to keep them under control. I ingest Metformin daily to control my blood sugar by reducing the glucose production from my liver.

Last time I looked, I was not a plant.

Plants can photosynthesize, so they have a surplus of carbs which they can use for structural purposes etc.

Oh, and one very important additional point: a large class of animals -- insects -- contain lots of carbohydrates. Chitin contains a glucose polymer in which one hydroxyl group in each monomer is replaced by an acetylamine group (this can be embedded in a protein matrix, a kind of natural composite). This would tend to support the point above about nitrogen constraints, which would affect insects less than plants.

Rand, I think you're generalizing too far. You don't eat anything like a representative sampling of plants and animals. You eat very specific parts of each, chosen according to tradition and taste, and you shouldn't really talk as if your sampling is representative of the entire kingdom.

F'r example, when you speak of plants containing lots o' carbs, I'm guessing you're thinking of rice, wheat, oats, et cetera. But here you are only eating the energy-storage organ of the plant, or its seed. If you ate the entire rice or wheat plant (or even the entire kernel, e.g. brown rice instead of white), you'd get lots more protein. You can't compare eating the seeds of plants to eating the muscles of animals, which have a far different purpose. You could compare it to eating the fat of animals, or animal "seeds" (i.e. eggs).

If you make this more reasonable comparison, the difference in protein/carb/fat content is much less, since both plants and animals mostly store energy as fat, oil or complex carbs. Or else you could compare eating animal muscles to eating the equivalent parts of a plants -- their branches and leaves, like celery and cabbage, in which case, it turns out you're eating mostly protein in both cases.

Plants and animals do differ in the kind of fats they use. IIRC, animals use longer hydrocarbon chain fats which are more energy dense but harder to process. The reason, if any, may just be that animals have to move around, so there's a premium on stored energy density that immobile plants don't have to pay. Like comparing the preferred fuels of jet aircraft and oil tankers, in one case it's worth using a more awkward fuel supply if you can pack more energy into a lighter mass of it.

F'r example, when you speak of plants containing lots o' carbs, I'm guessing you're thinking of rice, wheat, oats, et cetera.

He's talking about cellulose and hemicellulose, which are the dominant structural materials in most higher plants.